Dictionary of Chemistry [6th Ed.]

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3

A Dictionary of

Chemistry

SIXTH EDITION

Edited by

JOHN DAINTITH


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Preface viiCredits viii

Dictionary 1

Atomic Theory Chronology 49

Biochemistry Chronology 70

Crystal Defects (Feature) 152

Explosives Chronology 217

Plastics Chronology 422

Polymers (Feature) 430

Appendices

The Greek alphabet 569

Fundamental constants 569

SI units 570

The electromagnetic spectrum 572

The periodic table 573

The chemical elements 574

Nobel prizes in chemistry 576

Useful websites 583

Contents


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Preface

This dictionary was originally derived from the Concise Science Dictionary, firstpublished by Oxford University Press in 1984 (fifth edition, retitled Dictionaryof Science, 2005). It consisted of all the entries relating to chemistry in thisdictionary, including physical chemistry, as well as many of the terms used inbiochemistry. Subsequent editions included special feature articles onimportant topics as well as several chronologies tracing the history of sometopics and short biographical entries on the chemists and other scientistswho have been responsible for the development of the subject. For this sixthedition the text has been fully revised and some entries have beensubstantially expanded. In addition over 350 new entries have been addedcovering all branches of the subject. The coverage of certain fields, inparticular biochemistry, forensic chemistry, and chemoinformatics, has beenexpanded. A further improvement has been the inclusion of about 90additional chemical structures.

An asterisk placed before a word used in an entry indicates that this word canbe looked up in the dictionary and will provide further explanation orclarification. However, not every word that appears in the dictionary has anasterisk placed before it. Some entries simply refer the reader to anotherentry, indicating either that they are synonyms or abbreviations or that theyare most conveniently explained in one of the dictionary’s longer articles orfeatures. Synonyms and abbreviations are usually placed within bracketsimmediately after the headword. Terms that are explained within an entryare highlighted by being printed in boldface type.

The more physical aspects of physical chemistry and the physics itself will befound in A Dictionary of Physics, which is a companion volume to thisdictionary. A Dictionary of Biology contains a more thorough coverage of thebiophysical and biochemical entries from the Dictionary of Science togetherwith the entries relating to biology.

SI units are used throughout this book and its companion volumes.J.D.

2007


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A

AAR See amino acid racemization.

AAS See atomic absorption spec-troscopy.

abherent See release agent.

ab-initio calculation A method ofcalculating atomic and molecularstructure directly from the Ürst prin-ciples of quantum mechanics, with-out using quantities derived fromexperiment (such as ionization ener-gies found by spectroscopy) as para-meters. Ab-initio calculations requirea large amount of numerical compu-tation; the amount of computingtime required increases rapidly asthe size of the atom or molecule in-creases. The development of comput-ing power has enabled the propertiesof both small and large molecules tobe calculated accurately, so that thisform of calculation can now replace*semi-empirical calculations. Ab-initio calculations can, for example,be used to determine the bondlengths and bond angles of moleculesby calculating the total energy of themolecule for a variety of moleculargeometries and Ünding which confor-mation has the lowest energy.

absolute 1. Not dependent on orrelative to anything else, e.g. *ab-solute zero. 2. Denoting a tempera-ture measured on an absolute scale,a scale of temperature based on ab-solute zero. The usual absolute scalenow is that of thermodynamic *tem-perature; its unit, the kelvin, was for-merly called the degree absolute (°A)and is the same size as the degreeCelsius. In British engineering prac-tice an absolute scale with Fahren-

heit-size degrees has been used: thisis the Rankine scale.

absolute alcohol See ethanol.

absolute conÜguration A way ofdenoting the absolute structure of anoptical isomer (see optical activity).Two conventions are in use: The d–lconvention relates the structure ofthe molecule to some reference mol-ecule. In the case of sugars and simi-lar compounds, the dextrorotatoryform of glyceraldehyde(HOCH2CH(OH)CHO), 2,3-dihydroxy-propanal) was used. The rule is asfollows. Write the structure of thismolecule down with the asymmetriccarbon in the centre, the –CHOgroup at the top, the –OH on theright, the –CH2OH at the bottom, andthe –H on the left. Now imagine thatthe central carbon atom is at the cen-tre of a tetrahedron with the fourgroups at the corners and that the –Hand –OH come out of the paper andthe –CHO and –CH2OH groups gointo the paper. The resulting three-dimensional structure was taken tobe that of d-glyceraldehyde andcalled d-glyceraldehyde. Any com-pound that contains an asymmetriccarbon atom having this conÜgura-tion belongs to the d-series. One hav-ing the opposite conÜgurationbelongs to the l-series. It is importantto note that the preÜxes d- and l- donot stand for dextrorotatory andlaevorotatory (i.e. they are not thesame as d- and l-). In fact the arbitraryconÜguration assigned to d-glycer-aldehyde is now known to be the cor-rect one for the dextrorotatory form,although this was not known at the


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time. However, all d-compounds arenot dextrorotatory. For instance, theacid obtained by oxidizing the –CHOgroup of glyceraldehyde is glycericacid (1,2-dihydroxypropanoic acid).By convention, this belongs to the d-series, but it is in fact laevorotatory;i.e. its name can be written as d-glyceric acid or l-glyceric acid. Toavoid confusion it is better to use +(for dextrorotatory) and – (for laevo-rotatory), as in d-(+)-glyceraldehydeand d-(–)-glyceric acid.

The d–l convention can also beused with alpha amino acids (com-pounds with the –NH2 group on thesame carbon as the –COOH group). Inthis case the molecule is imagined as

being viewed along the H–C bond be-tween the hydrogen and the asym-metric carbon atom. If the clockwiseorder of the other three groups is–COOH, –R, –NH2, the amino acid be-longs to the d-series; otherwise it be-longs to the l-series. This is known asthe CORN rule.

The r–s convention is a conventionbased on priority of groups attachedto the chiral carbon atom. The orderof priority is I, Br, Cl, SO3H, OCOCH3,OCH3, OH, NO2, NH2, COOCH3,CONH2, COCH3, CHO, CH2OH, C6H5,C2H5, CH3, H, with hydrogen lowest.The molecule is viewed with thegroup of lowest priority behind thechiral atom. If the clockwise arrange-

absolute configuration 2

a

!"#$ $

%&" !$' '"

Absolute configuration


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ment of the other three groups is indescending priority, the compoundbelongs to the r-series; if the de-scending order is anticlockwise it isin the s-series. d-(+)-glyceraldehyde isr-(+)-glyceraldehyde. See illustration.

absolute temperature See ab-solute; temperature.

absolute zero Zero of thermody-namic *temperature (0 kelvin) andthe lowest temperature theoreticallyattainable. It is the temperature atwhich the kinetic energy of atomsand molecules is minimal. It is equiv-alent to –273.15°C or –459.67°F. Seealso zero-point energy.

absorption 1. (in chemistry) Thetake up of a gas by a solid or liquid,or the take up of a liquid by a solid.Absorption differs from adsorptionin that the absorbed substance per-meates the bulk of the absorbingsubstance. 2. (in physics) The conver-sion of the energy of electromagneticradiation, sound, streams of particles,etc., into other forms of energy onpassing through a medium. A beamof light, for instance, passingthrough a medium, may lose inten-sity because of two effects: scatteringof light out of the beam, and absorp-tion of photons by atoms or mol-ecules in the medium. When aphoton is absorbed, there is a transi-tion to an excited state.

absorption coefÜcient 1. (inspectroscopy) The molar absorptioncoefÜcient (symbol ε) is a quantitythat characterizes the absorption oflight (or any other type of electro-magnetic radiation) as it passesthrough a sample of the absorbingmaterial. It has the dimensions of1/(concentration × length). ε is de-pendent on the frequency of the inci-dent light; its highest value occurswhere the absorption is most in-tense. Since absorption bands usually

spread over a range of values of thefrequency ν it is useful to deÜne aquantity called the integrated ab-sorption coefÜcient, A, which is theintegral of all the absorption coefÜ-cients in the band, i.e. A = ∫ε(ν)dν.This quantity characterizes the inten-sity of a transition. It was formerlycalled the extinction coefÜcient. Seealso beer–lambert law. 2. The vol-ume of a given gas, measured at stan-dard temperature and pressure, thatwill dissolve in unit volume of agiven liquid.

absorption indicator See adsorp-tion indicator.

absorption spectrum See spec-trum.

absorption tower A long verticalcolumn used in industry for absorb-ing gases. The gas is introduced atthe bottom of the column and theabsorbing liquid, often water, passesin at the top and falls down againstthe countercurrent of gas. The tow-ers are also known as scrubbers.

ABS plastic Any of a class ofplastics based on acrylonitrile–butadiene–styrene copolymers.

abstraction A chemical reactionthat involves bimolecular removal ofan atom or ion from a molecule. Anexample is the abstraction of hydro-gen from methane by reaction with aradical:

CH4 + X. → H3C. + HX.

abundance 1. The ratio of the totalmass of a speciÜed element in theearth’s crust to the total mass of theearth’s crust, often expressed as apercentage. For example, the abun-dance of aluminium in the earth’scrust is about 8%. 2. The ratio of thenumber of atoms of a particular iso-tope of an element to the total num-ber of atoms of all the isotopespresent, often expressed as a percent-

3 abundance

a


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age. For example, the abundance ofuranium-235 in natural uranium is0.71%. This is the natural abundance,i.e. the abundance as found in naturebefore any enrichment has takenplace.

ac Anticlinal. See torsion angle.

acac The symbol for the *acetyl-acetonato ligand, used in formulae.

accelerant A Ûammable materialused to start and spread a Üre incases of arson. Petrol and parafÜn arethe substances commonly used.Traces of accelerant are detectable bygas chromatography in forensicwork.

accelerator A substance that in-creases the rate of a chemical reac-tion, i.e. a catalyst.

acceptor 1. (in chemistry and bio-chemistry) A compound, molecule,ion, etc., to which electrons aredonated in the formation of a co-ordinate bond. 2. (in physics) A sub-stance that is added as an impurity toa *semiconductor because of its abil-ity to accept electrons from the va-lence bands, causing p-typeconduction by the mobile positiveholes left. Compare donor.

accessory pigment A *photosyn-thetic pigment that traps light en-ergy and channels it to chlorophyll a,the primary pigment, which initiatesthe reactions of photosynthesis. Ac-cessory pigments include thecarotenes and chlorophylls b, c, andd.

accumulator (secondary cell; stor-age battery) A type of *voltaic cellor battery that can be recharged bypassing a current through it from anexternal d.c. supply. The chargingcurrent, which is passed in the oppo-site direction to that in which thecell supplies current, reverses thechemical reactions in the cell. The

common types are the *lead–acid ac-cumulator and the *nickel–iron andnickel–cadmium accumulators. Seealso sodium–sulphur cell.

acenaphthene A colourless crys-talline aromatic compound, C12H10;m.p. 95°C; b.p. 278°C. It is an inter-mediate in the production of somedyes.

acetaldehyde See ethanal.

acetaldol See aldol reaction.

acetals Organic compounds formedby addition of alcohol molecules toaldehyde molecules. If one moleculeof aldehyde (RCHO) reacts with onemolecule of alcohol (R1OH) a hemiac-etal is formed (RCH(OH)OR1). Therings of aldose sugars are hemiac-etals. Further reaction with a secondalcohol molecule produces a full ac-etal (RCH(OR1)2). It is common torefer to both types of compound sim-ply as ‘acetals’. The formation of ac-etals is reversible; acetals can behydrolysed back to aldehydes inacidic solutions. In synthetic organicchemistry aldehyde groups are often

ac 4

a

1 2

Acenaphthene

Acetals


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converted into acetal groups to pro-tect them before performing otherreactions on different groups in themolecule. See also ketals.

A• Information about IUPAC nomenclature

acetamide See ethanamide.

acetanilide A white crystalline pri-mary amide of ethanoic acid,CH3CONHC6H5; r.d. 1.2; m.p. 114.3°C;b.p. 304°C. It is made by reactingphenylamine (aniline) with excessethanoic acid or ethanoic anhydrideand is used in the manufacture ofdyestuffs and rubber. The full sys-tematic name is N-phenyl-ethanamide.

acetate See ethanoate.

acetate process See rayon.

acetic acid See ethanoic acid.

acetic anhydride See ethanoicanhydride.

acetoacetic acid See 3-oxobu-tanoic acid.

acetoacetic ester See ethyl 3-oxobutanoate.

acetone See propanone.

acetone–chlor–haemin test (Wagenaar test) A *presumptive testfor blood in which a small amount ofacetone (propenal) is added to thebloodstain, followed by a drop of hy-drochloric acid. Haemoglobin pro-duces derivatives such as haematinand haemin, forming small charac-teristic crystals that can be identiÜedunder a microscope.

acetonitrile See ethanenitrile.

acetophenone See phenyl methylketone.

acetylacetonato The ion(CH3COCHCOCH3)–, functioning as abidentate ligand coordinating

through the two oxygen atoms. Informulae, the symbol acac is used.

acetylating agent See ethanoy-lating agent.

acetylation See acylation.

acetyl chloride See ethanoylchloride.

acetylcholine A substance that isreleased at some (cholinergic) nerveendings. Its function is to pass on anerve impulse to the next nerve (i.e.at a synapse) or to initiate muscularcontraction. Once acetylcholine hasbeen released, it has only a transitoryeffect because it is rapidly brokendown by the enzyme cholinesterase.

5 Acheson process

a

CH3 OCH2

CH2

NH CH3

O CH3 CH3+

Acetylcholine

acetyl coenzyme A (acetyl CoA) Acompound formed in the mitochon-dria when an acetyl group (CH3CO–),derived from the breakdown of fats,proteins, or carbohydrates (via *gly-colysis), combines with the thiolgroup (–SH) of *coenzyme A. AcetylCoA feeds into the energy generating*Kreb’s cycle and also plays a role inthe synthesis and oxidation of fattyacids.

acetylene See ethyne.

acetylenes See alkynes.

acetyl group See ethanoyl group.

acetylide See carbide.

Acheson process An industrialprocess for the manufacture ofgraphite by heating coke mixed withclay. The reaction involves the pro-duction of silicon carbide, whichloses silicon at 4150°C to leave


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graphite. The process was patentedin 1896 by the US inventor EdwardGoodrich Acheson (1856–1931).

achiral Describing a molecule thatdoes not contain a *chirality el-ement.

acid 1. A type of compound thatcontains hydrogen and dissociates inwater to produce positive hydrogenions. The reaction, for an acid HX, iscommonly written:

HX ˆ H+ + X–

In fact, the hydrogen ion (the proton)is solvated, and the complete reac-tion is:

HX + H2O ˆ H3O+ + X–

The ion H3O+ is the oxonium ion (orhydroxonium ion or hydronium ion).This deÜnition of acids comes fromthe Arrhenius theory. Such acids tendto be corrosive substances with asharp taste, which turn litmus redand give colour changes with other*indicators. They are referred to asprotonic acids and are classiÜed intostrong acids, which are almost com-pletely dissociated in water (e.g. sul-phuric acid and hydrochloric acid),and weak acids, which are only par-tially dissociated (e.g. ethanoic acidand hydrogen sulphide). The strengthof an acid depends on the extent towhich it dissociates, and is measuredby its *dissociation constant. See alsobase.2. In the Lowry–Brønsted theory ofacids and bases (1923), the deÜnitionwas extended to one in which anacid is a proton donor (a Brønstedacid), and a base is a proton acceptor(a Brønsted base). For example, in

HCN + H2O ˆ H3O+ + CN–

the HCN is an acid, in that it donatesa proton to H2O. The H2O is acting asa base in accepting a proton. Simi-larly, in the reverse reaction H3O+ isan acid and CN– a base. In such reac-

tions, two species related by loss orgain of a proton are said to be conju-gate. Thus, in the reaction aboveHCN is the conjugate acid of the baseCN–, and CN– is the conjugate base ofthe acid HCN. Similarly, H3O+ is theconjugate acid of the base H2O. Anequilibrium, such as that above, is acompetition for protons between anacid and its conjugate base. A strongacid has a weak conjugate base, andvice versa. Under this deÜnitionwater can act as both acid and base.Thus in

NH3 + H2O ˆ NH4+ + OH–

the H2O is the conjugate acid of OH–.The deÜnition also extends the ideaof acid–base reaction to solventsother than water. For instance, liquidammonia, like water, has a high di-electric constant and is a good ioniz-ing solvent. Equilibria of the type

NH3 + Na+Cl– ˆ Na+NH2– + HCl

can be studied, in which NH3 andHCl are acids and NH2

– and Cl– aretheir conjugate bases.3. A further extension of the idea ofacids and bases was made in theLewis theory (G. N. Lewis, 1923). Inthis, a Lewis acid is a compound oratom that can accept a pair of elec-trons and a Lewis base is one thatcan donate an electron pair. ThisdeÜnition encompasses ‘traditional’acid–base reactions. In

HCl + NaOH → NaCl + H2Othe reaction is essentially

H+ + :OH– → H:OHi.e. donation of an electron pair byOH–. But it also includes reactionsthat do not involve ions, e.g.

H3N: + BCl3 → H3NBCl3in which NH3 is the base (donor) andBCl3 the acid (acceptor). The Lewistheory establishes a relationship be-tween acid–base reactions and *oxi-

achiral 6

a


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dation–reduction reactions. See hsabprinciple.

See also aqua acid; hydroxoacid;oxoacid.

acid anhydrides (acyl anhydrides)Compounds that react with water toform an acid. For example, carbondioxide reacts with water to give car-bonic acid:

CO2(g) + H2O(aq) ˆ H2CO3(aq)A particular group of acid anhydridesare anhydrides of carboxylic acids.They have a general formula of thetype R.CO.O.CO.R′, where R and R′are alkyl or aryl groups. For example,the compound ethanoic anhydride(CH3.CO.O.CO.CH3) is the acid anhy-dride of ethanoic (acetic) acid. Or-ganic acid anhydrides can beproduced by dehydrating acids (ormixtures of acids). They are usuallymade by reacting an acyl halide withthe sodium salt of the acid. Theyreact readily with water, alcohols,phenols, and amines and are used in*acylation reactions.A• Information about IUPAC nomenclature

7 acid rain

a

R

O

OH

R

O

OH

R

O

R

O

O

Acid anhydride

acid–base indicator Seeindicator.

acid dissociation constant Seedissociation.

acid dye See dyes.

acid halides See acyl halides.

acidic 1. Describing a compound

that is an acid. 2. Describing a solu-tion that has an excess of hydrogenions. 3. Describing a compound thatforms an acid when dissolved inwater. Carbon dioxide, for example,is an acidic oxide.

acidic hydrogen (acid hydrogen) Ahydrogen atom in an *acid thatforms a positive ion when the aciddissociates. For instance, inmethanoic acid

HCOOH ˆ H+ + HCOO–

the hydrogen atom on the carboxy-late group is the acidic hydrogen (theone bound directly to the carbonatom does not dissociate).

acidimetry Volumetric analysisusing standard solutions of acids todetermine the amount of base pre-sent.

acidity constant See dissociation.

acid rain Precipitation having a pHvalue of less than about 5.0, whichhas adverse effects on the fauna andÛora on which it falls. Rainwater typ-ically has a pH value of 5.6, due tothe presence of dissolved carbondioxide (forming carbonic acid). Acidrain results from the emission intothe atmosphere of various pollutantgases, in particular sulphur dioxideand various oxides of nitrogen,which originate from the burning offossil fuels and from car exhaustfumes, respectively. These gases dis-solve in atmospheric water to formsulphuric and nitric acids in rain,snow, or hail (wet deposition). Alter-natively, the pollutants are depositedas gases or minute particles (dry de-position). Both types of acid deposi-tion affect plant growth – bydamaging the leaves and impairingphotosynthesis and by increasing theacidity of the soil, which results inthe leaching of essential nutrients.This acid pollution of the soil also


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leads to acidiÜcation of water drain-ing from the soil into lakes andrivers, which become unable to sup-port Üsh life. Lichens are particularlysensitive to changes in pH and can beused as indicators of acid pollution.

acid salt A salt of a polybasic acid(i.e. an acid having two or moreacidic hydrogens) in which not allthe hydrogen atoms have been re-placed by positive ions. For example,the dibasic acid carbonic acid (H2CO3)forms acid salts (hydrogencarbonates)containing the ion HCO3

–. Some saltsof monobasic acids are also known asacid salts. For instance, the com-pound potassium hydrogendiÛuoride,KHF2, contains the ion [F...H–F]–, inwhich there is hydrogen bonding be-tween the Ûuoride ion F– and a hy-drogen Ûuoride molecule.

acid value A measure of theamount of free acid present in a fat,equal to the number of milligrams ofpotassium hydroxide needed to neu-tralize this acid. Fresh fats containglycerides of fatty acids and very lit-tle free acid, but the glycerides de-compose slowly with time and theacid value increases.

acridine A colourless crystallineheterocyclic compound, C12H9N; m.p.110°C. The ring structure is similarto that of anthracene, with threefused rings, the centre ring contain-ing a nitrogen heteroatom. Severalderivatives of acridine (such as acri-dine orange) are used as dyes or bio-logical stains.

acid salt 8

a

N

Acridine

Acrilan A tradename for a syntheticÜbre. See acrylic resins.

acrolein See propenal.

acrylamide An inert gel (polyacry-lamide) employed as a medium in*electrophoresis. It is used particu-larly in the separation of macromole-cules, such as nucleic acids andproteins.

acrylate See propenoate.

acrylic acid See propenoic acid.

acrylic resins Synthetic resinsmade by polymerizing esters or otherderivatives of acrylic acid (propenoicacid). Examples are poly(propenoni-trile) (e.g. Acrilan), and poly(methyl 2-methylpropenoate) (polymethylmethacrylate, e.g. Perspex).

acrylonitrile See propenonitrile.

ACT See activated-complex theory.

actinic radiation Electromagneticradiation that is capable of initiatinga chemical reaction. The term is usedespecially of ultraviolet radiation andalso to denote radiation that will af-fect a photographic emulsion.

actinides See actinoids.

actinium Symbol Ac. A silveryradioactive metallic element belong-ing to group 3 (formerly IIIA) of theperiodic table; a.n. 89; mass numberof most stable isotope 227 (half-life21.7 years); m.p. 1050 ± 50°C; b.p.3200°C (estimated). Actinium–227 oc-curs in natural uranium to an extentof about 0.715%. Actinium–228 (half-life 6.13 hours) also occurs in nature.There are 22 other artiÜcial isotopes,all radioactive and all with very shorthalf-lives. Its chemistry is similar tothat of lanthanum. Its main use is asa source of alpha particles. The el-ement was discovered by A. Debiernein 1899.

A• Information from the WebElements site


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actinium series See radioactiveseries.

actinoid contraction A smoothdecrease in atomic or ionic radiuswith increasing proton numberfound in the *actinoids.

actinoids (actinides) A series of el-ements in the *periodic table, gener-ally considered to range in atomicnumber from thorium (90) to lawren-cium (103) inclusive. The actinoids allhave two outer s-electrons (a 7s2

conÜguration), follow actinium, andare classiÜed together by the fact thatincreasing proton number corre-sponds to Ülling of the 5f level. Infact, because the 5f and 6d levels areclose in energy the Ülling of the 5f or-bitals is not smooth. The outer elec-tron conÜgurations are as follows:89 actinium (Ac) 6d17s2

90 thorium (Th) 6d27s2

91 protactinium (Pa) 5f26d17s2

92 uranium (Ur) 5f36d7s2

93 neptunium (Np) 5f57s2 (or5f46d17s2)94 plutonium (Pu) 5f67s2

95 americium (Am) 5f77s2

96 curium (Cm) 5f76d1s2

97 berkelium (Bk) 5f86d7s2 (or 5f97s2)98 californium (Cf) 5f107s2

99 einsteinium (Es) 5f117s2

100 fermium (Fm) 5f127s2

101 mendelevium (Md) 5f137s2

102 nobelium (Nb) 5f147s2

103 lawrencium (Lw) 5f146d1s2

The Ürst four members (Ac to Ur)occur naturally. All are radioactiveand this makes investigation difÜcultbecause of self-heating, short life-times, safety precautions, etc. Likethe *lanthanoids, the actinoids showa smooth decrease in atomic andionic radius with increasing protonnumber. The lighter members of theseries (up to americium) have f-elec-trons that can participate in bonding,unlike the lanthanoids. Conse-quently, these elements resemble the

transition metals in forming coordi-nation complexes and displayingvariable valency. As a result of in-creased nuclear charge, the heaviermembers (curium to lawrencium)tend not to use their inner f-electronsin forming bonds and resemble thelanthanoids in forming compoundscontaining the M3+ ion. The reasonfor this is pulling of these inner elec-trons towards the centre of the atomby the increased nuclear charge.Note that actinium itself does nothave a 5f electron, but it is usuallyclassiÜed with the actinoids becauseof its chemical similarities. See alsotransition elements.

actinometer See actinometry.

actinometry The measurement ofthe intensity of electromagnetic radi-ation. An instrument that measuresthis quantity is called an actinometer.Recent actinometers use the *photo-electric effect but earlier instrumentsdepended either on the Ûuorescenceproduced by the radiation on ascreen or on the amount of chemicalchange induced in some suitable sub-stance. Different types of actinome-ter have different names according tothe type of radiation they measure. Apyroheliometer measures the inten-sity of radiation from the sun. Apyranometer measures the intensityof radiation that reaches the surfaceof the earth after being scattered bymolecules or objects suspended inthe atmosphere. A pyrogeometermeasures the difference between theoutgoing infrared radiation from theearth and the incoming radiationfrom the sun that penetrates theearth’s atmosphere.

action potential The change inelectrical potential that occurs acrossa cell membrane during the passageof a nerve impulse. As an impulsetravels in a wavelike manner along

9 action potential

a


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the axon of a nerve, it causes a local-ized and transient switch in electricpotential across the cell membranefrom –60 mV (the resting potential)to +45 mV. The change in electric po-tential is caused by an inÛux ofsodium ions. Nervous stimulation ofa muscle Übre has a similar effect.

action spectrum A graphical plotof the efÜciency of electromagneticradiation in producing a photochemi-cal reaction against the wavelengthof the radiation used. For example,the action spectrum for photosynthe-sis using light shows a peak in the re-gion 670–700 nm. This correspondsto a maximum absorption in the ab-sorption spectrum of chlorophylls inthis region.

activated adsorption *Adsorp-tion that involves an activation en-ergy. This occurs in certain cases ofchemisorption.

activated alumina See aluminiumhydroxide.

activated charcoal See charcoal.

activated complex See activated-complex theory.

activated-complex theory (ACT)A theory enabling the rate constantsin chemical reactions to be calcu-lated using statistical thermodynam-ics. The events assumed to be takingplace can be shown in a diagramwith the potential energy as the ver-tical axis, while the horizontal axis,called the reaction coordinate, repre-sents the course of the reaction. Astwo reactants A and B approach eachother, the potential energy rises to amaximum. The collection of atomsnear the maximum is called the acti-vated complex. After the atoms haverearranged in the chemical reaction,the value of the potential energy fallsas the products of the reaction areformed. The point of maximum po-

tential energy is called the transitionstate of the reaction, as reactantspassing through this state becomeproducts. In ACT, it is assumed thatthe reactants are in equilibrium withthe activated complex, and that thisdecomposes along the reaction coor-dinate to give the products. ACT wasdeveloped by the US chemist HenryEyring and colleagues in the 1930s.See also eyring equation.

activated sludge process Asewage and waste-water treatment.The sludge produced after primarytreatment is pumped into aerationtanks, where it is continuouslystirred and aerated, resulting in theformation of small aggregates of sus-pended colloidal organic mattercalled Ûoc. Floc contains numerousslime-forming and nitrifying bac-teria, as well as protozoans, whichdecompose organic substances in thesludge. Agitation or air injectionmaintains high levels of dissolvedoxygen, which helps to reduce the*biochemical oxygen demand.Roughly half the sewage in Britain istreated using this method.

activation analysis An analyticaltechnique that can be used to detectmost elements when present in asample in milligram quantities (orless). In neutron activation analysisthe sample is exposed to a Ûux ofthermal neutrons in a nuclear reac-tor. Some of these neutrons are cap-tured by nuclides in the sample toform nuclides of the same atomicnumber but a higher mass number.These newly formed nuclides emitgamma radiation, which can be usedto identify the element present bymeans of a gamma-ray spectrometer.Activation analysis has also been em-ployed using charged particles, suchas protons or alpha particles.

activation energy Symbol Ea. The

action spectrum 10

a


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minimum energy required for achemical reaction to take place. In areaction, the reactant moleculescome together and chemical bondsare stretched, broken, and formed inproducing the products. During thisprocess the energy of the system in-creases to a maximum, then decreasesto the energy of the products (see il-lustration). The activation energy isthe difference between the maximumenergy and the energy of the reac-tants; i.e. it is the energy barrier thathas to be overcome for the reactionto proceed. The activation energy de-termines the way in which the rateof the reaction varies with tempera-ture (see arrhenius equation). It isusual to express activation energiesin joules per mole of reactants. Anactivation energy greater than 200 KJmol-1 suggests that a bond has beencompletely broken in forming thetransition state (as in the SN1 reac-tion). A lower Ügure suggests incom-plete breakage (as in the SN2 reaction).See also activated-complex theory.

11 activity

a

ener

gy products

Ea∆H

reactants

Activation energy

activator 1. A substance that in-creases the activity of a catalyst; forexample, a substance that – by bind-ing to an *allosteric site on an en-zyme – enables the active site of theenzyme to bind to the substrate. 2.Any compound that potentiates theactivity of a drug or other foreignsubstance in the body.

active mass See mass action.

active site (active centre) 1. A siteon the surface of a catalyst at whichactivity occurs. 2. The site on thesurface of an *enzyme molecule that

binds the substrate molecule. Theproperties of an active site are deter-mined by the three-dimensionalarrangement of the polypeptidechains of the enzyme and their con-stituent amino acids. These governthe nature of the interaction thattakes place and hence the degree ofsubstrate speciÜcity and susceptibil-ity to *inhibition.

activity 1. Symbol a. A thermody-namic function used in place of con-centration in equilibrium constantsfor reactions involving nonidealgases and solutions. For example, ina reaction

A ˆ B + Cthe true equilibrium constant isgiven by

K = aBaC/aAwhere aA, aB, and aC are the activitiesof the components, which functionas concentrations (or pressures) cor-rected for nonideal behaviour. Activ-ity coefÜcients (symbol γ) are deÜnedfor gases by γ = a/p (where p is pres-sure) and for solutions by γ = aX(where X is the mole fraction). Thus,the equilibrium constant of a gas re-action has the form

Kp = γBpBγCpC/γApA

The equilibrium constant of a reac-tion in solution is

Kc = γBXBγCXC/γAXA

The activity coefÜcients thus act ascorrection factors for the pressuresor concentrations. The activity isgiven by an equation

µ = µŠ + RT ln awhere µ is chemical potential See alsofugacity.2. Symbol A. The number of atoms ofa radioactive substance that disinte-grate per unit time. The speciÜc ac-tivity (a) is the activity per unit massof a pure radioisotope. See radiationunits.


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activity series See electromotiveseries.

acyclic Describing a compound that does not have a ring in its mol-ecules.

acyl anhydrides See acid anhy-drides.

acylation The process of introduc-ing an acyl group (RCO–) into a com-pound. The usual method is to reactan alcohol with an acyl halide or acarboxylic acid anhydride; e.g.

RCOCl + R′OH → RCOOR′ + HClThe introduction of an acetyl group(CH3CO–) is acetylation, a processused for protecting –OH groups in or-ganic synthesis.

acyl Üssion The breaking of thecarbon–oxygen bond in an acylgroup. It occurs in the hydrolysis ofan *ester to produce an alcohol and acarboxylic acid.

acylglycerols See glycerides.

acyl group A group of the typeRCO–, where R is an organic group.An example is the acetyl groupCH3CO–.

acyl halides (acid halides) Organiccompounds containing the group–CO.X, where X is a halogen atom(see formula). Acyl chlorides, for in-stance, have the general formulaRCOCl. The group RCO– is the acylgroup. In systematic chemical nomen-clature acyl-halide names end in thesufÜx -oyl; for example, ethanoylchloride, CH3COCl. Acyl halides reactreadily with water, alcohols, phenols,and amines and are used in *acyla-tion reactions. They are made byreplacing the –OH group in a car-boxylic acid by a halogen using ahalogenating agent such as PCl5.


adamantane A colourless crys-talline hydrocarbon C10H16; m.p.269°C. It is found in certain petro-leum fractions. The structure con-tains three symmetrically fusedcyclohexane rings.

activity series 12

a

Adams catalyst A dark brownpowder, a hydrated form of platinum(IV) oxide (PtO2), produced by heatingchloroplatinic acid (H2PtCl6) withsodium nitrate (NaNO3). Platinum ni-trate is produced, and this decom-poses to Platinum (IV) oxide withevolution of NO2 and oxygen. It isused in hydrogenations of alkenes toalkanes, nitro compounds to aminos,and ketones to alcohols. The actualcatalyst is not the oxide but Ünely di-vided *platinum black, which formsduring the hydrogenation reaction.

addition polymerization Seepolymerization.

addition reaction A chemical re-action in which one molecule adds toanother. Addition reactions occurwith unsaturated compounds con-taining double or triple bonds, andmay be *electrophilic or *nucle-ophilic. An example of electrophilicaddition is the reaction of hydrogenchloride with an alkene, e.g.

HCl + CH2:CH2 → CH3CH2ClAn example of nucleophilic additionis the addition of hydrogen cyanideacross the carbonyl bond in aldehy-des to form *cyanohydrins. Addi-tion–elimination reactions are ones inwhich the addition is followed by

H

H

H

H

Adamantane


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elimination of another molecule (seecondensation reaction).

additive A substance added to an-other substance or material to im-prove its properties in some way.Additives are often present in smallamounts and are used for a variety ofpurposes, as in preventing corrosion,stabilizing polymers, and preservingand improving foods (see food addi-tive).

adduct A compound formed by anaddition reaction. The term is usedparticularly for compounds formedby coordination between a Lewis acid(acceptor) and a Lewis base (donor).See acid.

adenine A *purine derivative. It isone of the major component bases of*nucleotides and the nucleic acids*DNA and *RNA.

13 adiabatic demagnetization

a

NH

CH

N

N

CH

N

NH2

Adenine

N

O

N

N

OH

OH

N

NH2

OH

Adenosine

adenosine A nucleoside compris-ing one adenine molecule linked to ad-ribose sugar molecule. The phos-

phate-ester derivatives of adenosine,AMP, ADP, and *ATP, are of funda-mental biological importance as car-riers of chemical energy.

adenosine diphosphate (ADP) Seeatp.

adenosine monophosphate(AMP) See atp.

adenosine triphosphate See atp.

adhesive A substance used for join-ing surfaces together. Adhesives aregenerally colloidal solutions, whichset to gels. There are many types in-cluding animal glues (based on colla-gen), vegetable mucilages, andsynthetic resins (e.g. *epoxy resins).

adiabatic approximation An ap-proximation used in *quantum me-chanics when the time dependenceof parameters, such as the internu-clear distance between atoms in amolecule, is slowly varying. This ap-proximation means that the solutionof the *Schrödinger equation at onetime goes continuously over to thesolution at a later time. It was formu-lated by Max *Born and the Sovietphysicist Vladimir AlexandrovichFock (1898–1974) in 1928. The*Born–Oppenheimer approximationis an example of the adiabatic ap-proximation.

adiabatic demagnetization Atechnique for cooling a paramagneticsalt, such as potassium chrome alum,to a temperature near *absolute zero.The salt is placed between the polesof an electromagnet and the heatproduced during magnetization is re-moved by liquid helium. The salt isthen isolated thermally from the sur-roundings and the Üeld is switchedoff; the salt is demagnetized adiabati-cally and its temperature falls. This isbecause the demagnetized state,being less ordered, involves more en-ergy than the magnetized state. The


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extra energy can come only from theinternal, or thermal, energy of thesubstance.

adiabatic process Any processthat occurs without heat entering orleaving a system. In general, an adia-batic change involves a fall or rise intemperature of the system. For exam-ple, if a gas expands under adiabaticconditions, its temperature falls(work is done against the retreatingwalls of the container). The adiabaticequation describes the relationshipbetween the pressure (p) of an idealgas and its volume (V), i.e. pVγ = K,where γ is the ratio of the principalspeciÜc *heat capacities of the gasand K is a constant.

adipic acid See hexanedioic acid.

ADP See atp.

adrenaline (epinephrine) A hor-mone, produced by the medulla ofthe adrenal glands, that increasesheart activity, improves the powerand prolongs the action of muscles,and increases the rate and depth ofbreathing to prepare the body for‘fright, Ûight, or Üght’. At the sametime it inhibits digestion and excre-tion.

adiabatic process 14

a

adsorbate A substance that is ad-sorbed on a surface.

adsorbent A substance on the sur-face of which a substance is ad-sorbed.

adsorption The formation of alayer of gas, liquid, or solid on thesurface of a solid or, less frequently,of a liquid. There are two types de-pending on the nature of the forcesinvolved. In chemisorption a singlelayer of molecules, atoms, or ions isattached to the adsorbent surface bychemical bonds. In physisorption ad-sorbed molecules are held by theweaker *van der Waals’ forces. Ad-sorption is an important feature ofsurface reactions, such as corrosion,and heterogeneous catalysis. Theproperty is also utilized in adsorption*chromatography.

adsorption indicator (absorptionindicator) A type of indicator used in reactions that involve precipita-tion. The yellow dye Ûuorescein is acommon example, used for the reac-tion

NaCl(aq) + AgNO3(aq) → AgCl(s) +NaNO3(aq)

As silver nitrate solution is added to the sodium chloride, silver chlo-ride precipitates. As long as Cl– ionsare in excess, they adsorb on theprecipitate particles. At the endpoint, no Cl– ions are left in solutionand negative Ûuorescein ions arethen adsorbed, giving a pink colourto the precipitate. The technique is sometimes known as Fajan’smethod.

adsorption isotherm An equationthat describes how the amount of asubstance adsorbed onto a surface de-pends on its pressure (if a gas) or itsconcentration (if in a solution), at aconstant temperature. Several ad-sorption isotherms are used in sur-face chemistry including the *BETisotherm and the *Langmuir ad-sorption isotherm. The differentisotherms correspond to different as-sumptions about the surface and theadsorbed molecules.

OH

OH

N H

CH3OH

Adrenaline


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adulterant See cutting agent.

aerogel A low-density porous trans-parent material that consists of morethan 90% air. Usually based on metaloxides or silica, aerogels are used asdrying agents and insulators.

aerosol A colloidal dispersion of asolid or liquid in a gas. The com-monly used aerosol sprays contain aninert propellant liqueÜed under pres-sure. *ChloroÛuorocarbons, such asdichlorodiÛuoromethane, are com-monly used in aerosol cans. This usehas been criticized on the groundsthat these compounds persist in theatmosphere and may lead to deple-tion of the *ozone layer.

AES See atomic emission spec-troscopy.

A-factor See arrhenius equation.

afÜnity chromatography A bio-chemical technique for purifying nat-ural polymers, especially proteins. Itfunctions by attaching a speciÜc lig-and by covalent bonding to an insolu-ble inert support. The ligand has tohave a speciÜc afÜnity for the poly-mer, so that when a solution contain-ing the ligand is passed down acolumn of the material it is speciÜ-cally retarded and thus separatedfrom any contaminating molecules.An example of a suitable ligand isthe substrate of an enzyme, providedthat it does not change irreversiblyduring the chromatography.

aÛatoxin A poisonous compound,C15H12O6, produced by the fungus As-pergillus Ûavus. It is extremely toxic tofarm animals and can cause liver can-cer in humans. It may occur as a con-taminant of stored cereal crops,cotton seed, and, especially, peanuts.There are four isomeric forms.

AFM See atomic force microscope.

agar An extract of certain species of

red seaweeds that is used as a gellingagent in microbiological culturemedia, foodstuffs, medicines, andcosmetic creams and jellies. Nutrientagar consists of a broth made frombeef extract or blood that is gelledwith agar and used for the cultiva-tion of bacteria, fungi, and somealgae.

agarose A carbohydrate polymerthat is a component of agar. It is usedin chromatography and electrophore-sis.

agate A variety of *chalcedony thatforms in rock cavities and has a pat-tern of concentrically arranged bandsor layers that lie parallel to the cavitywalls. These layers are frequently al-ternating tones of brownish-red.Moss agate does not show the samebanding and is a milky chalcedonycontaining mosslike or dendritic pat-terns formed by inclusions of man-ganese and iron oxides. Agates areused in jewellery and for ornamentalpurposes.

agitator A bladelike instrumentused in fermenters and *bioreactorsto mix the medium continuously inorder to maintain the rate of oxygentransfer and to help keep the cells insuspension.

air See earth’s atmosphere.

15 air

a

N

O

N

N

OH

OH

N

NH2

OH

Aflatoxin


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air pollution (atmospheric pollu-tion) The release into the atmos-phere of substances that cause avariety of harmful effects to the nat-ural environment. Most air pollu-tants are gases that are released intothe troposphere, which extendsabout 8 km above the surface of theearth. The burning of fossil fuels, forexample in power stations, is a majorsource of air pollution as this processproduces such gases as sulphur diox-ide and carbon dioxide. Released intothe atmosphere, both these gases arethought to contribute to the green-house effect. Sulphur dioxide and ni-trogen oxides, released in carexhaust fumes, are air pollutants thatare responsible for the formation of*acid rain; nitrogen oxides also con-tribute to the formation of *photo-chemical smog. See also ozone layer;pollution.

alabaster See gypsum.

alanine See amino acid.

albumin (albumen) One of a groupof globular proteins that are solublein water but form insoluble coagu-lates when heated. Albumins occurin egg white, blood, milk, and plants.Serum albumins, which constituteabout 55% of blood plasma protein,help regulate the osmotic pressureand hence plasma volume. They alsobind and transport fatty acids. α-lac-talbumin is one of the proteins inmilk.

alcoholic fermentation See fer-mentation.

alcohols Organic compounds thatcontain the –OH group. In systematicchemical nomenclature alcoholnames end in the sufÜx -ol. Examplesare methanol, CH3OH, and ethanol,C2H5OH. Primary alcohols have twohydrogen atoms on the carbonjoined to the –OH group (i.e. they

contain the group –CH2–OH); sec-ondary alcohols have one hydrogenon this carbon (the other two bondsbeing to carbon atoms, as in(CH3)2CHOH); tertiary alcohols haveno hydrogen on this carbon (as in(CH3)3COH): see formulae. The differ-ent types of alcohols may differ inthe way they react chemically. Forexample, with potassium dichro-mate(VI) in sulphuric acid the follow-ing reactions occur:primary alcohol → aldehyde → car-boxylic acidsecondary alcohol → ketonetertiary alcohol – no reaction

Other characteristics of alcoholsare reaction with acids to give *estersand dehydration to give *alkenes or*ethers. Alcohols that have two –OHgroups in their molecules are diols(or dihydric alcohols), those withthree are triols (or trihydric alcohols),etc.A• Information about IUPAC nomenclature

air pollution 16

a

H

H HC

OH

HC

CH3 CH3

OH

COH

tertiary alcohol (2-methylpropan-2-ol)

_

__

__

__

__

__

_primary alcohol (methanol)

secondary alcohol (propan-2-ol)

CH3CH3

CH3

Alcohols

aldehydes Organic compoundsthat contain the group –CHO (thealdehyde group; i.e. a carbonyl group(C=O) with a hydrogen atom boundto the carbon atom). In systematicchemical nomenclature, aldehydenames end with the sufÜx -al. Exam-ples of aldehydes are methanal(formaldehyde), HCOH, and ethanal(acetaldehyde), CH3CHO. Aldehydes


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are formed by oxidation of primary*alcohols; further oxidation yieldscarboxylic acids. They are reducingagents and tests for aldehydes in-clude *Fehling’s test and *Tollensreagent. Aldehydes have certain char-acteristic addition and condensationreactions. With sodium hydrogensul-phate(IV) they form addition com-pounds of the type [RCOH(SO3)H]–Na+. Formerly these were known asbisulphite addition compounds. Theyalso form addition compounds withhydrogen cyanide to give *cyanohy-drins and with alcohols to give *ac-etals and undergo condensationreactions to yield *oximes, *hydra-zones, and *semicarbazones. Aldehy-des readily polymerize. See alsoketones.A• Information about IUPAC nomenclature

17 alicyclic compound

a

R C

O

H

aldehyde group

Aldehyde

aldohexose See monosaccharide.

aldol See aldol reaction.

aldol reaction A reaction of alde-hydes of the type

2RCH2CHO ˆRCH2CH(OH)CHRCHO

where R is a hydrocarbon group. Theresulting compound is a hydroxy-aldehyde, i.e. an aldehyde–alcohol oraldol, containing alcohol (–OH) andaldehyde (–CHO) groups on adjacentcarbon atoms. The reaction is base-catalysed, the Ürst step being the for-mation of a carbanion of the typeRHC–CHO, which adds to the car-bonyl group of the other aldehydemolecule. For the carbanion to form,the aldehyde must have a hydrogen

atom on the carbon next to the car-bonyl group.

Aldols can be further converted toother products; in particular, theyare a source of unsaturated aldehy-des. For example, the reaction ofethanal gives 3-hydroxybutenal (ac-etaldol):

2CH3CHO ˆ CH3CH(OH)CH2CHOThis can be further dehydrated to 2-butenal (crotonaldehyde):

CH3CH(OH)CH2CHO → H2O +CH3CH:CHCHO

aldose See monosaccharide.

aldosterone A hormone producedby the adrenal glands that controlsexcretion of sodium by the kidneysand thereby maintains the balance ofsalt and water in the body Ûuids.

aldoximes Compounds formed byreaction between hydroxylamine andan aldehyde

RCOH + H2NOH → RCH:NOH + H2OIf R is an aliphatic group, the al-doxime is generally a liquid or low-melting solid. If R is aromatic, thealdoxime is a crystalline solid. Al-doximes have a planar structure andcan exist in two isomeric forms. Inthe syn form, the OH group is on thesame side of the double bond as theH. In the anti form the OH and H areon opposite sides. Typically, aliphaticaldehydes give anti aldoximes; aro-matic aldehydes give syn aldoximes.

algin (alginic acid) A complex poly-saccharide occurring in the cell wallsof the brown algae (Phaeophyta).Algin strongly absorbs water to forma viscous gel. It is produced commer-cially from a variety of species ofLaminaria and from Macrocystis pyriferain the form of alginates, which areused mainly as a stabilizer and tex-turing agent in the food industry.

alicyclic compound A compound


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that contains a ring of atoms and isaliphatic. Cyclohexane, C6H12, is anexample.

aliphatic compounds Organiccompounds that are *alkanes,*alkenes, or *alkynes or their deriva-tives. The term is used to denotecompounds that do not have the spe-cial stability of *aromatic com-pounds. All noncyclic organiccompounds are aliphatic. Cyclicaliphatic compounds are said to bealicyclic.

alizarin An orange-red compound,C14H8O4. The compound is a deriva-tive of *anthraquinone, with hy-droxyl groups substituted at the 1and 2 positions. It is an importantdyestuff producing red or violet*lakes with metal hydroxide.Alizarin occurs naturally as the glu-coside in madder. It can be synthe-sized by heating anthraquinone withsodium hydroxide.

alkali A *base that dissolves inwater to give hydroxide ions.

alkali metals (group 1 elements)The elements of group 1 (formerlyIA) of the *periodic table: lithium(Li), sodium (Na), potassium (K),rubidium (Rb), caesium (Cs), andfrancium (Fr). All have a characteris-tic electron conÜguration that is anoble gas structure with one outer s-electron. They are typical metals (inthe chemical sense) and readily losetheir outer electron to form stableM+ ions with noble-gas conÜgura-tions. All are highly reactive, withthe reactivity (i.e. metallic character)increasing down the group. There isa decrease in ionization energy fromlithium (520 kJ mol–1) to caesium(380 kJ mol–1). The second ionizationenergies are much higher and diva-lent ions are not formed. Other prop-erties also change down the group.Thus, there is an increase in atomic

and ionic radius, an increase in den-sity, and a decrease in melting andboiling point. The standard electrodepotentials are low and negative, al-though they do not show a regulartrend because they depend both onionization energy (which decreasesdown the group) and the hydrationenergy of the ions (which increases).

All the elements react with water(lithium slowly; the others violently)and tarnish rapidly in air. They canall be made to react with chlorine,bromine, sulphur, and hydrogen. Thehydroxides of the alkali metals arestrongly alkaline (hence the name)and do not decompose on heating.The salts are generally soluble. Thecarbonates do not decompose onheating, except at very high tempera-tures. The nitrates (except forlithium) decompose to give the ni-trite and oxygen:

2MNO3(s) → 2MNO2(s) + O2(g)Lithium nitrate decomposes to theoxide. In fact lithium shows a num-ber of dissimilarities to the othermembers of group 1 and in manyways resembles magnesium (see diag-onal relationship). In general, thestability of salts of oxo acids in-creases down the group (i.e. with in-creasing size of the M+ ion). Thistrend occurs because the smallercations (at the top of the group) tendto polarize the oxo anion more effec-tively than the larger cations at thebottom of the group.

alkalimetry Volumetric analysisusing standard solutions of alkali todetermine the amount of acid pre-sent.

alkaline 1. Describing an alkali. 2. Describing a solution that has anexcess of hydroxide ions (i.e. a pHgreater than 7).

alkaline-earth metals (group 2elements) The elements of group 2

aliphatic compounds 18

a


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(formerly IIA) of the *periodic table:beryllium (Be), magnesium (Mg), cal-cium (Ca), strontium (Sr), and barium(Ba). The elements are sometimes re-ferred to as the ‘alkaline earths’, al-though strictly the ‘earths’ are theoxides of the elements. All have acharacteristic electron conÜgurationthat is a noble-gas structure with twoouter s-electrons. They are typicalmetals (in the chemical sense) andreadily lose both outer electrons toform stable M2+ ions; i.e. they arestrong reducing agents. All are reac-tive, with the reactivity increasingdown the group. There is a decreasein both Ürst and second ionizationenergies down the group. Althoughthere is a signiÜcant difference be-tween the Ürst and second ionizationenergies of each element, com-pounds containing univalent ions arenot known. This is because the diva-lent ions have a smaller size andlarger charge, leading to higherhydration energies (in solution) orlattice energies (in solids). Conse-quently, the overall energy changefavours the formation of divalentcompounds. The third ionizationenergies are much higher than thesecond ionization energies, and tri-valent compounds (containing M3+)are unknown.

Beryllium, the Ürst member of thegroup, has anomalous properties be-cause of the small size of the ion; itsatomic radius (0.112 nm) is much lessthan that of magnesium (0.16 nm).From magnesium to radium there isa fairly regular increase in atomicand ionic radius. Other regularchanges take place in moving downthe group from magnesium. Thus,the density and melting and boilingpoints all increase. Beryllium, on theother hand, has higher boiling andmelting points than calcium and itsdensity lies between those of calciumand strontium. The standard elec-

trode potentials are negative andshow a regular small decrease frommagnesium to barium. In some waysberyllium resembles aluminium (seediagonal relationship).

All the metals are rather less reac-tive than the alkali metals. Theyreact with water and oxygen (beryl-lium and magnesium form a protec-tive surface Ülm) and can be made toreact with chlorine, bromine, sul-phur, and hydrogen. The oxides andhydroxides of the metals show theincreasing ionic character in movingdown the group: beryllium hydrox-ide is amphoteric, magnesium hy-droxide is only very slightly solublein water and is weakly basic, calciumhydroxide is sparingly soluble anddistinctly basic, strontium and bar-ium hydroxides are quite soluble andbasic. The hydroxides decompose onheating to give the oxide and water:

M(OH)2(s) → MO(s) + H2O(g)The carbonates also decompose onheating to the oxide and carbon diox-ide:

MCO3(s) → MO(s) + CO2(g)The nitrates decompose to give theoxide:

2M(NO3)2(s) → 2MO(s) + 4NO2(g) +O2(g)

As with the *alkali metals, the stabil-ity of salts of oxo acids increasesdown the group. In general, salts ofthe alkaline-earth elements are solu-ble if the anion has a single charge(e.g. nitrates, chlorides). Most saltswith a doubly charged anion (e.g. car-bonates, sulphates) are insoluble. Thesolubilities of salts of a particularacid tend to decrease down thegroup. (Solubilities of hydroxides in-crease for larger cations.)

alkaloid One of a group of nitroge-nous organic compounds, mostly de-rived from plants, and having diversepharmacological properties. They are

19 alkaloid

a


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biosynthesized from amino acids andclassiÜed according to some struc-tural feature. A simple classiÜcationis into:the pyridine group (e.g. coniine, nico-tine)

the tropine group (e.g. atropine, co-caine)

the quinoline group (e.g. quinine,strychnine, brucine)

the isoquinoline group (e.g. mor-phine, codeine)

the phenylethylamine group (e.g.methamphetamine, mescaline,ephedrine)

the indole group (e.g. tryptamine, ly-sergic acid)

the purine group (e.g. caffeine, theo-bromine, theophylline)

alkanal An aliphatic aldehyde.

alkanes (parafÜns) Saturated hy-drocarbons with the general formulaCnH2n+2. In systematic chemicalnomenclature alkane names end inthe sufÜx -ane. They form a *homolo-gous series (the alkane series)methane (CH4), ethane (C2H6),propane (C3H8), butane (C4H10), pen-tane (C5H12), etc. The lower membersof the series are gases; the high-mo-lecular weight alkanes are waxysolids. Alkanes are present in naturalgas and petroleum. They can bemade by heating the sodium salt of acarboxylic acid with soda lime:

RCOO–Na+ + Na+OH– → Na2CO3 +RH

Other methods include the *Wurtzreaction and *Kolbe’s method. Gener-ally the alkanes are fairly unreactive.They form haloalkanes with halo-gens when irradiated with ultravioletradiation.A• Information about IUPAC nomenclature• Further details about IUPAC

nomenclature

alkanol An aliphatic alcohol.

alkenes (oleÜnes; oleÜns) Unsatu-rated hydrocarbons that contain oneor more double carbon–carbon bondsin their molecules. In systematicchemical nomenclature alkenenames end in the sufÜx -ene. Alkenesthat have only one double bond forma homologous series (the alkene se-ries) starting ethene (ethylene),CH2:CH2, propene, CH3CH:CH2, etc.The general formula is CnH2n. Highermembers of the series show iso-merism depending on position of thedouble bond; for example, butene(C4H8) has two isomers, which are (1)but-1-ene (C2H5CH:CH2) and (2) but-2-ene (CH3CH:CHCH3): see formulae.Alkenes can be made by dehydrationof alcohols (passing the vapour overhot pumice):

RCH2CH2OH – H2O → RCH:CH2

alkanal 20

a

CH2

CH

CH2

CH3

CH3

CH

CH

CH3

but-1-ene

but-2-ene

Alkenes

An alternative method is the removalof a hydrogen atom and halogenatom from a haloalkane by potas-sium hydroxide in hot alcoholic solu-tion:

RCH2CH2Cl + KOH → KCl + H2O +RCH:CH2

Alkenes typically undergo *addi-tion reactions to the double bond.They can be tested for by the *Baeyertest. See also hydrogenation; oxoprocess; ozonolysis; zieglerprocess.A• Information about IUPAC nomenclature

alkoxides Compounds formed by


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reaction of alcohols with sodium orpotassium metal. Alkoxides are salt-like compounds containing the ionR–O–.

alkyd resin A type of *polyesterresin used in paints and other sur-face coating. The original alkydresins were made by copolymerizingphthalic anhydride with glycerol, togive a brittle cross-linked polymer.The properties of such resins can bemodiÜed by adding monobasic acidsor alcohols during the polymeriza-tion.

alkylation A chemical reactionthat introduces an *alkyl group intoan organic molecule. The *Friedel–Crafts reaction results in alkylationof aromatic compounds.

alkylbenzenes Organic com-pounds that have an alkyl groupbound to a benzene ring. The sim-plest example is methylbenzene(toluene), CH3C6H5. Alkyl benzenescan be made by the *Friedel–Craftsreaction.

alkyl group A group obtained byremoving a hydrogen atom from analkane, e.g. methyl group, CH3–, de-rived from methane.

alkyl halides See haloalkanes.

alkynes (acetylenes) Unsaturatedhydrocarbons that contain one ormore triple carbon–carbon bonds intheir molecules. In systematic chemi-cal nomenclature alkyne names endin the sufÜx -yne. Alkynes that haveonly one triple bond form a *ho-mologous series: ethyne (acetylene),CH≡CH, propyne, CH3CH≡CH, etc.They can be made by the action ofpotassium hydroxide in alcohol solu-tion on haloalkanes containing halo-gen atoms on adjacent carbon atoms;for example:

RCHClCH2Cl + 2KOH → 2KCl +2H2O + RCH≡CH

Like *alkenes, alkynes undergo addi-tion reactions.


allenes Compounds that containthe group >C=C=C<, in which threecarbon atoms are linked by two adja-cent double bonds. The outer carbonatoms are each linked to two otheratoms or groups by single bonds. Thesimplest example is 1,2-propadiene,CH2CCH2. Allenes are *dienes withtypical reactions of alkenes. Underbasic conditions, they often convertto alkynes. In an allene, the twodouble bonds lie in planes that areperpendicular to each other. Con-sequently, in an allene of the typeR1R2C:C:CR3R4, the groups R1 and R2 lie in a plane perpendicular to the plane containing R3 and R4.Under these circumstances, the mol-ecule is chiral and can show opticalactivity.

allosteric enzyme An enzymethat has two structurally distinctforms, one of which is active and theother inactive. In the active form, thequaternary structure (see protein) ofthe enzyme is such that a substratecan interact with the enzyme at theactive site (see enzyme–substratecomplex). The conformation of thesubstrate-binding site becomes al-tered in the inactive form and inter-action with the substrate is notpossible. Allosteric enzymes tend tocatalyse the initial step in a pathwayleading to the synthesis of molecules.The end product of this synthesis canact as a feedback inhibitor (see inhibi-tion) and the enzyme is converted tothe inactive form, thereby control-ling the amount of product synthe-sized.

allosteric site A binding site onthe surface of an enzyme other thanthe *active site. In noncompetitive

21 allosteric site

a


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*inhibition, binding of the inhibitorto an allosteric site inhibits the activ-ity of the enzyme. In an *allostericenzyme, the binding of a regulatorymolecule to the allosteric sitechanges the overall shape of the en-zyme, either enabling the substrateto bind to the active site or prevent-ing the binding of the substrate.

allotropy The existence of el-ements in two or more differentforms (allotropes). In the case of oxy-gen, there are two forms: ‘normal’dioxygen (O2) and ozone, or trioxy-gen (O3). These two allotropes havedifferent molecular conÜgurations.More commonly, allotropy occurs be-cause of different crystal structuresin the solid, and is particularly preva-lent in groups 14, 15, and 16 of theperiodic table. In some cases, the al-lotropes are stable over a tempera-ture range, with a deÜnite transitionpoint at which one changes into theother. For instance, tin has two al-lotropes: white (metallic) tin stableabove 13.2°C and grey (nonmetallic)tin stable below 13.2°C. This form ofallotropy is called enantiotropy. Car-bon also has two allotropes – dia-mond and graphite – althoughgraphite is the stable form at all tem-peratures. This form of allotropy, inwhich there is no transition tempera-ture at which the two are in equilib-rium, is called monotropy. See alsopolymorphism.

allowed bands See energy bands.

allowed transition A transitionbetween two electronic states al-lowed according to *selection rulesassociated with group theory. Theprobability of a transition betweenstates m and n produced by the inter-action of electromagnetic radiationwith an atomic system is propor-tional to the square of the magnitudeof the matrix elements of the electric

allotropy 22

a dipole moment. If this quantity is notzero, the transition is an allowedtransition; if it is zero the transitionis a *forbidden transition as a dipoletransition. It may, however, be an al-lowed transition for magnetic dipoleor quadrupole-moment transitions,which have much smaller transitionprobabilities and consequently givemuch weaker lines in the spectrum.

alloy A material consisting of twoor more metals (e.g. brass is an alloyof copper and zinc) or a metal and anonmetal (e.g. steel is an alloy of ironand carbon, sometimes with othermetals included). Alloys may be com-pounds, *solid solutions, or mixturesof the components.

alloy steels See steel.

allyl alcohol See propenol.

allyl group See propenyl group.

Alnico A tradename for a series ofalloys, containing iron, aluminium,nickel, cobalt, and copper, used tomake permanent magnets.

alpha helix The most commonform of secondary structure in *pro-teins, in which the polypeptide chainis coiled into a helix. The helicalstructure is held in place by weak hy-drogen bonds between the N–H andC=O groups in successive turns of thehelix (see illustration). Compare betasheet.

alpha-iron See iron.

alphamethyltryptamine (AMT) Asynthetic derivative of *tyrptamine

NH

CH2

NH2

CH3

Alphamethyltryptamine


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with stimulant and hallucinogenicproperties. It is used illegally as aclub drug.

alpha-naphthol test A biochemi-cal test to detect the presence of car-bohydrates in solution, also knownas Molisch’s test (after the Austrianchemist H. Molisch (1856–1937), whodevised it). A small amount of alco-holic alpha-naphthol is mixed withthe test solution and concentratedsulphuric acid is poured slowly downthe side of the test tube. A positivereaction is indicated by the forma-tion of a violet ring at the junction ofthe liquids.

alpha particle A helium nucleusemitted by a larger nucleus duringthe course of the type of radioactivedecay known as alpha decay. As a he-lium nucleus consists of two protonsand two neutrons bound together asa stable entity the loss of an alphaparticle involves a decrease in *nu-cleon number of 4 and decrease of 2in the *atomic number, e.g. thedecay of a uranium–238 nucleus intoa thorium–234 nucleus. A stream of

alpha particles is known as an alpha-ray or alpha-radiation.

alternant Describing a conjugatedmolecule in which the atoms can bedivided into two sets of alternateatoms such that no atom has a directlink to another atom in the same set.Naphthalene, for example, has an al-ternant conjugated system.

alum See aluminium potassiumsulphate; alums.

alumina See aluminium oxide; alu-minium hydroxide.

aluminate A salt formed when alu-minium hydroxide or γ-alumina isdissolved in solutions of strong bases,such as sodium hydroxide. Alumi-nates exist in solutions containingthe aluminate ion, commonly writ-ten [Al(OH)4]–. In fact the ion proba-bly is a complex hydrated ion andcan be regarded as formed from a hy-drated Al3+ ion by removal of four hy-drogen ions:

[Al(H2O)6]3+ + 4OH– → 4H2O +[Al(OH)4(H2O)2]–

Other aluminates and polyalumi-nates, such as [Al(OH)6]3– and

23 aluminate

a

•••

•••

• • •

OOOHHH

CCCCCC

NNN

R

NNN

HHHHHH

OOO

HHH

CCC

CCCNNN

HHHOOO

CCC

R

R

HHHOOO

CCCNNN

HHHCCC

R

hydrogen bond= amino-acid side chain

•••

OOO

Alpha helix


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[(HO)3AlOAl(OH)3]2–, are also present.See also aluminium hydroxide.

aluminium Symbol Al. A silvery-white lustrous metallic element be-longing to *group 3 (formerly IIIB) ofthe periodic table; a.n. 13; r.a.m.26.98; r.d. 2.7; m.p. 660°C; b.p.2467°C. The metal itself is highly re-active but is protected by a thintransparent layer of the oxide, whichforms quickly in air. Aluminium andits oxide are amphoteric. The metalis extracted from puriÜed bauxite(Al2O3) by electrolysis; the mainprocess uses a *Hall–Heroult cell butother electrolytic methods are underdevelopment, including conversionof bauxite with chlorine and electrol-ysis of the molten chloride. Pure alu-minium is soft and ductile but itsstrength can be increased by work-hardening. A large number of alloysare manufactured; alloying elementsinclude copper, manganese, silicon,zinc, and magnesium. Its lightness,strength (when alloyed), corrosion re-sistance, and electrical conductivity(62% of that of copper) make it suit-able for a variety of uses, includingvehicle and aircraft construction,building (window and door frames),and overhead power cables. Al-though it is the third most abundantelement in the earth’s crust (8.1% byweight) it was not isolated until 1825by H. C. *Oersted.


aluminium acetate See alu-minium ethanoate.

aluminium chloride A whitishsolid, AlCl3, which fumes in moist airand reacts violently with water (togive hydrogen chloride). It is knownas the anhydrous salt (hexagonal; r.d.2.44 (fused solid); m.p. 190°C (2.5atm.); sublimes at 178°C) or the hexa-hydrate AlCl3.6H2O (rhombic; r.d.

aluminium 24

aCl

Cl Cl

Cl

Al Al

Cl

Cl

Aluminium chloride

2.398; loses water at 100°C), both ofwhich are deliquescent. Aluminiumchloride may be prepared by passinghydrogen chloride or chlorine overhot aluminium or (industrially) bypassing chlorine over heated alu-minium oxide and carbon. The chlo-ride ion is polarized by the smallpositive aluminium ion and thebonding in the solid is intermediatebetween covalent and ionic. In theliquid and vapour phases dimer mol-ecules exist, Al2Cl6, in which thereare chlorine bridges making coordi-nate bonds to aluminium atoms (seeformula). The AlCl3 molecule can alsoform compounds with other mol-ecules that donate pairs of electrons(e.g. amines or hydrogen sulphide);i.e. it acts as a Lewis *acid. At hightemperatures the Al2Cl6 molecules inthe vapour dissociate to (planar)AlCl3 molecules. Aluminium chlorideis used commercially as a catalyst inthe cracking of oils. It is also a cata-lyst in certain other organic reac-tions, especially the Friedel–Craftsreaction.

aluminium ethanoate (aluminiumacetate) A white solid, Al(OOCCH3)3,which decomposes on heating, isvery slightly soluble in cold water,and decomposes in warm water. Thenormal salt, Al(OOCCH3)3, can onlybe made in the absence of water (e.g.ethanoic anhydride and aluminiumchloride at 180°C); in water it formsthe basic salts Al(OH)(OOCCH3)2 andAl2(OH)2(OOCCH3)4. The reaction ofaluminium hydroxide with ethanoicacid gives these basic salts directly.The compound is used extensively indyeing as a mordant, particularly in


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combination with aluminium sul-phate (known as red liquor); in thepaper and board industry for sizingand hardening; and in tanning. Itwas previously used as an antisepticand astringent.

aluminium hydroxide A whitecrystalline compound, Al(OH)3; r.d.2.42–2.52. The compound occurs nat-urally as the mineral gibbsite (mono-clinic). In the laboratory it can beprepared by precipitation fromsolutions of aluminium salts. Suchsolutions contain the hexaquo-aluminium(III) ion with six watermolecules coordinated, [Al(H2O)6]3+.In neutral solution this ionizes:

[Al(H2O)6]3+ ˆ H+ + [Al(H2O)5OH]2+

The presence of a weak base such asS2– or CO3

2– (by bubbling hydrogensulphide or carbon dioxide throughthe solution) causes further ioniza-tion with precipitation of aluminiumhydroxide

[Al(H2O)6]3+(aq) → Al(H2O)3(OH)3(s) +3H+(aq)

The substance contains coordinatedwater molecules and is more cor-rectly termed hydrated aluminiumhydroxide. In addition, the precipi-tate has water molecules trapped init and has a characteristic gelatinousform. The substance is amphoteric.In strong bases the *aluminate ion isproduced by loss of a further proton:

Al(H2O)3(OH)3(s) + OH–(aq) ˆ[Al(H2O)2(OH)4]–(aq) + H2O(l)

On heating, the hydroxide trans-forms to a mixed oxide hydroxide,AlO.OH (rhombic; r.d. 3.01). This sub-stance occurs naturally as diasporeand boehmite. Above 450°C it trans-forms to γ-alumina.

In practice various substances canbe produced that are mixed crys-talline forms of Al(OH)3, AlO.OH, andaluminium oxide (Al2O3) with watermolecules. These are known as hy-

drated alumina. Heating the hydratedhydroxide causes loss of water, andproduces various activated aluminas,which differ in porosity, number ofremaining –OH groups, and particlesize. These are used as catalysts (par-ticularly for organic dehydration re-actions), as catalyst supports, and inchromatography. Gelatinous freshlyprecipitated aluminium hydroxidewas formerly widely used as a mor-dant for dyeing and calico printingbecause of its ability to form insolu-ble coloured *lakes with vegetabledyes. See also aluminium oxide.

aluminium oxide (alumina) Awhite or colourless oxide of alu-minium occurring in two mainforms. The stable form α-alumina(r.d. 3.97; m.p. 2015°C; b.p. 2980 ±60°C) has colourless hexagonal orrhombic crystals; γ-alumina (r.d.3.5–3.9) transforms to the α-form onheating and is a white microcrys-talline solid. The compound occursnaturally as corundum or emery inthe α-form with a hexagonal-close-packed structure of oxide ions withaluminium ions in the octahedral in-terstices. The gemstones ruby andsapphire are aluminium oxidecoloured by minute traces ofchromium and cobalt respectively. Anumber of other forms of aluminiumoxide have been described (β-, δ-, andζ-alumina) but these contain alkali-metal ions. There is also a short-livedspectroscopic suboxide AlO. Thehighly protective Ülm of oxideformed on the surface of aluminiummetal is yet another structural varia-tion, being a defective rock-salt form(every third Al missing).

Pure aluminium oxide is obtainedby dissolving the ore bauxite insodium hydroxide solution; impuri-ties such as iron oxides remain in-soluble because they are notamphoteric. The hydrated oxide is

25 aluminium oxide

a


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precipitated by seeding with materialfrom a previous batch and this isthen roasted at 1150–1200°C to givepure α-alumina, or at 500–800°C togive γ-alumina. The bonding in alu-minium hydroxide is not purely ionicdue to polarization of the oxide ion.Although the compound might beexpected to be amphoteric, α-alu-mina is weakly acidic, dissolving inalkalis to give solutions containingaluminate ions; it is resistant to acidattack. In contrast γ-alumina is typi-cally amphoteric dissolving both inacids to give aluminium salts and inbases to give aluminates. α-aluminais one of the hardest materialsknown (silicon carbide and diamondare harder) and is widely used as anabrasive in both natural (corundum)and synthetic forms. Its refractorynature makes alumina brick an idealmaterial for furnace linings and alu-mina is also used in cements forhigh-temperature conditions. See alsoaluminium hydroxide.

aluminium potassium sulphate(potash alum; alum) A white orcolourless crystalline compound,Al2(SO4)3.K2SO4.24H2O; r.d. 1.757;loses 18H2O at 92.5°C; becomes anhy-drous at 200°C. It forms cubic or oc-tahedral crystals that are soluble incold water, very soluble in hot water,and insoluble in ethanol and acetone.The compound occurs naturally asthe mineral kalinite. It is a doublesalt and can be prepared by recrystal-lization from a solution containingequimolar quantities of potassiumsulphate and aluminium sulphate. Itis used as a mordant for dyeing andin the tanning and Ünishing ofleather goods (for white leather). Seealso alums.

aluminium sulphate A white orcolourless crystalline compound,Al2(SO4)3, known as the anhydrouscompound (r.d. 2.71; decomposes at

770°C) or as the hydrate Al2(SO)3.18H2O (monoclinic; r.d. 1.69; loseswater at 86.5°C). The anhydrous saltis soluble in water and slightly solu-ble in ethanol; the hydrate is verysoluble in water and insoluble inethanol. The compound occurs natu-rally in the rare mineral alunogenite(Al2(SO)3.18H2O). It may be preparedby dissolving aluminium hydroxideor china clays (aluminosilicates) in sulphuric acid. It decomposes onheating to sulphur dioxide, sulphurtrioxide, and aluminium oxide. Its so-lutions are acidic because of hydroly-sis.

Aluminium sulphate is commer-cially one of the most important alu-minium compounds; it is used insewage treatment (as a Ûocculatingagent) and in the puriÜcation ofdrinking water, the paper industry,and in the preparation of mordants.It is also a Üre-prooÜng agent. Alu-minium sulphate is often wronglycalled alum in these industries.

aluminium trimethyl Seetrimethylaluminium.

alums A group of double salts withthe formula A2SO4.B2(SO4)3.24H2O,where A is a monovalent metal and Ba trivalent metal. The original exam-ple contains potassium and alu-minium (called potash alum orsimply alum); its formula is oftenwritten AlK(SO4)2.12H2O (aluminiumpotassium sulphate-12-water). Ammo-nium alum is AlNH4(SO4)2.12H2O,chrome alum is KCr(SO4)2.12H2O (seepotassium chromium sulphate), etc.The alums are isomorphous and canbe made by dissolving equivalentamounts of the two salts in waterand recrystallizing. See also alu-minium sulphate.

alunogenite A mineral form ofhydrated *aluminium sulphate,Al2(SO4)3.18H2O.

aluminium potassium sulphate 26

a


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amalgam An alloy of mercury withone or more other metals. Most met-als form amalgams (iron and plat-inum are exceptions), which may beliquid or solid. Some contain deÜniteintermetallic compounds, such asNaHg2.

amatol A high explosive consistingof a mixture of ammonium nitrateand trinitrotoluene.

ambident Describing a chemicalspecies that has two alternative reac-tive centres such that reaction at onecentre stops or inhibits reaction atthe other. An example is the *eno-late ion in which electrophilic attackcan occur at either the oxygen atomor at the beta-carbon atom.

ambidentate Describing a ligandthat can coordinate at two differentsites. For example, the NO2 moleculecan coordinate through the N atom(the nitro ligand) or through an Oatom (the nitrido ligand). Complexesthat differ only in the way the ligandcoordinates display linkage isomerism.

ambo- A preÜx used to indicate thata substance is present as a mixture of racemic diastereoisomers in un-speciÜed proportions. For example, ifl-alanine is reacted with dl-leucine,the resulting dipeptide can be de-

scribed as l-alanyl-ambo-leucine, toindicate the mixture.

americium Symbol Am. A radio-active metallic transuranic elementbelonging to the *actinoids; a.n. 95;mass number of most stable isotope243 (half-life 7.95 × 103 years); r.d.13.67 (20°C); m.p. 994 ± 4°C; b.p.2607°C. Ten isotopes are known. Theelement was discovered by G. T.Seaborg and associates in 1945, whoobtained it by bombarding ura-nium–238 with alpha particles.


amethyst The purple variety of themineral *quartz. It is found chieÛy inBrazil, the Urals (Soviet Union), Ari-zona (USA), and Uruguay. The colouris due to impurities, especially ironoxide. It is used as a gemstone.

amides 1. Organic compounds con-taining the group –CO.NH2 (theamide group). Compounds contain-ing this group are primary amides.Secondary and tertiary amides canalso exist, in which the hydrogenatoms on the nitrogen are replacedby one or two other organic groupsrespectively. Simple examples of pri-mary amides are ethanamide,CH3CONH2, and propanamide,C2H5CONH2. They are made by heat-

27 amides

a

HN

H

H

CH3N

H

H

CH3N

CH3

H

CH3N

CH3

CH3

ammonia

primary amine (methylamine)

secondary amine(dimethylamine)

tertiary amine(trimethylamine)

amino group

imino group

Amines


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amidol 28

aC

O

NH2

R amide group

Amides

ing the ammonium salt of the corre-sponding carboxylic acid. Amides canalso be made by reaction of ammonia(or an amine) with an acyl halide. Seealso hofmann’s reaction. 2. Inor-ganic compounds containing the ionNH2

–, e.g. KNH2 and Cd(NH2)2. Theyare formed by the reaction of ammo-nia with electropositive metals.


amidol See aminophenol.

amination A chemical reaction inwhich an amino group (–NH2) is in-troduced into a molecule. Examplesof amination reaction include the re-action of halogenated hydrocarbonswith ammonia (high pressure andtemperature) and the reduction ofnitro compounds and nitriles.

amines Organic compounds de-rived by replacing one or more of thehydrogen atoms in ammonia by or-ganic groups (see illustration). Pri-mary amines have one hydrogenreplaced, e.g. methylamine, CH3NH2.They contain the functional group–NH2 (the amino group). Secondaryamines have two hydrogens replaced,e.g. methylethylamine, CH3(C2H5)NH.Tertiary amines have all three hydro-gens replaced, e.g. trimethylamine,(CH3)3N. Amines are produced by thedecomposition of organic matter.They can be made by reducing nitrocompounds or amides. See also imines.


of primary amines• Information about IUPAC nomenclature

of secondary and tertiary amines

amine salts Salts similar to ammo-nium salts in which the hydrogenatoms attached to the nitrogen arereplaced by one or more organicgroups. Amines readily form salts byreaction with acids, gaining a protonto form a positive ammonium ion,They are named as if they were sub-stituted derivatives of ammoniumcompounds; for example, dimethy-lamine ((CH3)2NH) will react withhydrogen chloride to give dimethyl-ammonium chloride, which is anionic compound [(CH3)2NH2]+Cl–.When the amine has a common non-systematic name the sufÜx -ium canbe used; for example, phenylamine(aniline) would give [C6H5NH3]+Cl–,known as anilinium chloride. For-merly, such compounds were some-times called hydrochlorides, e.g.aniline hydrochloride with the for-mula C6H5NH2.HCl.

Salts formed by amines are crys-talline substances that are readilysoluble in water. Many insoluble *alkaloids (e.g. quinine and atropine)are used medicinally in the form ofsoluble salts (‘hydrochlorides’). If al-kali (sodium hydroxide) is added tosolutions of such salts the free amineis liberated.

If all four hydrogen atoms of anammonium salt are replaced by or-ganic groups a quaternary ammo-nium compound is formed. Suchcompounds are made by reacting ter-tiary amines with halogen com-pounds; for example, trimethylamine((CH3)3N) with chloromethane(CH3Cl) gives tetramethylammoniumchloride, (CH3)4N+Cl–. Salts of thistype do not liberate the free aminewhen alkali is added, and quaternaryhydroxides (such as (CH3)4N+OH–) canbe isolated. Such compounds arestrong alkalis, comparable to sodiumhydroxide.

amino acid Any of a group of


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water-soluble organic compoundsthat possess both a carboxyl (–COOH)and an amino (–NH2) group attachedto the same carbon atom, called theα-carbon atom. Amino acids can berepresented by the general formulaR–CH(NH2)COOH. R may be hydrogenor an organic group and determinesthe properties of any particularamino acid. Through the formationof peptide bonds, amino acids join to-gether to form short chains (*pep-tides) or much longer chains(*polypeptides). Proteins are com-posed of various proportions of about20 commonly occurring amino acids(see table on pp 30–31). The sequenceof these amino acids in the proteinpolypeptides determines the shape,properties, and hence biological roleof the protein. Some amino acidsthat never occur in proteins are nev-ertheless important, e.g. ornithineand citrulline, which are intermedi-ates in the urea cycle.

Plants and many microorganismscan synthesize amino acids from sim-ple inorganic compounds, but ani-mals rely on adequate supplies intheir diet. The *essential amino acidsmust be present in the diet whereasothers can be manufactured fromthem.


amino acid racemization (AAR)A dating technique used in archaeol-ogy based on the relative amounts ofthe optical isomers of an amino acidin a sample. In most organisms, thel-isomer of the amino acid is the oneproduced by metabolism. When theorganism dies, this isomer slowlyconverts into the d-form, and eventu-ally an equilibrium is reached inwhich the two forms are present inequal amounts. Measuring the pro-portions of the l- and d-forms in asample can, in principle, give an esti-

mate of the time since death. Not allamino acids racemize at the samerate, and the rate of the process de-pends on other factors such as mois-ture and temperature. Most work hasbeen done using leucine or asparticacid.

A particular application in forensicscience involves measuring the d/lratio of aspartic acid in the dentineof teeth. Once a tooth has fullyformed, the dentine is isolated by theenamel and then racemization takesplace in the living subject at a fairlyconstant temperature and moisturelevel. Measuring the ratio gives afairly good estimate of the age of thesubject (rather than the time sincedeath).

aminobenzene See phenylamine.

amino group See amines.

aminophenol Any of various or-ganic compounds used as reducingagents, especially as photographicdevelopers, and for making dyes. Ex-amples include amidol (the dihydro-chloride of 2,4-diaminophenol),metol (the hemisulphate of 4-methyl-aminophenol) and rhodinol (4-methyl-aminophenol).

α-aminotoluene See benzylamine.

ammine A coordination *complexin which the ligands are ammoniamolecules. An example of an am-mine is the tetraamminecopper(II)ion [Cu(NH3)4]2+.

ammonia A colourless gas, NH3,with a strong pungent odour; r.d.0.59 (relative to air); m.p. –77.7°C;b.p. –33.35°C. It is very soluble inwater and soluble in alcohol. Thecompound may be prepared in thelaboratory by reacting ammoniumsalts with bases such as calcium hy-droxide, or by the hydrolysis of a ni-tride. Industrially it is made by the*Haber process and over 80 million

29 ammonia

a


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30

a

!

" #" #

# $

# %

" "

& '

&

&

& &

! ( (

! ) )

!" )" *

++


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tonnes per year are used either di-rectly or in combination. Major usesare the manufacture of nitric acid,ammonium nitrate, ammoniumphosphate, and urea (the last three asfertilizers), explosives, dyestuffs andresins.

Liquid ammonia has some similar-ity to water as it is hydrogen bondedand has a moderate dielectric con-

stant, which permits it to act as anionizing solvent. It is weakly self-ionized to give ammonium ions,NH4

+ and amide ions, NH2–. It also

dissolves electropositive metals togive blue solutions, which are be-lieved to contain solvated electrons.Ammonia is extremely soluble inwater giving basic solutions that con-tain solvated NH3 molecules and

31 ammonia

a

! , ,

!" -

- -

! . .

!" . /

!" ." 0

! 1 1

!

2"3"

The amino acids occurring in proteins


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small amounts of the ions NH4+ and

OH–. The combustion of ammonia inair yields nitrogen and water. In thepresence of catalysts NO, NO2, andwater are formed; this last reaction isthe basis for the industrial produc-tion of nitric acid. Ammonia is agood proton acceptor (i.e. it is a base)and gives rise to a series of ammo-nium salts, e.g.

NH3 + HCl → NH4+ + Cl–.

It is also a reducing agent.The participation of ammonia in

the *nitrogen cycle is a most impor-tant natural process. Nitrogen-Üxingbacteria are able to achieve similarreactions to those of the Haberprocess, but under normal conditionsof temperature and pressure. Theserelease ammonium ions, which areconverted by nitrifying bacteria intonitrite and nitrate ions.

ammoniacal Describing a solutionin which the solvent is aqueous am-monia.

ammonia clock A form of atomicclock in which the frequency of aquartz oscillator is controlled by the vibrations of excited ammoniamolecules. The ammonia molecule(NH3) consists of a pyramid with a ni-trogen atom at the apex and one hy-drogen atom at each corner of thetriangular base. When the moleculeis excited, once every 20.9 microsec-onds the nitrogen atom passesthrough the base and forms a pyra-mid the other side: 20.9 microsec-onds later it returns to its originalposition. This vibration back andforth has a frequency of 23 870 hertzand ammonia gas will only absorbexcitation energy at exactly this fre-quency. By using a crystal oscillatorto feed energy to the gas and a suit-able feedback mechanism, the oscil-lator can be locked to exactly thisfrequency.

ammonia–soda process Seesolvay process.

ammonium alum See alums.

ammonium bicarbonate See am-monium hydrogencarbonate.

ammonium carbonate A colour-less or white crystalline solid,(NH4)2CO3, usually encountered asthe monohydrate. It is very soluble incold water. The compound decom-poses slowly to give ammonia, water,and carbon dioxide. Commercial ‘am-monium carbonate’ is a double saltof ammonium hydrogencarbonateand ammonium aminomethanoate(carbamate), NH4HCO3.NH2COONH4.This material is manufactured byheating a mixture of ammoniumchloride and calcium carbonate andrecovering the product as a sublimedsolid. It readily releases ammoniaand is the basis of sal volatile. It isalso used in dyeing and wool prepa-ration and in baking powders.

ammonium chloride (sal ammo-niac) A white or colourless cubicsolid, NH4Cl; r.d. 1.53; sublimes at340°C. It is very soluble in water andslightly soluble in ethanol but insolu-ble in ether. It may be prepared byfractional crystallization from a solu-tion containing ammonium sulphateand sodium chloride or ammoniumcarbonate and calcium chloride. Puresamples may be made directly by thegas-phase reaction of ammonia andhydrogen chloride. Because of itsease of preparation it can be manu-factured industrially alongside anyplant that uses or produces ammo-nia. The compound is used in drycells, metal Ünishing, and in thepreparation of cotton for dyeing andprinting.

ammonium hydrogencarbonate(ammonium bicarbonate) A whitecrystalline compound, NH4HCO3. It is

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formed naturally as a decay productof nitrogenous matter and is madecommercially by various methods:the action of carbon dioxide andsteam on a solution of ammoniumcarbonate; heating commercial am-monium carbonate (which alwayscontains some hydrogencarbonate);and the interaction of ammonia, car-bon dioxide, and water vapour. It isused in some *baking powders andmedicines.

ammonium ion The monovalentcation NH4

+. It may be regarded asthe product of the reaction of ammo-nia (a Lewis base) with a hydrogenion. The ion has tetrahedral symme-try. The chemical properties of am-monium salts are frequently verysimilar to those of equivalent alkali-metal salts.

ammonium nitrate A colourlesscrystalline solid, NH4NO3; r.d. 1.72;m.p. 169.6°C; b.p. 210°C. It is verysoluble in water and soluble inethanol. The crystals are rhombicwhen obtained below 32°C andmonoclinic above 32°C. It may bereadily prepared in the laboratory bythe reaction of nitric acid with aque-ous ammonia. Industrially, it is man-ufactured by the same reaction usingammonia gas. Vast quantities of am-monium nitrate are used as fertiliz-ers (over 20 million tonnes per year)and it is also a component of someexplosives.

ammonium sulphate A whiterhombic solid, (NH4)2SO4; r.d. 1.77;decomposes at 235°C. It is very solu-ble in water and insoluble in ethanol.It occurs naturally as the mineralmascagnite. Ammonium sulphatewas formerly manufactured from the‘ammoniacal liquors’ produced dur-ing coal-gas manufacture but is nowproduced by the direct reaction be-tween ammonia gas and sulphuric

acid. It is decomposed by heating torelease ammonia (and ammoniumhydrogensulphate) and eventuallywater, sulphur dioxide, and ammo-nia. Vast quantities of ammoniumsulphate are used as fertilizers.

ammonium thiocyanate Acolourless, soluble crystalline com-pound, NH4NCS. It is made by theaction of hydrogen cyanide on am-monium sulphide or from ammoniaand carbon disulphide in ethanol. Onheating, it turns into its isomerthiourea, SC(NH2)2. Its solutions givea characteristic blood-red colour withiron(III) compounds and so are em-ployed as a test for ferric iron. Am-monium thiocyanate is used as arapid Üxative in photography and asan ingredient in making explosives.

amorphous Describing a solid thatis not crystalline; i.e. one that has nolong-range order in its lattice. Manypowders that are described as ‘amor-phous’ in fact are composed ofmicroscopic crystals, as can bedemonstrated by X-ray diffraction.*Glasses are examples of true amor-phous solids.

amount of substance Symbol n.A measure of the number of entitiespresent in a substance. The speciÜedentity may be an atom, molecule,ion, electron, photon, etc., or anyspeciÜed group of such entities. Theamount of substance of an element,for example, is proportional to thenumber of atoms present. For all en-tities, the constant of proportionalityis the *Avogadro constant. The SI unitof amount of substance is the *mole.

AMP See atp; cyclic amp.

ampere Symbol A. The SI unit ofelectric current. The constant currentthat, maintained in two straight par-allel inÜnite conductors of negligiblecross section placed one metre apart

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in a vacuum, would produce a forcebetween the conductors of 2 × 10–7

N m–1. This deÜnition replaced theearlier international ampere deÜnedas the current required to deposit0.001 118 00 gram of silver from a so-lution of silver nitrate in one second.The unit is named after the Frenchphysicist André Marie Ampère (1775–1836).

ampere-hour A practical unit ofelectric charge equal to the chargeÛowing in one hour through a con-ductor passing one ampere. It isequal to 3600 coulombs.

ampere-turn The SI unit of magne-tomotive force equal to the magneto-motive force produced when acurrent of one ampere Ûows throughone turn of a magnetizing coil.

amperometric titration Amethod of determining the chemicalcomposition of a solution by measur-ing the current passing through acell containing the solution; the po-tential is held constant during thetitration for both the indicator andreference electrodes, with changes inthe current being measured. The cur-rent Ûowing through the cell is meas-ured as a function of the amount ofsubstance being titrated.

amphetamine A drug, 1-phenyl-2-aminopropane (or a derivative of thiscompound), that stimulates the cen-tral nervous system by causing therelease of the transmitters noradren-aline and dopamine from nerve end-ings. It inhibits sleep, suppresses theappetite, and has variable effects onmood; prolonged use can lead to ad-diction.

amphetamines A group of syn-thetic stimulants, with structuresbased in phenyl amines. Ampheta-mine itself has the formula

C6H5CH2CH(NH2)CH3.

Methamphetamine has a methylgroup on the amino group, i.e.

C6H5CH2CH(NHCH3)CH3. The hydrochloride of methampheta-mine can be crystallized giving a par-ticularly powerful form known as‘ice’ or crystal meth. Other examplesof amphetamines are *ecstasy and*mescaline. In the UK, ampheta-mines are class B controlled sub-stances (or class A if prepared forinjection).

amphiboles A large group of rock-forming metasilicate minerals. Theyhave a structure of silicate tetrahedralinked to form double endless chains,in contrast to the single chains of the*pyroxenes, to which they areclosely related. They are present inmany igneous and metamorphicrocks. The amphiboles show a widerange of compositional variation butconform to the general formula:X2–3Y5Z8O22(OH)2, where X = Ca, Na,K, Mg, or Fe2+; Y = Mg, Fe2+, Fe3+, Al,Ti, or Mn; and Z = Si or Al. The hy-droxyl ions may be replaced by F, Cl,or O. Most amphiboles are mono-clinic, including:cummingtonite, (Mg,Fe2+)7(Si8O22)-(OH)2; tremolite, Ca2Mg5(Si8O22)-(OH,F)2; actinolite, Ca2(Mg,Fe2+)5-(Si8O22)(OH,F)2; *hornblende,NaCa2(Mg,Fe2+,Fe3+,Al)5((Si,Al)8O22)-(OH,F)2; edenite, NaCa2(Mg,Fe2+)5-(Si7AlO22)(OH,F)2; and riebeckite,Na2,Fe3

2+(Si8O22)(OH,F)2. Anthophyl-lite, (Mg,Fe2+)7(Si8O22)(OH,F)2, andgedrite, (Mg,Fe2+)6Al(Si,Al)8O22)-(OH,F)2, are orthorhombic amphi-boles.

amphibolic pathway A biochemi-cal pathway that serves both anabolicand catabolic processes. An impor-tant example of an amphibolic path-way is the *Krebs cycle, whichinvolves both the catabolism of car-bohydrates and fatty acids and the

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synthesis of anabolic precursors foramino-acid synthesis.

amphiphilic Describing a moleculethat has both hydrophilic and hy-drophobic parts, as in *detergents.

amphiprotic See amphoteric; sol-vent.

ampholyte A substance that canact as either an acid, in the presenceof a strong base, or a base, when inthe presence of a strong acid.

ampholyte ion See zwitterion.

amphoteric Describing a com-pound that can act as both an acidand a base (in the traditional sense ofthe term). For instance, aluminiumhydroxide is amphoteric: as a baseAl(OH)3 it reacts with acids to formaluminium salts; as an acid H3AlO3 itreacts with alkalis to give *alumi-nates. Oxides of metals are typicallybasic and oxides of nonmetals tendto be acidic. The existence of ampho-teric oxides is sometimes regarded asevidence that an element is a *metal-loid. Compounds such as the aminoacids, which contain both acidic andbasic groups in their molecules, canalso be described as amphoteric. Sol-vents, such as water, that can bothdonate and accept protons are usu-ally described as amphiprotic (see sol-vent).

AMT See alphamethyltryptamine.

a.m.u. See atomic mass unit.

amylase Any of a group of closelyrelated enzymes that degrade starch,glycogen, and other polysaccharides.Plants contain both α- and β-amylases; the name diastase is givento the component of malt containingβ-amylase, important in the brewingindustry. Animals possess only α-amylases, found in pancreatic juice(as pancreatic amylase) and also (inhumans and some other species) in

saliva (as salivary amylase or ptyalin).Amylases cleave the long polysaccha-ride chains, producing a mixture ofglucose and maltose.

amyl group Formerly, any of sev-eral isomeric groups with the for-mula C5H11–.

amylopectin A *polysaccharidecomprising highly branched chainsof glucose molecules. It is one of theconstituents (the other being amy-lose) of *starch.

amylose A *polysaccharide consist-ing of linear chains of between 100and 1000 linked glucose molecules.Amylose is a constituent of *starch.In water, amylose reacts with iodineto give a characteristic blue colour.

anabolic steroids Steroid hor-mones related to the male sex hor-mone testosterone. They promotethe development of masculine char-acteristics and increase musclegrowth. Anabolic steroids have beenused medically for various conditionsbut are also used illegally by sports-men and women and by body-builders. They have a number ofdeleterious side effects and are a con-trolled drug in the UK and manyother countries. Their use is bannedby nearly all major sports regulatingbodies. They can be detected in bloodand urine by gas chromatography–mass spectroscopy.

anabolism The metabolic synthesisof proteins, fats, and other con-stituents of living organisms frommolecules or simple precursors. Thisprocess requires energy in the formof ATP. Drugs that promote suchmetabolic activity are described asanabolic. See metabolism. Compare ca-tabolism.

Analar reagent A chemicalreagent of high purity with known

35 Analar reagent

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contaminants for use in chemicalanalyses.

analyser A device, used in the *po-larization of light, that is placed inthe eyepiece of a *polarimeter to ob-serve plane-polarized light. Theanalyser, which may be a *Nicolprism or *Polaroid, can be orientedin different directions to investigatein which plane an incoming wave ispolarized or if the light is plane po-larized. If there is one direction fromwhich light does not emerge fromthe analyser when it is rotated, theincoming wave is plane polarized. Ifthe analyser is horizontal when ex-tinction of light takes place, the po-larization of light must have been inthe vertical plane. The intensity of abeam of light transmitted through ananalyser is proportional to cos2θ,where θ is the angle between theplane of polarization and the planeof the analyser. Extinction is said tobe produced by ‘crossing’ the *polar-izer and analyser.

analysis The determination of thecomponents in a chemical sample.Qualitative analysis involves deter-mining the nature of a pure un-known compound or the compoundspresent in a mixture. Various chemi-cal tests exist for different elementsor types of compound, and system-atic analytical procedures can beused for mixtures. Quantitativeanalysis involves measuring the pro-portions of known components in amixture. Chemical techniques forthis fall into two main classes: *volu-metric analysis and *gravimetricanalysis. In addition, there are numer-ous physical methods of qualitativeand quantitative analysis, includingspectroscopic techniques, mass spec-trometry, polarography, chromatog-raphy, activation analysis, etc.

analyser 36

a anchimeric assistance See neigh-bouring-group participation.

Andrews titration A titrationused to estimate amounts of reduc-ing agents. The reducing agent beingestimated is dissolved in concen-trated hydrochloric acid and titratedwith a solution of potassium iodate.A drop of tetrachloromethane isadded to the solution. The end pointof the titration is reached when theiodine colour disappears from thislayer. This is due to the reducingagent being oxidized and the iodatebeing reduced to ICl. This reaction in-volves a four-electron change.

ANFO Ammonium nitrate–fuel oil.A mixture used extensively as a blast-ing agent in mining and quarrying.The proportions are approximately94% ammonium nitrate and 6% fueloil. ANFO has been used in terroristattacks (e.g. an attack on the MurrahFederal Building, Oklahoma City, in1995).

angle-resolved photoelectronspectroscopy (ARPES) A techniquefor studying the composition andstructure of surfaces by measuringboth the kinetic energy and angulardistribution of photoelectronsejected from a surface by electromag-netic radiation. See also photoelec-tron spectroscopy.

anglesite A mineral form of*lead(II) sulphate, PbSO4.

angle strain The departure of abond angle from its normal value.The effects of angle strain are oftenapparent in aliphatic ring com-pounds. For example, in cyclopen-tane, C5H10, the angles between thebonds of the ring differ from the nor-mal tetrahedral angle (109° 28′),which is found in cyclohexane. Thisring form of angle strain is oftencalled Baeyer strain.


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angstrom Symbol Å. A unit oflength equal to 10–10 metre. It wasformerly used to measure wave-lengths and intermolecular distancesbut has now been replaced by thenanometre. 1 Å = 0.1 nanometre. Theunit is named after A. J. *Ångström.

Angström, Anders Jonas (1814–74) Swedish astronomer and physi-cist who became professor of physicsat the University of Uppsala from1858 until his death. He workedmainly with emission *spectra,demonstrating the presence of hydro-gen in the sun. He also worked outthe wavelengths of *Fraunhofer lines.Since 1905 spectral wavelengthshave been expressed in *angstroms.

angular momentum A propertyof a rotating body. In the case of arigid rotating body the angular mo-mentum is given by Iω, where I is the*moment of inertia of the body andω is its angular velocity. The quan-tum theory of angular momentum isclosely associated with the *rotationgroup and has important applica-tions in the electronic structure ofatoms and diatomic molecules and*rotational spectroscopy of mol-ecules. Electron spin is a type of an-gular momentum.

anharmonicity The extent towhich the oscillation of an oscillatordiffers from simple harmonic mo-tion. In molecular vibrations the an-harmonicity is very small near theequilibrium position, becomes largeas the vibration moves away fromthe equilibrium position, and is verylarge as dissociation is approached.Anharmonicity is taken into accountin molecular vibrations by adding ananharmonicity term to the potentialenergy function of the molecule. For a harmonic oscillator the poten-tial energy function U is given by U = f(r – re)2 where r is the inter-

atomic distance, re is the equilibriuminteratomic distance, and f is a con-stant. Anharmonicity is taken intoaccount by adding a cubic term g(r –re)3 to the quadratic term, where g ismuch smaller than f. Higher terms in(r – re) can be added to improve thedescription of anharmonicity.

anharmonic oscillator An oscil-lating system (in either classical me-chanics or *quantum mechanics)that is not oscillating in simple har-monic motion. In general, the prob-lem of an anharmonic oscillator isnot exactly soluble, although manysystems approximate to harmonic os-cillators and for such systems the*anharmonicity can be calculatedusing *perturbation theory.

anhydride A compound that pro-duces a given compound on reactionwith water. For instance, sulphur tri-oxide is the (acid) anhydride of sul-phuric acid SO3 + H2O → H2SO4.See also acid anhydrides.

anhydrite An important rock-forming anhydrous mineral form ofcalcium sulphate, CaSO4. It is chemi-cally similar to *gypsum but isharder and heavier and crystallizes inthe rhombic form (gypsum is mono-clinic). Under natural conditions an-hydrite slowly hydrates to formgypsum. It occurs chieÛy in whiteand greyish granular masses and isoften found in the caprock of certainsalt domes. It is used as a raw ma-terial in the chemical industry and inthe manufacture of cement and fer-tilizers.

anhydrous Denoting a chemicalcompound lacking water: appliedparticularly to salts lacking theirwater of crystallization.

aniline See phenylamine.

anilinium ion The ion C6H5NH3+,

derived from *phenylamine.

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animal charcoal See charcoal.

animal starch See glycogen.

anion A negatively charged *ion,i.e. an ion that is attracted to the*anode in *electrolysis. Comparecation.

anionic detergent See detergent.

anionic resin See ion exchange.

anisotropic Denoting a medium inwhich certain physical properties aredifferent in different directions.Wood, for instance, is an anisotropicmaterial: its strength along the graindiffers from that perpendicular tothe grain. Single crystals that are notcubic are anisotropic with respect tosome physical properties, such as thetransmission of electromagnetic radi-ation. Compare isotropic.

annealing A form of heat treat-

ment applied to a metal to soften it,relieve internal stresses and instabili-ties, and make it easier to work ormachine. It consists of heating themetal to a speciÜed temperature for aspeciÜed time, both of which dependon the metal involved, and then al-lowing it to cool slowly. It is appliedto both ferrous and nonferrous met-als and a similar process can be ap-plied to other materials, such asglass.

annelation See annulation.

annulation A type of chemical re-action in which a ring is fused to amolecule by formation of two newbonds. Sometimes the term annela-tion is used. See also cyclization.

annulenes Organic hydrocarbonsthat have molecules containing sim-ple single rings of carbon atoms

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[14]-Annulene

[18]-Annulene

H

[30]-Annulene

H

H H

H H

H H

H H

H H

H H

H H

H

H H

H

H

H

HH H

H

H

H

H H

H H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

HH

HH

HH

H

H

H HH

H H

Annulenes


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linked by alternating single and dou-ble bonds. Such compounds haveeven numbers of carbon atoms.*Cyclo-octatetraene, C8H8, is the nextin the series following benzene.Higher annulenes are usually re-ferred to by the number of carbonatoms in the ring, as in [10]-annu-lene, C10H10, [12]-annulene, C12H12,etc. The lower members are not sta-ble as a result of the interactions be-tween hydrogen atoms inside thering. This is true even for moleculesthat have the necessary number of pielectrons to be *aromatic com-pounds. Thus, [10]-annulene has 4n +2 pi electrons with n = 2, but is notaromatic because it is not planar.[14]-annulene also has a suitablenumber of pi electrons to be aro-matic (n = 3) but is not planar be-cause of interaction between theinner hydrogens.

The compound [18]-annulene islarge enough to be planar and obeysthe Hückel rule (4n + 2 = 18, with n =4). It is a brownish red fairly stablereactive solid. NMR evidence showsthat it has aromatic character. Theannulene with n = 7, [30]-annulene,can also exist in a planar form but ishighly unstable. See also pseudoaro-matic.

anode A positive electrode. In*electrolysis anions are attracted tothe anode. In an electronic vacuumtube it attracts electrons from the*cathode and it is therefore from theanode that electrons Ûow out of thedevice. In these instances the anodeis made positive by external means;however in a *voltaic cell the anodeis the electrode that spontaneouslybecomes positive and therefore at-tracts electrons to it from the exter-nal circuit.

anode sludge See electrolytic re-fining.

anodizing A method of coating ob-jects made of aluminium with a pro-tective oxide Ülm, by making themthe anode in an electrolytic bath con-taining an oxidizing electrolyte. An-odizing can also be used to produce a decorative Ünish by formation of an oxide layer that can absorb acoloured dye.

anomeric effect If a pyranose ringhas an electronegative substituent atthe anomeric carbon then this sub-stituent is more likely to take theaxial conÜguration than the equator-ial conÜguration. This is a conse-quence of a general effect (thegeneralized anomeric effect) that in achain of atoms X–C–Y–C, in which Yand X are atoms with nonbondingelectron pairs (e.g. F, O, N), the syn-clinal conformation is more likely.For example, in the compoundCH3–O–CH2Cl, rotation can occuralong the O–C bond. The conforma-tion in which the chlorine atom iscloser to the methyl group is pre-ferred (the lone pairs on the oxygenatom act as groups in determiningthe conformation).

anomers Diastereoisomers of cyclicforms of sugars or similar moleculesdiffering in the conÜguration at theC1 atom of an aldose or the C2 atomof a ketose. This atom is called theanomeric carbon. Anomers are desig-nated α if the conÜguration at theanomeric carbon is the same as thatat the reference asymmetric carbonin a Fischer projection. If the conÜgu-ration differs the anomer is desig-nated β. The α- and β-forms ofglucose are examples of anomers (seemonosaccharide).

anoxic reactor A *bioreactor inwhich the organisms being culturedare anaerobes or in which the reac-tion being exploited does not requireoxygen.

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anthocyanin One of a group of*Ûavonoid pigments. Anthocyaninsoccur in various plant organs and areresponsible for many of the blue,red, and purple colours in plants(particularly in Ûowers).

anthracene A white crystallinesolid, C14H10; r.d. 1.28; m.p. 215.8°C;b.p. 341.4°C. It is an aromatic hydro-carbon with three fused rings (seeformula), and is obtained by the dis-tillation of crude oils. The main useis in the manufacture of dyes.

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O

O

1

2

3

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8

Anthraquinone

anthracite See coal.

anthraquinone A colourless crys-talling *quinone; m.p. 154°C. It maybe prepared by reacting benzenewith phthalic anhydride. The com-

pound is the basis of a range ofdyestuffs. *Alizarin is an example.

anti 1. See torsion angle. 2. Seee–z convention.

antiaromatic See pseudoaromatic.

antibiotics Substances that destroyor inhibit the growth of microorgan-isms, particularly disease-producingbacteria. Antibiotics are obtainedfrom microorganisms (especiallymoulds) or synthesized. Commonantibiotics include the penicillins,streptomycin, and the tetracyclines.They are used to treat various infec-tions but tend to weaken the body’snatural defence mechanisms and cancause allergies. Overuse of antibioticscan lead to the development of resis-tant strains of microorganisms.

antibonding orbital See orbital.

anticlinal See torsion angle.

antiferromagnetism See magnet-ism.

antiÛuorite structure A crystalstructure for ionic compounds of thetype M2X, which is the same as theÛuorite (CaF2) structure except thatthe positions of the anions andcations are reversed, i.e. the cationsoccupy the F– sites and the anions oc-cupy the Ca2+ sites. Examples of theantiÛuorite structure are given byK2O and K2S. See fluorite structure.

antifoaming agent A substancethat prevents a foam from forming,employed in such processes as elec-troplating and paper-making, and inwater for boilers. They are com-pounds that are absorbed strongly bythe liquid (usually water) but lack theproperties that allow foaming. Sub-stances used include polyamides,polysiloxanes and silicones.

antifreeze A substance added tothe liquid (usually water) in the cool-ing systems of internal-combustion

Anthracene

O

R

R

R

R

R

R

R

+

Anthocyanin


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engines to lower its freezing point sothat it does not solidify at sub-zerotemperatures. The commonest an-tifreeze is *ethane-1,2-diol (ethyleneglycol).

antigorite See serpentine.

anti-isomer See isomerism.

antiknock agent A petrol additivethat inhibits preignition (‘knocking’)in internal-combustion engines. Anti-knock agents work by retardingcombustion chain reactions. Thecommonest, lead(IV) tetraethyl,causes environmental pollution, andits use is being discouraged.

antimonic compounds Com-pounds of antimony in its +5 oxida-tion state; e.g. antimonic chloride isantimony(V) chloride (SbCl5).

antimonous compounds Com-pounds of antimony in its +3 oxida-tion state; e.g. antimonous chloride isantimony(III) chloride (SbCl3).

antimony Symbol Sb. An elementbelonging to *group 15 (formerly VB)of the periodic table; a.n. 51; r.a.m.121.75; r.d. 6.68; m.p. 630.5°C; b.p.1750°C. Antimony has several al-lotropes. The stable form is a bluish-white metal. Yellow antimony andblack antimony are unstable non-metallic allotropes made at low tem-peratures. The main source isstibnite (Sb2S3), from which anti-mony is extracted by reduction withiron metal or by roasting (to give theoxide) followed by reduction withcarbon and sodium carbonate. Themain use of the metal is as an alloy-ing agent in lead-accumulator plates,type metals, bearing alloys, solders,Britannia metal, and pewter. It is alsoan agent for producing pearlitic castiron. Its compounds are used inÛame-prooÜng, paints, ceramics,enamels, glass dyestuffs, and rubbertechnology. The element will burn in

air but is unaffected by water or di-lute acids. It is attacked by oxidizingacids and by halogens. It was Ürst re-ported by Tholden in 1450.A• Information from the WebElements site

antioxidants Substances that slowthe rate of oxidation reactions. Vari-ous antioxidants are used to preservefoodstuffs and to prevent the deterio-ration of rubber, synthetic plastics,and many other materials. Some an-tioxidants act as chelating agents tosequester the metal ions that catalyseoxidation reactions. Others inhibitthe oxidation reaction by removingoxygen free radicals. Naturally occur-ring antioxidants include *vitamin Eand β-carotene; they limit the celland tissue damage caused by foreignsubstances, such as toxins and pollu-tants, in the body.

antiparallel spins Neighbouringspinning electrons in which the*spins, and hence the magnetic mo-ments, of the electrons are aligned inthe opposite direction. Under somecircumstances the interactions be-tween magnetic moments in atomsfavour *parallel spins, while underother conditions they favour antipar-allel spins. The case of antiferromag-netism (see magnetism) is an exampleof a system with antiparallel spins.

antiperiplanar See torsion angle.

anti-Stokes radiation Electro-magnetic radiation occurring in the*Raman effect, which is of muchlower intensity than the Rayleighscattering in which the frequency ofthe radiation is higher than that ofthe incident light, i.e. displaced to-wards shorter wavelengths. If the fre-quency of the original light is ν, thefrequency of the anti-Stokes radia-tion is ν + νk, where νk is the fre-quency of the rotation or vibration ofthe molecule. The spectral lines asso-

41 anti-Stokes radiation

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ciated with anti-Stokes radiation arecalled anti-Stokes lines. The quantumtheory of the Raman effect providesan explanation for the intensity ofthe anti-Stokes line being much lessthan the intensity of the Stokes line.

ap Antiperiplanar. See torsionangle.

apatite A complex mineral form of*calcium phosphate, Ca5(PO4)3-(OH,F,Cl); the commonest of thephosphate minerals. It has a hexago-nal structure and occurs widely as anaccessory mineral in igneous rocks(e.g. pegmatite) and often in regionaland contact metamorphic rocks, es-pecially limestone. Large depositsoccur in the Kola Peninsula, Russia. Itis used in the production of fertiliz-ers and is a major source of phospho-rus. The enamel of teeth is composedchieÛy of apatite.

apical Designating certain bondsand positions in a bipyramidal orpyramidal structure. For example, ina trigonal bipyramid the two bondsaligned through the central atom arethe apical bonds and the groups at-tached to these bonds are in apicalpositions. The three other bonds andpositions are equatorial. Similarly, ina square pyramidal structure thebond perpendicular to the base ofthe pyramid is the apical bond. Thefour other coplanar bonds (and posi-tions) are said to be basal.

aprotic See solvent.

aqua acid A type of acid in whichthe acidic hydrogen is on a watermolecule coordinated to a metal ion.For example,

Al(OH2)63+ + H2O →Al(OH2)5(OH)2+ + H3O+

aqua regia A mixture of concen-trated nitric acid and concentratedhydrochloric acid in the ratio 1:3 re-spectively. It is a very powerful oxi-

dizing mixture and will dissolve allmetals (except silver, which forms aninsoluble chloride) including suchnoble metals as gold and platinum,hence its name (‘royal water’). Nitro-syl chloride (NOCl) is believed to beone of the active constituents.

aquation The process in whichwater molecules solvate or form co-ordination complexes with ions.

aqueous Describing a solution inwater.

aquo ion A hydrated positive ionpresent in a crystal or in solution.

arachidonic acid An unsaturatedfatty acid, CH3(CH2)3(CH2CH:CH)4-(CH2)3COOH, that is essential forgrowth in mammals. It can be syn-thesized from *linoleic acid. Arachi-donic acid acts as a precursor toseveral biologically active com-pounds, including prostaglandins,and plays an important role in mem-brane production and fat metabo-lism. The release of arachidonic acidfrom membrane phospholipids istriggered by certain hormones.

arachno structure See borane.

aragonite A rock-forming anhy-drous mineral form of calcium car-bonate, CaCO3. It is much less stablethan *calcite, the commoner form ofcalcium carbonate, from which itmay be distinguished by its greaterhardness and speciÜc gravity. Overtime aragonite undergoes recrystal-lization to calcite. Aragonite occursin cavities in limestone, as a depositin limestone caverns, as a precipitatearound hot springs and geysers, andin high-pressure low-temperaturemetamorphic rocks; it is also foundin the shells of a number of molluscsand corals and is the main con-stituent of pearls. It is white orcolourless when pure but the pres-

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ence of impurities may tint it grey,blue, green, or pink.

arene complex A complex inwhich an aromatic ring is bound to ametal atom by its pi-electrons. Exam-ples of arene complexes are the*sandwich compounds (C6H6)2Cr and(C5H5)2Fe.

arenes Aromatic hydrocarbons,such as benzene, toluene, and naph-thalene.

argentic compounds Compoundsof silver in its higher (+2) oxidationstate; e.g. argentic oxide is silver(II)oxide (AgO).

argentite A sulphide ore of silver,Ag2S. It crystallizes in the cubic sys-tem but most commonly occurs inmassive form. It is dull grey-black incolour but bright when Ürst cut andoccurs in veins associated with othersilver minerals. Important depositsoccur in Mexico, Peru, Chile, Bolivia,and Norway.

argentous compounds Com-pounds of silver in its lower (+1) oxi-dation state; e.g. argentous chlorideis silver(I) chloride.

arginine See amino acid.

argon Symbol Ar. A monatomicnoble gas present in air (0.93%); a.n.18; r.a.m. 39.948; d. 0.00178 g cm–3;m.p. –189°C; b.p. –185°C. Argon isseparated from liquid air by frac-tional distillation. It is slightly solu-ble in water, colourless, and has nosmell. Its uses include inert atmos-pheres in welding and special-metalmanufacture (Ti and Zr), and (whenmixed with 20% nitrogen) in gas-Ülledelectric-light bulbs. The element isinert and has no true compounds.Lord *Rayleigh and Sir William*Ramsey identiÜed argon in 1894.


Arndt–Eisert systhesis A methodof converting a carboxylic acid intothe next higher homologue acid (orone of its derivatives). Diazomethaneis used to insert a CH2 group.

aromatic compound An organiccompound that contains a benzenering in its molecules or that haschemical properties similar to ben-zene. Aromatic compounds are un-saturated compounds, yet they donot easily partake in addition reac-tions. Instead they undergo elec-trophilic substitution.

Benzene, the archetypal aromaticcompound, has an hexagonal ring ofcarbon atoms and the classical for-mula (the Kekulé structure) wouldhave alternating double and singlebonds. In fact all the bonds in ben-zene are the same length intermedi-ate between double and single C–Cbonds. The properties arise becausethe electrons in the π-orbitals are de-localized over the ring, giving anextra stabilization energy of150 kJ mol–1 over the energy of aKekulé structure. The condition forsuch delocalization is that a com-pound should have a planar ringwith (4n + 2) pi electrons – this isknown as the Hückel rule. Aromaticbehaviour is also found in hetero-cyclic compounds such as pyridine.Aromatic character can be detectedby the presence of a ring currentusing NMR. See also annulenes; non-benzenoid aromatic; pseudoaro-matic.A• Information about IUPAC nomenclature

aromaticity The property charac-teristic of *aromatic compounds.

ARPES See angle-resolved photo-electron spectroscopy.

Arrhenius, Svante (August)(1859–1927) Swedish physicalchemist who Ürst demonstrated that

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electrolytes conduct because of thepresence of ions. He also worked onkinetics and proposed the *Arrhe-nius equation. He was the Ürst to pre-dict the greenhouse effect resultingfrom carbon dioxide. Arrhenius wasawarded the 1903 Nobel Prize forchemistry.

Arrhenius equation An equationof the form

k = Aexp(–Ea/RT)where k is the rate constant of agiven reaction and Ea the *activationenergy. A is a constant for a given re-action, called the pre-exponentialfactor (or A-factor). Often the equa-tion is written in logarithmic form

lnk = lnA – Ea/RTA graph of lnk against 1/T is a straightline with a gradient –Ea/R and an in-tercept on the lnk axis of lnA. It isnamed after Svante *Arrhenius.

Arrhenius theory See acid.

arsenate(III) See arsenic(iii) oxide.

arsenate(V) See arsenic(v) oxide.

arsenic Symbol As. A metalloid el-ement of *group 15 (formerly VB) ofthe periodic table; a.n. 33; r.a.m.74.92; r.d. 5.7; sublimes at 613°C. Ithas three allotropes – yellow, black,and grey. The grey metallic form isthe stable and most common one.Over 150 minerals contain arsenicbut the main sources are as impuri-ties in sulphide ores and in the min-erals orpiment (As2S3) and realgar(As4S4). Ores are roasted in air toform arsenic oxide and then reducedby hydrogen or carbon to metallic ar-senic. Arsenic compounds are used ininsecticides and as doping agents insemiconductors. The element is in-cluded in some lead-based alloys topromote hardening. Confusion canarise because As4O6 is often sold aswhite arsenic. Arsenic compoundsare accumulative poisons. The el-

ement will react with halogens, con-centrated oxidizing acids, and hot al-kalis. Albertus Magnus is believed tohave been the Ürst to isolate the el-ement in 1250.


arsenic acid See arsenic(v) oxide.

arsenic(III) acid See arsenic(iii)oxide.

arsenic hydride See arsine.

arsenic(III) oxide (arsenic trioxide;arsenious oxide; white arsenic) Awhite or colourless compound,As4O6, existing in three solid forms.The commonest has cubic or octahe-dral crystals (r.d. 3.87; sublimes at193°C) and is soluble in water,ethanol, and alkali solutions. It oc-curs naturally as arsenolite. A vitre-ous form can be prepared by slowcondensation of the vapour (r.d.3.74); its solubility in cold water ismore than double that of the cubicform. The third modiÜcation, whichoccurs naturally as claudetite, hasmonoclinic crystals (r.d. 4.15). Ar-senic(III) oxide is obtained commer-cially as a byproduct from thesmelting of nonferrous sulphide ores;it may be produced in the laboratoryby burning elemental arsenic in air.The structure of the molecule is simi-lar to that of P4O6, with a tetrahedralarrangement of As atoms edge linkedby oxygen bridges. Arsenic(III) oxideis acidic; its solutions were formerlycalled arsenious acid (technically, ar-senic(III) acid). It forms arsenate(III)salts (formerly called arsenites). Ar-senic(III) oxide is extremely toxic andis used as a poison for vermin; tracedoses are used for a variety of medici-nal purposes. It is also used for pro-ducing opalescent glasses andenamels.

arsenic(V) oxide (arsenic oxide) A

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white amorphous deliquescent solid,As2O5; r.d. 4.32; decomposes at315°C. It is soluble in water andethanol. Arsenic(V) oxide cannot beobtained by direct combination of ar-senic and oxygen; it is usually pre-pared by the reaction of arsenic withnitric acid followed by dehydrationof the arsenic acid thus formed. Itreadily loses oxygen on heating togive arsenic(III) oxide. Arsenic(V)oxide is acidic, dissolving in water togive arsenic(V) acid (formerly calledarsenic acid), H3AsO4; the acid is trib-asic and slightly weaker than phos-phoric acid and should be visualizedas (HO)3AsO. It gives arsenate(V) salts(formerly called arsenates).

arsenic trioxide See arsenic(iii)oxide.

arsenious acid See arsenic(iii)oxide.

arsenious oxide See arsenic(iii)oxide.

arsenite See arsenic(iii) oxide.

arsenolite A mineral form of *ar-senic(III) oxide, As4O6.

arsine (arsenic hydride) A colourlessgas, AsH3; m.p. –116.3°C; b.p. –55°C.It is soluble in water, chloroform,and benzene. Liquid arsine has a rela-tive density of 1.69. Arsine is pro-duced by the reaction of mineralacids with arsenides of electroposi-tive metals or by the reduction ofmany arsenic compounds usingnascent hydrogen. It is extremelypoisonous and, like the hydrides ofthe heavier members of group 15(formerly VB), is readily decomposedat elevated temperatures (around260–300°C). Like ammonia and phos-phine, arsine has a pyramidal struc-ture.

Arsine gas has a very importantcommercial application in the pro-duction of modern microelectronic

components. It is used in a dilute gasmixture with an inert gas and itsready thermal decomposition is ex-ploited to enable other growing crys-tals to be doped with minute tracesof arsenic to give n-type semiconduc-tors.

artinite A mineral form of basic*magnesium carbonate,MgCO3.Mg(OH)2.3H2O.

aryl group A group obtained by re-moving a hydrogen atom from anaromatic compound, e.g. phenylgroup, C6H5–, derived from benzene.

aryne A compound that can be re-garded as formed from an arene byremoving two adjacent hydrogenatoms to convert a double bond intoa triple bond. Arynes are transient in-termediates in a number of reactions.The simplest example is *benzyne.

asbestos Any one of a group ofÜbrous amphibole minerals (amosite,crocidolite (blue asbestos), tremolite,anthophyllite, and actinolite) or theÜbrous serpentine mineral chrysotile.Asbestos has widespread commercialuses because of its resistance to heat,chemical inertness, and high electri-cal resistance. The Übres may bespun and woven into Üreproof clothfor use in protective clothing, cur-tains, brake linings, etc., or mouldedinto blocks. Since the 1970s short as-bestos Übres have been recognized asa cause of asbestosis, a serious lungdisorder, and mesothelioma, a fatalform of lung cancer. These concernshave limited its use and imposedmany safety procedures when it isused. Canada is the largest producerof asbestos; others include Russia,South Africa, Zimbabwe, and China.

ascorbic acid See vitamin c.

asparagine See amino acid.

aspartic acid See amino acid.

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asphalt Bitumen. See petroleum.

aspirin (acetylsalicylic acid) anacetylated form of *salicylic acid (1-hydroxybenzoic acid), used exten-sively as a medicinal drug; r.d. 1.4;m.p. 138–140°C; b.p. 140°C (with de-composition). It was Ürst marketed in1899 as a analgesic. The acid can beobtained from willow bark and it haslong been known that the bark couldbe used for pain relief and for the re-duction of fever. The name salicylicacid comes from the botanical nameof the willow (Salix alba). Aspirin actsby suppressing the production of cer-tain hormones (prostaglandins andthromboxanes) by inhibiting the en-zyme cyclooxygenase (COX). Conse-quently, it is known as a ‘COXinhibitor’. It is used for the treatmentof arthritis and to reduce body tem-perature. It also acts as an anticoagu-lant in the blood and small doses aretaken regularly to reduce the risk ofheart attack. A common side effect ofhigh doses is stomach bleeding andstomach ulcers. Aspirin is made in-dustrially from phenol, which withconcentrated sodium hydroxide andcarbon dioxide gives sodium phenox-ide:

C6H5OH + NaOH → C6H5O– Na+ +H2O.

The phenoxide ion undergoes elec-trophilic substitution to give sodiumsalicylate:

C6H5O– + CO2 + Na+ → C6H4(OH)COO– Na+

With acid, this forms salicylic acid,which can be acetylated in the orthoposition with ethanoic anhydride.

associated liquid See association.

association The combination ofmolecules of one substance withthose of another to form chemicalspecies that are held together byforces weaker than normal chemical

bonds. For example, ethanol andwater form a mixture (an associatedliquid) in which hydrogen bondingholds the different molecules to-gether.

astatine Symbol At. A radioactive*halogen element; a.n. 85; r.a.m. 211;m.p. 302°C; b.p. 337°C. It occurs nat-urally by radioactive decay from ura-nium and thorium isotopes. Astatineforms at least 20 isotopes, the moststable astatine–210 has a half-life of8.3 hours. It can also be produced byalpha bombardment of bismuth–200.Astatine is stated to be more metallicthan iodine; at least 5 oxidationstates are known in aqueous solu-tions. It will form interhalogen com-pounds, such as AtI and AtCl. Theexistence of At2 has not yet been es-tablished. The element was synthe-sized by nuclear bombardment in1940 by D. R. Corson, K. R. MacKen-zie, and E. Segré at the University ofCalifornia.


Aston, Francis William(1877–1945) British chemist andphysicist, who until 1910 worked atMason College (later BirminghamUniversity) and then with J. J. *Thom-son at Cambridge University. In 1919Aston designed the mass spectro-graph (see mass spectroscopy), forwhich he was awarded the NobelPrize for chemistry in 1922. With ithe discovered the *isotopes of neon,and was thus able to explain noninte-gral atomic weights.

astrochemistry The study of mol-ecules in interstellar space. Interstel-lar molecules are usually detected by their spectra in the radio, micro-wave, or infrared regions of the elec-tromagnetic spectrum. To date, over140 different molecules have beendetected. Of special interest in astro-

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chemistry is the way in which thesemolecules are formed and the way in which they interact with clouds ofinterstellar dust.A• The website of the astrochemistry work

group of the International AstronomicalUnion

asymmetric atom See optical ac-tivity.

asymmetric induction The pref-erential formation of one particularenantiomer or diastereoisomer in areaction as a result of a chiral el-ement in one of the reactants or inthe catalyst used.

asymmetric top See moment ofinertia.

atactic polymer See polymer.

ATLEED (automated tensor low-energy electron diffraction) A formof *LEED (low-energy electron dif-fraction) in which the informationobtained can readily be stored andanalysed using a computer.

atmolysis The separation of a mix-ture of gases by means of their differ-ent rates of diffusion. Usually,separation is effected by allowing thegases to diffuse through the walls ofa porous partition or membrane.

atmosphere 1. (atm.) A unit ofpressure equal to 101 325 pascals.This is equal to 760.0 mmHg. The ac-tual *atmospheric pressure Ûuctuatesaround this value. The unit is usuallyused for expressing pressures well inexcess of standard atmospheric pres-sure, e.g. in high-pressure chemicalprocesses. 2. See earth’s atmos-phere.

atmospheric pressure The pres-sure exerted by the weight of the airabove it at any point on the earth’ssurface. At sea level the atmospherewill support a column of mercury

about 760 mm high. This decreaseswith increasing altitude. The stan-dard value for the atmospheric pres-sure at sea level in SI units is 101 325pascals.

atom The smallest part of an el-ement that can exist chemically.Atoms consist of a small dense nu-cleus of protons and neutrons sur-rounded by moving electrons. Thenumber of electrons equals the num-ber of protons so the overall chargeis zero. The electrons are consideredto move in circular or elliptical orbits(see bohr theory) or, more accu-rately, in regions of space around thenucleus (see orbital).

The electronic structure of an atomrefers to the way in which the elec-trons are arranged about the nucleus,and in particular the *energy levelsthat they occupy. Each electron canbe characterized by a set of fourquantum numbers, as follows:(1) The principal quantum number ngives the main energy level and hasvalues 1, 2, 3, etc. (the higher thenumber, the further the electronfrom the nucleus). Traditionally,these levels, or the orbits correspond-ing to them, are referred to as shellsand given letters K, L, M, etc. The K-shell is the one nearest the nucleus.The maximum number of electronsin a given shell is 2n2.(2) The orbital quantum number l,which governs the angular momen-tum of the electron. The possible val-ues of l are (n – 1), (n – 2), … 1, 0.Thus, in the Ürst shell (n = 1) the elec-trons can only have angular momen-tum zero (l = 0). In the second shell (n= 2), the values of l can be 1 or 0, giv-ing rise to two subshells of slightlydifferent energy. In the third shell (n= 3) there are three subshells, with l= 2, 1, or 0. The subshells are de-noted by letters s(l = 0), p(l = 1), d(l =2), f(l = 3). The number of electrons in

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each subshell is written as a super-script numeral to the subshell sym-bol, and the maximum number ofelectrons in each subshell is s2, p6,d10, and f14. The orbital quantumnumber is sometimes called the az-imuthal quantum number.(3) The magnetic quantum number m,which governs the energies of elec-trons in an external magnetic Üeld.This can take values of +l, +(l – 1), …1, 0, –1, … –(l – 1), –l. In an s-subshell(i.e. l = 0) the value of m = 0. In a p-subshell (l = 1), m can have values +1, 0, and –1; i.e. there are three p-orbitals in the p-subshell, usuallydesignated px, py, and pz. Under nor-mal circumstances, these all have the same energy level.(4) The spin quantum number ms,which gives the spin of the individ-ual electrons and can have the values+½ or –½.

According to the *Pauli exclusionprinciple, no two electrons in theatom can have the same set of quan-tum numbers. The numbers deÜnethe quantum state of the electron,and explain how the electronic struc-tures of atoms occur. See Chronol-ogy: Atomic Theory.

atomic absorption spectroscopy(AAS) An analytical technique inwhich a sample is vaporized and thenonexcited atoms absorb electromag-netic radiation at characteristic wave-lengths.

atomic clock An apparatus forstandardizing time based on periodicphenomena within atoms or mol-ecules. See ammonia clock; caesiumclock.

atomic emission spectroscopy(AES) An analytical technique inwhich a sample is vaporized and theatoms present are detected by theiremission of electromagnetic radia-tion at characteristic wavelengths.

atomic force microscope (AFM)A type of microscope in which asmall probe, consisting of a tiny chipof diamond, is held on a spring-loaded cantilever in contact with thesurface of the sample. The probe ismoved slowly across the surface andthe tracking force between the tipand the surface is monitored. Theprobe is raised and lowered so as tokeep this force constant, and aproÜle of the surface is produced.Scanning the probe over the samplegives a computer-generated contourmap of the surface. The instrumentis similar to the scanning tunnellingmicroscope, but uses mechanicalforces rather than electrical signals.It can resolve individual moleculesand, unlike the scanning tunnellingmicroscope, can be used with non-conducting samples, such as biologi-cal specimens.

atomicity The number of atoms ina given molecule. For example, oxy-gen (O2) has an atomicity of 2, ozone(O3) an atomicity of 3, benzene (C6H6)an atomicity of 12, etc.

atomic mass unit (a.m.u.) A unitof mass used to express *relativeatomic masses. It is equal to 1/12 ofthe mass of an atom of the isotopecarbon–12 and is equal to 1.660 33 ×10–27 kg. This unit superseded boththe physical and chemical mass unitsbased on oxygen–16 and is some-times called the uniÜed mass unit orthe dalton.

atomic number (proton number)Symbol Z. The number of protons inthe nucleus of an atom. The atomicnumber is equal to the number ofelectrons orbiting the nucleus in aneutral atom.

atomic orbital See orbital.

atomic volume The relative

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atomic mass of an element dividedby its density.

atomic weight See relativeatomic mass.

atom-probe Üeld-ion micros-copy A technique for identifying in-

dividual atoms on surfaces. In theatom-probe *Üeld-ionization micro-scope (FIM) there is a hole in theÛuorescent screen, with which theFIM image of an adsorbed atom isbrought into coincidence. The gascausing the imaging is removed. The

49 atom-probe Üeld-ion microscopy

aATOMIC THEORY

c.430 BC Greek natural philosopher Empedocles (d. c. 430 BC) proposes that allmatter consists of four elements: earth, air, fire, and water.

c.400 BC Greek natural philosopher Democritus of Abdera (c. 460–370 BC)proposes that all matter consists of atoms.

c.306 BC Greek philosopher Epicurus (c. 342–270 BC) champions Democritus’atomic theory.

1649 French philosopher Pierre Gassendi (1592–1655) proposes an atomictheory (having read Epicurus).

1803 John Dalton proposes Dalton’s atomic theory.

1897 J. J. Thomson discovers the electron.

1904 J. J. Thomson proposes his ‘plum pudding’ model of the atom, withelectrons embedded in a nucleus of positive charges. Japanese physicist Hantaro Nagaoka (1865–1950) proposes a ‘Saturn’model of the atom with a central nucleus having a ring of manyelectrons.

1911 New Zealand-born physicist Ernest Rutherford (1871–1937) discovers theatomic nucleus.

1913 Niels Bohr proposes model of the atom with a central nucleussurrounded by orbiting electrons.British physicist Henry Moseley (1887–1915) equates the positive chargeon the nucleus with its atomic number. Frederick Soddy discovers isotopes.

1916 German physicist Arnold Sommerfield (1868–1951) modifies Bohr’smodel of the atom specifying elliptical orbits for the electrons.

1919 Ernest Rutherford discovers the proton.

1920 Ernest Rutherford postulates the existence of the neutron.

1926 Erwin Schrödinger proposes a wave-mechanical model of the atom (withelectrons represented as wave trains).

1932 British physicist James Chadwick (1891–1974) discovers the neutron. Werner Heisenberg proposes a model of the atomic nucleus in whichprotons and neutrons exchange electrons to achieve stability.

1939 Niels Bohr proposes a ‘liquid drop’ model of the atomic nucleus.

1948 German-born US physicist Marie Goeppert-Meyer (1906–72) andGerman physicist Hans Jensen (1907–73) independently propose the‘shell’ structure of the nucleus.

1950 US physicist Leo Rainwater (1917–86) combines the ‘liquid-drop’ and‘shell’ models of the nucleus into a single theory.


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adsorbed atom is removed as an ionby a pulse of potential difference.The atom then passes in the same di-rection as the gas ions, through thehole in the screen. This enables theatom to be identiÜed by a mass spec-trometer behind the screen. Atom-probe FIM identiÜes both the typeand the position of the atom and canbe used to observe atomic processes,such as evaporation, with the pulseused for analysis lasting about 2nanoseconds.

ATP (adenosine triphosphate) A nu-cleotide that is of fundamental im-portance as a carrier of chemicalenergy in all living organisms. It con-sists of adenine linked to d-ribose (i.e.adenosine); the d-ribose componentbears three phosphate groups, lin-early linked together by covalentbonds (see formula). These bonds canundergo hydrolysis to yield either amolecule of ADP (adenosine diphos-phate) and inorganic phosphate or amolecule of AMP (adenosine mono-phosphate) and pyrophosphate. Boththese reactions yield a large amountof energy (about 30.6 kJ mol–1) that isused to bring about such biologicalprocesses as muscle contraction, theactive transport of ions and mol-ecules across cell membranes, andthe synthesis of biomolecules. Thereactions bringing about these pro-cesses often involve the enzyme-catalysed transfer of the phosphate

group to intermediate substrates.Most ATP-mediated reactions requireMg2+ ions as *cofactors.

ATP is regenerated by the rephos-phorylation of AMP and ADP usingthe chemical energy obtained fromthe oxidation of food. This takesplace during *glycolysis and the*Krebs cycle but, most signiÜcantly,is also a result of the reduction–oxidation reactions of the *electrontransport chain, which ultimately re-duces molecular oxygen to water (ox-idative phosphorylation).

atropine A poisonous crystalline al-kaloid, C17H23NO3; m.p. 118–119°C. Itcan be extracted from deadly night-shade and other solanaceous plantsand is used in medicine to treat colic,to reduce secretions, and to dilatethe pupil of the eye.

atropisomers Conformers thathave highly restricted rotation abouta single bond and can consequentlybe separated as distinct species. Thiscan occur, for example, in the case ofsubstituted biphenyls with largegroups at the orthopositions of therings.

ATRS See attenuated total re-flectance spectroscopy.

attenuated total reÛectancespectroscopy (ATRS) A variation ofinfrared spectroscopy in which the IRsource is reÛected from the sample

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–O O

O–

O

P

O–

O

P

O–

O

PO O CH2 O

OH OH

ribose

N

N

N

N

NH2

adenine

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and absorption occurs only in thesurface layer. ATRS is used in foren-sic science for analysis of thin layers(e.g. paint).

atto- Symbol a. A preÜx used in themetric system to denote 10–18. For ex-ample, 10–18 second = 1 attosecond(as).

attractor The set of points in phasespace to which the representativepoint of a dissipative system (i.e. onewith internal friction) tends as thesystem evolves. The attractor can be:a single point; a closed curve (a limitcycle), which describes a system withperiodic behaviour; or a strange at-tractor, in which case the system ex-hibits *chaos.

Aufbau principle A principle thatgives the order in which orbitals areÜlled in successive elements in theperiodic table. The order of Ülling is1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p,6s, 4f, 5d, 6p, 7s, 5f, 6d. See atom.

Auger effect The ejection of anelectron from an atom as a result ofthe de-excitation of an excited elec-tron within the atom. An electron isÜrst ejected from an atom by a pho-ton, electron impact, ion impact, orsome other process, thus creating avacancy. In the subsequent re-arrangement of the electronic struc-ture of the atom, an electron from ahigher energy level falls into the va-cancy. This process is associated withexcess energy, which is released bythe ejection of a second electron(rather than by emission of a pho-ton). This second electron is calledthe Auger electron. Auger spectros-copy is a form of electron spectros-copy using this effect to study theenergy levels of ions. It is also a formof analysis and can be used to iden-tify the presence of elements in sur-face layers of solids. The effect was

discovered by the French physicistPierre Auger (1899–1994) in 1925.

Auger electron See auger effect.

auric compounds Compounds ofgold in its higher (+3) oxidation state;e.g. auric chloride is gold(III) chloride(AuCl3).

aurous compounds Compoundsof gold in its lower (+1) oxidationstate; e.g. aurous chloride is gold(I)chloride (AuCl).

austenite See steel.

autocatalysis *Catalysis in whichone of the products of the reaction isa catalyst for the reaction. Reactionsin which autocatalysis occurs have acharacteristic S-shaped curve for re-action rate against time – the reac-tion starts slowly and increases asthe amount of catalyst builds up,falling off again as the products areused up.

autoclave A strong steel vesselused for carrying out chemical reac-tions, sterilizations, etc., at high tem-perature and pressure.

automated tensor low-energyelectron diffraction See atleed.

autoprotolysis A transfer of a hy-drogen ion (H+) between molecules ofan amphiprotic *solvent, one mol-ecule acting as a Brønsted acid andthe other as a Brønsted base. It oc-curs in the autoionization of water.

autoprotolysis constant Seeionic product.

autoradiography An experimen-tal technique in which a radioactivespecimen is placed in contact with(or close to) a photographic plate, soas to produce a record of the distribu-tion of radioactivity in the specimen.The Ülm is darkened by the ionizingradiation from radioactive parts ofthe sample. Autoradiography has a

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number of applications, particularlyin the study of living tissues andcells.

auxochrome A group in a dye mol-ecule that inÛuences the colour dueto the *chromophore. Auxochromesare groups, such as –OH and –NH2,containing lone pairs of electronsthat can be delocalized along withthe delocalized electrons of the chro-mophore. The auxochrome inten-siÜes the colour of the dye. Formerly,the term was also used of suchgroups as –SO2O–, which make themolecule soluble and affect its appli-cation.

Avogadro, Amedeo (1776–1856)Italian chemist and physicist. In 1811he published his hypothesis (see avo-gadro’s law), which provided amethod of calculating molecularweights from vapour densities. Theimportance of the work remainedunrecognized, however, until cham-pioned by Stanislao Cannizzaro(1826–1910) in 1860.

Avogadro constant Symbol NA orL. The number of atoms or moleculesin one *mole of substance. It has thevalue 6.022 1367(36) × 1023. Formerlyit was called Avogadro’s number.

Avogadro’s law Equal volumes ofall gases contain equal numbers ofmolecules at the same pressure andtemperature. The law, often calledAvogadro’s hypothesis, is true onlyfor ideal gases. It was Ürst proposedin 1811 by Amedeo *Avogadro.

axial See ring conformations.

Axilrod–Teller formula An ex-pression for three-body interactionsin intermolecular forces. These di-pole dispersion terms, called Axilrod–Teller terms (named after B. M. Axilrod and E. Teller, who dis-covered them in 1943) are of impor-tance in the third virial coefÜcient.

At low temperatures, the Axilrod–Teller terms can have a comparablesize to two-body interactions, makingthem of importance to such proper-ties of liquids as pressure and surfacetension. The Axilrod–Teller formulafor the dispersion energy V of threeclosed-shell atoms A, B, and C isgiven by

V = – C6/rAB6 – C6/rBC

6 – C6/rCA6 +

C′/(rABrBCrCA)3,where C6 is a coefÜcient dependingon the identity of the atoms, C′ is acoefÜcient depending on anotherconstant and the angles between theatoms, and the r terms give distancesbetween the indicated atoms.

AX spectrum A general pattern ofthe nuclear magnetic resonance spec-trum of a molecule AX, where A andX are both spin-½ nuclei. If X is spinα, the spin–spin interaction betweenthe nuclei of A and X results in oneline in the spectrum of A beingshifted by J/2 from the frequency ofprecession it has in the absence ofcoupling. If X is spin β, the change infrequency of precession of A is –J/2.Thus, single lines in the resonancesof both A and X are split into dou-blets, with splittings J. The A reso-nance in molecules of the type AnX isalso a doublet with the splitting J.

azeotrope (azeotropic mixture; con-stant-boiling mixture) A mixture oftwo liquids that boils at constantcomposition; i.e. the composition ofthe vapour is the same as that of theliquid. Azeotropes occur because ofdeviations in Raoult’s law leading toa maximum or minimum in the*boiling-point–composition diagram.When the mixture is boiled, thevapour initially has a higher propor-tion of one component than is pre-sent in the liquid, so the proportionof this in the liquid falls with time.Eventually, the maximum and mini-

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mum point is reached, at which thetwo liquids distil together withoutchange in composition. The composi-tion of an azeotrope depends on thepressure.

azeotropic distillation A tech-nique for separating components ofan azeotrope by adding a third liquidto form a new azeotrope with one ofthe original components. It is mostcommonly used to separate ethanolfrom water, adding benzene to asso-ciate with the ethanol.

azides Compounds containing theion N3

– or the group –N3.

azimuthal quantum number Seeatom.

azine An organic heterocyclic com-pound containing a six-memberedring formed from carbon and nitro-gen atoms. Pyridine is an examplecontaining one nitrogen atom(C5H5N). Diazines have two nitrogenatoms in the ring (e.g. C4H4N2), andisomers exist depending on the rela-tive positions of the nitrogen atoms.Triazines contain three nitrogenatoms.


azo compounds Organic com-pounds containing the group –N=N–linking two other groups. They can

be formed by reaction of a diazo-nium ion with a benzene ring.A• Information about IUPAC nomenclature

azo dye See dyes.

azoimide See hydrogen azide.

azulene 1. A blue crystalline com-pound, C10H8; m.p. 99°C. It contains aÜve-membered ring fused to a seven-membered ring and has aromaticproperties. When heated it is con-verted into naphthalene. 2. Any of anumber of blue oils that can be pro-duced by distilling or heating essen-tial oils from plants.

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azurite A secondary mineral con-sisting of hydrated basic copper car-bonate, Cu3(OH)2(CO3)2, inmonoclinic crystalline form. It is generally formed in the upper zoneof copper ore deposits and often oc-curs with *malachite. Its intenseazure-blue colour made it formerlyimportant as a pigment. It is a minorore of copper and is used as a gem-stone.


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B

Babbit metal Any of a group of re-lated alloys used for making bear-ings. They consist of tin containingantimony (about 10%) and copper(1–2%), and often lead. The originalalloy was invented in 1839 by the USinventor Isaac Babbit (1799–1862).

Babo’s law The vapour pressure ofa liquid is decreased when a solute isadded, the amount of the decreasebeing proportional to the amount ofsolute dissolved. The law was discov-ered in 1847 by the German chemistLambert Babo (1818–99). See alsoraoult’s law.

backbiting A rearrangement thatcan occur in some *polymerizationreactions involving free radicals. Aradical that has an unpaired electronat the end of the chain changes intoa radical with the unpaired electronelsewhere along the chain, the newradical being more stable than theone from which it originates. For ex-ample, the radical

RCH2CH2CH2CH2CH2CH2.

may change intoRCH2CH.CH2CH2CH2CH3

The rearrangement is equivalent to a hydrogen atom being transferredwithin the molecule. The new un-paired electron initiates furtherpolymerization, with the produc-tion of polymers with butyl(CH3CH2CH2CH2–) side chains.

back donation A form of chemicalbonding in which a *ligand forms asigma bond to an atom or ion by do-nating a pair of electrons, and thecentral atom donates electrons backby overlap of its d-orbitals withempty p- or d-orbitals on the ligand.It occurs, for example, in metal car-bonyls.

back e.m.f. An electromotive forcethat opposes the main current Ûowin a circuit. For example, in an elec-tric cell, *polarization causes a backe.m.f. to be set up by chemicalmeans.

background radiation Low inten-sity *ionizing radiation present on

Backbiting

RCH2

CH2

CH2

CH2CH2

H2C

RCH

CH2

CH2

CH2CH2

CH3

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CH2

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the surface of the earth and in theatmosphere as a result of cosmicradiation and the presence of radio-isotopes in the earth’s rocks, soil, and atmosphere. The radioisotopesare either natural or the result of nu-clear fallout or waste gas from powerstations. Background counts must betaken into account when measuringthe radiation produced by a speciÜedsource.

back titration A technique in *vol-umetric analysis in which a knownexcess amount of a reagent is addedto the solution to be estimated. Theunreacted amount of the addedreagent is then determined by titra-tion, allowing the amount of sub-stance in the original test solution tobe calculated.

bacteriocidal Capable of killingbacteria. Common bacteriocides aresome antibiotics, antiseptics, and dis-infectants.

bacteriorhodopsin A membrane-bound protein of the halophilic (salt-resistant) bacterium Halobacteriumhalobium. When activated by light, itpumps protons out of the cell; thiscreates a concentration gradient,which enables ATP to be synthesized.Bacteriorhodopsin is composed ofseven α-helix segments, which spanthe membrane and are joined to-gether by short amino-acid chains. Itcontains the prosthetic group retinal,which is also found in the pigmentrhodopsin in the rod cells of verte-brates.

Baeyer, Adolf von (1835–1917)German organic chemist noted forhis work on organic synthesis. Hesynthesized and determined thestructure of indigo and also synthe-sized some of the Ürst barbiturates.In his work on carbon rings he for-mulated his strain theory. Bayer was

awarded the 1905 Nobel Prize forchemistry.

Baeyer strain See angle strain.

Baeyer test A test for unsaturatedcompounds in which potassium per-manganate is used. Alkenes, for ex-ample, are oxidised to glycols, andthe permanganate loses its colour:

3R2C=CR2 + 2KMnO4 + 4H2O →2MnO2 + 2KOH + 3R2COHR2COH

Baeyer–Villiger reaction A re-arrangement reaction, sometimesknown as the Dakin reaction, com-monly used in organic synthesis inwhich a ketone reacts with a peroxyacid to form an ester. For example,

R–CO–R → R–CO–O–RThe reaction is equivalent to the in-sertion of an oxygen atom next tothe ketone’s carbonyl (>C=O) group.Meta-chloroperbenzoic acid (m-CPBA;ClC6H4.CO.O.OH) and triÛuoro-perethanoic acid (CF3.CO.O.OH) aretypical peroxy acids employed in thereaction. It was discovered by Adolfvon *Baeyer and the Germanchemist V. Villiger in 1899.

Bakelite A trade name for certain*phenol–formaldehyde resins, Ürstintroduced in 1909 by the Belgian–US chemist Leo Hendrik Baekeland(1863–1944).

baking powder A mixture of pow-dered compounds added to dough orcake mixture to make it rise in cook-ing. It is used as a substitute for yeastin bread-making. Baking powdersconsist of a source of carbon dioxide,such as sodium hydrogencarbonateor ammonium hydrogencarbonate,and an acidic substance such as cal-cium hydrogenphosphate, potassiumhydrogentartrate (cream of tartar), orsodium hydrogenphosphate. In thehot wet mixture, the acid releasesbubbles of carbon dioxide gas from

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the hydrogencarbonate, which makethe mixture rise.

baking soda See sodium hydro-gencarbonate.

balance An accurate weighing de-vice. The simple beam balance con-sists of two pans suspended from acentrally pivoted beam. Knownmasses are placed on one pan andthe substance or body to be weighedis placed in the other. When thebeam is exactly horizontal the twomasses are equal. An accurate labora-tory balance weighs to the nearesthundredth of a milligram. Speciallydesigned balances can be accurate toa millionth of a milligram. Moremodern substitution balances use thesubstitution principle. In this cali-brated weights are removed from thesingle lever arm to bring the singlepan suspended from it into equilib-rium with a Üxed counter weight.The substitution balance is more ac-curate than the two-pan device andenables weighing to be carried outmore rapidly. In automatic electronicbalances, mass is determined not bymechanical deÛection but by elec-tronically controlled compensationof an electric force. A scanner moni-tors the displacement of the pansupport generating a current propor-tional to the displacement. This cur-rent Ûows through a coil forcing thepan support to return to its originalposition by means of a magneticforce. The signal generated enablesthe mass to be read from a digitaldisplay. The mass of the empty con-tainer can be stored in the balance’scomputer memory and automaticallydeducted from the mass of the con-tainer plus its contents.

Balmer series See hydrogen spec-trum.

banana bond Informal name forthe type of electron-deÜcient three-

centre bond holding the B–H–Bbridges in *boranes and similar com-pounds.

band spectrum See spectrum.

band theory See energy bands.

bar A c.g.s. unit of pressure equal to 106 dynes per square centimetreor 105 pascals (approximately750 mmHg or 0.987 atmosphere).The millibar (100 Pa) is commonlyused in meteorology.

Barbier–Wieland degradationThe stepwise degradation of a car-boxylic acid to the next lower homo-logue. First the ester is convertedinto a tertiary alcohol using a Grig-nard reagent (PhMgX) and acid (HX):

RCH2COOCH3 → RCH2C(OH)Ph2.The secondary alcohol is then dehy-drated using ethanoic anhydride(CH3COOCOCH3) to give an alkene:

RCH2C(OH)Ph2 → RCH=CPh2.The alkene is oxidized with chromicacid:

RCH=CPh2 → RCOOH + Ph2CO.The result is conversion of an acidRCH2COOH to the lower acidRCOOH.

barbiturate Any one of a group ofdrugs derived from barbituric acidthat have a depressant effect on thecentral nervous system. Barbiturateswere originally used as sedatives andsleeping pills but their clinical use isnow limited due to their toxic side-effects; prolonged use can lead to ad-diction. SpeciÜc barbiturates inclinical use include phenobarbitone,for treating epilepsy, and methohexi-tane sodium, used as an anaesthetic.

Barfoed’s test A biochemical testto detect monosaccharide (reducing)sugars in solution, devised by theSwedish physician Christen Barfoed(1815–99). Barfoed’s reagent, a mix-ture of ethanoic (acetic) acid and cop-

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per(II) acetate, is added to the test so-lution and boiled. If any reducingsugars are present a red precipitateof copper(II) oxide is formed. The re-action will be negative in the pres-ence of disaccharide sugars as theyare weaker reducing agents.

barite See barytes.

barium Symbol Ba. A silvery-whitereactive element belonging to *group2 (formerly IIA) of the periodic table;a.n. 56; r.a.m. 137.34; r.d. 3.51; m.p.725°C; b.p. 1640°C. It occurs as theminerals barytes (BaSO4) andwitherite (BaCO3). Extraction is byhigh-temperature reduction of bar-ium oxide with aluminium or siliconin a vacuum, or by electrolysis offused barium chloride. The metal isused as a getter in vacuum systems.It oxidizes readily in air and reactswith ethanol and water. Soluble bar-ium compounds are extremely poiso-nous. It was Ürst identiÜed in 1774 byKarl *Scheele, and was extracted byHumphry *Davy in 1808.A• Information from the WebElements site

barium bicarbonate See bariumhydrogencarbonate.

barium carbonate A white insolu-ble compound, BaCO3; r.d. 4.43. It de-composes on heating to give bariumoxide and carbon dioxide:

BaCO3(s) → BaO(s) + CO2(g)The compound occurs naturally asthe mineral witherite and can be pre-pared by adding an alkaline solutionof a carbonate to a solution of a bar-ium salt. It is used as a raw materialfor making other barium salts, as aÛux for ceramics, and as a raw ma-terial in the manufacture of certaintypes of optical glass.

barium chloride A white com-pound, BaCl2. The anhydrous com-pound has two crystalline forms: an

α form (monoclinic; r.d. 3.856),which transforms at 962°C to a βform (cubic; r.d. 3.917; m.p. 963°C;b.p. 1560°C). There is also a dihy-drate, BaCl2.2H2O (cubic; r.d. 3.1),which loses water at 113°C. It is pre-pared by dissolving barium carbonate(witherite) in hydrochloric acid andcrystallizing out the dihydrate. Thecompound is used in the extractionof barium by electrolysis.

barium hydrogencarbonate (bar-ium bicarbonate) A compound,Ba(HCO3)2, which is only stable in so-lution. It can be formed by the actionof carbon dioxide on a suspension ofbarium carbonate in cold water:

BaCO3(s) + CO2(g) + H2O(l) →Ba(HCO3)2(aq)

On heating, this reaction is reversed.

barium hydroxide (baryta) Awhite solid, Ba(OH)2, sparingly solu-ble in water. The common form isthe octahydrate, Ba(OH)2.8H2O; mon-oclinic; r.d. 2.18; m.p. 78°C. It can beproduced by adding water to bariummonoxide or by the action of sodiumhydroxide on soluble barium com-pounds and is used as a weak alkaliin volumetric analysis.

barium oxide A white or yellowishsolid, BaO, obtained by heating bar-ium in oxygen or by the thermal de-composition of barium carbonate ornitrate; cubic; r.d. 5.72; m.p. 1923°C;b.p. 2000°C. When barium oxide isheated in oxygen the peroxide, BaO2,is formed in a reversible reactionthat was once used as a method forobtaining oxygen (the Brin process).Barium oxide is now used in themanufacture of lubricating-oil addi-tives.

barium peroxide A dense off-white solid, BaO2, prepared by care-fully heating *barium oxide inoxygen; r.d. 4.96; m.p. 450°C. It is

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used as a bleaching agent. Withacids, hydrogen peroxide is formedand the reaction is used in the labo-ratory preparation of hydrogen per-oxide.

barium sulphate An insolublewhite solid, BaSO4, that occurs natu-rally as the mineral *barytes (orheavy spar) and can be prepared as aprecipitate by adding sulphuric acidto barium chloride solution; r.d. 4.50;m.p. 1580°C. The rhombic formchanges to a monoclinic form at1149°C. It is used as a raw materialfor making other barium salts, as apigment extender in surface coatingmaterials (called blanc Üxe), and in the glass and rubber industries.Barium compounds are opaque to X-rays, and a suspension of the sul-phate in water is used in medicine toprovide a contrast medium for X-raysof the stomach and intestine. Al-though barium compounds are ex-tremely poisonous, the sulphate issafe to use because it is very insolu-ble.

barrel A measurement of volume,widely used in the chemical industry,equal to 35 UK gallons (approxi-mately 159 litres).

Bartlett, Neil (1932– ) British in-organic chemist who prepared oxy-gen hexachloroplatinate in 1961 and,in 1962, xenon hexachloroplatine –the Ürst compound of a noble gas.

Barton, Sir Derek (HaroldRichard) (1918–98) British organicchemist who worked on steroids andother natural products. He is notedfor his investigations into the confor-mation of organic compounds and itseffect on chemical properties. Bartonwas awarded the 1969 Nobel Prizefor chemistry (with O. Hassell(1897–1981)).

baryta See barium hydroxide.

barytes (barite) An orthorhombicmineral form of *barium sulphate,BaSO4; the chief ore of barium. It isusually white but may also be yellow,grey, or brown. Large deposits occurin Andalusia, Spain, and in the USA.

basal See apical.

basalt A Üne-grained basic igneousrock. It is composed chieÛy of cal-cium-rich plagioclase feldspar andpyroxene; other minerals presentmay be olivine, magnetite, and ap-atite. Basalt is the commonest type oflava.

base A compound that reacts with aprotonic acid to give water (and asalt). The deÜnition comes from theArrhenius theory of acids and bases.Typically, bases are metal oxides, hy-droxides, or compounds (such as am-monia) that give hydroxide ions inaqueous solution. Thus, a base maybe either: (1) An insoluble oxide orhydroxide that reacts with an acid,e.g.

CuO(s) + 2HCl(aq) → CuCl2(aq) +H2O(l)

Here the reaction involves hydrogenions from the acid

CuO(s) + 2H+(aq) → H2O(l) +Cu2+(aq)

(2) A soluble hydroxide, in whichcase the solution contains hydroxideions. The reaction with acids is a re-action between hydrogen ions andhydroxide ions:

H+ + OH– → H2O(3) A compound that dissolves inwater to produce hydroxide ions. For example, ammonia reacts as fol-lows:

NH3(g) + H2O(l) ˆ NH4+(aq) + –OH

Similar reactions occur with organic*amines (see also nitrogenous base;amine salts). A base that dissolves inwater to give hydroxide ions is called

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an alkali. Ammonia and sodium hy-droxide are common examples.

The original Arrhenius deÜnition ofa base has been extended by theLowry–Brønsted theory and by theLewis theory. See acid.

base dissociation constant Seedissociation.

base metal A common relativelyinexpensive metal, such as iron orlead, that corrodes, oxidizes, or tar-nishes on exposure to air, moisture,or heat, as distinguished from pre-cious metals, such as gold and silver.

base pairing Pairing of nucleotidebases by hydrogen bonding. Basepairing holds together the twostrands in the double helix structureof *DNA. The purine adenine pairswith the pyrimidine thymine and thepurine guanine pairs with the pyrim-idine cytosine. See also chargaff’srule.

base unit A unit that is deÜned ar-bitrarily rather than being deÜned bysimple combinations of other units.For example, the ampere is a baseunit in the SI system deÜned in termsof the force produced between twocurrent-carrying conductors, whereasthe coulomb is a derived unit,deÜned as the quantity of chargetransferred by one ampere in onesecond.

basic 1. Describing a compoundthat is a base. 2. Describing a solu-tion containing an excess of hydrox-ide ions; alkaline.

basic dye See dyes.

basicity constant See dissocia-tion.

basic-oxygen process (BOPprocess) A high-speed method ofmaking high-grade steel. It originatedin the Linz–Donawitz (L–D) process.Molten pig iron and scrap are

charged into a tilting furnace, similarto the Bessemer furnace except thatit has no tuyeres. The charge is con-verted to steel by blowing high-pressure oxygen onto the surface ofthe metal through a water-cooledlance. The excess heat produced en-ables up to 30% of scrap to be incor-porated into the charge. The processhas largely replaced the Bessemerand open-hearth processes.

basic salt A compound that can beregarded as being formed by replac-ing some of the oxide or hydroxideions in a base by other negative ions.Basic salts are thus mixed salt–oxides(e.g. bismuth(III) chloride oxide,BiOCl) or salt–hydroxides (e.g. lead(II)chloride hydroxide, Pb(OH)Cl).

basic slag *Slag formed from abasic Ûux (e.g. calcium oxide) in ablast furnace. The basic Ûux is usedto remove acid impurities in the oreand contains calcium silicate, phos-phate, and sulphide. If the phospho-rus content is high the slag can beused as a fertilizer.

bathochromic shift A shift of aspectral band to longer wavelengthsas a result of substitution in a mol-ecule or a change in the conditions.Compare hypsochromic shift.

battery A number of electric cellsjoined together. The common carbattery, or *accumulator, usuallyconsists of six secondary cells con-nected in series to give a total e.m.f.of 12 volts. A torch battery is usuallya dry version of the *Leclanché cell,two of which are often connected inseries. Batteries may also have cellsconnected in parallel, in which casethey have the same e.m.f. as a singlecell, but their capacity is increased,i.e. they will provide more totalcharge. The capacity of a battery isusually speciÜed in ampere-hours,

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the ability to supply 1 A for 1 hr, orthe equivalent.

bauxite The chief ore of alu-minium, consisting of hydrous alu-minium oxides and aluminouslaterite. It is a claylike amorphousmaterial formed by the weatheringof silicate rocks under tropical condi-tions. The chief producers are Aus-tralia, Guinea, Jamaica, Russia, Brazil,and Surinam.

b.c.c. Body-centred cubic. See cubiccrystal.

beam balance See balance.

Beattie–Bridgman equation An*equation of state that relates thepressure, volume, and temperatureof a gas and the gas constant. TheBeattie–Bridgman equation uses em-pirical constants to take into accountthe reduction in the effective num-ber of molecules due to various typesof molecular aggregation. It is givenby

P = RT(1 – ε)(V + B)/V2 – A/V2,where P is the pressure, T is the ther-modynamic temperature, V is thevolume, R is the gas constant, and A,B, and ε are constants related to Üveempirical constants A0, B0, a, b, and cby: A = A0(1 – a/V), B = B0(1 – b/V), andε = c/VT3.

Beckmann rearrangement Thechemical conversion of a ketone*oxime into an *amide, usually usingsulphuric acid as a catalyst. The reac-tion, used in the manufacture ofnylon and other polyamides, isnamed after the German chemistErnst Beckmann (1853–1923).

Beckmann thermometer A ther-mometer for measuring smallchanges of temperature (see illustra-tion). It consists of a mercury-in-glassthermometer with a scale coveringonly 5 or 6°C calibrated in hun-dredths of a degree. It has two mer-

cury bulbs, the range of temperatureto be measured is varied by runningmercury from the upper bulb intothe larger lower bulb. It is used par-ticularly for measuring *depressionof freezing point or *elevation ofboiling point of liquids when soluteis added, in order to Ünd relative mo-lecular masses.

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reservoir foradjusting range

scale for measuringtemperature change

stem

Beckmann thermometer

becquerel Symbol Bq. The SI unitof activity (see radiation units). Theunit is named after the discoverer ofradioactivity A. H. *Becquerel.

Becquerel, Antoine Henri(1852–1908) French physicist. Hisearly researches were in optics, thenin 1896 he accidentally discovered*radioactivity in Ûuorescent salts ofuranium. Three years later heshowed that it consists of chargedparticles that are deÛected by a mag-netic Üeld. For this work he wasawarded the 1903 Nobel Prize forphysics, which he shared with Pierreand Marie *Curie.

Beer–Lambert law A law relating


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the reduction in luminous intensityof light passing through a material tothe length of the light’s path throughthe material: i.e.

log(I/I0) = –ε[J]l,where ε is the molar *absorptioncoefÜcient, I is the intensity afterpassing through a sample of length l,I0 is the incident intensity, and [J] isthe molar concentration of species J.The Beer–Lambert law was formedempirically; however, it can be de-rived on the basis that the loss in in-tensity dI is proportional to thethickness dl of the sample, the con-centration [J], and the intensity I(since the rate of absorption is pro-portional to the intensity). TheBeer–Lambert law means that the in-tensity of light (or any other form ofelectromagnetic radiation) passingthrough a sample diminishes expo-nentially with the concentration andthe thickness of the sample (for agiven wave number).

beet sugar See sucrose.

Beilstein’s test A test for the pres-ence of a halogen (chlorine, bromine,or iodine) in an organic compound. Apiece of copper wire or gauze is pre-heated strongly in the oxidizingÛame of a Bunsen burner (until theÛame is no longer green) and the testsubstance placed on the wire orgauze, which is re-heated. A greenÛame indicates the presence of ahalogen.

bell metal A type of *bronze usedin casting bells. It consists of 60–85%copper alloyed with tin, often withsome zinc and lead included.

Belousov–Zhabotinskii reactionSee b–z reaction.

Bénard cell A structure associatedwith a layer of liquid that is conÜnedby two horizontal parallel plates, inwhich the lateral dimensions are

much larger than the width of thelayer. Before heating the liquid is ho-mogeneous. However, if after heatingfrom below the temperatures of theplates are T1 and T2, at a critical valueof the temperature gradient ∆T = T1 –T2 the liquid abruptly starts to con-vect. The liquid spontaneously orga-nizes itself into a set of convectionrolls, i.e. the liquid goes round in aseries of ‘cells’, called Bénard cells.

Benedict’s test A biochemical testto detect reducing sugars in solution,devised by the US chemist S. R. Bene-dict (1884–1936). Benedict’s reagent –a mixture of copper(II) sulphate and aÜltered mixture of hydrated sodiumcitrate and hydrated sodium carbon-ate – is added to the test solution andboiled. A high concentration of re-ducing sugars induces the formationof a red precipitate; a lower concen-tration produces a yellow precipitate.Benedict’s test is a more sensitive al-ternative to *Fehling’s test.

beneÜciation (ore dressing) Theseparation of an ore into the valuablecomponents and the waste material(gangue). This may be achieved by anumber of processes, includingcrushing, grinding, magnetic separa-tion, froth Ûotation, etc. The dressedore, consisting of a high proportionof valuable components, is thenready for smelting or some otherreÜning process.

bent sandwich See sandwich com-pound.

benzaldehyde See benzenecar-baldehyde.

benzene A colourless liquid hydro-carbon, C6H6; r.d. 0.88; m.p. 5.5°C;b.p. 80.1°C. It is now made fromgasoline from petroleum by catalyticreforming (formerly obtained fromcoal tar). Benzene is the archetypal*aromatic compound. It has an un-

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benzenecarbaldehyde 62

b

saturated molecule, yet will not read-ily undergo addition reactions. Onthe other hand, it does undergo sub-stitution reactions in which hydro-gen atoms are replaced by otheratoms or groups. This behaviour oc-curs because of delocalization of p-electrons over the benzene ring, andall the C–C bonds in benzene areequivalent and intermediate inlength between single and doublebonds. It can be regarded as a reso-nance hybrid of Kekulé and Dewarstructures (see formulae). In formu-lae it can be represented by a hexa-gon with a ring inside it.

benzenecarbaldehyde (benzalde-hyde) A yellowish volatile oily liq-uid, C6H5CHO; r.d. 1.04; m.p. –26°C;b.p. 178.1°C. The compound occursin almond kernels and has an al-mond-like smell. It is made frommethylbenzene (by conversion todichloromethyl benzene, C6H5CHCl2,followed by hydrolysis). Benzenecar-baldehyde is used in Ûavourings, per-fumery, and the dyestuffs industry.

benzenecarbonyl chloride (ben-zoyl chloride) A colourless liquid,C6H5COCl; r.d. 1.21; m.p. 0°C; b.p.197.2°C. It is an *acyl halide, used tointroduce benzenecarbonyl groupsinto molecules. See acylation.

benzenecarbonyl group (benzoylgroup) The organic group C6H5CO–.

benzenecarboxylate (benzoate) Asalt or ester of benzenecarboxylicacid.

benzenecarboxylic acid (benzoic

acid) A white crystalline compound,C6H5COOH; r.d. 1.27; m.p. 122.4°C;b.p. 249°C. It occurs naturally insome plants and is used as a foodpreservative. Benzenecarboxylic acidhas a carboxyl group bound directlyto a benzene ring. It is a weak car-boxylic acid (Ka = 6.4 × 10–5 at 25°C),which is slightly soluble in water. Italso undergoes substitution reactionson the benzene ring.

benzene-1,4-diol (hydroquinone;quinol) A white crystalline solid,C6H4(OH)2; r.d. 1.33; m.p. 170°C; b.p.285°C. It is used in making dyes. Seealso quinhydrone electrode.

benzene hexacarboxylic acid Seemellitic acid.

benzene hexachloride (BHC) Acrystalline substance, C6H6Cl6, madeby adding chlorine to benzene. It isused as a pesticide and, like *DDT,concern has been expressed at its en-vironmental effects.

benzenesulphonic acid A colour-less deliquescent solid, C6H5SO2OH,m.p. 43–44°C, usually found as anoily liquid. It is made by treating ben-zene with concentrated sulphuricacid. Its alkyl derivatives are used as*detergents.

benzfuran (coumarone) A crys-talline aromatic compound, C8H6O. It is a heterocyclic compound hav-ing a benzene ring fused to a Üve-membered *furan ring.

benzil 1,2-diphenylethan-1,2-dione.See benzilic acid rearrangement.

Kekulé structures Dewar structures

Benzene


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63 benzoylation

b

benzilic acid rearrangement Anorganic rearrangement reaction inwhich benzil (1,2-diphenylethan-1,2-dione) is treated with hydroxide andthen acid to give benzilic acid (2-hy-droxy-2,2-diphenylethanoic acid):

C6H5.CO.CO.C6H5 →(C6H5)2C(OH).COOH

In the reaction a phenyl group(C6H5–) migrates from one carbonatom to another. The reaction wasdiscovered in 1828 by Justus von*Liebig; it was the Ürst rearrange-ment reaction to be described.

benzoate See benzenecarboxylate.

benzodiazepines A group of re-lated psychotic drugs that affect thecentral nervous system. They areused medically in the treatment ofanxiety, insomnia, convulsions, andalcohol withdrawal. All are addictiveand available only as prescriptiondrugs in the UK. Common examplesare *diazepam (Valium) and *Ûuni-trazepam (Rohypnol).

benzoic acid See benzenecar-boxylic acid.

benzoin A colourless crystallinecompound, C6H5CHOHCOC6H5; m.p.

137°C. It is a condensation product of*benzenecarbaldehyde (benzalde-hyde), made by the action of sodiumcyanide on benzenecarbaldehyde inalcoholic solution. It also occurs natu-rally as the resin of a tropical tree. Itis both a secondary alcohol and a ke-tone, and gives reactions characteris-tic of both types of compound.

benzopyrene A crystalline aro-matic hydrocarbon, C20H12; m.p.179°C. It is found in coal tar and ishighly carcinogenic.

1

2 3

4

5

6

789

10

11

12

2a

5a

7a8a

12a

Benzopyrene

Benzilic acid rearrangement

Benzfuran isomers

benzoquinone See cyclohexadi-ene-1,4-dione.

benzoylation A chemical reactionin which a benzoyl group (benzene-carbonyl group, C6H5CO) is intro-duced into a molecule. See acylation.


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benzoyl chloride 64

b

benzoyl chloride See benzenecar-bonyl chloride.

benzoylecgonine (BZ) A primarymetabolite of cocaine, used in drugtesting. It can be detected in theurine up to 48 hours after taking co-caine. BZ is tested for by immunoas-say or by gas chromatography/massspectrometry.

benzoyl group See benzenecar-bonyl group.

benzpyrene A pale yellow solid,C20H12, m.p. 179°C, whose moleculesconsist of Üve fused benzene rings. Itoccurs in tars from coal and tobaccosmoke and is a *carcinogen.

benzvalene A valence isomer ofbenzene, C6H6, with a bridged struc-ture.

benzyl alcohol See phenyl-methanol.

benzylamine (α-aminotoluene,phenylmethylamine) A colourless liq-uid, C6H5CH2NH2; r.d. 0.981; b.p.185°C. It behaves in the same way asprimary aliphatic amines.

benzyne (1,2-didehydrobenzene) Ahighly reactive short-lived com-pound, C6H4, having a hexagonalring of carbon atoms containing twodouble bonds and one triple bond.Benzyne, which is the simplest exam-ple of an *aryne, is thought to be anintermediate in a number of reac-tions.

CH

CHCH

CH

Benzyne

Bergius, Friedrich Karl Rudolf(1884–1949) German organicchemist. While working with Fritz

*Haber in Karlsruhe, he become in-terested in reactions at high pres-sures. In 1912 he devised anindustrial process for making lighthydrocarbons by the high-pressurehydrogenation of coal or heavy oil.The work earned him a share of the1931 Nobel Prize for chemistry withCarl Bosch (1874–1940). The Bergiusprocess proved important for supply-ing petrol for the German war effortin World War II.

Bergius process A process formaking hydrocarbon mixtures (forfuels) from coal by heating powderedcoal mixed with tar and iron(III)oxide catalyst at 450°C under hydro-gen at a pressure of about 200 atmos-pheres. In later developments of theprocess, the coal was suspended inliquid hydrocarbons and other cata-lysts were used. The process was de-veloped by Friedrich *Bergius duringWorld War I as a source of motor fuel.

berkelium Symbol Bk. A radio-active metallic transuranic elementbelonging to the *actinoids; a.n. 97;mass number of the most stable iso-tope 247 (half-life 1.4 × 103 years); r.d.(calculated) 14. There are eightknown isotopes. It was Ürst producedby G. T. Seborg and associates in1949 by bombarding americium–241with alpha particles.A• Information from the WebElements site

Berry mechanism A mechanismby which ligands in trigonal bipyra-midal complexes can interchange be-tween axial and equatorial positions.It involves intermediate formation ofa square pyramidal conformation. Itis also known as the Berry pseudo-rotation.

Berthelot, Marcellin (Pierre Eu-gène) (1827–1907) French chemistwho pioneered organic synthesis,producing many organic compounds


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from simple starting materials.Berthelot was also one of the Ürst toinvestigate thermochemistry.

Berthelot equation An *equationof state that relates the pressure, vol-ume, and temperature of a gas andthe gas constant. It is given by:

PV = RT[1 + 9PTc(1 – 6Tc2/T2)/128PcT],

where P is the pressure, V is the vol-ume, R is the gas constant, T is thethermodynamic temperature, and Tcand Pc are the critical temperatureand pressure of the gas. The Berth-elot equation can be derived fromthe *Clapeyron–Clausius equation.

Berthollide compound A solidcompound with slight variations inchemical composition (see nonstoi-chiometric compound). Berthollidecompounds are named after theFrench inorganic chemist ClaudeLouis Berthollet (1748–1822), who at-tacked the law of constant composi-tion.

beryl A hexagonal mineral form ofberyllium aluminium silicate,Be3Al2Si6O18; the chief ore of beryl-lium. It may be green, blue, yellow,or white and has long been used as agemstone. Beryl occurs throughoutthe world in granite and pegmatites.*Emerald, the green gem variety, oc-curs more rarely and is of greatvalue. Important sources of beryl arefound in Brazil, Madagascar, and theUSA.

beryllate A compound formed insolution when beryllium metal, orthe oxide or hydroxide, dissolves in

strong alkali. The reaction (for themetal) is often written

Be + 2OH–(aq) → BeO22–(aq) + H2(g)

The ion BeO22– is the beryllate ion. In

fact, as with the *aluminates, theions present are probably hydroxyions of the type Be(OH)42– (thetetrahydroxoberyllate(II) ion) to-gether with polymeric ions.

beryllia See beryllium oxide.

beryllium Symbol Be. A grey metal-lic element of *group 2 (formerly IIA)of the periodic table; a.n. 4; r.a.m.9.012; r.d. 1.85; m.p. 1278°C; b.p.2970°C. Beryllium occurs as beryl(3BeO.Al2O3.6SiO2) and chrysoberyl(BeO.Al2O3). The metal is extractedfrom a fused mixture of BeF2/NaF byelectrolysis or by magnesium reduc-tion of BeF2. It is used to manufac-ture Be–Cu alloys, which are used innuclear reactors as reÛectors andmoderators because of their low ab-sorption cross section. Berylliumoxide is used in ceramics and in nu-clear reactors. Beryllium and its com-pounds are toxic and can causeserious lung diseases and dermatitis.The metal is resistant to oxidation byair because of the formation of anoxide layer, but will react with dilutehydrochloric and sulphuric acids.Beryllium compounds show high co-valent character. The element wasisolated independently by F. *Wöhlerand A. A. Bussy in 1828.


beryllium bronze A hard, strong

65 beryllium bronze

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Berry mechanism


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type of *bronze containing about 2%beryllium, in addition to copper andtin.

beryllium hydroxide A whitecrystalline compound, Be(OH)2, pre-cipitated from solutions of berylliumsalts by adding alkali. Like the oxide,it is amphoteric and dissolves in ex-cess alkali to give *beryllates.

beryllium oxide (beryllia) An in-soluble solid compound, BeO; hexag-onal; r.d. 3.01; m.p. 2530°C; b.p.3900°C. It occurs naturally as bromel-lite, and can be made by burningberyllium in oxygen or by the de-composition of beryllium carbonateor hydroxide. It is an important am-photeric oxide, reacting with acids toform salts and with alkalis to formcompounds known as *beryllates.Beryllium oxide is used in theproduction of beryllium and beryllium–copper refractories, tran-sistors, and integrated circuits.

Berzelius, Jöns Jacob (1779–1848)Swedish chemist. After moving toStockholm he worked with miningchemists and, with them, discoveredseveral elements, including cerium(1803), selenium (1817), lithium(1818), thorium (1828), and vanadium(1830). He produced the Ürst accuratetable of atomic weights and was ex-tremely inÛuential in the generaldevelopment of 19th-centurychemistry.

Bessemer process A process forconverting *pig iron from a *blastfurnace into *steel. The molten pigiron is loaded into a refractory-linedtilting furnace (Bessemer converter)at about 1250°C. Air is blown intothe furnace from the base and*spiegel is added to introduce thecorrect amount of carbon. Impurities(especially silicon, phosphorus, andmanganese) are removed by the con-verter lining to form a slag. Finally

the furnace is tilted so that themolten steel can be poured off. Inthe modern VLN (very low nitrogen)version of this process, oxygen andsteam are blown into the furnace inplace of air to minimize the absorp-tion of nitrogen from the air by thesteel. The process is named after theBritish engineer Sir Henry Bessemer(1813–98), who announced it in 1856.See also basic-oxygen process.

beta decay A type of radioactivedecay in which an unstable atomicnucleus changes into a nucleus of thesame mass number but different pro-ton number. The change involves theconversion of a neutron into a pro-ton with the emission of an electronand an antineutrino (n → p + e– +

_ν )

or of a proton into a neutron withthe emission of a positron and a neu-trino (p → n + e+ + ν). An example isthe decay of carbon–14:

146C → 14

7N + e– + _ν

The electrons or positrons emittedare called beta particles and streamsof beta particles are known as betaradiation.

beta-iron A nonmagnetic allotropeof iron that exists between 768°Cand 900°C.

beta particle See beta decay.

beta sheet (β-pleated sheet) Aform of secondary structure in *pro-teins in which extended polypeptidechains lie parallel to each other andare linked by hydrogen bonds be-tween the N–H and C=O groups (seeillustration overleaf). Beta sheetsoccur in many globular proteins andlink polypeptides of the same type incertain Übrous proteins, includingÜbroin (the protein of silk).

BET isotherm An isotherm thattakes account of the possibility thatthe monolayer in the *Langmuir ad-sorption isotherm can act as a sub-

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67 binary acid

b

strate for further adsorption. TheBET isotherm (named after S.Brunauer, P. Emmett, and E. Teller)has the form:

V/Vmon = cz/(1 – z)[1 – (1 – c)z],

where z = p/p* (p* is the vapour pres-sure above a macroscopically thicklayer of liquid on the surface), Vmon isthe volume that corresponds to thesurface being covered by a mono-layer, V and p are the volume andpressure of the gas respectively, and cis a constant. In the BET isotherm,the isotherm rises indeÜnitely athigh pressures (in contrast to theLangmuir isotherm). It provides auseful approximation over someranges of pressure but underesti-mates adsorption for low pressuresand overestimates adsorption forhigh pressures.

BHC See benzene hexachloride.

bicarbonate See hydrogencarbon-ate.

bicarbonate of soda See sodiumhydrogencarbonate.

bifurcation A phenomenon in dy-namical systems in which the num-ber of solutions for a type ofbehaviour suddenly changes whenone of the parameters deÜning thesystem reaches a critical value. Bifur-cations can be depicted in a bifurca-tion diagram in which one axis of agraph is a variable of the system andthe other axis is a parameter that canbring about bifurcations. In manysystems a whole series of bifurca-tions occur, called bifurcation cas-cades. In certain cases thesebifurcation cascades can lead tochaotic reactions.

bimolecular reaction A step in achemical reaction that involves twomolecules. See molecularity.

binary Describing a compound oralloy formed from two elements.

binary acid An *acid in which theacidic hydrogen atom(s) are bound di-

HHH

NNN

O

CCCHHH

R

NNN

R

HHH

R

H

CCC

OOO

O

CCC

R

H

HHHNNN

O

CCC

hydrogen bondamino-acid side chain

CCC

O

HHH

R

R

NNN

HHH

O

CCC

CCC

RHHH

HHH

OOO

CCCCCC

NNN HHH

NNN NNN

CCCN

HHH

OOOR

H

NNNH

H

O

C

R

OOO

CCC

R

CCC

NNN

HHH

R

HHH

O

CCC

R

CCC

OOO

HHH

NNN

CCC

OOO

R

HHH

NNN

HHH

R

HHH CCC

NNN

HHH

OR

C

OOO

H

N

NNN

•••

•••

•••

••••••

••••••

HHH

•••

H

•••

• • •

R

CCC

CCC

CCC

HHH

CCC

CCCCCC

CCC

CCC

HHH

CCC

CCC

HHH

CCC

CCC

=

Beta sheet


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rectly to an atom other than oxygen.Examples are hydrogen chloride(HCl) and hydrogen sulphide (H2S).Such compounds are sometimescalled hydracids. Compare oxoacid.

binding site An area on the sur-face of a molecule that combineswith another molecule. Binding siteson enzymes can be *active sites or*allosteric sites.

bioaccumulation An increase inthe concentration of chemicals, suchas pesticides, in organisms that livein environments contaminated by awide variety of organic compounds.These compounds are not usually de-composed in the environment (i.e.they are not biodegradable) or metab-olized by the organisms, so that theirrate of absorption and storage isgreater than their rate of excretion.The chemicals are normally stored infatty tissues. *DDT is known as a per-sistent pesticide, as it is not easilybroken down and bioaccumulatesalong food chains, so that increasingconcentrations occur in individual or-ganisms at each trophic level.

bioactivation A metabolic processin which a product that is chemicallyreactive is produced from a relativelyinactive precursor.

biochemical fuel cell A systemthat exploits biological reactions forthe conversion of biomass (chemicalenergy) to electricity (electrical en-ergy). One potential application is thegeneration of electricity from indus-trial waste and sewage. Methyl-trophic organisms (i.e. organisms thatuse methane or methanol as theirsole carbon sources) are being inves-tigated for their potential use in bio-chemical fuel cells.

biochemical oxygen demand(BOD) The amount of oxygen takenup by microorganisms that decom-

pose organic waste matter in water.It is therefore used as a measure ofthe amount of certain types of or-ganic pollutant in water. BOD is cal-culated by keeping a sample of watercontaining a known amount of oxy-gen for Üve days at 20°C. The oxygencontent is measured again after thistime. A high BOD indicates the pres-ence of a large number of microor-ganisms, which suggests a high levelof pollution.

biochemistry The study of thechemistry of living organisms, espe-cially the structure and function oftheir chemical components (princi-pally proteins, carbohydrates, lipids,and nucleic acids). Biochemistry hasadvanced rapidly with the develop-ment, from the mid-20th century, ofsuch techniques as chromatography,X-ray diffraction, radioisotopic la-belling, and electron microscopy.Using these techniques to separateand analyse biologically importantmolecules, the steps of the metabolicpathways in which they are involved(e.g. glycolysis) have been deter-mined. This has provided someknowledge of how organisms obtainand store energy, how they manufac-ture and degrade their biomolecules,and how they sense and respond totheir environment. See Chronology.

biodiesel See biofuel.

bioelement Any chemical elementthat is found in the molecules andcompounds that make up a living or-ganism. In the human body the mostcommon bioelements (in decreasingorder of occurrence) are oxygen, car-bon, hydrogen, nitrogen, calcium,and phosphorus. Other bioelementsinclude sodium, potassium, magne-sium, and copper. See essential el-ement.

bioenergetics The study of theÛow and the transformations of en-

binding site 68

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ergy that occur in living organisms.Typically, the amount of energy thatan organism takes in (from food orsunlight) is measured and dividedinto the amount used for growth ofnew tissues; that lost through death,wastes, and (in plants) transpiration;and that lost to the environment asheat (through respiration).

biofuel A gaseous, liquid, or solidfuel that contains an energy contentderived from a biological source. Theorganic matter that makes up livingorganisms provides a potentialsource of trapped energy that is be-ginning to be exploited to supply theever-increasing energy demandaround the world. An example of abiofuel is rapeseed oil, which can beused in place of diesel fuel in mod-iÜed engines. The methyl ester ofthis oil, rapeseed methyl ester (RME),can be used in unmodiÜed diesel en-gines and is sometimes known asbiodiesel. Other biofuels include*biogas and *gasohol.

biogas A mixture of methane andcarbon dioxide resulting from theanaerobic decomposition of suchwaste materials as domestic, indus-trial, and agricultural sewage. The de-composition is carried out bymethanogenic bacteria; these oblig-ate anaerobes produce methane, themain component of biogas, whichcan be collected and used as an en-ergy source for domestic processes,such as heating, cooking, and light-ing. The production of biogas is car-ried out in special digesters, whichare widely used in China and India.As well as providing a source of fuel,these systems also enable sewage,which contains pathogenic bacteria,to be digested, thereby removing thedanger to humans that could other-wise result from untreated domesticand agricultural waste.

bioinorganic chemistry Biochem-istry involving compounds that con-tain metal atoms or ions. Twocommon examples of bioinorganiccompounds are haemoglobin (whichcontains iron) and chlorophyll(which contains magnesium). Manyenzymes contain metal atoms andbioinorganics are important in anumber of biochemical processes, in-cluding oxygen transport, electrontransfer, and protein folding.

bioluminescence The emission of light without heat (see lumi-nescence) by living organisms. Thephenomenon occurs in glow-wormsand ÜreÛies, bacteria and fungi, andin many deep-sea Üsh (among oth-ers); in animals it may serve as ameans of protection (e.g. by disguis-ing the shape of a Üsh) or speciesrecognition or it may provide matingsignals. The light is produced duringthe oxidation of a compound calledluciferin (the composition of whichvaries according to the species), thereaction being catalysed by an en-zyme, luciferase. Bioluminescencemay be continuous (e.g. in bacteria)or intermittent (e.g. in ÜreÛies).

biomarker A normal metabolitethat, when present in abnormal con-centrations in certain body Ûuids,can indicate the presence of a partic-ular disease or toxicological condi-tion. For example, abnormalconcentrations of glucose in theblood can be indicative of diabetesmellitus (see insulin).

biomolecule Any molecule that isinvolved in the maintenance andmetabolic processes of living organ-isms (see metabolism). Biomoleculesinclude carbohydrate, lipid, protein,nucleic acid, and water molecules;some biomolecules are macromole-cules.

biopolymer A polymer that occurs

69 biopolymer

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BIOCHEMISTRY

1833 French chemist Anselme Payen (1795–1871) discovers diastase (the firstenzyme to be discovered).

1836 Theodor Schwann discovers the digestive enzyme pepsin.

c.1860 Louis Pasteur demonstrates fermentation is caused by ‘ferments’ in yeastsand bacteria.

1869 German biochemist Johann Friedrich Miescher (1844–95) discovers nucleicacid.

1877 Pasteur’s ‘ferments’ are designated as enzymes.

1890 German chemist Emil Fischer (1852–1919) proposes the ‘lock-and-key’mechanism to explain enzyme action.

1901 Japanese chemist Jokichi Takamine (1854–1922) isolates adrenaline (the firsthormone to be isolated).

1903 German biologist Eduard Buchner (1860–1917) discovers the enzymezymase (causing fermentation).

1904 British biologist Arthur Harden (1865–1940) discovers coenzymes.

1909 Russian-born US biochemist Phoebus Levene (1869–1940) identifies ribose inRNA.

1921 Canadian physiologist Frederick Banting (1891–1941) and US physiologistCharles Best (1899–1978) isolate insulin.

1922 Alexander Fleming discovers the enzyme lysozyme.

1925 Russian-born British biologist David Keilin (1887–1963) discoverscytochrome.

1926 US biochemist James Sumner (1877–1955) crystallizes urease (the firstenzyme to be isolated).

1929 German chemist Hans Fischer (1881–1945) determines the structure ofhaem (in haemoglobin). K. Lohman isolates ATP from muscle.

1930 US biochemist John Northrop (1891–1987) isolates the stomach enzymepepsin.

1932 Swedish biochemist Hugo Theorell (1903–82) isolates the muscle proteinmyoglobin.

1937 Hans Krebs discovers the Krebs cycle.

1940 German-born US biochemist Fritz Lipmann (1899–1986) proposes that ATPis the carrier of chemical energy in many cells.

1943 US biochemist Britton Chance (1913– ) discovers how enzymes work (byforming an enzyme–substrate complex).

1952 US biologist Alfred Hershey (1908–97) proves that DNA carries geneticinformation.

1953 Francis Crick and James Watson discover the structure of DNA.

1955 Frederick Sanger discovers the amino acid sequence of insulin.


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71

b1956 US biochemist Arthur Kornberg (1918– ) discovers the enzyme DNApolymerase.US molecular biologist Paul Berg (1926– ) identifies the nucleic acid laterknown as transfer RNA.

1957 British biologist Alick Isaacs (1921–67) discovers interferon.

1959 Austrian-born British biochemist Max Perutz (1914–2002) determines thestructure of haemoglobin.

1960 South African-born British molecular biologist Sydney Brenner (1927– ) and French biochemist François Jacob (1920– ) discover messenger RNA.

1961 British biochemist Peter Mitchell (1920–92) proposes the chemiosmotictheory. Brenner and Crick discover that the genetic code consists of a series of basetriplets.

1969 US biochemist Gerald Edelman (1929– ) discovers the amino acid sequenceof immunoglobulin G.

1970 US virologists Howard Temin (1934–94) and David Baltimore (1938– )discover the enzyme reverse transcriptase.US molecular biologist Hamilton Smith (1931– ) discovers restrictionenzymes.

1973 US biochemists Stanley Cohen (1935– ) and Herbert Boyer (1936– ) userestriction enzymes to produce recombinant DNA.

1977 Sanger determines the complete base sequence of DNA in bacteriophageφX174.

1984 British biochemist Alec Jeffreys (1950– ) devises DNA profiling.

1985 US biochemist Kary Mullis (1944– ) invents the polymerase chain reactionfor amplifying DNA.

1986 US pharmacologists Robert Furchgott (1916– ) and Louis Ignarro(1941– ) demonstrate the importance of nitric oxide as a signal molecule inthe blood vascular system.

1988 US biochemist Peter Agre (1949– ) identifies a water-channel protein(aquaporin) in the plasma membrane of cells.

1994 Beginnings of DNA chip technology.

1998 US biochemist Roderick MacKinnon (1956– ) reveals detailed three-dimensional structure of potassium-ion channel in brain cells.

2001 US molecular biologist Harry Noller and colleagues produce first detailed X-ray crystallographic image of a complete ribosome.

2002 First synthetic virus created by Eckard Wimmer and associates, based onhuman poliovirus.

2004 A team led by David L. Spector produces the first real-time imaging of genetranscription in a living cell, using different fluorescent markers to tag nucleicacids and proteins.


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naturally, such as a *polysaccharide,*protein, or *nucleic acid.

bioreactor (industrial fermenter) Alarge stainless steel tank used togrow producer microorganisms inthe industrial production of enzymesand other chemicals. After the tankis steam-sterilized, an inoculum ofthe producer cells is introduced intoa medium that is maintained byprobes at optimum conditions oftemperature, pressure, pH, and oxy-gen levels for enzyme production. Anagitator (stirrer) mixes the medium,which is constantly aerated. It is es-sential that the culture medium issterile and contains the appropriatenutritional requirements for themicroorganism. When the nutrientshave been utilized the product is sep-arated; if the product is an extracellu-lar compound the medium can beremoved during the growth phase ofthe microorganisms, but an intracel-lular product must be harvestedwhen the batch culture growth stops.Some bioreactors are designed forcontinuous culture.

biosensor A device that uses an im-mobilized agent to detect or measurea chemical compound. The agents in-clude enzymes, antibiotics, or-ganelles, or whole cells. A reactionbetween the immobilized agent andthe molecule being analysed is trans-duced into an electronic signal. Thissignal may be produced in responseto the presence of a reaction product,the movement of electrons, or theappearance of some other factor (e.g.light). Biosensors are being used in-creasingly in diagnostic tests: theseallow quick, sensitive, and speciÜcanalysis of a wide range of biologicalproducts, including antibiotics, vita-mins, and other important biomole-cules (such as glucose), as well as thedetermination of certain *xenobi-

otics, such as synthetic organic com-pounds.

biosynthesis The production ofmolecules by a living cell, which isthe essential feature of *anabolism.

biotechnology The developmentof techniques for the application ofbiological processes to the produc-tion of materials of use in medicineand industry. For example, the pro-duction of antibiotics, cheese, andwine rely on the activity of variousfungi and bacteria. Genetic engineer-ing can modify bacterial cells to syn-thesize completely new substances,e.g. hormones, vaccines, monoclonalantibodies, etc., or introduce noveltraits into plants or animals.

biotin A vitamin in the *vitamin Bcomplex. It is the *coenzyme for var-ious enzymes that catalyse the incor-poration of carbon dioxide intovarious compounds. Adequateamounts are normally produced bythe intestinal bacteria; other sourcesinclude cereals, vegetables, milk, andliver.

biotite An important rock-formingsilicate mineral, a member of the*mica group of minerals, in commonwith which it has a sheetlike crystalstructure. It is usually black, darkbrown, or green in colour.

bipy See dipyridyl.

bipyramid See complex.

bipyridyl See dipyridyl.

biradical (diradical) A radical thathas two unpaired electrons at differ-ent points in the molecule, so thatthe radical centres are independentof each other.

birefringence See double refrac-tion.

Birge–Sponer extrapolation Amethod used to calculate the heat of

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dissociation of a molecule by extrap-olation from observed band spectra.The dissociation energy D0 is equal tothe sum of the vibrational quanta ∆G,where ∆G represents the energy be-tween two successive vibrationalstates. This means that D0 is approxi-mately equal to the area under thecurve of ∆G plotted against the vibra-tional quantum number v. If the Ürstfew vibrational quanta are observed,an approximate value of D0 can beobtained by a linear extrapolation ofthis curve. If a sufÜcient number ofvibrational quanta have been ob-served, a considerable improvementin the value of D0 can be obtained bytaking the curvature of the ∆G curveinto account by quadratic (and/or)higher terms. The technique was putforward by R. T. Birge and H. Sponerin 1926.

Birkeland–Eyde process Aprocess for the Üxation of nitrogenby passing air through an electric arc to produce nitrogen oxides. It was introduced in 1903 by the Nor-wegian chemists Kristian Birkeland(1867–1913) and Samuel Eyde(1866–1940). The process is economiconly if cheap hydroelectricity is avail-able.

bisecting See conformation.

bismuth Symbol Bi. A white crys-talline metal with a pinkish tinge be-longing to *group 15 (formerly VB) ofthe periodic table; a.n. 83; r.a.m.208.98; r.d. 9.78; m.p. 271.3°C; b.p.1560°C. The most important ores arebismuthinite (Bi2S3) and bismite(Bi2O3). Peru, Japan, Mexico, Bolivia,and Canada are major producers. Themetal is extracted by carbon reduc-tion of its oxide. Bismuth is the mostdiamagnetic of all metals and itsthermal conductivity is lower thanany metal except mercury. The metalhas a high electrical resistance and a

high Hall effect when placed in mag-netic Üelds. It is used to make low-melting-point casting alloys with tinand cadmium. These alloys expandon solidiÜcation to give clear replica-tion of intricate features. It is alsoused to make thermally activatedsafety devices for Üre-detection andsprinkler systems. More recent appli-cations include its use as a catalystfor making acrylic Übres, as a con-stituent of malleable iron, as a car-rier of uranium–235 fuel in nuclearreactors, and as a specialized thermo-couple material. Bismuth compounds(when lead-free) are used for cosmet-ics and medical preparations. It is at-tacked by oxidizing acids, steam (athigh temperatures), and by moisthalogens. It burns in air with a blueÛame to produce yellow oxide fumes.C. G. Junine Ürst demonstrated that itwas different from lead in 1753.A• Information from the WebElements site

bisphosphonates (diphospho-nates) A class of medical drugs usedin the treatment of osteoporosis andother conditions that involve fragilebones. The bisphosphonates attackosteoclasts (i.e. the bone cells thatbreak down bone tissue). The generalformula is O3P–C(R1R3)–PO3. The sidechain R1 is a simple group (–H, –OH,–Cl). R2 is usually a longer chain (e.g.–S–C6H4–Cl or –(CH2)5–NH2.

bistability The ability of a systemto exist in two steady states. Bistabil-ity is a necessary condition for oscil-lations to occur in chemicalreactions. The two steady states of abistable system are not states of ther-modynamic equilibrium, as they areassociated with conditions far fromequilibrium. Chemical reactions in-volving bistability jump suddenlyfrom one state to the other statewhen a certain concentration of oneof the participants in the reaction is

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reached. The effect is analogous tothe phenomenon of supercooling, i.e.the cooling of a liquid below itsfreezing point without freezing. Seealso oscillating reaction.

bisulphate See hydrogensulphate.

bisulphite See hydrogensulphite;aldehydes.

bite angle See chelate.

bittern The solution of salts re-maining when sodium chloride iscrystallized from sea water.

bitumen See petroleum.

bituminous coal See coal.

bituminous sand See oil sand.

biuret test A biochemical test todetect proteins in solution, namedafter the substance biuret (H2NCON-HCONH2), which is formed whenurea is heated. Sodium hydroxide ismixed with the test solution anddrops of 1% copper(II) sulphate solu-tion are then added slowly. A positiveresult is indicated by a violet ring,caused by the reaction of *peptidebonds in the proteins or peptides.Such a result will not occur in thepresence of free amino acids.

bivalent (divalent) Having a va-lency of two.

Black, Joseph (1728–99) Britishchemist and physician, born inFrance. He studied at Glasgow andEdinburgh, where his thesis (1754)contained the Ürst accurate descrip-tion of the chemistry of carbon diox-ide. In 1757 he discovered latentheat, and was the Ürst to distinguishbetween heat and temperature.

blackdamp (choke damp) Air leftdepleted in oxygen following the ex-plosion of Üredamp in a mine.

black lead See carbon.

blanc Üxe See barium sulphate.

blast furnace A furnace for smelt-ing iron ores, such as haematite(Fe2O3) or magnetite (Fe3O4), to make*pig iron. The furnace is a tall refrac-tory-lined cylindrical structure that ischarged at the top with the dressedore (see beneficiation), coke, and aÛux, usually limestone. The conver-sion of the iron oxides to metalliciron is a reduction process in whichcarbon monoxide and hydrogen arethe reducing agents. The overall reac-tion can be summarized thus:

Fe3O4 + 2CO + 2H2 → 3Fe + 2CO2 +2H2O

The CO is obtained within the fur-nace by blasting the coke with hotair from a ring of tuyeres about two-thirds of the way down the furnace.The reaction producing the CO is:

2C + O2 → 2COIn most blast furnaces hydrocarbons(oil, gas, tar, etc.) are added to theblast to provide a source of hydrogen.In the modern direct-reductionprocess the CO and H2 may be pro-duced separately so that the reduc-tion process can proceed at a lowertemperature. The pig iron producedby a blast furnace contains about 4%carbon and further reÜning is usuallyrequired to produce steel or cast iron.

blasting gelatin A high explosivemade from nitroglycerine and guncotton (cellulose nitrate).

bleaching powder A white solidregarded as a mixture of calciumchlorate(I), calcium chloride, and cal-cium hydroxide. It is prepared on alarge scale by passing chlorine gasthrough a solution of calcium hy-droxide. Bleaching powder is sold onthe basis of available chlorine, whichis liberated when it is treated with adilute acid. It is used for bleachingpaper pulps and fabrics and for steril-izing water.

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blende A naturally occurring metalsulphide, e.g. zinc blende ZnS.

Bloch’s theorem A theorem relat-ing to the *quantum mechanics ofcrystals stating that the wave func-tion ψ for an electron in a periodicpotential has the form ψ(r) =exp(ik•r)U(r), where k is the wavevector, r is a position vector, and U(r)is a periodic function that satisÜesU(r + R) = U(r), for all vectors R of theBravais lattice of the crystal. Block’stheorem is interpreted to mean thatthe wave function for an electron ina periodic potential is a plane wavemodulated by a periodic function.This explains why a free-electronmodel has some success in describingthe properties of certain metals al-though it is inadequate to give aquantitative description of the prop-erties of most metals. Block’s theo-rem was formulated by theGerman-born US physicist FelixBloch (1905–83) in 1928. See also en-ergy band.

block See periodic table.

block copolymer See polymer.

blood pigment Any one of agroup of metal-containing colouredprotein compounds whose functionis to increase the oxygen-carrying ca-pacity of blood.

blue vitriol See copper(ii)sulphate.

boat See ring conformations.

boat conformation See confor-mation.

BOD See biochemical oxygen de-mand.

body-centred cubic (b.c.c.) Seecubic crystal.

boehmite A mineral form of amixed aluminium oxide and hydrox-ide, AlO.OH. It is named after the

German scientist J. Böhm. See alu-minium hydroxide.

Bohr, Niels Henrik David (1885–1962) Danish physicist. In 1913 hepublished his explanation of howatoms, with electrons orbiting acentral nucleus, achieve stability byassuming that their angular momen-tum is quantized. Movement of elec-trons from one orbit to another isaccompanied by the absorption oremission of energy in the form oflight, thus accounting for the seriesof lines in the emission *spectrum ofhydrogen. For this work Bohr wasawarded the 1922 Nobel Prize forphysics. See bohr theory.

bohrium Symbol Bh. A radioactive*transactinide element; a.n. 107. Itwas Ürst made in 1981 by Peter Arm-bruster and a team in Darmstadt,Germany, by bombarding bismuth-209 nuclei with chromium-54 nuclei.Only a few atoms of bohrium haveever been detected.A• Information from the WebElements site

Bohr magneton See magneton.

Bohr theory The theory publishedin 1913 by Niels *Bohr to explain theline spectrum of hydrogen. He as-sumed that a single electron of massm travelled in a circular orbit of ra-dius r, at a velocity v, around a posi-tively charged nucleus. The angularmomentum of the electron wouldthen be mvr. Bohr proposed that elec-trons could only occupy orbits inwhich this angular momentum hadcertain Üxed values, h/2π, 2h/2π,3h/2π,…nh/2π, where h is the Planckconstant. This means that the angu-lar momentum is quantized, i.e. canonly have certain values, each ofwhich is a multiple of n. Each permit-ted value of n is associated with anorbit of different radius and Bohr as-sumed that when the atom emitted

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or absorbed radiation of frequency ν,the electron jumped from one orbitto another; the energy emitted or ab-sorbed by each jump is equal to hν.This theory gave good results in pre-dicting the lines observed in thespectrum of hydrogen and simpleions such as He+, Li2+, etc. The idea ofquantized values of angular momen-tum was later explained by the wavenature of the electron. Each orbit hasto have a whole number of wave-lengths around it; i.e. nλ = 2πr, whereλ is the wavelength and n a wholenumber. The wavelength of a particleis given by h/mv, so nh/mv = 2πr,which leads to mvr = nh/2π. Modernatomic theory does not allow sub-atomic particles to be treated in thesame way as large objects, and Bohr’sreasoning is somewhat discredited.However, the idea of quantized angu-lar momentum has been retained.

boiling point (b.p.) The tempera-ture at which the saturated vapourpressure of a liquid equals the exter-nal atmospheric pressure. As a conse-quence, bubbles form in the liquidand the temperature remains con-stant until all the liquid has evapo-rated. As the boiling point of a liquiddepends on the external atmosphericpressure, boiling points are usuallyquoted for standard atmosphericpressure (760 mmHg = 101 325 Pa).

boiling-point–composition dia-gram A graph showing how theboiling point and vapour composi-tion of a mixture of two liquids de-pends on the composition of themixture. The abscissa shows therange of compositions from 100% Aat one end to 100% B at the other.The diagram has two curves: thelower one gives the boiling points (ata Üxed pressure) for the differentcompositions. The upper one is plot-ted by taking the composition ofvapour at each temperature on the

boiling-point curve. The two curveswould coincide for an ideal mixture,but generally they are different be-cause of deviations from *Raoult’slaw. In some cases, they may show amaximum or minimum and coincideat some intermediate composition,explaining the formation of*azeotropes.

boiling-point elevation See elevation of boiling point.

Boltzmann, Ludwig Eduard(1844–1906) Austrian physicist. Heheld professorships in Graz, Vienna,Munich, and Leipzig, where heworked on the kinetic theory ofgases (see maxwell–boltzmann dis-tribution) and on thermodynamics(see boltzmann equation). He suf-fered from depression and commit-ted suicide.

Boltzmann constant Symbol k.The ratio of the universal gas con-stant (R) to the Avogadro constant(NA). It may be thought of thereforeas the gas constant per molecule:

k = R/NA = 1.380 658(12) × 10–23 J K–1

It is named after Ludwig *Boltzmann.

Boltzmann equation An equationused in the study of a collection ofparticles in *nonequilibrium statisti-cal mechanics, particularly theirtransport properties. The Boltzmannequation describes a quantity calledthe distribution function, f, whichgives a mathematical description ofthe state and how it is changing. Thedistribution function depends on aposition vector r, a velocity vector v,and the time t; it thus provides a sta-tistical statement about the positionsand velocities of the particles at anytime. In the case of one species ofparticle being present, Boltzmann’sequation can be written

∂f/∂t + a.(∂f/∂v) + v.(∂f/∂r) = (∂f/∂t)coll, where a is the acceleration of bodies

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between collisions and (∂f/∂t)coll is therate of change of f(r,v,t) due to colli-sions. The Boltzmann equation canbe used to calculate *transportcoefÜcients, such as conductivity. Theequation was proposed by Ludwig*Boltzmann in 1872.

Boltzmann formula An equationconcerning the entropy S of a systemthat derives from statistical mechan-ics. It states that entropy is related tothe number W of distinguishableways in which the equation S = klnW, where k is the Boltzmann con-stant, can describe the system. It ex-presses in quantitative terms theconcept that entropy is a measure ofthe disorder of a system. It was dis-covered in the late 19th century byLudwig *Boltzmann while he wasstudying statistical mechanics.

bomb calorimeter An apparatusused for measuring heats of combus-tion (e.g. caloriÜc values of fuels andfoods). It consists of a strong con-tainer in which the sample is sealedwith excess oxygen and ignited elec-trically. The heat of combustion atconstant volume can be calculatedfrom the resulting rise in tempera-ture.

bond See chemical bond.

bond dissociation energy Seebond energy.

bond energy An amount of energyassociated with a bond in a chemicalcompound. It is obtained from theheat of atomization. For instance, inmethane the bond energy of the C–Hbond is one quarter of the enthalpyof the process

CH4(g) → C(g) + 4H(g)Bond energies (or bond enthalpies)can be calculated from the standardenthalpy of formation of the com-pound and from the enthalpies of at-omization of the elements. Energies

calculated in this way are called aver-age bond energies or bond–energyterms. They depend to some extenton the molecule chosen; the C–Hbond energy in methane will differslightly from that in ethane. Thebond dissociation energy is a differ-ent measurement, being the energyrequired to break a particular bond;e.g. the energy for the process:

CH4(g) → CH3•(g) + H•(g)

bond enthalpy See bond energy.

bonding orbital See orbital.

bond order A value indicating thedegree of bonding between twoatoms in a molecule relative to a sin-gle bond. Bond orders are theoreticalvalues depending on the way the cal-culation is done. For example, inethane the bond order of the carbon-carbon bond is 1. In ethene, the bondorder is 2. In benzene the bond orderas calculated by molecular orbitaltheory is 1.67.

bone black See charcoal.

borane (boron hydride) Any of agroup of compounds of boron andhydrogen, many of which can be pre-pared by the action of acid on mag-nesium boride (MgB2). Others aremade by pyrolysis of the products ofthis reaction in the presence of hy-drogen and other reagents. They areall volatile, reactive, and oxidizereadily in air, some explosively so.The boranes are a remarkable groupof compounds in that their structurescannot be described using the con-ventional two-electron covalent bondmodel (see electron-deficient com-pound). The simplest example is di-

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borane (B2H6): see formula. Other bo-ranes include B4H10, B5H9, B5H11,B6H10, and B10H4. The larger boranemolecules have open or closed poly-hedra of boron atoms. In addition,there is a wide range of borane deriv-atives containing atoms of other el-ements, such as carbon andphosphorus. Borohydride ions of thetype B6H6

2– also exist. Boranes andborohydride ions are classiÜed ac-cording to their structure. Thosewith a complete polyhedron are saidto have a closo-structure. Those inwhich the polyhedron is incompleteby loss of one vertex have a nido-structure (from the Greek for ‘nest’).Those with open structures by re-moval of two or more vertices havean arachno structure (from the Greekfor ‘spider’). See also wade’s rules.

borate Any of a wide range of ioniccompounds that have negative ionscontaining boron and oxygen (seeformulae). Lithium borate, for exam-ple, contains the simple anionB(OH)4–. Most borates, however, areinorganic polymers with rings,chains, or other networks based onthe planar BO3 group or the tetra-

hedral BO3(OH) group. ‘Hydrated’ bo-rates are ones containing –OHgroups; many examples occur natu-rally. Anhydrous borates, which con-tain BO3 groups, can be made bymelting together boric acid andmetal oxides.

borax (disodium tetraborate-10-water) A colourless monoclinic solid,Na2B4O7.10H2O, soluble in water andvery slightly soluble in ethanol; mon-oclinic; r.d. 1.73; loses 8H2O at 75°C;loses 10H2O at 320°C. The formulagives a misleading impression of thestructure. The compound containsthe ion [B4O5(OH)4]2– (see borate).Attempts to recrystallize this com-pound above 60.8°C yield the penta-hydrate. The main sources are theborate minerals kernite (Na2B4O7.4H2O) and tincal (Na2B4O7.10H2O).The ores are puriÜed by carefullycontrolled dissolution and recrystal-lization. On treatment with mineralacids borax gives boric acid.

Borax is a very important sub-stance in the glass and ceramics in-dustries as a raw material for makingborosilicates. It is also important as ametallurgical Ûux because of the abil-

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B B

O OB

O–

B3O3–6

as in Na3B3O6

BO–

O BO–

O B

O–

(BO2)n–n as in CaB2O4

HO BO

O

B–

OH

OB OH

B–O

OH

O

[B4O5(OH)4]2–

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ity of molten borates to dissolvemetal oxides. In solution it partiallyhydrolyses to boric acid and can thusact as a buffer. For this reason it isused as a laundry pre-soak. It is usedmedicinally as a mild alkaline anti-septic and astringent for the skin andmucous membranes.

Disodium tetraborate is the sourceof many industrially important boroncompounds, such as barium borate(fungicidal paints), zinc borate (Üre-retardant additive in plastics), andboron phosphate (heterogeneous acidcatalyst in the petrochemicals indus-try).

borax-bead test A simple labora-tory test for certain metal ions insalts. A small amount of the salt ismixed with borax and a molten beadformed on the end of a piece of plat-inum wire. Certain metals can beidentiÜed by the colour of the beadproduced in the oxidizing and reduc-ing parts of a Bunsen Ûame. For ex-ample, iron gives a bead that is redwhen hot and yellow when cold inthe oxidizing Ûame and a green beadin the reducing Ûame.

borazon See boron nitride.

Bordeaux mixture A mixture ofcopper(II) sulphate and calcium hy-droxide in water, used as a fungicide.

boric acid Any of a number ofacids containing boron and oxygen.Used without qualiÜcation the termapplies to the compound H3BO3(which is also called orthoboric acidor, technically, trioxoboric(III) acid).This is a white or colourless solidthat is soluble in water and ethanol;triclinic; r.d. 1.435; m.p. 169°C. It oc-curs naturally in the condensatefrom volcanic steam vents (sufÜoni).Commercially, it is made by treatingborate minerals (e.g. kernite,Na2B4O7.4H2O) with sulphuric acidfollowed by recrystallization.

In the solid there is considerablehydrogen bonding between H3BO3molecules resulting in a layer struc-ture, which accounts for the easycleavage of the crystals. H3BO3 mol-ecules also exist in dilute solutionsbut in more concentrated solutionspolymeric acids and ions are formed(e.g. H4B2O7; pyroboric acid ortetrahydroxomonoxodiboric(III) acid).The compound is a very weak acidbut also acts as a Lewis *acid in ac-cepting hydroxide ions:

B(OH)3 + H2O ˆ B(OH)4– + H+

If solid boric acid is heated it loseswater and transforms to another acidat 300°C. This is given the formulaHBO2 but is in fact a polymer(HBO2)n. It is called metaboric acid or,technically, polydioxoboric(III) acid.

Boric acid is used in the manufac-ture of glass (borosilicate glass),glazes and enamels, leather, paper,adhesives, and explosives. It is widelyused (particularly in the USA) in de-tergents, and because of the ability offused boric acid to dissolve othermetal oxides it is used as a Ûux inbrazing and welding. Because of itsmild antiseptic properties it is usedin the pharmaceutical industry andas a food preservative.

boric anhydride See boron(iii)oxide.

boric oxide See boron(iii) oxide.

boride A compound of boron witha metal. Most metals form at leastone boride of the type MB, MB2, MB4,MB6, or MB12. The compounds have avariety of structures; in particular,the hexaborides contain clusters ofB6 atoms. The borides are all hardhigh-melting materials with metal-like conductivity. They can be madeby direct combination of the el-ements at high temperatures (over2000°C) or, more usually, by high-temperature reduction of a mixture

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of the metal oxide and boron oxideusing carbon or aluminium. Chemi-cally, they are stable to nonoxidizingacids but are attacked by strong oxi-dizing agents and by strong alkalis.Magnesium boride (MgB2) is unusualin that it can be hydrolysed to bo-ranes. Industrially, metal borides areused as refractory materials. Themost important are CrB, CrB2, TiB2,and ZnB2. Generally, they are fabri-cated using high-temperature pow-der metallurgy, in which the articleis produced in a graphite die at over2000°C and at very high pressure.Items are pressed as near to Ünalshape as possible as machining re-quires diamond cutters and is ex-tremely expensive.

Born, Max (1882–1970) German-born British physicist who wasawarded the 1954 Nobel Prize forphysics (with W. Bothe) for his workon statistical mechanics. With*Heisenberg he also developed ma-trix mechanics.

Born–Haber cycle A cycle of reac-tions used for calculating the latticeenergies of ionic crystalline solids.For a compound MX, the lattice en-ergy is the enthalpy of the reaction

M+(g) + X–(g) → M+X–(s) ∆HL

The standard enthalpy of formationof the ionic solid is the enthalpy ofthe reaction

M(s) + ½X2(g) → M+X–(s) ∆Hf

The cycle involves equating this en-thalpy (which can be measured) tothe sum of the enthalpies of a num-ber of steps proceeding from the el-ements to the ionic solid. The stepsare:(1) Atomization of the metal:

M(s) → M(g) ∆H1

(2) Atomization of the nonmetal:½X2(g) → X(g) ∆H2

(3) Ionization of the metal:

M(g) → M+(g) + e ∆H3

This is obtained from the ionizationpotential.(4) Ionization of the nonmetal:

X(g) + e → X–(g) ∆H4

This is the electron afÜnity.(5) Formation of the ionic solids:

M+(g) + X–(g) → M+X–(s) ∆HL

Equating the enthalpies gives:∆Hf = ∆H1 + ∆H2 + ∆H3 + ∆H4 + ∆HL

from which ∆HL can be found. It isnamed after the German physicistMax Born (1882–1970) and Fritz*Haber.

bornite An important ore of coppercomposed of a mixed copper–ironsulphide, Cu5FeS4. Freshly exposedsurfaces of the mineral are a metallicreddish-brown but a purplish irides-cent tarnish soon develops – hence itis popularly known as peacock ore.Bornite is mined in Chile, Peru, Bo-livia, Mexico, and the USA.

Born–Oppenheimer approxima-tion An *adiabatic approximationused in molecular and solid-statephysics in which the motion ofatomic nuclei is taken to be so muchslower than the motion of electronsthat, when calculating the motionsof electrons, the nuclei can be takento be in Üxed positions. This approxi-mation was justiÜed using perturba-tion theory by Max Born and the USphysicist Julius Robert Oppenheimer(1904–67) in 1927.

borohydride ions See borane.

boron Symbol B. An element of*group 13 (formerly IIIB) of the peri-odic table; a.n. 5; r.a.m. 10.81; r.d.2.34–2.37 (amorphous); m.p. 2300°C;b.p. 2550°C. It forms two allotropes;amorphous boron is a brown powderbut metallic boron is black. Themetallic form is very hard (9.3 onMohs’ scale) and is a poor electrical

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conductor at room temperature. Atleast three crystalline forms are pos-sible; two are rhombohedral and theother tetragonal. The element isnever found free in nature. It occursas orthoboric acid in volcanic springsin Tuscany, as borates in kernite(Na2B4O7.4H2O), and as colemanite(Ca2B6O11.5H2O) in California. Sam-ples usually contain isotopes in theratio of 19.78% boron–10 to 80.22%boron–11. Extraction is achieved byvapour-phase reduction of borontrichloride with hydrogen on electri-cally heated Ülaments. Amorphousboron can be obtained by reducingthe trioxide with magnesium pow-der. Boron when heated reacts withoxygen, halogens, oxidizing acids,and hot alkalis. It is used in semicon-ductors and in Ülaments for special-ized aerospace applications.Amorphous boron is used in Ûares,giving a green coloration. The iso-tope boron–10 is used in nuclear re-actor control rods and shields. Theelement was discovered in 1808 bySir Humphry *Davy and by J. L. *Gay-Lussac and L. J. Thenard.


boron carbide A black solid, B4C,soluble only in fused alkali; it is ex-tremely hard, over 9½ on Mohs’scale; rhombohedral; r.d. 2.52; m.p.2350°C; b.p. >3500°C. Boron carbideis manufactured by the reduction ofboric oxide with petroleum coke inan electric furnace. It is used largelyas an abrasive, but objects can alsobe fabricated using high-temperaturepowder metallurgy. Boron nitride isalso used as a neutron absorber be-cause of its high proportion ofboron–10.

boron hydride See borane.

boron nitride A solid, BN, insolu-ble in cold water and slowly decom-

posed by hot water; r.d. 2.25 (hexago-nal); sublimes above 3000°C. Boronnitride is manufactured by heatingboron oxide to 800°C on an acid-solu-ble carrier, such as calcium phos-phate, in the presence of nitrogen orammonia. It is isoelectronic with car-bon and, like carbon, it has a veryhard cubic form (borazon) and asofter hexagonal form; unlikegraphite this is a nonconductor. It isused in the electrical industrieswhere its high thermal conductivityand high resistance are of especialvalue.

boron(III) oxide (boric anhydride;boric oxide; diboron trioxide) Aglassy solid, B2O3, that gradually ab-sorbs water to form boric acid. It hassome amphoteric characteristics andforms various salts.

boron trichloride A colourlessfuming liquid, BCl3, which reactswith water to give hydrogen chlorideand boric acid; r.d. 1.349; m.p.–107°C; b.p. 12.5°C. Boron trichlorideis prepared industrially by theexothermic chlorination of boroncarbide at above 700°C, followed byfractional distillation. An alternative,but more expensive, laboratorymethod is the reaction of dry chlo-rine with boron at high temperature.Boron trichloride is a Lewis *acid,forming stable addition compoundswith such donors as ammonia andthe amines and is used in the labora-tory to promote reactions that liber-ate these donors. The compound isimportant industrially as a source ofpure boron (reduction with hydro-gen) for the electronics industry. It isalso used for the preparation of bo-ranes by reaction with metal hy-drides.

borosilicate Any of a large numberof substances in which BO3 and SiO4units are linked to form networks

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with a wide range of structures.Borosilicate glasses are particularlyimportant; the addition of boron tothe silicate network enables the glassto be fused at lower temperaturesthan pure silica and also extends theplastic range of the glass. Thus suchglasses as Pyrex have a wider rangeof applications than soda glasses (nar-row plastic range, higher thermal ex-pansion) or silica (much highermelting point). Borosilicates are alsoused in glazes and enamels and inthe production of glass wools.

Bosch process See haber process.

Bose–Einstein statistics Seequantum statistics.

boson A particle or system thatobeys Bose–Einstein statistics (seequantum statistics). Combiningquantum mechanics with special rel-ativity theory gives a result that aboson has to have an integer spin. A*photon is an example of a boson.

bottled gas Gas supplied underpressure in metal cylinders. The termincludes pressurized gas (e.g. oxygenand nitrogen cylinders) and gasesliqueÜed under pressure (e.g. liquidbutane for use as a fuel). Colour con-ventions are used to identify the typeof gas or, in some cases, the speciÜcgas. The colour indicating the con-tents is that of the shoulder of thecylinder at the top. The convention isnot international, and practice differsin different countries. In the UK, theconvention is:Yellow for toxic or corrosive gasesRed for Ûammable gasesLight blue for oxidizing gasesMaroon for acetyleneDark green for argonGrey for carbon dioxideBrown for heliumBlue for nitrous oxideBlack for nitrogenWhite for oxygen

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A• Information on colour coding of gas

containers

bound state A system in whichtwo (or more) parts are bound to-gether in such a way that energy isrequired to split them. An exampleof a bound state is a *moleculeformed from two (or more) *atoms.

bowsprit See ring conformations.

Boyle, Robert (1627–91) Englishchemist and physicist, born in Ire-land. After moving to Oxford in 1654he worked on gases, using an airpump made by Robert Hooke. In1662 he discovered *Boyle’s law. Inchemistry he worked on *Ûame testsand acid-base *indicators. Boyle isgenerally regarded as the person whoestablished chemistry as a modernsubject, distinct from alchemy. Hewas the Ürst to give a deÜnition of achemical element.

Boyle’s law The volume (V) of agiven mass of gas at a constant tem-perature is inversely proportional toits pressure (p), i.e. pV = constant.This is true only for an *ideal gas.This law was discovered in 1662 byRobert *Boyle. On the continent ofEurope it is known as Mariotte’s lawafter E. Mariotte (1620–84), who dis-covered it independently in 1676. Seealso gas laws.

Brackett series See hydrogenspectrum.

Bragg, Sir William Henry(1862–1942) British physicist, whowith his son Sir William LawrenceBragg (1890–1971), was awarded the1915 Nobel Prize for physics for theirpioneering work on X-ray crystallog-raphy. He also constructed an X-rayspectrometer for measuring thewavelengths of X-rays. In the 1920s,while director of the Royal Institu-


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tion in London, he initiated X-ray dif-fraction studies of organic molecules.

Bragg peak A peak in the scatter-ing pattern in X-ray diffraction of acrystal. The intensity of Bragg peaksis proportional to the square of thenumber of the scatterers. If X-rayscattering from a solid producesBragg peaks this indicates that thesolid has long-range order. Braggpeaks are named after Sir WilliamLawrence *Bragg, who discoveredthem in 1912. The scattering of X-rays from a set of planes in a crystalthat gives rise to Bragg peaks iscalled Bragg scattering.

Bragg’s law When a beam of X-rays (wavelength λ) strikes a crystalsurface in which the layers of atomsor ions are separated by a distance d,the maximum intensity of thereÛected ray occurs when sinθ =nλ/2d, where θ (known as the Braggangle) is the complement of theangle of incidence and n is an inte-ger. The law enables the structure ofmany crystals to be determined. Itwas discovered in 1912 by SirWilliam Lawrence Bragg.

branched chain See chain.

brass A group of alloys consistingof copper and zinc. A typical yellowbrass might contain about 67% cop-per and 33% zinc.

Bravais lattice An inÜnite array oflattice points. A Bravais lattice canhave only fourteen space groups di-vided into seven crystal systems. It isnamed after Auguste Bravais(1811–63).

Bremsstrahlung (German: brakingradiation) The *X-rays emitted whena charged particle, especially a fastelectron, is rapidly slowed down, aswhen it passes through the electricÜeld around an atomic nucleus. TheX-rays cover a whole continuous

range of wavelengths down to a min-imum value, which depends on theenergy of the incident particles.Bremsstrahlung are produced by ametal target when it is bombardedby electrons.

brewing The process by whichbeer is made. Fermentation of sugarsfrom barley grain by the yeasts Sac-charomyces cerevisiae and S. uvarum (orS. carlsbergenesis) produces alcohol. Inthe Ürst stage the barley grain issoaked in water, a process known asmalting. The grain is then allowed togerminate and the natural enzymesof the grain (the amylases and themaltases) convert the starch to mal-tose and then to glucose. The nextstage is kilning or roasting, in whichthe grains are dried and crushed. Thecolour of a beer depends on the tem-perature used for this process: thehigher the temperature, the darkerthe beer. In the next stage, mashing,the crushed grain is added to waterat a speciÜc temperature and any re-maining starch is converted to sugar;the resultant liquid is the raw ma-terial of brewing, called wort. Theyeast is then added to the wort toconvert the sugar to alcohol, fol-lowed by hops, which give beer itscharacteristic Ûavour. Hops are thefemale Ûowers of the vine Humuluslupulus; they contain resins (humu-lones, cohumulones, and adhumu-lones) that give beer its distinctivebitter taste.

bridge An atom or group joiningtwo other atoms in a molecule. Seealuminium chloride; borane.

brighteners Substances added todetergents or used to treat textiles orpaper in order to brighten thecolours or, particularly, to enhancewhiteness. Blueing agents are used inlaundries to give a slight blue cast towhite material in order to counteract

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yellowing. Fluorescent brightenersare compounds that absorb visible orultraviolet radiation and Ûuoresce inthe blue region of the optical spec-trum.

Brillouin zone A cell in a recipro-cal lattice. The Ürst Brillouin zone isthe cell of smallest volume enclosedby those planes that are perpendicu-lar bisectors of reciprocal lattice vec-tors. Higher Brillouin zones alsoexist. Brillouin zones are used in thetheory of energy levels in a periodicpotential, as in a crystal. They arenamed after the French physicistLéon Brillouin (1889–1969), who in-troduced them into the theory ofcrystals in 1930.

Brinell hardness A scale for meas-uring the hardness of metals intro-duced around 1900 by the Swedishmetallurgist Johann Brinell (1849–1925). A small chromium-steel ball ispressed into the surface of the metalby a load of known weight. The ratioof the mass of the load in kilogramsto the area of the depression formedin square millimetres is the Brinellnumber.

Brin process A process formerlyused for making oxygen by heatingbarium oxide in air to form the per-oxide and then heating the peroxideat higher temperature (>800°C) toproduce oxygen

2BaO2 → 2BaO + O2

Britannia metal A silvery alloyconsisting of 80–90% tin, 5–15% anti-mony, and sometimes small percent-ages of copper, lead, and zinc. It isused in bearings and some domesticarticles.

British thermal unit (Btu) The Im-perial unit of heat, being originallythe heat required to raise the tem-perature of 1lb of water by 1°F. 1 Btuis now deÜned as 1055.06 joules.

bromate A salt or ester of a bromicacid.

bromic(I) acid (hypobromous acid)A yellow liquid, HBrO. It is a weakacid but a strong oxidizing agent.

bromic(V) acid A colourless liquid,HBrO3, made by adding sulphuricacid to barium bromate. It is a strongacid.

bromide See halide.

bromination A chemical reactionin which a bromine atom is intro-duced into a molecule. See also halo-genation.

bromine Symbol Br. A *halogen el-ement; a.n. 35; r.a.m. 79.909; r.d.3.13; m.p. –7.2°C; b.p. 58.78°C. It is ared volatile liquid at room tempera-ture, having a red-brown vapour.Bromine is obtained from brines inthe USA (displacement with chlo-rine); a small amount is obtainedfrom sea water in Anglesey. Largequantities are used to make 1,2-di-bromoethane as a petrol additive. Itis also used in the manufacture ofmany other compounds. Chemically,it is intermediate in reactivity be-tween chlorine and iodine. It formscompounds in which it has oxidationstates of 1, 3, 5, or 7. The liquid isharmful to human tissue and thevapour irritates the eyes and throat.The element was discovered in 1826by Antoine Balard.


bromoethane (ethyl bromide) Acolourless Ûammable liquid, C2H5Br;r.d. 1.46; m.p. –119°C; b.p. 38.4°C. Itis a typical *haloalkane, which canbe prepared from ethene and hydro-gen bromide. Bromoethane is used asa refrigerant.

bromoform Seetribromomethane; haloforms.

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bromomethane (methyl bromide)A colourless volatile nonÛammableliquid, CH3Br; r.d. 1.68; m.p. –93°C;b.p. 3.56°C. It is a typical *halo-alkane.

N-bromosuccinimide (NBS) Acrystalline solid, C4O2NBr, used ex-tensively as a reagent for elec-trophilic addition of bromine. It actsby producing a small constant supplyof bromine in solution

C4O2NBr + H+ + Br– → C4O2NH +Br2.

85 brusselator

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NOO

Br

N-bromosuccinimide

bromothymol blue An acid–base*indicator that is yellow in acid solu-tions and blue in alkaline solutions.It changes colour over the pH range6–8.

Brønsted, Johannes Nicolaus(1879–1947) Danish physical chemist.He worked on thermochemistry andelectrochemistry and is best knownfor the Lowry–Brønsted theory of*acids and bases, which he proposed(independently of Lowry) in 1923.

Brønsted acid See acid.

Brønsted base See acid.

bronze Any of a group of alloys ofcopper and tin, sometimes with leadand zinc present. The amount of tinvaries from 1% to 30%. The alloy ishard and easily cast and extensivelyused in bearings, valves, and othermachine parts. Various improvedbronzes are produced by addingother elements; for instance, phos-phor bronzes contain up to 1% phos-phorus. In addition certain alloys of

copper and metals other than tin arecalled bronzes – aluminium bronze isa mixture of copper and aluminium.Other special bronzes include *bellmetal, *gun metal, and *berylliumbronze.

Brownian movement The contin-uous random movement of micro-scopic solid particles (of about 1micrometre in diameter) when sus-pended in a Ûuid medium. First ob-served by the British botanist RobertBrown (1773–1858) in 1827 whenstudying pollen particles, it was origi-nally thought to be the manifestationof some vital force. It was later recog-nized to be a consequence of bom-bardment of the particles by thecontinually moving molecules of theliquid. The smaller the particles themore extensive is the motion. The ef-fect is also visible in particles ofsmoke suspended in a still gas.

A• Robert Brown’s original paper• Jean Perrin’s paper

brown-ring test A test for ionic ni-trates. The sample is dissolved andiron(II) sulphate solution added in atest tube. Concentrated sulphuricacid is then added slowly so that itforms a separate layer. A brown ring(of Fe(NO)SO4) at the junction of theliquids indicates a positive result.

brucite A mineral form of *magne-sium hydroxide, Mg(OH)2.

brusselator A type of chemical re-action mechanism that leads to an*oscillating reaction. It involves theconversion of reactants A and B intoproducts C and B by a series of foursteps:

A → X

2X + Y → 3Y

B + X → Y + C

X → D


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Autocatalysis occurs as in the*Lotka–Volterra mechanism and the*oregonator. If the concentrations ofA and B are maintained constant, theconcentrations of X and Y oscillatewith time. A graph of the concentra-tion of X against that of Y is a closedloop (the limit cycle of the reaction).The reaction settles down to thislimit cycle whatever the initial con-centrations of X and Y, i.e. the limitcycle is an *attractor for the system.The reaction mechanism is namedafter the city of Brussels, where theresearch group that discovered it isbased.

Buchner funnel A type of funnelwith an internal perforated tray onwhich a Ûat circular Ülter paper canbe placed, used for Ültering by suc-tion. It is named after the Germanchemist Eduard Buchner (1860–1917).

buckminsterfullerene A form ofcarbon composed of clusters of 60carbon atoms bonded together in apolyhedral structure composed ofpentagons and hexagons (see illustra-tion). Originally it was identiÜed in1985 in products obtained by Üring ahigh-power laser at a graphite target.It can be made by an electric arcstruck between graphite electrodesin an inert atmosphere. The mol-ecule, C60, was named after the USarchitect Richard Buckminster Fuller(1895–1983) because of the resem-blance of the structure to the geo-desic dome, which Fuller invented.The molecules are informally calledbuckyballs; more formally, the sub-stance itself is also called fullerene.The substance is a yellow crystallinesolid (fullerite), soluble in benzene.

Various fullerene derivatives areknown in which organic groups areattached to carbon atoms on thesphere. In addition, it is possible toproduce novel enclosure compounds

by trapping metal ions within the C60cage. Some of these have semicon-ducting properties. The electric-arcmethod of producing C60 also leadsto a smaller number of fullerenessuch as C70, which have less symmet-rical molecular structures. It is alsopossible to produce forms of carbonin which the atoms are linked in acylindrical, rather than spherical,framework with a diameter of a fewnanometres. They are known asbuckytubes (or nanotubes).A• Information about IUPAC nomenclature

and representation of fullerenes andrelated compounds

Buchner funnel 86

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Buckminsterfullerene

buckyball See buckminster-fullerene.

buckytube See buckminster-fullerene.

buffer A solution that resistschange in pH when small amounts ofan acid or alkali are added over a cer-tain range or when the solution is di-luted. Acidic buffers consist of aweak acid with a salt of the acid. Thesalt provides the negative ion A–,which is the conjugate base of theacid HA. An example is carbonic acid


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and sodium hydrogencarbonate.Basic buffers have a weak base and a salt of the base (to provide theconjugate acid). An example is am-monia solution with ammoniumchloride.

In an acidic buffer, for example,molecules HA and ions A– are pre-sent. When acid is added most of theextra protons are removed by thebase:

A– + H+ → HAWhen base is added, most of theextra hydroxide ions are removed byreaction with undissociated acid:

OH– + HA → A– + H2OThus, the addition of acid or basechanges the pH very little. The hy-drogen-ion concentration in a bufferis given by the expression

Ka = [H+] = [A–]/[HA]i.e. it depends on the ratio of conju-gate base to acid. As this is not al-tered by dilution, the hydrogen-ionconcentration for a buffer does notchange much during dilution.

In the laboratory, buffers are usedto prepare solutions of known stablepH. Natural buffers occur in living or-ganisms, where the biochemical re-actions are very sensitive to changein pH. The main natural buffers areH2CO3/HCO3

– and H2PO4–/HPO4

2–.Buffer solutions are also used inmedicine (e.g. in intravenous injec-tions), in agriculture, and in manyindustrial processes (e.g. dyeing,fermentation processes, and the food industry).

bumping Violent boiling of a liquidcaused by superheating so that bub-bles form at a pressure above atmos-pheric pressure. It can be preventedby putting pieces of porous pot in the liquid to enable bubbles ofvapour to form at the normal boil-ing point.

buna rubber A type of syntheticrubber based in polymerization ofbutadiene (buta-1,3-diene). The namecomes from Bu (for butadiene) andNa (for sodium, which was used as acatalyst in the original polymeriza-tion reaction). An improved form,known as Buna-S was developed bycopolymerizing butadiene withstyrene. In 1934, Buna-N was in-vented, in which the styrene was re-placed by acrylonitrile, giving aproduct with better oil resistance (seenitrile rubber).

Bunsen, Robert Wilhelm(1811–99) German chemist, who heldprofessorships at Kassel, Marburg,and Heidelberg. His early researcheson arsenic-containing compoundscost him an eye in an explosion. Hethen turned to gas analysis and spec-troscopy, enabling him and *Kirch-hoff to discover the elements*caesium (1860) and *rubidium(1861). He also popularized the use ofthe *Bunsen burner and developedthe *Bunsen cell.

Bunsen burner A laboratory gasburner having a vertical metal tubeinto which the gas is led, with a holein the side of the base of the tube toadmit air. The amount of air can beregulated by a sleeve on the tube.When no air is admitted the Ûame isluminous and smoky. With air, it hasa faintly visible hot outer part (theoxidizing part) and an inner bluecone where combustion is incom-plete (the cooler reducing part of theÛame). The device is named afterRobert *Bunsen, who used a similardevice (without a regulating sleeve)in 1855.

Bunsen cell A *primary cell de-vised by Robert *Bunsen consistingof a zinc cathode immersed in dilutesulphuric acid and a carbon anodeimmersed in concentrated nitric acid.

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The electrolytes are separated by aporous pot. The cell gives an e.m.f. ofabout 1.9 volts.

burette A graduated glass tubewith a tap at one end leading to aÜne outlet tube, used for deliveringknown volumes of a liquid (e.g. intitration).

buta-1,3-diene (butadiene) Acolourless gaseous hydrocarbon,CH2:CHCH:CH2; m.p. –109°C; b.p.–4.5°C. It is made by catalytic dehy-drogenation of butane (from petro-leum or natural gas) and polymerizedin the production of synthetic rub-bers. The compound is a conjugated*diene in which the electrons in thepi orbitals are partially delocalizedover the whole molecule. It can havetrans and cis forms, the latter takingpart in *Diels–Alder reactions.

butanal (butyraldehyde) A colour-less Ûammable liquid aldehyde,C3H7CHO; r.d. 0.8; m.p. –99°C; b.p.75.7°C.

butane A gaseous hydrocarbon,C4H10; d. 0.58 g cm–3; m.p. –138°C;b.p. 0°C. Butane is obtained frompetroleum (from reÜnery gas or by cracking higher hydrocarbons).The fourth member of the *alkaneseries, it has a straight chain ofcarbon atoms and is isomeric with 2-methylpropane (CH3CH(CH3)CH3,formerly called isobutane). It can eas-ily be liqueÜed under pressure and issupplied in cylinders for use as a fuelgas. It is also a raw material for mak-ing buta-1,3-diene (for synthetic rub-ber).

butanedioic acid (succinic acid) Acolourless crystalline fatty acid,(CH2)2(COOH)2; r.d. 1.6; m.p. 185°C;b.p. 235°C. A weak carboxylic acid, itis produced by fermentation of sugaror ammonium tartrate and used as asequestrant and in making dyes. It

occurs in living organisms as an in-termediate in metabolism, especiallyin the *Krebs cycle.

burette 88

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CH2

CH2

O

OH

OH

O

Butanedioic acid

butanoic acid (butyric acid) Acolourless liquid water-soluble acid,C3H7COOH; r.d. 0.96; b.p. 163°C. It isa weak acid (Ka = 1.5 × 10–5 mol dm–3

at 25°C) with a rancid odour. Its es-ters are present in butter and inhuman perspiration. The acid is usedto make esters for Ûavourings andperfumery.

butanol Either of two aliphaticalcohols with the formula C4H9OH.Butan-1-ol, CH3(CH2)3OH, is aprimary alcohol; r.d. 0.81; m.p.–89.5°C; b.p. 117.3°C. Butan-2-ol,CH3CH(OH)C2H5, is a secondary al-cohol; r.d. 0.81; m.p. –114.7°C; b.p.100°C. Both are colourless volatileliquids obtained from butane and areused as solvents.

butanone (methyl ethyl ketone) Acolourless Ûammable water-solubleliquid, CH3COC2H5; r.d. 0.8; m.p.–86.4°C; b.p. 79.6°C. It can be madeby the catalytic oxidation of butaneand is used as a solvent.

butene (butylene) Either of twoisomers with the formula C4H8: 1-butene (CH3CH2CH:CH2), which ismade by passing 1-butanol vapourover heated alumina, and but-2-ene(CH3CH:CHCH3), which is made byheating 2-butanol with sulphuricacid. They are unpleasant-smellinggases used in the manufacture ofpolymers. The isomer 2-methyl-propane ((CH3)2C:CH2) was formerly


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89 B–Z reaction

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called isobutene or isobutylene. Seealkenes.

butenedioic acid Either of two isomers with the formulaHCOOHC:CHCOOH. Both com-pounds can be regarded as deriva-tives of ethene in which a hydrogenatom on each carbon has been re-placed by a –COOH group. The com-pounds show cis–trans isomerism.The trans form is fumaric acid (r.d.1.64; sublimes at 165°C) and the cisform is maleic acid (r.d. 1.59; m.p.139–140°C). Both are colourless crys-talline compounds used in makingsynthetic resins. The cis form israther less stable than the trans formand converts to the trans form at120°C. Unlike the trans form it caneliminate water on heating to form acyclic anhydride containing a–CO.O.CO– group (maleic anhydride).Fumaric acid is an intermediate inthe *Krebs cycle.

cis-butenedioic acid(maleic acid)

trans-butenedioic acid(fumaric acid)

Butenedioic acid

Butler–Volmer equation Anequation for the rate of an electro-chemical reaction; it describes thecurrent density at an electrode interms of the overpotential. The But-ler–Volmer equation is given by:

j = ja – jc = je[exp(1 – α)Fη/RT – exp(– αF/RT)],

where ja and jc are the individualcathode and anode currents respec-tively, and je is the equilibrium cur-rent, called the exchange currentdensity. By deÜnition

je = jce = jae, where jce is the equilibrium cathodecurrent and jae is the equilibriumanode current. F is the Faraday con-stant, η is the overpotential, R is thegas constant, T is the thermodynamictemperature, and α is a quantitycalled the *transfer coefÜcient.

butterÛy effect See chaos.

butylene See butene.

butyl group The organic groupCH3(CH2)3–.

butyl rubber A type of syntheticrubber obtained by copolymerizing2-methylpropene (CH2:C(CH3)CH3;isobutylene) and methylbuta-1,3-diene (CH2:C(CH3)CH:CH2, isoprene).Only small amounts of isoprene(about 2 mole %) are used. The rub-ber can be vulcanized. Large amounts were once used for tyreinner tubes.

butyraldehyde See butanal.

butyric acid See butanoic acid.

by-product A compound formedduring a chemical reaction at thesame time as the main product. Com-mercially useful by-products are ob-tained from a number of industrialprocesses. For example, calcium chlo-ride is a by-product of the *Solvayprocess for making sodium carbon-ate. Propanone is a by-product in themanufacture of *phenol.

BZ See benzoylecgonine.

B–Z reaction (Belousov–Zhabotin-skii reaction) A chemical reactionthat shows a periodic colour changebetween magenta and blue with a pe-riod of about one minute. It occurswith a mixture of sulphuric acid,potassium bromate(V), cerium sul-phate, and propanedioic acid. Thecolour change is caused by alternat-ing oxidation–reductions in which


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cerium changes its oxidation state(Ce3+ gives a magenta solution whileCe4+ gives a blue solution). The B–Zreaction is an example of a chemical*oscillating reaction – a reaction in

which there is a regular periodicchange in the concentration of oneor more reactants. The mechanism ishighly complicated, involving a largenumber of steps. See brusselator.

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C

cacodyl An oily liquid,(CH3)2AsAs(CH3)2. It has a characteris-tic odour of garlick and is poisonous.Cacodyl was one of the Ürst organo-metallic compounds to be synthe-sized (by heating arsenic withpotassium acetate). The group(CH3)2As– is the cacodyl group.

cadmium Symbol Cd. A soft bluishmetal belonging to *group 12 (for-merly IIB) of the periodic table; a.n.48; r.a.m. 112.41; r.d. 8.65; m.p.320.9°C; b.p. 765°C. The element’sname is derived from the ancientname for calamine, zinc carbonateZnCO3, and it is usually found associ-ated with zinc ores, such as spha-lerite (ZnS), but does occur as themineral greenockite (CdS). Cadmiumis usually produced as an associateproduct when zinc, copper, and leadores are reduced. Cadmium is used inlow-melting-point alloys to make sol-ders, in Ni–Cd batteries, in bearingalloys, and in electroplating (over50%). Cadmium compounds are usedas phosphorescent coatings in TVtubes. Cadmium and its compoundsare extremely toxic at low concentra-tions; great care is essential wheresolders are used or where fumes areemitted. It has similar chemical prop-erties to zinc but shows a greater ten-dency towards complex formation.The element was discovered in 1817by F. Stromeyer.A• Information from the WebElements site

cadmium cell See weston cell.

cadmium sulphide A water-insoluble compound, CdS; r.d. 4.82. It occurs naturally as the mineral

greenockite and is used as a pigmentand in semiconductors and Ûuores-cent materials.

caesium Symbol Cs. A soft silvery-white metallic element belonging to*group 1 (formerly IA) of the periodictable; a.n. 55; r.a.m. 132.905; r.d.1.88; m.p. 28.4°C; b.p. 678°C. It oc-curs in small amounts in a numberof minerals, the main source beingcarnallite (KCl.MgCl2.6H2O). It is ob-tained by electrolysis of molten cae-sium cyanide. The natural isotope iscaesium–133. There are 15 otherradioactive isotopes. Caesium–137(half-life 33 years) is used as a gammasource. As the heaviest alkali metal,caesium has the lowest ionization po-tential of all elements, hence its usein photoelectric cells, etc.A• Information from the WebElements site

caesium chloride structure Atype of ionic crystal structure inwhich the anions are at the eight cor-ners of a cubic unit cell with onecation at the centre of the cell. It canequivalently by described as cationsat the corners of the cell with ananion at the centre. Each type of ionhas a coordination number of 8. Ex-amples of compounds with thisstructure are CsCl, CsBr, CsI, CsCN,CuZn, and NH4Cl.

caesium clock An *atomic clockthat depends on the energy differ-ence between two states of the cae-sium–133 nucleus when it is in amagnetic Üeld. In one type, atoms ofcaesium–133 are irradiated withradio-frequency radiation, whose fre-quency is chosen to correspond to


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the energy difference between thetwo states. Some caesium nuclei ab-sorb this radiation and are excited tothe higher state. These atoms aredeÛected by a further magnetic Üeld,which causes them to hit a detector.A signal from this detector is fedback to the radio-frequency oscillatorto prevent it drifting from the reso-nant frequency of 9 192 631 770hertz. In this way the device is lockedto this frequency with an accuracybetter than 1 part in 1013. The cae-sium clock is used in the *SI unitdeÜnition of the second.

caffeine (1,3,7-trimethylxanthine)An alkaloid, C8H10N4O2; m.p. 235°C;sublimes at 176°C. It is a stimulantand diuretic and is present in cof-fee, tea, and some soft drinks. Seemethylxanthines.

cage compound See clathrate.

cage effect An effect occurring incertain condensed-phase reactions inwhich fragments are formed andtheir diffusion is hindered by a sur-rounding ‘cage’ of molecules. The ini-tial fragments are consequently morelikely to recombine or to react to-gether to form new products.

Cahn–Ingold–Prelog system Seecip system.

calamine A mineral form of zinccarbonate, ZnCO3 (smithsonite), al-though in the USA the same name isgiven to a hydrated zinc silicate(hemimorphite). The calamine usedmedicinally in lotions for treatingsunburn and other skin conditions isbasic zinc carbonate coloured pinkwith a trace of iron(III) oxide.

calcination The formation of a cal-cium carbonate deposit from hardwater. See hardness of water.

calcinite A mineral form of *potas-sium hydrogencarbonate, KHCO3.

calcite One of the most commonand widespread minerals, consistingof crystalline calcium carbonate,CaCO3. Calcite crystallizes in therhombohedral system; it is usuallycolourless or white and has a hard-ness of 3 on the Mohs’ scale. It hasthe property of double refraction,which is apparent in Iceland spar –the transparent variety of calcite. It isan important rock-forming mineraland is a major constituent in lime-stones, marbles, and carbonatites.

calcium Symbol Ca. A soft greymetallic element belonging to*group 2 (formerly IIA) of the peri-odic table; a.n. 20; r.a.m. 40.08; r.d.1.54; m.p. 839°C; b.p. 1484°C. Cal-cium compounds are common in theearth’s crust; e.g. limestone and mar-ble (CaCO3), gypsum (CaSO4.2H2O),and Ûuorite (CaF2). The element is ex-tracted by electrolysis of fused cal-cium chloride and is used as a getterin vacuum systems and a deoxidizerin producing nonferrous alloys. It isalso used as a reducing agent in theextraction of such metals as thorium,zirconium, and uranium.

Calcium is an essential element forliving organisms, being required fornormal growth and development. Inanimals it is an important con-stituent of bones and teeth and ispresent in the blood, being requiredfor muscle contraction and othermetabolic processes. In plants it is aconstituent (in the form of calciumpectate) of the middle lamella.


calcium acetylide See calcium di-carbide.

calcium bicarbonate See calciumhydrogencarbonate.

calcium carbide See calcium di-carbide.

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calcium carbonate A white solid,CaCO3, which is only sparingly solu-ble in water. Calcium carbonatedecomposes on heating to give *cal-cium oxide (quicklime) and carbondioxide. It occurs naturally as theminerals *calcite (rhombohedral; r.d.2.71) and *aragonite (rhombic; r.d.2.93). Rocks containing calcium car-bonate dissolve slowly in acidiÜedrainwater (containing dissolved CO2)to cause temporary hardness. In thelaboratory, calcium carbonate is pre-cipitated from *limewater by carbondioxide. Calcium carbonate is used inmaking lime (calcium oxide) and isthe main raw material for the*Solvay process.

calcium chloride A white deli-quescent compound, CaCl2, which issoluble in water; r.d. 2.15; m.p.782°C; b.p. >1600°C. There are anumber of hydrated forms, includingthe monohydrate, CaCl2.H2O, the di-hydrate, CaCl2.2H2O (r.d. 0.84), andthe hexahydrate, CaCl2.6H2O (trigo-nal; r.d. 1.71; the hexahydrate loses4H2O at 30°C and the remaining2H2O at 200°C). Large quantities of itare formed as a byproduct of the*Solvay process and it can be pre-pared by dissolving calcium carbon-ate or calcium oxide in hydrochloricacid. Crystals of the anhydrous saltcan only be obtained if the hydratedsalt is heated in a stream of hydrogenchloride. Solid calcium chloride isused in mines and on roads to reducedust problems, whilst the molten saltis the electrolyte in the extraction ofcalcium. An aqueous solution of cal-cium chloride is used in refrigerationplants.

calcium cyanamide A colourlesssolid, CaCN2, which sublimes at1300°C. It is prepared by heating cal-cium dicarbide at 800°C in a streamof nitrogen:

CaC2(s) + N2(g) → CaCN2(s) + C(s)

The reaction has been used as amethod of Üxing nitrogen in coun-tries in which cheap electricity isavailable to make the calcium dicar-bide (the cyanamide process). Cal-cium cyanamide can be used as afertilizer because it reacts with waterto give ammonia and calcium car-bonate:

CaCN2(s) + 3H2O(l) → CaCO3(s) +2NH3(g)

It is also used in the production ofmelamine, urea, and certain cyanidesalts.

calcium dicarbide (calciumacetylide; calcium carbide; carbide) Acolourless solid compound, CaC2;tetragonal; r.d. 2.22; m.p. 450°C; b.p.2300°C. In countries in which elec-tricity is cheap it is manufactured byheating calcium oxide with eithercoke or ethyne at temperaturesabove 2000°C in an electric arc fur-nace. The crystals consist of Ca2+ andC2

– ions arranged in a similar way tothe ions in sodium chloride. Whenwater is added to calcium dicarbide,the important organic raw materialethyne (acetylene) is produced:

CaC2(s) + 2H2O(l) → Ca(OH)2(s) +C2H2(g)

calcium Ûuoride A white crys-talline solid, CaF2; r.d. 3.2; m.p.1360°C; b.p. 2500°C. It occurs natu-rally as the mineral *Ûuorite (orÛuorspar) and is the main source ofÛuorine. See also fluorite structure.

calcium hydrogencarbonate (cal-cium bicarbonate) A compound,Ca(HCO3)2, that is stable only in solu-tion and is formed when water con-taining carbon dioxide dissolvescalcium carbonate:

CaCO3(s) + H2O(l) + CO2(g) →Ca(HCO3)2(aq)

It is the cause of temporary *hard-ness in water, because the calcium

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ions react with soap to give scum.Calcium hydrogencarbonate is unsta-ble when heated and decomposes togive solid calcium carbonate. This ex-plains why temporary hardness is re-moved by boiling and the formationof ‘scale’ in kettles and boilers.

calcium hydroxide (slaked lime) Awhite solid, Ca(OH)2, which dissolvessparingly in water (see limewater);hexagonal; r.d. 2.24. It is manufac-tured by adding water to calciumoxide, a process that evolves muchheat and is known as slaking. It isused as a cheap alkali to neutralizethe acidity in certain soils and in themanufacture of mortar, whitewash,bleaching powder, and glass.

calcium nitrate A white deliques-cent compound, Ca(NO3)2, that isvery soluble in water; cubic; r.d. 2.50;m.p. 561°C. It can be prepared byneutralizing nitric acid with calciumcarbonate and crystallizing it fromsolution as the tetrahydrateCa(NO3)2.4H2O, which exists in twomonoclinic crystalline forms (α, r.d.1.9; β, r.d. 1.82). There is also a trihy-drate, Ca(NO3)2.3H2O. The anhydroussalt can be obtained from the hy-drate by heating but it decomposeson strong heating to give the oxide,nitrogen dioxide, and oxygen. Cal-cium nitrate is sometimes used as anitrogenous fertilizer.

calcium octadecanoate (calciumstearate) An insoluble white salt,Ca(CH3(CH2)16COO)2, which is formedwhen soap is mixed with water con-taining calcium ions and is the scumproduced in hard-water regions.

calcium oxide (quicklime) A whitesolid compound, CaO, formed byheating calcium in oxygen or by thethermal decomposition of calciumcarbonate; cubic; r.d. 3.35; m.p.2580°C; b.p. 2850°C. On a large scale,calcium carbonate in the form of

limestone is heated in a tall tower(lime kiln) to a temperature above550°C:

CaCO3(s) ˆ CaO(s) + CO2(g)Although the reaction is reversible,the carbon dioxide is carried away bythe upward current through the kilnand all the limestone decomposes.Calcium oxide is used to make cal-cium hydroxide, as a cheap alkali fortreating acid soil, and in extractivemetallurgy to produce a slag with theimpurities (especially sand) presentin metal ores.

calcium phosphate(V) A whiteinsoluble powder, Ca3(PO4)2; r.d. 3.14.It is found naturally in the mineral*apatite, Ca5(PO4)3(OH,F,Cl), and asrock phosphate. It is also the mainconstituent of animal bones. Calciumphosphate can be prepared by mix-ing solutions containing calcium ionsand hydrogenphosphate ions in thepresence of an alkali:

HPO42– + OH– → PO4

3– + H2O

3Ca2+ + 2PO43– → Ca3(PO4)2

It is used extensively as a fertilizer.The compound was formerly calledcalcium orthophosphate (see phos-phates).

calcium stearate See calcium oc-tadecanoate.

calcium sulphate A white solidcompound, CaSO4; r.d. 2.96; 1450°C.It occurs naturally as the mineral*anhydrite, which has a rhombicstructure, transforming to a mono-clinic form at 200°C. More com-monly, it is found as the dihydrate,*gypsum, CaSO4.2H2O (monoclinic;r.d. 2.32). When heated, gypsumloses water at 128°C to give thehemihydrate, 2CaSO4.H2O, betterknown as *plaster of Paris. Calciumsulphate is sparingly soluble in waterand is a cause of permanent *hard-ness of water. It is used in the manu-

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facture of certain paints, ceramics,and paper. The naturally occurringforms are used in the manufacture ofsulphuric acid.

Calgon Tradename for a water-softening agent. See hardness ofwater.

caliche A mixture of salts found indeposits between gravel beds in theAtacama and Tarapaca regions ofChile. They vary from 4 m to 15 cmthick and were formed by periodicleaching of soluble salts during wetgeological epochs, followed by dryingout of inland seas in dry periods.They are economically important as asource of nitrates. A typical composi-tion is NaNO3 17.6%, NaCl 16.1%,Na2SO4 6.5%, CaSO4 5.5%, MgSO43.0%, KNO3 1.3%, Na2B4O7 0.94%,KClO3 0.23%, NaIO3 0.11%, sand andgravel to 100%.

californium Symbol Cf. A radio-active metallic transuranic elementbelonging to the *actinoids; a.n. 98;mass number of the most stable iso-tope 251 (half-life about 700 years).Nine isotopes are known; cali-fornium–252 is an intense neutronsource, which makes it useful in neu-tron *activation analysis and poten-tially useful as a radiation source inmedicine. The element was Ürst pro-duced by Glenn Seaborg (1912–99)and associates in 1950.A• Information from the WebElements site

calixarenes Compounds that havemolecules with a cuplike structure(the name comes from the Greekcalix, cup). The simplest, has fourphenol molecules joined by four–CH2– groups into a ring (formingthe base of the ‘cup’). The four phe-nol hexagons point in the same di-rection to form a cavity that can bindsubstrate molecules. Interest hasbeen shown in the potential ability

of calixarene molecules to mimic en-zyme action.

calmodulin A protein, consistingof 148 amino-acid residues, that actsas a receptor for calcium ions inmany calcium-regulated processes inboth animal and plant cells. Calmo-dulin mediates reactions catalysed bymany enzymes.

calomel See mercury(i) chloride.

calomel half cell (calomel elec-trode) A type of half cell in whichthe electrode is mercury coated withcalomel (HgCl) and the electrolyte isa solution of potassium chloride andsaturated calomel. The standard elec-trode potential is –0.2415 volt (25°C).In the calomel half cell the reactionsare

HgCl(s) ˆ Hg+(aq) + Cl–(aq)

Hg+(aq) + e ˆ Hg(s)The overall reaction is

HgCl(s) + e ˆ Hg(s) + Cl–(aq)This is equivalent to a Cl2(g)|Cl–(aq)half cell.

calorie The quantity of heat re-quired to raise the temperature of 1gram of water by 1°C (1 K). The calo-rie, a c.g.s. unit, is now largely re-placed by the *joule, an *SI unit. 1calorie = 4.186 8 joules.

Calorie (kilogram calorie; kilocalo-rie) 1000 calories. This unit is still inlimited use in estimating the energyvalue of foods, but is obsolescent.

caloriÜc value The heat per unitmass produced by complete combus-tion of a given substance. CaloriÜcvalues are used to express the energyvalues of fuels; usually these are ex-pressed in megajoules per kilogram(MJ kg–1). They are also used to meas-ure the energy content of foodstuffs;i.e. the energy produced when thefood is oxidized in the body. Theunits here are kilojoules per gram

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(kJ g–1), although Calories (kilocalo-ries) are often still used in nontechni-cal contexts. CaloriÜc values aremeasured using a *bomb calorime-ter.

calorimeter Any of various devicesused to measure thermal properties,such as caloriÜc values or heats ofchemical reactions. See also bombcalorimeter.

Calvin, Melvin (1911–97) US bio-chemist. After World War II, at theLawrence Radiation Laboratory,Berkeley, he investigated the light-in-dependent reactions of *photosyn-thesis. Using radioactive carbon-14 tolabel carbon dioxide he discoveredthe *Calvin cycle, for which he wasawarded the 1961 Nobel Prize forchemistry.

Calvin cycle The metabolic path-way of the light-independent stage of*photosynthesis, which occurs in thestroma of the chloroplasts. The path-way was elucidated by Melvin*Calvin and his co-workers and in-volves the Üxation of carbon dioxideand its subsequent reduction to car-bohydrate. During the cycle, carbondioxide combines with *ribulose bis-phosphate, through the mediation ofthe enzyme ribulose bisphosphatecarboxylase, to form an unstable six-carbon compound that breaksdown to form two molecules of thethree-carbon compound glycerate 3-phosphate. This is converted toglyceraldehyde 3-phosphate, which isused to regenerate ribulose bisphos-phate and to produce glucose andfructose.

calx A metal oxide formed by heat-ing an ore in air.

camphor A white crystalline cyclicketone, C10H16O; r.d. 0.99; m.p.179°C; b.p. 204°C. It was formerly ob-tained from the wood of the For-

mosan camphor tree, but can now besynthesized. The compound has acharacteristic odour associated withits use in mothballs. It is a plasticizerin celluloid.

calorimeter 96

cCH3CH3CH3

H3C C CH3H3C C CH3H3C C CH3

COCOCO

CCC

CH2CH2CH2CCCHHH

H2CH2CH2C

H2CH2CH2C

Camphor

Canada balsam A yellow-tintedresin used for mounting specimensin optical microscopy. It has similaroptical properties to glass.

candela Symbol Cd. The *SI unit ofluminous intensity equal to the lumi-nous intensity in a given direction ofa source that emits monochromaticradiation of frequency 540 × 1012 Hzand has a radiant intensity in that di-rection of 1/683 watt per steradian.

cane sugar See sucrose.

cannabinoids A large group ofstructurally related phenolic com-pounds found in the plant Cannabissativa. The main one is *tetrahydro-cannabinol (THC), which is the com-pound responsible for the affects ofcannabis. It affects cannabinoid re-ceptors found in the brain (and alsoin the spleen). The name ‘cannabi-noid’ is also applied to structurallyunrelated compounds found natu-rally in animal tissue and having anaffect on the cannabinoid receptors.These endocannabinoids (endog-enous cannabinoids) are believed toact as ‘messengers’ between cells.

cannabis An illegal drug producedfrom the plant Cannabis sativa. Thedried inÛorescences of the plant areknown as marijuana and the thickresin produced from the plant is


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known as hashish. The normalmethod of using cannabis is tosmoke it, although it can also betaken in certain foods. Cannabis is aclass C controlled substance in theUK. It contains a large number of re-lated compounds known as herbal*cannabinoids. The main active com-ponent is *tetrahydrocannabinol(THC).

Cannizzaro, Stanislao(1826–1910) Italian chemist. He dis-covered the *Cannizzaro reaction in1853. His main contribution to chem-istry was his recognition of the valid-ity of *Avogadro’s law and hisdeduction that common gases suchas H2 and O2 are diatomic molecules.This led to the establishment of a re-liable system of atomic and molecu-lar weights.

Cannizzaro reaction A reaction ofaldehydes to give carboxylic acidsand alcohols. It occurs in the pres-ence of strong bases with aldehydesthat do not have alpha hydrogenatoms. For example, benzenecar-baldehyde gives benzenecarboxylicacid and benzyl alcohol:

2C6H5CHO → C6H5COOH +C6H5CH2OH

Aldehydes that have alpha hydrogenatoms undergo the *aldol reaction in-stead. The Cannizzaro reaction is anexample of a *disproportionation. Itwas discovered by Stanislao *Canniz-zaro.

canonical form One of the possi-ble structures of a molecule that to-gether form a *resonance hybrid.

capillary A tube of small diameter.

capillary electrophoresis (CE) Atechnique for investigating mixturesof charged species. In capilliary zoneelectrophoresis (CZE), the sample isintroduced into a Üne capillary tube,with each end of the capillary placed

in a reservoir containing an elec-trolyte (e.g. a buffer solution). Thesource reservoir contains a positiveelectrode and the destination reser-voir contains a negative electrode. Ahigh potential difference is main-tained between the electrodes. Com-ponents of the sample Ûow throughthe buffer solution in the capillaryunder the inÛuence of the electricÜeld. Their mobility depends on theircharge, size, and shape, and the com-ponents separate as they movethrough the tube. They are detectedclose to the end of the capillary, usu-ally by ultraviolet absorption. Capil-liary gel electrophoresis (CGE) is usedfor separating large charged species,such as DNA fragments. The capil-liary is Ülled with a gel and separa-tion depends on the size of thespecies. In CE, a graph of detectoroutput against time is known as anelectropherogram. Individual compo-nents can be identiÜed by their reten-tion times in the capillary. Thetechnique can be highly sensitiveand is widely used in forensic labora-tories.

capillary gel electrophoresis(CGE) See capillary electro-phoresis.

capillary zone electrophoresis(CZE) See capillary electro-phoresis.

capric acid See decanoic acid.

caproic acid See hexanoic acid.

caprolactam (6-hexanelactam) Awhite crystalline substance,

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C6H11NO; r.d. 1.02; m.p. 69–71°C; b.p.139°C. It is a *lactam containing the–NH.CO– group with Üve CH2 groupsmaking up the rest of the seven-membered ring. Caprolactam is usedin making *nylon.

caprylic acid See octanoic acid.

capture Any of various processes inwhich a system of particles absorbsan extra particle. There are severalexamples in atomic and nuclearphysics. For instance, a positive ionmay capture an electron to give aneutral atom or molecule. Similarly,a neutral atom or molecule capturingan electron becomes a negative ion.An atomic nucleus may capture aneutron to produce a different (oftenunstable) nucleus. Another type ofnuclear capture is the process inwhich the nucleus of an atom ab-sorbs an electron from the innermostorbit (the K shell) to transform into adifferent nucleus. In this process(called K capture) the atom is left inan excited state and generally decaysby emission of an X-ray photon.

Radiative capture is any suchprocess in which the capture resultsin an excited state that decays byemission of photons. A common ex-ample is neutron capture to yield anexcited nucleus, which decays byemission of a gamma ray.

carat 1. A measure of Üneness (pu-rity) of gold. Pure gold is described as24-carat gold. 14-carat gold contains14 parts in 24 of gold, the remainderusually being copper. 2. A unit ofmass equal to 0.200 gram, used tomeasure the masses of diamonds andother gemstones.

carbamide See urea.

carbanion An organic ion with anegative charge on a carbon atom;i.e. an ion of the type R3C–. Carban-ions are intermediates in certain

types of organic reaction (e.g. the*aldol reaction).

carbazole A white crystalline com-pound found with anthracene,C12H9N; m.p. 238°C; b.p. 335°C. It isused in the manufacture of dyestuffs.

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1

2

3

4

5

6

7

8

9

4a5a

9a 9b

Carbazole

carbene A species of the type R2C:,in which the carbon atom has twoelectrons that do not form bonds.Methylene, :CH2, is the simplest ex-ample. Carbenes are highly reactiveand exist only as transient intermedi-ates in certain organic reactions.They attack double bonds to give cy-clopropane derivatives. They alsocause insertion reactions, in whichthe carbene group is inserted be-tween the carbon and hydrogenatoms of a C–H bond:

C–H + :CR2 → C–CR2–H

carbenium ion See carbocation.

carbide Any of various compoundsof carbon with metals or other moreelectropositive elements. True car-bides contain the ion C4– as in Al4C3.These are saltlike compounds givingmethane on hydrolysis, and were for-merly called methanides. Com-pounds containing the ion C2

2– arealso saltlike and are known as dicar-bides. They yield ethyne (acetylene)on hydrolysis and were formerlycalled acetylides. The above types ofcompound are ionic but have par-tially covalent bond character, butboron and silicon form true covalentcarbides, with giant molecular struc-tures. In addition, the transition met-als form a range of interstitial


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carbides in which the carbon atomsoccupy interstitial positions in themetal lattice. These substances aregenerally hard materials with metal-lic conductivity. Some transition met-als (e.g. Cr, Mn, Fe, Co, and Ni) haveatomic radii that are too small toallow individual carbon atoms in theinterstitial holes. These form car-bides in which the metal lattice isdistorted and chains of carbon atomsexist (e.g. Cr3C2, Fe3C). Such com-pounds are intermediate in characterbetween interstitial carbides andionic carbides. They give mixtures ofhydrocarbons on hydrolysis withwater or acids.

carbocation An ion with a positivecharge that is mostly localized on acarbon atom. There are two types:

Carbonium ions have Üve bonds tothe carbon atom and a completeouter shell of eight electrons. A sim-ple example is the ion CH5

+, whichhas a trigonal bipyramidal shape.Ions of this type are transientspecies. They can be produced byelectron impact and detected bymass spectroscopy.

Carbenium ions have three bondsto the carbon atom and are planar,with six outer electrons and a vacantp-orbital. Ions of this type are inter-mediates in a number of organic re-actions (for example, in the SN1mechanism of *nucleophilic substitu-tion). Certain carbenium ions are sta-bilized by delocalization of thecharge. An example is the orange-redsalt (C6H5)3C+Cl-. Carbenium ions canbe produced by *superacids.

carbocyclic See cyclic.

carbohydrate One of a group oforganic compounds based on thegeneral formula Cx(H2O)y. The sim-plest carbohydrates are the *sugars(saccharides), including glucose andsucrose. *Polysaccharides are carbo-

hydrates of much greater molecularweight and complexity; examples arestarch, glycogen, and cellulose. Car-bohydrates perform many vital rolesin living organisms. Sugars, notablyglucose, and their derivatives are es-sential intermediates in the conver-sion of food to energy. Starch andother polysaccharides serve as energystores in plants. Cellulose, lignin, andothers form the supporting cell wallsand woody tissue of plants. Chitin isa structural polysaccharide found inthe body shells of many invertebrateanimals.A• Information about IUPAC nomenclature

carbolic acid See phenol.

carbon Symbol C. A nonmetallic el-ement belonging to *group 14 (for-merly IVB) of the periodic table; a.n.6; r.a.m. 12.011; m.p. ∼3550°C; b.p.∼4827°C. Carbon has three main al-lotropic forms (see allotropy).

*Diamond (r.d. 3.52) occurs natu-rally and can be produced syntheti-cally. It is extremely hard and hashighly refractive crystals. The hard-ness of diamond results from the co-valent crystal structure, in whicheach carbon atom is linked by cova-lent bonds to four others situated atthe corners of a tetrahedron. TheC–C bond length is 0.154 nm and thebond angle is 109.5°.

Graphite (r.d. 2.25) is a soft blackslippery substance (sometimes calledblack lead or plumbago). It occursnaturally and can also be made bythe *Acheson process. In graphitethe carbon atoms are arranged in lay-ers, in which each carbon atom issurrounded by three others to whichit is bound by single or double bonds.The layers are held together by muchweaker van der Waals’ forces. Thecarbon–carbon bond length in thelayers is 0.142 nm and the layers are0.34 nm apart. Graphite is a good

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conductor of heat and electricity. Ithas a variety of uses including electri-cal contacts, high-temperature equip-ment, and as a solid lubricant.Graphite mixed with clay is the ‘lead’in pencils (hence its alternativename). The third crystalline allotropeis fullerite (see buckminster-fullerene). There are also severalamorphous forms of carbon, such as*carbon black and *charcoal.

There are two stable isotopes ofcarbon (proton numbers 12 and 13)and four radioactive ones (10, 11, 14,15). Carbon–14 is used in *carbondating.

Carbon forms a large number ofcompounds because of its uniqueability to form stable covalent bondswith other carbon atoms and alsowith hydrogen, oxygen, nitrogen,and sulphur atoms, resulting in theformation of a variety of compoundscontaining chains and rings of car-bon atoms.


carbon assimilation The incorpo-ration of carbon from atmosphericcarbon dioxide into organic mol-ecules. This process occurs during*photosynthesis. See carbon cycle.

carbonate A salt of carbonic acidcontaining the carbonate ion, CO3

2–.The free ion has a plane triangularstructure. Metal carbonates may beionic or may contain covalentmetal–carbonate bonds (complex car-bonates) via one or two oxygenatoms. The carbonates of the alkalimetals are all soluble but other car-bonates are insoluble; they all reactwith mineral acids to release carbondioxide.

carbonate minerals A group ofcommon rock-forming minerals con-taining the anion CO3

2– as the funda-mental unit in their structure. The

most important carbonate mineralsare *calcite, *dolomite, and *magne-site. See also aragonite.

carbonation The solution of car-bon dioxide in a liquid under pres-sure.

carbon bisulphide See carbondisulphide.

carbon black A Üne carbon powdermade by burning hydrocarbons in in-sufÜcient air. It is used as a pigmentand a Üller (e.g. for rubber).

carbon cycle 1. One of the majorcycles of chemical elements in theenvironment. Carbon (as carbondioxide) is taken up from the atmos-phere and incorporated into the tis-sues of plants in *photosynthesis. Itmay then pass into the bodies of ani-mals as the plants are eaten. Duringthe respiration of plants, animals,and organisms that bring about de-composition, carbon dioxide is re-turned to the atmosphere. Thecombustion of fossil fuels (e.g. coaland peat) also releases carbon diox-ide into the atmosphere. See illustra-tion. 2. (in physics) A series ofnuclear reactions in which four hy-drogen nuclei combine to form a he-lium nucleus with the liberation ofenergy, two positrons, and two neu-trinos. The process is believed to bethe source of energy in many starsand to take place in six stages. In thisseries carbon–12 acts as if it were acatalyst, being reformed at the end ofthe series:

126C + 11H → 13

7N + γ137N → 13

6C + e+ + νe

136C + 11H → 14

7N + γ147N + 11H → 15

8O + γ158O → 15

7N + e+ + νe

157N + 11H → 12

6C + 42He.

carbon dating (radiocarbon dating)

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A method of estimating the ages ofarchaeological specimens of biologi-cal origin. As a result of cosmic radia-tion a small number of atmosphericnitrogen nuclei are continuouslybeing transformed by neutron bom-bardment into radioactive nuclei ofcarbon–14:

147N + n → 14

6C + pSome of these radiocarbon atomsÜnd their way into living trees andother plants in the form of carbondioxide, as a result of photosynthesis.When the tree is cut down photosyn-thesis stops and the ratio of radiocar-bon atoms to stable carbon atomsbegins to fall as the radiocarbon de-cays. The ratio 14C/12C in the speci-men can be measured and enablesthe time that has elapsed since thetree was cut down to be calculated.The method has been shown to giveconsistent results for specimens upto some 40 000 years old, though itsaccuracy depends upon assumptionsconcerning the past intensity of thecosmic radiation. The technique was

developed by Willard F. Libby(1908–80) and his coworkers in1946–47.

carbon dioxide A colourlessodourless gas, CO2, soluble in water,ethanol, and acetone; d. 1.977 g dm–3

(0°C); m.p. –56.6°C; b.p. –78.5°C. It oc-curs in the atmosphere (0.04% by vol-ume) but has a short residence timein this phase as it is both consumedby plants during *photosynthesis andproduced by respiration and by com-bustion. It is readily prepared in thelaboratory by the action of diluteacids on metal carbonates or of heaton heavy-metal carbonates. Carbondioxide is a by-product from themanufacture of lime and from fer-mentation processes.

Carbon dioxide has a small liquidrange and liquid carbon dioxide isproduced only at high pressures. Themolecule CO2 is linear with each oxy-gen making a double bond to the car-bon. Chemically, it is unreactive andwill not support combustion. It dis-

101 carbon dioxide

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carbon dioxide inthe atmosphere

combustion respiration respiration indecomposers

respiration photosynthesisorganiccompoundsin animals

carbon compoundsin dead organicmatter

death

deathdeathdeath

organic compoundsin green plants

fossilizationfossilizationfossilization feedingfeedingfeeding

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solves in water to give *carbonicacid.

Large quantities of solid carbondioxide (dry ice) are used in processesrequiring large-scale refrigeration. Itis also used in Üre extinguishers as adesirable alternative to water formost Üres, and as a constituent ofmedical gases as it promotes exhala-tion. It is also used in carbonateddrinks.

The level of carbon dioxide in theatmosphere has increased by some12% in the last 100 years, mainly be-cause of extensive burning of fossilfuels and the destruction of largeareas of rain forest. This has beenpostulated as the main cause of theaverage increase of 0.5°C in globaltemperatures over the same period,through the *greenhouse effect.Steps are now being taken to preventfurther increases in atmospheric CO2concentration and subsequent globalwarming.

carbon disulphide (carbon bisul-phide) A colourless highly refractiveliquid, CS2, slightly soluble in waterand soluble in ethanol and ether; r.d.1.261; m.p. –110°C; b.p. 46.3°C. Purecarbon disulphide has an etherealodour but the commercial product iscontaminated with a variety of othersulphur compounds and has a mostunpleasant smell. It was previouslymanufactured by heating a mixtureof wood, sulphur, and charcoal; mod-ern processes use natural gas and sul-phur. Carbon disulphide is anexcellent solvent for oils, waxes, rub-ber, sulphur, and phosphorus, but itsuse is decreasing because of its hightoxicity and its Ûammability. It isused for the preparation of xanthatesin the manufacture of viscose yarns.

carbon Übres Fibres of carbon inwhich the carbon has an orientedcrystal structure. Carbon Übres aremade by heating textile Übres and

are used in strong composite ma-terials for use at high temperatures.

carbonic acid A dibasic acid,H2CO3, formed in solution when car-bon dioxide is dissolved in water:

CO2(aq) + H2O(l) ˆ H2CO3(aq)The acid is in equilibrium with dis-solved carbon dioxide, and also disso-ciates as follows:

H2CO3 ˆ H+ + HCO3–

Ka = 4.5 × 10–7 mol dm–3

HCO3– ˆ CO3

2– + H+

Ka = 4.8 × 10–11 mol dm–3

The pure acid cannot be isolated,although it can be produced in ethersolution at –30°C. Carbonic acid givesrise to two series of salts: the *car-bonates and the *hydrogencarbon-ates.

carbonium ion See carbocation.

carbonize (carburize) To change anorganic compound into carbon byheating, or to coat something withcarbon in this way.

carbon monoxide A colourlessodourless gas, CO, sparingly solublein water and soluble in ethanol andbenzene; d. 1.25 g dm–3 (0°C); m.p.–199°C; b.p. –191.5°C. It is Ûammableand highly toxic. In the laboratory itcan be made by the dehydration ofmethanoic acid (formic acid) usingconcentrated sulphuric acid. Industri-ally it is produced by the oxidation ofnatural gas (methane) or (formerly)by the water-gas reaction. It isformed by the incomplete combus-tion of carbon and is present in car-exhaust gases.

It is a neutral oxide, which burnsin air to give carbon dioxide, and is agood reducing agent, used in a num-ber of metallurgical processes. It hasthe interesting chemical property of forming a range of transitionmetal carbonyls, e.g. Ni(CO)4. Car-bon monoxide is able to use vacant

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p-orbitals in bonding with metals;the stabilization of low oxidationstates, including the zero state, is aconsequence of this. This also ac-counts for its toxicity, which is dueto the binding of the CO to the ironin haemoglobin, thereby blockingthe uptake of oxygen.

103 carbyne

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O C__ C__ C__ O__O C__ O__

C O__ carbon monoxide

carbon dioxide

tricarbon dioxide(carbon suboxide)

Carbon monoxide

C

O

OH

R carboxyl group

Carboxylic acid

carbon suboxide See tricarbondioxide.

carbon tetrachloride See tetra-chloromethane.

carbonyl chloride (phosgene) Acolourless gas, COCl2, with an odourof freshly cut hay. It is used in or-ganic chemistry as a chlorinatingagent, and was formerly used as awar gas.

carbonyl compound A compoundcontaining the carbonyl group >C=O.Aldehydes, ketones, and carboxylicacids are examples of organic car-bonyl compounds. Inorganic car-bonyls are complexes in whichcarbon monoxide has coordinated toa metal atom or ion, as in *nickelcarbonyl, Ni(CO)4. See also ligand.

carbonyl group The group >C=O,found in aldehydes, ketones, car-boxylic acids, amides, etc., and in in-organic carbonyl complexes (seecarbonyl compound).

carboranes Compounds similar tothe *boranes, but with one or moreboron atoms replaced by carbonatoms.

carborundum See silicon carbide.

carboxyhaemoglobin The highlystable product formed when *haemo-

globin combines with carbon monox-ide. Carbon monoxide competes withoxygen for haemoglobin, with whichit binds strongly: the afÜnity ofhaemoglobin for carbon monoxide is250 times greater than that for oxy-gen. This reduces the availability ofhaemoglobin for combination with(and transport of) oxygen and ac-counts for the toxic effects of carbonmonoxide on the respiratory system.

carboxylate An anion formedfrom a *carboxylic acid. For example,ethanoic acid gives rise to theethanoate ion, CH3COO–.

carboxyl group The organic group–COOH, present in *carboxylic acids.

carboxylic acids Organic com-pounds containing the group –CO.OH(the carboxyl group; i.e. a carbonylgroup attached to a hydroxyl group).In systematic chemical nomenclaturecarboxylic-acid names end in thesufÜx -oic, e.g. ethanoic acid,CH3COOH. They are generally weakacids. Many long-chain carboxylicacids occur naturally as esters in fatsand oils and are therefore alsoknown as *fatty acids. See also glyc-erides.


carburize See carbonize.

carbylamine reaction See iso-cyanide test.

carbyne A transient species of thetype R–C≡, with three nonbondingelectrons on the carbon atom. For-


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merly, carbynes were called methyli-dynes.

carcinogen Any agent that pro-duces cancer, e.g. tobacco smoke,certain industrial chemicals, andionizing radiation (such as X-rays and ultraviolet rays).

Carius method A method of deter-mining the amount of sulphur andhalogens in an organic compound, byheating the compound in a sealedtube with silver nitrate in concen-trated nitric acid. The compound isdecomposed and silver sulphide andhalides are precipitated, separated,and weighed.

carnallite A mineral consisting of ahydrated mixed chloride of potas-sium and magnesium, KCl.MgCl2.6H2O.

carnauba wax A natural wax ob-tained from the leaves of the copaibapalm of South America. It is ex-tremely hard and brittle and is usedin varnishes and to add hardness andlustre to other waxes in polishes.

Carnot, Nicolas Léonard Sadi(1796–1832) French physicist, whoÜrst worked as a military engineer.He then turned to scientiÜc researchand in 1824 published his analysis ofthe efÜciency of heat engines. Thekey to this analysis is the thermody-namic *Carnot cycle and the even-tual introduction of the idea of*entropy in thermodynamics. Hedied at an early age from cholera.

Carnot cycle The most efÜcientcycle of operations for a reversible*heat engine. Published in 1824 byNicolas *Carnot, it consists of fouroperations on the working substancein the engine (see illustration):a. Isothermal expansion at thermody-

namic temperature T1 with heat q1

taken in.

b. Adiabatic expansion with a fall oftemperature to T2.

c. Isothermal compression at temper-ature T2 with heat q2 given out.

d. Adiabatic compression with a riseof temperature back to T1.

According to the Carnot principle, theefÜciency of any reversible heat en-gine depends only on the tempera-ture range through which it works,rather than the properties of theworking substances. In any reversibleengine, the efÜciency (η) is the ratioof the work done (W) to the heatinput (q1), i.e. η = W/q1. As, accordingto the Ürst law of *thermodynamics,W = q1 – q2, it follows that η =(q1 – q2)/q1. For the Kelvin tempera-ture scale, q1/q2 = T1/T2 and η =(T1 – T2)/T1. For maximum efÜciencyT1 should be as high as possible andT2 as low as possible.

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adiabaticcompression

isothermalcompression

adiabaticexpansion

isothermalexpansion

T1q1

T2q2

V

P

Carnot cycle

carnotite A radioactive mineralconsisting of hydrated uraniumpotassium vanadate,K2(UO2)2(VO4)2.nH2O. It varies in col-our from bright yellow to lemon- orgreenish-yellow. It is a source of ura-nium, radium, and vanadium. Thechief occurrences are in the ColoradoPlateau, USA; Radium Hill, Australia;and Katanga, Zaïre.

Carnot principle See carnotcycle.


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Caro’s acid See peroxo-sulphuric(vi) acid.

carotene A member of a class of*carotenoid pigments. Examples areβ-carotene and lycopene, which col-our carrot roots and ripe tomatofruits respectively. α- and β-caroteneyield vitamin A when they are bro-ken down during animal digestion.

carotenoid Any of a group of yel-low, orange, red, or brown plant pig-ments chemically related to terpenes.Carotenoids are responsible for thecharacteristic colour of many plantorgans, such as ripe tomatoes, car-rots, and autumn leaves. They alsoabsorb light energy and pass this onto chlorophyll molecules in the light-dependent reactions of photosynthe-sis.

Carothers, Wallace Hume(1896–1937) US industrial chemist,who joined the Du Pont companywhere he worked on polymers. In1931 he produced *neoprene, a syn-thetic rubber. His greatest successcame in 1935 with the discovery ofthe polyamide that came to beknown as *nylon. Carothers, whosuffered from depression, committedsuicide.

carrageenan A naturally occurringpolysaccharide isolated from redalgae. The polymer is composed of d-galactose units, many of which aresulphated. K-carrageenan is a gellingagent with uses similar to those of*agar.

carrier gas The gas that carries thesample in *gas chromatography.

carrier molecule 1. A moleculethat plays a role in transporting elec-trons through the *electron trans-port chain. Carrier molecules areusually proteins bound to a nonpro-tein group; they can undergo oxida-tion and reduction relatively easily,

thus allowing electrons to Ûowthrough the system. 2. A lipid-soluble molecule that can bind tolipid-insoluble molecules and trans-port them across membranes. Carriermolecules have speciÜc sites that in-teract with the molecules they trans-port. Several different molecules maycompete for transport by the samecarrier.

Carr–Purcell–Meiboom–Gill se-quence See cpmg sequence.

CARS (coherent anti-Stokes Ramanspectroscopy) A form of Raman spec-troscopy (see raman effect) enablingthe intensity of Raman transitions tobe increased. In this technique twolaser beams with different frequen-cies pass through a sample producingelectromagnetic radiation of severalfrequencies. It is possible to adjustthe frequency of the lasers so thatone of the frequencies correspondsto that of a Stokes line from the sam-ple and the coherent emission hasthe frequency of the anti-Stokes linewith a high intensity. CARS enablesRaman spectra to be obtained evenin the presence of other radiation.One application of CARS is to obtainRaman spectra from bodies in Ûames.Using this technique temperatures indifferent parts of the Ûame can be es-timated from the intensity of thetransitions.

cascade liqueÜer An apparatusfor liquefying a gas of low *criticaltemperature. Another gas, alreadybelow its critical temperature, isliquiÜed and evaporated at a reducedpressure in order to cool the Ürst gasto below its critical temperature. Inpractice a series of steps is oftenused, each step enabling the criticaltemperature of the next gas to bereached.

cascade process Any process thattakes place in a number of steps, usu-

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ally because the single step is tooinefÜcient to produce the desired re-sult. For example, in various ura-nium-enrichment processes theseparation of the desired isotope isonly poorly achieved in a singlestage; to achieve better separationthe process has to be repeated anumber of times, in a series, withthe enriched fraction of one stagebeing fed to the succeeding stage forfurther enrichment. Another exam-ple of cascade process is that operat-ing in a *cascade liqueÜer.

case hardening The hardening ofthe surface layer of steel, used fortools and certain mechanical compo-nents. The commonest method is tocarburize the surface layer by heat-ing the metal in a hydrocarbon or bydipping the red hot metal intomolten sodium cyanide. Diffusion ofnitrogen into the surface layer toform nitrides is also used.

casein One of a group of phos-phate-containing proteins (phospho-proteins) found in milk. Caseins areeasily digested by the enzymes ofyoung mammals and represent amajor source of phosphorus.

CAS registry A database of chemi-cal compounds, certain mixtures,and biological sequences maintainedby the Chemical Abstracts Service ofthe American Chemical Society. Inthe registry every entry has a uniqueCAS registry number (CASRN). Thishas three parts: up to six digits, fol-lowed by two digits, followed by onedigit. For example, the CAS numberof water is 7732-18-5. The Ünal digitis a check number. CAS registrynumbers are used for searchingchemical databases. The size of theregistry is immense, with over 32million substances and 59 million se-quences. It identiÜes every chemicalthat has been described in the litera-

ture since 1957 and around 50 000new numbers are added each week.

A• Further information from the CAS site• A free service for Ünding CAS numbers

from CambridgeSoft

cassiterite A yellow, brown, orblack form of tin(IV) oxide, SnO2,that forms tetragonal, often twinned,crystals; the principal ore of tin. It oc-curs in hydrothermal veins and meta-somatic deposits associated with acidigneous rocks and in alluvial (placer)deposits. The chief producers areMalaysia, Indonesia, Democratic Re-public of Congo, and Nigeria.

cast iron A group of iron alloyscontaining 1.8 to 4.5% of carbon. It isusually cast into speciÜc shapes readyfor machining, heat treatment, or as-sembly. It is sometimes produced di-rect from the *blast furnace or itmay be made from remelted *pigiron.

Castner–Kellner cell An elec-trolytic cell used industrially for theproduction of *sodium hydroxide.The usually iron cell is Ülled withbrine (sodium chloride solution) andemploys liquid mercury as the cath-ode. The sodium liberated thereforms an amalgam with the mercury,which is run off and reacted withwater to give sodium hydroxide (andhydrogen); the mercury is then re-used. Chlorine gas produced at theanode is another valuable by-product.

castor oil A pale-coloured oil ex-tracted from the castor-oil plant. Itcontains a mixture of glycerides of fatty acids, the predominant acid being ricinoleic acid,C17H32(OH)COOH. It is used as a *drying oil in paints and varnishesand medically as a laxative.

catabolism The metabolic break-down of large molecules in living or-

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ganisms to smaller ones, with the re-lease of energy. Respiration is an ex-ample of a catabolic series ofreactions. See metabolism. Compareanabolism.

catalysis The process of changingthe rate of a chemical reaction by useof a *catalyst.

catalyst A substance that increasesthe rate of a chemical reaction with-out itself undergoing any permanentchemical change. Catalysts that havethe same phase as the reactants arehomogeneous catalysts (e.g. *en-zymes in biochemical reactions ortransition-metal complexes used inthe liquid phase for catalysing or-ganic reactions). Those that have adifferent phase are hetereogeneouscatalysts (e.g. metals or oxides usedin many industrial gas reactions). Thecatalyst provides an alternative path-way by which the reaction can pro-ceed, in which the activation energyis lower. It thus increases the rate atwhich the reaction comes to equilib-rium, although it does not alter theposition of the equilibrium. The cata-lyst itself takes part in the reactionand consequently may undergo phys-ical change (e.g. conversion into pow-der). In certain circumstances, verysmall quantities of catalyst can speedup reactions. Most catalysts are alsohighly speciÜc in the type of reactionthey catalyse, particularly enzymesin biochemical reactions. Generally,the term is used for a substance thatincreases reaction rate (a positive cat-alyst). Some reactions can be sloweddown by negative catalysts (see inhi-bition).

catalytic converter A device usedin the exhaust systems of motor vehi-cles to reduce atmospheric pollution.The three main pollutants producedby petrol engines are: unburnt hydro-carbons, carbon monoxide produced

by incomplete combustion of hydro-carbons, and nitrogen oxides pro-duced by nitrogen in the air reactingwith oxygen at high engine tempera-tures. Hydrocarbons and carbonmonoxide can be controlled by ahigher combustion temperature anda weaker mixture. However, thehigher temperature and greater avail-ability of oxygen arising from thesemeasures encourage formation ofnitrogen oxides. The use of three-way catalytic converters solves thisproblem by using platinum andpalladium catalysts to oxidize thehydrocarbons and the CO andrhodium catalysts to reduce the ni-trogen oxides back to nitrogen. Thesethree-way catalysts require that theair–fuel ratio is strictly stochiometric.Some catalytic converters promoteoxidation reactions only, leaving thenitrogen oxides unchanged. Three-way converters can reduce hydrocar-bons and CO emissions by some 85%,at the same time reducing nitrogenoxides by 62%.

catalytic cracking See cracking.

catalytic rich gas process See crgprocess.

cataphoresis See electrophoresis.

catechol See 1,2-dihydroxy-benzene.

catecholamine Any of a class ofamines that possess a catechol(C6H4(OH)2) ring. Including*dopamine, *adrenaline, and *nora-drenaline, they function as neuro-transmitters and/or hormones.

catenane A type of compound con-sisting of two or more large ringsthat are interlocked like the links ofa chain. In a catenane, there is nochemical bonding between the rings;the rings are held together by *me-chanical bonding.

catenation 1. The formation of

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chains of atoms in chemical com-pounds. 2. The formation of a *cate-nane compound by mechanicalbonding.

cathetometer A telescope ormicroscope Ütted with crosswires inthe eyepiece and mounted so that itcan slide along a graduated scale.Cathetometers are used for accuratemeasurement of lengths without me-chanical contact. The microscopetype is often called a travelling micro-scope.

cathine (β-hydroxyamphetamine)An alkaloid, C9H13NO. It may con-tribute to the stimulant activity of*khat.

cathinone (β-ketoamphetamine)An alkaloid, C9H11NO. It is the mainactive ingredient in fresh *khat.

cathode A negative electrode. In*electrolysis cations are attracted tothe cathode. In vacuum electronic de-vices electrons are emitted by thecathode and Ûow to the *anode. It istherefore from the cathode that elec-trons Ûow into these devices. How-ever, in a primary or secondary cellthe cathode is the electrode thatspontaneously becomes negative dur-ing discharge, and from which there-fore electrons emerge.

cathodic protection See sacrifi-cial protection.

cation A positively charged ion, i.e.an ion that is attracted to the cath-ode in *electrolysis. Compare anion.

cationic detergent Seedetergent.

cationic dye See dyes.

cationic resin See ion exchange.

caustic Describing a substance thatis strongly alkaline (e.g. caustic soda).

caustic potash See potassium hy-droxide.

caustic soda See sodium hydrox-ide.

Cavendish, Henry (1731–1810)British chemist and physicist, born inFrance. Although untrained, his in-heritance from his grandfather, theDuke of Devonshire, enabled him tolive as a recluse and study science. Inhis experiments with gases (1766), hecorrectly distinguished between hy-drogen and carbon dioxide. In 1784he showed that sparking mixtures ofhydrogen and oxygen produced purewater, showing that water is not anelement. Also in sparking experi-ments with air, Cavendish recog-nised a small percentage unchanged(later identiÜed as noble gases).Cavendish also did signiÜcant workin physics.

c.c.p. Cubic close packing. See closepacking.

CD See circular dichroism.

CD spectrum (circular dichroismspectrum) The spectrum obtained byplotting the variable IR – IL againstfrequency of the incident electro-magnetic radiation, where IR and IL

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are the absorption intensities forright- and left-circularly polarizedlight, respectively. One application ofCD spectroscopy is to determine theconÜgurations of complexes of transi-tion metals. If the CD spectra of simi-lar transition metal complexes(including similarity of geometry) aretaken, the features of their CD spec-tra are also similar.

CE See capillary electrophoresis.

celestine A mineral form of stron-tium sulphate, SrSO4.

cell 1. A system in which two elec-trodes are in contact with an elec-trolyte. The electrodes are metal orcarbon plates or rods or, in somecases, liquid metals (e.g. mercury). Inan *electrolytic cell a current froman outside source is passed throughthe electrolyte to produce chemicalchange (see electrolysis). In a*voltaic cell, spontaneous reactionsbetween the electrodes and elec-trolyte(s) produce a potential differ-ence between the two electrodes.

Voltaic cells can be regarded asmade up of two *half cells, eachcomposed of an electrode in contactwith an electrolyte. For instance, azinc rod dipped in zinc sulphate solu-tion is a Zn|Zn2+ half cell. In such asystem zinc atoms dissolve as zincions, leaving a negative charge onthe electrode

Zn(s) → Zn2+(aq) + 2eThe solution of zinc continues untilthe charge build-up is sufÜcient toprevent further ionization. There isthen a potential difference betweenthe zinc rod and its solution. Thiscannot be measured directly, sincemeasurement would involve makingcontact with the electrolyte, therebyintroducing another half cell (seeelectrode potential). A rod of cop-per in copper sulphate solution com-prises another half cell. In this case

the spontaneous reaction is one inwhich copper ions in solution takeelectrons from the electrode and aredeposited on the electrode as copperatoms. In this case, the copper ac-quires a positive charge.

The two half cells can be con-nected by using a porous pot for theliquid junction (as in the *Daniellcell) or by using a salt bridge. The re-sulting cell can then supply currentif the electrodes are connectedthrough an external circuit. The cellis written

Zn(s)|Zn2+(aq)|Cu2+(aq)|CuE = 1.10 V

Here, E is the e.m.f. of the cell equalto the potential of the right-handelectrode minus that of the left-handelectrode for zero current. Note that‘right’ and ‘left’ refer to the cell aswritten. Thus, the cell could be writ-ten

Cu(s)|Cu2+(aq)|Zn2+(aq)|Zn(s)E = –1.10 V

The overall reaction for the cell isZn(s) + Cu2+(aq) → Cu(s) +

Zn2+(aq)This is the direction in which the cellreaction occurs for a positive e.m.f.

The cell above is a simple exampleof a chemical cell; i.e. one in whichthe e.m.f. is produced by a chemicaldifference. Concentration cells arecells in which the e.m.f. is caused bya difference of concentration. Thismay be a difference in concentrationof the electrolyte in the two halfcells. Alternatively, it may be an elec-trode concentration difference (e.g.different concentrations of metal inan amalgam, or different pressures ofgas in two gas electrodes). Cells arealso classiÜed into cells withouttransport (having a single electrolyte)and with transport (having a liquidjunction across which ions are trans-ferred). Various types of voltaic cell

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exist, used as sources of current,standards of potential, and experi-mental set-ups for studying electro-chemical reactions. See also dry cell;primary cell; secondary cell;lithium battery.2. See kerr effect (for Kerr cell).

cellophane Reconstituted cellulosein the form of a thin transparentsheet, made by extruding a viscouscellulose xanthate solution through aÜne slit into a bath of acid (seerayon). It is commonly used as awrapping material, especially forfoodstuffs, but is being replaced bypolypropene because of its Ûamma-bility.

cellular plastic See expanded plas-tic.

cellular plastics Solid syntheticmaterials having an open structure.A common example is rigid *poly-styrene foam used in insulation andpackaging.

celluloid A transparent highlyÛammable substance made from cel-lulose nitrate with a camphor plasti-cizer. It was formerly widely used asa thermoplastic material, especiallyfor Ülm (a use now discontinuedowing to the Ûammability of cellu-loid).

cellulose A polysaccharide thatconsists of a long unbranched chainof glucose units. It is the main con-

stituent of the cell walls of all plants,many algae, and some fungi and isresponsible for providing the rigidityof the cell wall. It is an importantconstituent of dietary Übre. TheÜbrous nature of extracted cellulosehas led to its use in the textile indus-try for the production of cotton,artiÜcial silk, etc.

cellulose acetate See celluloseethanoate.

cellulose ethanoate (cellulose ac-etate) A compound prepared bytreating cellulose (cotton linters orwood pulp) with a mixture ofethanoic anhydride, ethanoic acid,and concentrated sulphuric acid. Cel-lulose in the cotton is ethanoylatedand when the resulting solution istreated with water, celluloseethanoate forms as a Ûocculent whitemass. It is used in lacquers, nonshat-terable glass, varnishes, and as a Übre(see also rayon).

cellulose nitrate A highly Ûamma-ble material made by treating cellu-lose (wood pulp) with concentratednitric acid. Despite the alternativename nitrocellulose, the compound isin fact an ester (containing CONO2groups), not a nitro compound(which would contain C–NO2). It isused in explosives (as guncotton) andcelluloid.

Celsius scale A *temperature scalein which the Üxed points are the

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temperatures at standard pressure ofice in equilibrium with water (0°C)and water in equilibrium with steam(100°C). The scale, between these twotemperatures, is divided in 100 de-grees. The degree Celsius (°C) is equalin magnitude to the *kelvin. Thisscale was formerly known as thecentigrade scale; the name was ofÜ-cially changed in 1948 to avoid con-fusion with a hundredth part of agrade. It is named after the Swedishastronomer Anders Celsius (1701–44),who devised the inverted form ofthis scale (ice point 100°, steam point0°) in 1742.

cement Any of various substancesused for bonding or setting to a hardmaterial. Portland cement is a mix-ture of calcium silicates and alumi-nates made by heating limestone(CaCO3) with clay (containing alumi-nosilicates) in a kiln. The product isground to a Üne powder. Whenmixed with water it sets in a fewhours and then hardens over alonger period of time due to the for-mation of hydrated aluminates andsilicates.

cementation Any metallurgicalprocess in which the surface of ametal is impregnated by some othersubstance, especially an obsoleteprocess for making steel by heatingbars of wrought iron to red heat forseveral days in a bed of charcoal. Seealso case hardening.

cementite See steel.

centi- Symbol c. A preÜx used inthe metric system to denote one hun-dredth. For example, 0.01 metre = 1centimetre (cm).

centigrade scale See celsiusscale.

centrifugal pump See pump.

centrifuge A device in which solidor liquid particles of different densi-

ties are separated by rotating them ina tube in a horizontal circle. Thedenser particles tend to move alongthe length of the tube to a greater ra-dius of rotation, displacing thelighter particles to the other end.

ceramics Inorganic materials, suchas pottery, enamels, and refractories.Ceramics are metal silicates, oxides,nitrides, etc.

cerebroside Any one of a class of*glycolipids in which a single sugarunit is bound to a sphingolipid (seephospholipid). The most commoncerebrosides are galactocerebrosides,containing the sugar group galactose;they are found in the plasma mem-branes of neural tissue and are abun-dant in the myelin sheaths ofneurones.

cerium Symbol Ce. A silvery metal-lic element belonging to the *lan-thanoids; a.n. 58; r.a.m. 140.12; r.d.6.77 (20°C); m.p. 799°C; b.p. 3426°C.It occurs in allanite, bastnasite,cerite, and monazite. Four isotopesoccur naturally: cerium–136, –138,–140, and –142; Üfteen radioisotopeshave been identiÜed. Cerium is usedin mischmetal, a rare-earth metalcontaining 25% cerium, for use inlighter Ûints. The oxide is used in the glass industry. It was discoveredby Martin Klaproth (1743–1817) in1803.


cermet A composite material con-sisting of a ceramic in combinationwith a sintered metal, used when ahigh resistance to temperature, cor-rosion, and abrasion is needed.

cerussite An ore of lead consistingof lead carbonate, PbCO3. It is usuallyof secondary origin, formed by theweathering of *galena. Pure cerussiteis white but the mineral may be grey

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due to the presence of impurities. Itforms well-shaped orthorhombiccrystals. It occurs in the USA, Spain,and SW Africa.

cetane See hexadecane.

cetane number A number thatprovides a measure of the ignitioncharacteristics of a Diesel fuel whenit is burnt in a standard Diesel en-gine. It is the percentage of cetane(hexadecane) in a mixture of cetaneand 1-methylnaphthalene that hasthe same ignition characteristics asthe fuel being tested. Compare oc-tane number.

CFC See chlorofluorocarbon.

CGE Capillary gel electrophoresis.See capillary electrophoresis.

c.g.s. units A system of *unitsbased on the centimetre, gram, andsecond. Derived from the metric sys-tem, it was badly adapted to use withthermal quantities (based on the in-consistently deÜned *calorie) andwith electrical quantities (in whichtwo systems, based respectively onunit permittivity and unit permeabil-ity of free space, were used). For sci-entiÜc purposes c.g.s. units have nowbeen replaced by *SI units.

chain A line of atoms of the sametype in a molecule. In a straight chainthe atoms are attached only to singleatoms, not to groups. Propane, for in-stance, is a straight-chain alkane,CH3CH2CH3, with a chain of threecarbon atoms. A branched chain isone in which there are side groupsattached to the chain. Thus, 3-ethyl-octane, CH3CH2CH(C2H5)C5H11, is abranched-chain alkane in whichthere is a side chain (C2H5) attachedto the third carbon atom. A closedchain is a *ring of atoms in a mol-ecule; otherwise the molecule has anopen chain.

Chain, Sir Ernst Boris (1906–79)

German-born British biochemist,who began his research career atCambridge University in 1933. Twoyears later he joined *Florey at Ox-ford, where they isolated andpuriÜed *penicillin. They also devel-oped a method of producing the drugin large quantities and carried out itsÜrst clinical trials. The two menshared the 1945 Nobel Prize for phys-iology or medicine with penicillin’sdiscoverer, Alexander *Fleming.

chain reaction A reaction that isself-sustaining as a result of the prod-ucts of one step initiating a subse-quent step. Chemical chain reactionsusually involve free radicals as inter-mediates. An example is the reactionof chlorine with hydrogen initiatedby ultraviolet radiation. A chlorinemolecule is Ürst split into atoms:

Cl2 → Cl• + Cl•These react with hydrogen as follows

Cl• + H2 → HCl + H•

H• + Cl2 → HCl + Cl• etc.Combustion and explosion reactionsinvolve similar free-radical chain re-actions.

chair See ring conformations.

chair conformation See confor-mation.

chalcedony A mineral consistingof a microcrystalline variety of*quartz. It occurs in several forms,including a large number of semi-precious gemstones; for example,sard, carnelian, jasper, onyx, chryso-prase, agate, and tiger’s-eye.

chalcogens See group 16 elements.

chalconides Binary compoundsformed between metals and group 16elements; i.e. oxides, sulphides, se-lenides, and tellurides.

chalcopyrite (copper pyrites) Abrassy yellow mineral consisting of a

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mixed copper–iron sulphide, CuFeS2,crystallizing in the tetragonal sys-tem; the principal ore of copper. It issimilar in appearance to pyrite andgold. It crystallizes in igneous rocksand hydrothermal veins associatedwith the upper parts of acid igneousintrusions. Chalcopyrite is the mostwidespread of the copper ores, occur-ring, for example, in Cornwall (UK),Sudbury (Canada), Chile, Tasmania(Australia), and Rio Tinto (Spain).

chalk A very Üne-grained whiterock composed of the skeletal re-mains of microscopic sea creatures,such as plankton, and consistinglargely of *calcium carbonate(CaCO3). It is used in toothpaste andcosmetics. It should not be confusedwith blackboard ‘chalk’, which ismade from calcium sulphate.

change of phase (change of state)A change of matter in one physical*phase (solid, liquid, or gas) into an-other. The change is invariably ac-companied by the evolution orabsorption of energy.

chaos Unpredictable and appar-ently random behaviour arising in asystem that is governed by determin-istic laws (i.e. laws that, given thestate of the system at a given time,uniquely determine the state at anyother time). Chaotic dynamics arewidespread in nature: examples arethe turbulent Ûow of Ûuids, the dy-namics of planetary moons, oscilla-tions in electrical circuits, and thelong-term unpredictability of theweather. In such situations, chaosarises because the mathematicalequations expressing the (determinis-tic) laws in question are nonlinearand extremely sensitive to the initialconditions. It is impossible to specifydata at a given time to a sufÜcient de-gree of precision to be able to predictthe future behaviour of the system

because two pieces of initial data dif-fering by even a very small amountwill give widely different results at alater time. Originally, the theory wasintroduced to describe unpredictabil-ity in meteorology, as exempliÜed bythe butterÛy effect. It has been sug-gested that the dynamical equationsgoverning the weather are so sensi-tive to the initial data that whetheror not a butterÛy Ûaps its wings inone part of the world may make thedifference between a tornado occur-ring or not occurring in some otherpart of the world. In chemistry,chaos theory has been used to ex-plain oscillatory (clock) reactions,such as the *B–Z reaction, andchaotic reactions.

chaotic dynamics See chaos.

chaotic reaction A type of chemi-cal reaction in which the concentra-tions of reactants show chaoticbehaviour. This may occur when thereaction involves a large number ofcomplex interlinked steps. Undersuch conditions, it is possible for thereaction to display unpredictablechanges with time. See also bistabil-ity; oscillating reaction.

charcoal A porous form of carbonproduced by the destructive distilla-tion of organic material. Charcoalfrom wood is used as a fuel. All formsof charcoal are porous and are usedfor adsorbing gases and purifyingand clarifying liquids. There are sev-eral types depending on the source.Charcoal from coconut shells is a par-ticularly good gas adsorbent. Animalcharcoal (or bone black) is made byheating bones and dissolving out thecalcium phosphates and other min-eral salts with acid. It is used in sugarreÜning. Activated charcoal is char-coal that has been activated for ad-sorption by steaming or by heatingin a vacuum.

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Chargaff’s rule The principle thatin any sample of DNA the amount ofadenine equals the amount ofthymine and the amount of guanineequals the amount of cytosine. It is aconsequence of *base pairing. Therule was published in 1950 by theAustrian–American biochemist ErwinChargaff (1905–2002).

charge A property of some *ele-mentary particles that gives rise toan interaction between them andconsequently to the host of materialphenomena described as electrical.Charge occurs in nature in twoforms, conventionally described aspositive and negative in order to dis-tinguish between the two kinds of in-teraction between particles. Twoparticles that have similar charges(both negative or both positive) inter-act by repelling each other; two par-ticles that have dissimilar charges(one positive, one negative) interactby attracting each other.

The natural unit of negative chargeis the charge on an *electron, whichis equal but opposite in effect to thepositive charge on the proton. Large-scale matter that consists of equalnumbers of electrons and protons iselectrically neutral. If there is an ex-cess of electrons the body is nega-tively charged; an excess of protonsresults in a positive charge. A Ûow ofcharged particles, especially a Ûow of electrons, constitutes an electriccurrent. Charge is measured incoulombs, the charge on an electronbeing 1.602 × 10–19 coulombs.

charge carrier The entity thattransports electric charge in an elec-tric current. The nature of the carrierdepends on the type of conductor: inmetals, the charge carriers are elec-trons; in *semiconductors the carri-ers are electrons (n-type) or positive*holes (p-type); in gases the carriersare positive ions and electrons; in

electrolytes they are positive andnegative ions.

charge density 1. The electriccharge per unit volume of a mediumor body (volume charge density). 2. The electric charge per unit sur-face area of a body (surface chargedensity).

charge-transfer complex Achemical compound in which thereis weak coordination involving thetransfer of charge between two mol-ecules. An example is phenoquinone,in which the phenol and quinonemolecules are not held together byformal chemical bonds but are asso-ciated by transfer of charge betweenthe compounds’ aromatic ring sys-tems.

Charles, Jacques AlexandreCésar (1746–1823) French chemistand physicist, who became professorof physics at the Paris Conservatoiredes Arts et Métiers. He is best re-membered for discovering *Charles’law (1787), relating to the volumeand temperature of a gas. In 1783 hebecame the Ürst person to make anascent in a hydrogen balloon.

Charles’ law The volume of a Üxedmass of gas at constant pressure ex-pands by a constant fraction of itsvolume at 0°C for each Celsius de-gree or kelvin its temperature israised. For any *ideal gas the fractionis approximately 1/273. This can beexpressed by the equation V = V0(1 +t/273), where V0 is the volume at 0°Cand V is its volume at t°C. This isequivalent to the statement that thevolume of a Üxed mass of gas at con-stant pressure is proportional to itsthermodynamic temperature, V = kT,where k is a constant. The law re-sulted from experiments begunaround 1787 by Jacques *Charles butwas properly established only by themore accurate results published in

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1802 by Joseph *Gay-Lussac. Thus thelaw is also known as Gay-Lussac’slaw. An equation similar to thatgiven above applies to pressures forideal gases: p = p0(1 + t/273), a rela-tionship known as Charles’ law ofpressures. See also gas laws.

cheddite Any of a group of high ex-plosives made from nitro compoundsmixed with sodium or potassiumchlorate.

chelate An inorganic complex inwhich a *ligand is coordinated to ametal ion at two (or more) points, sothat there is a ring of atoms includ-ing the metal. The process is knownas chelation. Chelating agents areclassiÜed according to the number ofpoints at which they can coordinate.A ligand such as diaminoethane(H2N(CH2)2NH2), which can coordi-nate at two points, is said to bebidentate. The angle made betweentwo bonds coordinating to a metalatom is the bite angle of the ligand.See also sequestration.

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H2C

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CH2

NH2

Chelate

chelate effect The effect in whicha chelate complex is generally morestable than the analogous complexformed with monodentate ligands.For example, the complex ion [Cu(en)(OH2)4]2+ is more stable than the com-plex ion [Cu(NH3)2 (OH2)4]2+. Here, endenotes the bidentate ethylene di-amine (1,2-diaminoethane) ligand.The main cause of the chelate effectis the effect of reaction entropywhen the complex is formed. Thus,the reaction

[Cu(OH2)6]2+ + en → [Cu(en)(OH2)4 +2H2O

results in a net increase in the num-ber of molecules (from 2 to 3). Thereaction

[Cu(OH2)6]2+ + 2NH3 →Cu(NH3)2(OH2)4 + 2H2O

involves no net increase in the num-ber of molecules. As a result, thechelate reaction has a larger reactionentropy and is more favourable.

cheletropic reaction A type of ad-dition reaction in which a conjugatedmolecule forms two single bondsfrom terminal atoms of the conju-gated system to a single atom on an-other molecule to give a cyclicadduct. Cheletropic reactions aretypes of *pericyclic reaction.

ChemDraw A widely used chemi-cal drawing and modelling programproduced by CambridgeSoft.

A• The CambridgeSoft website, giving

details about ChemDraw and associatedsoftware

chemical bond A strong force ofattraction holding atoms together ina molecule or crystal. Typically chem-ical bonds have energies of about1000 kJ mol–1 and are distinguishedfrom the much weaker forces be-tween molecules (see van der waals’force). There are various types.Ionic (or electrovalent) bonds can beformed by transfer of electrons. Forinstance, the calcium atom has anelectron conÜguration of [Ar]4s2, i.e.it has two electrons in its outer shell.The chlorine atom is [Ne]3s23p5, withseven outer electrons. If the calciumatom transfers two electrons, one toeach chlorine atom, it becomes aCa2+ ion with the stable conÜgura-tion of an inert gas [Ar]. At the sametime each chlorine, having gainedone electron, becomes a Cl– ion, also


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with an inert-gas conÜguration [Ar].The bonding in calcium chloride isthe electrostatic attraction betweenthe ions.Covalent bonds are formed by shar-ing of valence electrons rather thanby transfer. For instance, hydrogenatoms have one outer electron (1s1).In the hydrogen molecule, H2, eachatom contributes 1 electron to thebond. Consequently, each hydrogenatom has control of 2 electrons – oneof its own and the second from theother atom – giving it the electronconÜguration of an inert gas [He]. Inthe water molecule, H2O, the oxygenatom, with six outer electrons, gainscontrol of an extra two electrons sup-plied by the two hydrogen atoms.This gives it the conÜguration [Ne].Similarly, each hydrogen atom gainscontrol of an extra electron from theoxygen, and has the [He] electronconÜguration.

A particular type of covalent bondis one in which one of the atoms sup-plies both the electrons. These areknown as coordinate (semipolar ordative) bonds, and written A→B,where the direction of the arrow de-notes the direction in which elec-trons are donated.

Covalent or coordinate bonds inwhich one pair of electrons is sharedare electron-pair bonds and areknown as single bonds. Atoms canalso share two pairs of electrons toform double bonds or three pairs intriple bonds. See orbital.

In a compound such as sodiumchloride, Na+Cl–, there is probablycomplete transfer of electrons informing the ionic bond (the bond issaid to be heteropolar). Alternatively,in the hydrogen molecule H–H, thepair of electrons is equally shared be-tween the two atoms (the bond is ho-mopolar). Between these twoextremes, there is a whole range ofintermediate bonds, which have both

ionic and covalent contributions.Thus, in hydrogen chloride, H–Cl,the bonding is predominantly cova-lent with one pair of electrons sharedbetween the two atoms. However,the chlorine atom is more elec-tronegative than the hydrogen andhas more control over the electronpair; i.e. the molecule is polarizedwith a positive charge on the hydro-gen and a negative charge on thechlorine, forming a *dipole. See alsobanana bond; hydrogen bond;metallic bond; multicentre bond;multiple bond.

chemical cell See cell.

chemical combination The com-bination of elements to give com-pounds. There are three laws ofchemical combination.(1) The law of constant compositionstates that the proportions of the el-ements in a compound are alwaysthe same, no matter how the com-pound is made. It is also called thelaw of constant proportions ordeÜnite proportions.(2) The law of multiple proportionsstates that when two elements A andB combine to form more than onecompound, then the masses of B thatcombine with a Üxed mass of A arein simple ratio to one another. Forexample, carbon forms two oxides.In one, 12 grams of carbon is com-bined with 16 grams of oxygen (CO);in the other 12 g of carbon is com-bined with 32 grams of oxygen (CO2).The oxygen masses combining with aÜxed mass of carbon are in the ratio16:32, i.e. 1:2.(3) The law of equivalent proportionsstates that if two elements A and Beach form a compound with a thirdelement C, then a compound of Aand B will contain A and B in the rel-ative proportions in which they reactwith C. For example, sulphur andcarbon both form compounds with

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hydrogen. In methane 12 g of carbonreact with 4 g of hydrogen. In hydro-gen sulphide, 32 g of sulphur reactwith 2 g of hydrogen (i.e. 64 g of S for4 g of hydrogen). Sulphur and carbonform a compound in which the C:Sratio is 12:64 (i.e. CS2). The law issometimes called the law of recipro-cal proportions.

chemical dating An absolute *dat-ing technique that depends on meas-uring the chemical composition of aspecimen. Chemical dating can beused when the specimen is known toundergo slow chemical change at aknown rate. For instance, phosphatein buried bones is slowly replaced byÛuoride ions from the ground water.Measurement of the proportion ofÛuorine present gives a rough esti-mate of the time that the bones havebeen in the ground. Another, moreaccurate, method depends on thefact that amino acids in living organ-isms are l-optical isomers. Afterdeath, these racemize and the age ofbones can be estimated by measuringthe relative amounts of d- and l-amino acids present. See amino acidracemization.

chemical engineering The studyof the design, manufacture, and oper-ation of plant and machinery in in-dustrial chemical processes.

chemical equation A way of de-noting a chemical reaction using thesymbols for the participating parti-cles (atoms, molecules, ions, etc.); forexample,

xA + yB → zC + wDThe single arrow is used for an irre-versible reaction; double arrows (ˆ)are used for reversible reactions.When reactions involve differentphases it is usual to put the phase inbrackets after the symbol (s = solid; l= liquid; g = gas; aq = aqueous). Thenumbers x, y, z, and w, showing the

relative numbers of molecules react-ing, are called the stoichiometriccoefÜcients. The sum of the coefÜ-cients of the reactants minus thesum of the coefÜcients of the prod-ucts (x + y – z – w in the example) isthe stoichiometric sum. If this is zerothe equation is balanced. Sometimesa generalized chemical equation isconsidered

ν1A1 + ν2A2 + … → … νnAn +νn+1An+1 …

In this case the reaction can be writ-ten ΣνiAi = 0, where the convention isthat stoichiometric coefÜcients arepositive for reactants and negativefor products. The stoichiometric sumis Σνi.

chemical equilibrium A re-versible chemical reaction in whichthe concentrations of reactants andproducts are not changing with timebecause the system is in thermody-namic equilibrium. For example, thereversible reaction

3H2 + N2 ˆ 2NH3

is in chemical equilibrium when therate of the forward reaction

3H2 + N2 → 2NH3

is equal to the rate of the back reac-tion

2NH3 → 3H2 + N2

See also equilibrium constant.

chemical equivalent See equiva-lent weight.

chemical fossil Any of various or-ganic compounds found in ancientgeological strata that appear to bebiological in origin and are assumedto indicate that life existed when therocks were formed. The presence ofchemical fossils in Archaean strataindicates that life existed over 3000million years ago.

chemically induced dynamic nu-clear polarization See cidnp.

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chemical markup language(CML) A markup language used toexpress chemical information. CML isan extension of XML (extensiblemarkup language), which is widelyused to store and transfer text. A par-ticular feature of XML is the use oftags to identify different types of in-formation. These are commonly inangle brackets. In the text version ofthis dictionary, for example, head-words are surrounded by the tags<hw>…</hw>, cross references areidentiÜed by <xr>…</xr>, etc. InCML, there are tags for many differ-ent types of chemical information.For example, structures can be repre-sented by tagged data giving theatoms and their coordinates in twoor three dimensions. Reactions canalso be indicated, as well as data forspectra and other properties.A• An FAQ produced by the Chemistry

Department, Cambridge University

chemical potential Symbol: µ. Fora given component in a mixture, thecoefÜcient ∂G/∂n, where G is theGibbs free energy and n the amountof substance of the component. Thechemical potential is the change inGibbs free energy with respect tochange in amount of the component,with pressure, temperature, andamounts of other components beingconstant. Components are in equilib-rium if their chemical potentials areequal.

chemical reaction A change inwhich one or more chemical el-ements or compounds (the reactants)form new compounds (the products).All reactions are to some extent re-versible; i.e. the products can alsoreact to give the original reactants.However, in many cases the extent ofthis back reaction is negligibly small,and the reaction is regarded as irre-versible.

chemical shift A change in thenormal wavelength of absorption oremission of electromagnetic wave-length in a process in which there isa nuclear energy change (as in the*Mössbauer effect and *nuclear mag-netic resonance) or a change in elec-tron energy levels in the inner shellsof an atom (as in X-ray *photoelec-tron spectroscopy).

chemical warfare The use of toxicchemical substances in warfare ormilitary operations. A large numberof chemicals have been designed orused for warfare, including pul-monary agents (chlorine, *carbonylchloride, *diphosgene), blisteragents, (*lewisite, *sulphur mustard,*nitrogen mustard), and the *nerveagents. Chemical warfare agents areclassiÜed as weapons of mass destruc-tion by the United Nations. See alsochemical weapons convention.

Chemical Weapons ConventionAn agreement dating from 1993 onthe production, stockpiling, and useof chemical weapons (see chemicalwarfare). It came into force in 1997and is administered by the Organiza-tion for the Prohibition of ChemicalWeapons (OPCW).A• The OPCW website, giving details of the

convention

chemiluminescence See lumines-cence.

chemiosmotic theory A theorypostulated by the British biochemistPeter Mitchell (1920–92) to explainthe formation of ATP in the mito-chondrial *electron transport chain.As electrons are transferred alongthe electron carrier system in theinner mitochondrial membrane, hy-drogen ions (protons) are activelytransported (via *hydrogen carriers)into the space between the inner andouter mitochondrial membranes,

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which thus contains a higher concen-tration of protons than the matrix.This creates an electrochemical gra-dient across the inner membrane,down which protons move back intothe matrix. This movement occursthrough special channels associatedwith ATP synthetase, the enzymethat catalyses the conversion of ADPto ATP, and is coupled with the phos-phorylation of ADP. A similar gradi-ent is created across the thylakoidmembranes of chloroplasts duringthe light-dependent reactions of*photosynthesis.

chemisorption See adsorption.

chemistry The study of the el-ements and the compounds theyform. Chemistry is mainly concernedwith effects that depend on the outerelectrons in atoms. See biochemistry;geochemistry; inorganic chem-istry; organic chemistry; physicalchemistry.

chemoinformatics The branch ofchemistry concerned with methodsof representing molecules and reac-tions and with the design and use ofdatabases for storing chemical infor-mation.

ChemSketch A commonly usedchemical drawing program for 2Dand 3D structures, copyright of Ad-vanced Chemistry Development, Inc.The program has certain additional

features including calculation of mo-lecular weight, calculation of per-centages of elements present, IUPACname generation, and viewing in Ras-Mol.A• Details and download from Advanced

Chemistry Development website

chert See flint.

Chile saltpetre A commercial min-eral largely composed of *sodium ni-trate from the caliche deposits inChile. Before the ammonia-oxidationprocess for nitrates most importedChilean saltpetre was used by thechemical industry; its principal usetoday is as an agricultural source ofnitrogen.

Chime A plug-in that allows chemi-cal structures to be displayed withina web page. The structures, whichare embedded as mol Üles, can beviewed interactively; i.e. they can berotated, expanded, exported, andrendered in *RasMol. The program isthe copyright of MDL InformationSystems, Inc., and is available free(subject to a license agreement).A• A downloadable version of Chime at

the MDL website (free registrationrequired)

china clay See kaolin.

Chinese white See zinc oxide.

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2H+ 2H+2H+ inner mitochondrialmembrane

NADHdehydrogenase

cytochromeb-c1 complex

cytochromeoxidase

mitochondrial matrix

6H+

3ADP + P

3 ATP

ATPaseATPaseATPase

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chirality The property of existingin left- and right-handed structuralforms. See optical activity.

chirality element The part of amolecule that makes it exist in left-and right-handed forms. In mostcases this is a chirality centre (i.e. anasymmetric atom). In certain casesthe element is a chirality axis. For ex-ample, in allenenes of the typeR1R2C=C=CR3R4 the C=C=C chain is achirality axis. Certain ring com-pounds may display chirality as a re-sult of a chirality plane in themolecule.

chirooptical spectroscopy Spec-troscopy making use of the proper-ties of chiral substances when theyinteract with polarized light of vari-ous wavelengths. Optical rotatorydispersion (the change of optical rota-tion with wavelength) and *circulardichroism are old examples of chi-rooptical spectroscopy. More recenttypes of chirooptical spectroscopy arevibrational optical rotatory disper-sion, vibrational circular dichroism.(VCD), and Raman optical activity(ROA), which are all the result of the interaction between chiral sub-stances and polarized infraredelectromagnetic radiation; thesetechniques are known as vibrationaloptical activity (VOA), as they are as-sociated with transitions in the vibra-tional energy levels in the electronicground state of a molecule. Chiro-optical spectroscopy is used in theanalysis of the structure of mol-ecules.

chitin A *polysaccharide compris-ing chains of N-acetyl-d-glucosamine,a derivative of glucose. Chitin isstructurally very similar to celluloseand serves to strengthen the support-ing structures of various inverte-brates. It also occurs in fungi.

chloral See trichloroethanal.

chloral hydrate See 2,2,2-trichloroethanediol.

chlorates Salts of the chloric acids;i.e. salts containing the ions ClO–

(chlorate(I) or hypochlorite), ClO2–

(chlorate(III) or chlorite), ClO3– (chlo-

rate(V)), or ClO4– (chlorate(VII) or per-

chlorate). When used withoutspeciÜcation of an oxidation state theterm ‘chlorate’ refers to a chlorate(V)salt.

chloric acid Any of the oxoacids ofchlorine: *chloric(I) acid, *chloric(III)acid, *chloric(V) acid, and*chloric(VII) acid. The term is com-monly used without speciÜcation ofthe oxidation state of chlorine tomean chloric(V) acid, HClO3.

chloric(I) acid (hypochlorous acid)A liquid acid that is stable only in so-lution, HOCl. It may be prepared bythe reaction of chlorine with an agi-tated suspension of mercury(II) oxide.Because the disproportionation ofthe ion ClO– is slow at low tempera-tures chloric(I) acid may be produced,along with chloride ions by the reac-tion of chlorine with water at 0°C. Athigher temperatures disproportiona-tion to the chlorate(V) ion, ClO3

–,takes place. Chloric(I) acid is a veryweak acid but is a mild oxidizingagent and is widely used as a bleach-ing agent.

chloric(III) acid (chlorous acid) Apale-yellow acid known only in solu-tion, HClO2. It is formed by the reac-tion of chlorine dioxide and waterand is a weak acid and an oxidizingagent.

chloric(V) acid (chloric acid) Acolourless unstable liquid, HClO3; r.d.1.2; m.p. <–20°C; decomposes at40°C. It is best prepared by the reac-tion of barium chlorate with sul-phuric acid although chloric(V) acidis also formed by the disproportiona-

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tion of chloric(I) acid in hot solutions.It is both a strong acid and a power-ful oxidizing agent; hot solutions ofthe acid or its salts have been knownto detonate in contact with readilyoxidized organic material.

chloric(VII) acid (perchloric acid)An unstable liquid acid, HClO4; r.d.1.76; m.p. –112°C; b.p. 39°C (50mmHg); explodes at about 90°C at at-mospheric pressure. There is also amonohydrate (r.d. 1.88 (solid), 1.77(liquid); m.p. 48°C; explodes at about110°C) and a dihydrate (r.d. 1.65; m.p.–17.8°C; b.p. 200°C). Commercialchloric(VII) acid is a water azeotrope,which is 72.5% HClO4, boiling at203°C. The anhydrous acid may beprepared by vacuum distillation ofthe concentrated acid in the presenceof magnesium perchlorate as a dehy-drating agent. Chloric(VII) acid isboth a strong acid and a strong oxi-dizing agent. It is widely used to de-compose organic materials prior toanalysis, e.g. samples of animal orvegetable matter requiring heavy-metal analysis.

chloride See halide.

chlorination 1. A chemical reac-tion in which a chlorine atom is in-troduced into a compound. Seehalogenation. 2. The treatment ofwater with chlorine to disinfect it.

chlorine Symbol Cl. A *halogen el-ement; a.n. 17; r.a.m. 35.453; d. 3.214g dm–3; m.p. –100.98°C; b.p. –34.6°C.It is a poisonous greenish-yellow gas and occurs widely in nature assodium chloride in seawater and ashalite (NaCl), carnallite (KCl.MgCl2.6H2O), and sylvite (KCl). It is manu-factured by the electrolysis of brineand also obtained in the *Downsprocess for making sodium. It hasmany applications, including thechlorination of drinking water,

bleaching, and the manufacture of alarge number of organic chemicals.

It reacts directly with many el-ements and compounds and is astrong oxidizing agent. Chlorinecompounds contain the element inthe 1, 3, 5, and 7 oxidation states. Itwas discovered by Karl Scheele in1774 and Humphry Davy conÜrmedit as an element in 1810.


chlorine dioxide A yellowish-redexplosive gas, ClO2; d. 3.09 g dm–3;m.p. –59.5°C; b.p. 9.9°C. It is solublein cold water but decomposed by hotwater to give chloric(VII) acid, chlo-rine, and oxygen. Because of its highreactivity, chlorine dioxide is bestprepared by the reaction of sodiumchlorate and moist oxalic acid at90°–100°C, as the product is then di-luted by liberated carbon dioxide.Commercially the gas is produced bythe reaction of sulphuric acid con-taining chloride ions with sulphurdioxide. Chlorine dioxide is widelyused as a bleach in Ûour milling andin wood pulping and also Ünds appli-cation in water puriÜcation.

chlorine monoxide See dichlo-rine oxide.

chlorite 1. See chlorates. 2. Agroup of layered silicate minerals,usually green or white in colour, thatare similar to the micas in structureand crystallize in the monoclinicsystem. Chlorites are composed ofcomplex silicates of aluminium,magnesium, and iron in combina-tion with water, with the formula(Mg,Al,Fe)12(Si,Al)8O20(OH)16. They aremost common in low-grade meta-morphic rocks and also occur as sec-ondary minerals in igneous rocks asalteration products of pyroxenes, am-phiboles, and micas. The term is de-

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rived from chloros, the Greek wordfor green.

chloroacetic acids See chloro-ethanoic acids.

chlorobenzene A colourless highlyÛammable liquid, C6H5Cl; r.d. 1.106;m.p. –45.43°C; b.p. 131.85°C. It is pre-pared by the direct chlorination ofbenzene using a halogen carrier (seefriedel–crafts reaction), or manu-factured by the *Raschig process. It is used mainly as an industrial sol-vent.

2-chlorobuta-1,3-diene (chloro-prene) A colourless liquid chlori-nated diene, CH2:CClCH:CH2; r.d.0.96; b.p. 59°C. It is polymerized tomake synthetic rubbers (e.g. neo-prene).

chlorocruorin A greenish iron-con-taining respiratory pigment that oc-curs in the blood of polychaeteworms. It closely resembles *haemo-globin.

chloroethane (ethyl chloride) Acolourless Ûammable gas, C2H5Cl;m.p. –136.4°C; b.p. 12.3°C. It is madeby reaction of ethene and hydrogenchloride and used in making leadtetraethyl for petrol.

chloroethanoic acids (chloroaceticacids) Three acids in which hydro-gen atoms in the methyl group ofethanoic acid have been replaced bychlorine atoms. They are: mono-chloroethanoic acid (CH2ClCOOH);dichloroethanoic acid (CHCl2COOH);trichloroethanoic acid (CCl3COOH).The presence of chlorine atoms inthe methyl group causes electronwithdrawal from the COOH groupand makes the chloroethanoic acidsstronger acids than ethanoic acid it-self. The Ka values (in moles dm–3 at25°C) are

CH3COOH 1.7 × 10–5

CH2ClCOOH 1.3 × 10–3

CHCl2COOH 5.0 × 10–2

CCl3COOH 2.3 × 10–1

chloroethene (vinyl chloride) Agaseous compound, CH2:CHCl; r.d.0.911; m.p. –153.8°C; b.p. –13.37°C. Itis made by chlorinating ethene togive dichloroethane, then removingHCl:

C2H4 + Cl2 → CH2ClCH2Cl →CH2CHCl

The compound is used in makingPVC.

chloroÛuorocarbon (CFC) A typeof compound in which some or all ofthe hydrogen atoms of a hydrocar-bon (usually an alkane) have been re-placed by chlorine and Ûuorineatoms. Most chloroÛuorocarbons arechemically unreactive and are stableat high temperatures. They are usedas aerosol propellants, refrigerants,and solvents, and in the manufactureof rigid packaging foam. A com-monly encountered commercialname for these compounds is freon,e.g. freon 12 is dichlorodiÛuoro-methane (CCl2F2). ChloroÛuoro-carbons, because of their chemicalinertness, can diffuse unchanged intothe upper atmosphere. Here, photo-chemical reactions cause them tobreak down and react with ozone (seeozone layer). For this reason, theiruse has been discouraged.

chloroform See trichloro-methane.

chloromethane (methyl chloride)A colourless Ûammable gas, CH3Cl;r.d. 0.916; m.p. –97.1°C; b.p. –24.2°C.It is a *haloalkane, made by directchlorination of methane and used asa local anaesthetic and refrigerant.

chlorophenol Any of the variouscompounds produced by chlorinatinga *phenol. Chlorophenols are fairlyacidic and have many uses, includingantiseptics, disinfectants, herbicides,

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insecticides, and wood preservatives,and in making dyes. They form con-densation polymers of the bakelitetype with methanal.

chlorophyll One of a number ofpigments, including (chlorophyll aand chlorophyll b), that are responsi-ble for the green colour of mostplants. Chlorophyll molecules are theprincipal sites of light absorption inthe light-dependent reactions of*photosynthesis. They are magne-sium-containing *porphyrins, chemi-cally related to *cytochrome and*haemoglobin.

123 chromatogram

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CHCH2 CH3

C2H5

CHCHCH

CH3

COCHCHCH

CCC

COOCH3COOCH3COOCH3

C20H39O

CCC O

CH2CH2CH2

CH2CH2CH2

CH3

H

H NNN NNN

MgMgMg

NNN NNN

CHCHCH

CH3

CHCHCH

hydrocarbon(phytol) chain

Chlorophyll

chloroplatinic acid A reddish crys-talline compound, H2PtCl6, made bydissolving platinum in aqua regia.

chloroprene See 2-chlorobuta-1,3-diene.

chlorosulphanes See disulphurdichloride.

chlorous acid See chloric(iii) acid.

choke damp See blackdamp.

cholecalciferol See vitamin d.

cholesteric crystal A type of *liq-

uid crystal in which molecules lie insheets at angles that change by asmall amount between each sheet.The pitch of the helix so formed issimilar to the wavelength of visiblelight. Cholesteric liquid crystalstherefore diffract light and havecolours that depend on the tempera-ture. (The word cholesteric is derivedfrom the Greek for bile solid.)

cholesterol A *sterol occurringwidely in animal tissues and also insome higher plants and algae. It canexist as a free sterol or esteriÜed witha long-chain fatty acid. Cholesterol isabsorbed through the intestine ormanufactured in the liver. It servesprincipally as a constituent of bloodplasma lipoproteins and of thelipid–protein complexes that formcell membranes. It is also importantas a precursor of various steroids, es-pecially the bile acids, sex hormones,and adrenocorticoid hormones. Thederivative 7-dehydrocholesterol isconverted to vitamin D3 by the actionof sunlight on skin. Increased levelsof dietary and blood cholesterol havebeen associated with atherosclerosis,a condition in which lipids accumu-late on the inner walls of arteries andeventually obstruct blood Ûow.

choline An amino alcohol,CH2OHCH2N(CH3)3OH. It occurswidely in living organisms as aconstituent of certain types ofphospholipids – the lecithins andsphingomyelins – and in the neuro-transmitter *acetylcholine. It issometimes classiÜed as a member of the vitamin B complex.

chromate A salt containing the ionCrO4

2–.

chromatogram A record obtainedby *chromatography. The term is ap-plied to the developed records of*paper chromatography and *thin-layer chromatography and also to the


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graphical record produced in *gaschromatography.

chromatography A technique foranalysing or separating mixtures ofgases, liquids, or dissolved sub-stances. The original technique (in-vented by the Russian botanistMikhail Tsvet in 1906) is a good ex-ample of column chromatography. Avertical glass tube is packed with anadsorbing material, such as alumina.The sample is poured into the col-umn and continuously washedthrough with a solvent (a processknown as elution). Different compo-nents of the sample are adsorbed todifferent extents and move down thecolumn at different rates. In Tsvet’soriginal application, plant pigmentswere used and these separated intocoloured bands in passing down thecolumn (hence the name chromatog-raphy). The usual method is to collectthe liquid (the eluate) as it passes outfrom the column in fractions.

In general, all types of chromatog-raphy involve two distinct phases –the stationary phase (the adsorbentmaterial in the column in the exam-ple above) and the moving phase (thesolution in the example). The separa-tion depends on competition for mol-ecules of sample between themoving phase and the stationaryphase. The form of column chro-matography above is an example ofadsorption chromatography, inwhich the sample molecules are ad-sorbed on the alumina. In partitionchromatography, a liquid (e.g. water)is Ürst absorbed by the stationaryphase and the moving phase is animmiscible liquid. The separation isthen by *partition between the twoliquids. In ion-exchange chromatog-raphy (see ion exchange), the processinvolves competition between differ-ent ions for ionic sites on the station-ary phase. *Gel Ültration is another

chromatographic technique in whichthe size of the sample molecules isimportant.

See also affinity chromatog-raphy; gas chromatography; high-performance liquid chromatogra-phy; paper chromatography; rfvalue; thin-layer chromatography.

chrome alum See potassiumchromium sulphate.

chrome iron ore A mixediron–chromium oxide, FeO.Cr2O3,used to make ferrochromium forchromium steels.

chrome red A basic lead chromate,PbO.PbCrO4, used as a red pigment.

chrome yellow Lead chromate,PbCrO4, used as a pigment.

chromic acid A hypothetical acid,H2CrO4, known only in chromatesalts.

chromic anhydride See chro-mium(vi) oxide.

chromic compounds Compoundscontaining chromium in a higher (+3or +6) oxidation state; e.g. chromicoxide is chromium(VI) oxide (CrO3).

chromite A spinel mineral,FeCr2O4; the principal ore of chro-mium. It is black with a metalliclustre and usually occurs in massiveform. It is a common constituent ofperidotites and serpentines. Thechief producing countries areTurkey, South Africa, Russia, thePhilippines, and Zimbabwe.

chromium Symbol Cr. A hard sil-very *transition element; a.n. 24;r.a.m. 52.00; r.d. 7.19; m.p. 1857°C;b.p. 2672°C. The main ore ischromite (FeCr2O4). The metal has abody-centred-cubic structure. It is ex-tracted by heating chromite withsodium chromate, from whichchromium can be obtained by elec-trolysis. Alternatively, chromite can

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be heated with carbon in an electricfurnace to give ferrochrome, whichis used in making alloy steels. Themetal is also used as a shiny decora-tive electroplated coating and in themanufacture of certain chromiumcompounds.

At normal temperatures the metalis corrosion-resistant. It reacts withdilute hydrochloric and sulphuricacids to give chromium(II) salts.These readily oxidize to the more sta-ble chromium(III) salts. Chromiumalso forms compounds with the +6oxidation state, as in chromates,which contain the CrO4

2– ion. The el-ement was discovered in 1797 byVauquelin.A• Information from the WebElements site

chromium dioxide See chro-mium(iv) oxide.

chromium(II) oxide A black insol-uble powder, CrO. Chromium(II)oxide is prepared by oxidizingchromium amalgam with air. At hightemperatures hydrogen reduces it tothe metal.

chromium(III) oxide (chromiumsesquioxide) A green crystallinewater-insoluble salt, Cr2O3; r.d. 5.21;m.p. 2435°C; b.p. 4000°C. It is ob-tained by heating chromium in astream of oxygen or by heating am-monium dichromate. The industrialpreparation is by reduction ofsodium dichromate with carbon.Chromium(III) oxide is amphoteric,dissolving in acids to give chro-mium(III) ions and in concentratedsolutions of alkalis to give chromites.It is used as a green pigment in glass,porcelain, and oil paint.

chromium(IV) oxide (chromiumdioxide) A black insoluble powder,CrO2; m.p. 300°C. It is prepared bythe action of oxygen onchromium(VI) oxide or chromium(III)

oxide at 420–450°C and 200–300 at-mospheres. The compound is unsta-ble.

chromium(VI) oxide (chromiumtrioxide; chromic anhydride) A redcompound, CrO3; rhombic; r.d. 2.70;m.p. 196°C. It can be made by carefuladdition of concentrated sulphuricacid to an ice-cooled concentratedaqueous solution of sodium dichro-mate with stirring. The mixture isthen Ültered through sintered glass,washed with nitric acid, then dried at120°C in a desiccator.

Chromium(VI) oxide is an ex-tremely powerful oxidizing agent,especially to organic matter; it im-mediately inÛames ethanol. It is anacidic oxide and dissolves in water toform ‘chromic acid’, a powerful oxi-dizing agent and cleansing Ûuid forglassware. At 400°C, chromium(VI)oxide loses oxygen to givechromium(III) oxide.

chromium potassium sulphateA red crystalline solid,K2SO4.Cr2(SO4)3.24H2O; r.d. 1.91. It isused as a mordant See also alums.

chromium sesquioxide Seechromium(iii) oxide.

chromium steel Any of a group of*stainless steels containing 8–25% ofchromium. A typical chromium steelmight contain 18% of chromium, 8%of nickel, and 0.15% of carbon.Chromium steels are highly resistantto corrosion and are used for cutlery,chemical plant, ball bearings, etc.

chromium trioxide Seechromium(vi) oxide.

chromophore A group causing col-oration in a *dye. Chromophores aregenerally groups of atoms having de-localized electrons.

chromous compounds Com-pounds containing chromium in its

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lower (+2) oxidation state; e.g. chro-mous chloride is chromium(II) chlo-ride (CrCl2).

chromyl chloride (chromium oxy-chloride) A dark red liquid, CrO2Cl2;r.d. 1.911; m.p. –96.5°C; b.p. 117°C. Itis evolved as a dark-red vapour on ad-dition of concentrated sulphuric acidto a mixture of solid potassiumdichromate and sodium chloride; itcondenses to a dark-red covalent liq-uid, which is immediately hydrolysedby solutions of alkalis to give the yel-low chromate. Since bromides andiodides do not give analogous com-pounds this is a speciÜc test forchloride ions. The compound is apowerful oxidizing agent, explodingon contact with phosphorus and in-Ûaming sulphur and many organiccompounds.

chrysotile See serpentine.

CI See colour index.

CIDNP (chemically induced dynamicnuclear polarization) A mechanismenabling nuclear spin to inÛuencethe direction of a chemical reaction.This can occur in certain cases, eventhough the gap between energy lev-els of nuclear spin states in a mag-netic Üeld is very much smaller thanthe dissociation energies of chemicalbonds. Two radicals, the electrons ofwhich have parallel spins, can onlycombine if the conversion of a *trip-let to a *singlet can take place. In amagnetic Üeld, a triplet has threenondegenerate states called T0, T+,and T–. For a triplet-to-singlet conver-sion to take place, one electron mustprecess faster than the other for asufÜcient time to enable a 180° phasedifference to develop. This differencein precession can arise when the nu-clear spin interacts with the electronon the radical, by means of hyperÜnecoupling.

cine-substitution A type of substi-tution reaction in which the enteringgroup takes a position on an atomadjacent to the atom to which theleaving group is attached. See alsotele-substitution.

cinnabar A bright red mineralform of mercury(II) sulphide, HgS,crystallizing in the hexagonal sys-tem; the principal ore of mercury. Itis deposited in veins and impregna-tions near recent volcanic rocks andhot springs. The chief sources in-clude Spain, Italy, and Yugoslavia.

cinnamic acid (3-phenylpropenoicacid) A white crystalline aromatic*carboxylic acid, C6H5CH:CHCOOH;r.d. 1.248 (trans isomer); m.p.135–136°C; b.p. 300°C. Esters of cin-namic acid occur in some essentialoils.

CIP system (Cahn–Ingold–Prelogsystem) A system for the unambigu-ous description of stereoisomers usedin the R–S convention (see absoluteconfiguration) and in the *E–Z con-vention. The system involves a se-quence rule for determining aconventional order of ligands. Therule is that the atom bonded directlyto the chiral centre or double bond isconsidered and the ligand in whichthis atom has the highest protonnumber takes precedence. So, for ex-ample, I takes precedence over Cl. Iftwo ligands have bonding atoms withthe same proton number, then sub-stituents are taken into account (withthe substituent of highest protonnumber taking precedence). Thus,–C2H5 has a higher precedence than–CH3. If a double (or triple) bond oc-curs to a substituent, then the sub-stituent is counted twice (or threetimes). An isotope of high nucleonnumber takes precedence over oneof lower nucleon number. Hydrogenalways has lowest priority in this sys-

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tem. For example, the sequence forsome common ligands is I, Br, Cl,SO3H, OCOCH3, OCH3, OH, NO2, NH2,COOCH3, CONH2, COCH3, CHO,CH2OH, C6H5, C2H5, CH3, H. The sys-tem was jointly developed by theBritish chemists Robert Sidney Cahn(1899–1981) and Sir Christopher KelkIngold (1893–1970) and the Bosnian–Swiss chemist Vladimir Prelog (1906–98).

circular birefringence A phenom-enon in which there is a differencebetween the refractive indices of themolecules of a substance for right-and left-circularly polarized light. Cir-cular birefringence depends on theway in which the electromagneticÜeld interacts with the molecule, andis affected by the handedness of themolecule, and hence its polarizabil-ity. If a molecule has the shape of ahelix, the polarizability is dependenton whether or not the electric Üeldof the electromagnetic Üeld rotates inthe same sense as the helix, thus giv-ing rise to circular birefringence.

circular dichroism (CD) The pro-duction of an elliptically polarizedwave when a linearly polarized lightwave passes through a substance thathas differences in the extinctioncoefÜcients for left- and right-handedpolarized light. The size of this effectis given by φ = π/λ(ηl – ηr), where φ isthe ellipticity of the beam thatemerges (in radians), λ is the wave-length of the light, and ηl and ηr arethe absorption indices of the left- andright-handed circularly polarizedlight, respectively. Circular dichro-ism is a property of optically activemolecules and is used to obtain infor-mation about proteins. See also cdspectrum.

circular polarization See polariza-tion of light.

cis See isomerism; torsion angle.

127 Clapeyron–Clausius equation

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cisplatin A platinum complex, cis-[PtCl2(NH3)2], used in cancer treat-ment to inhibit the growth oftumours. It acts by binding betweenstrands of DNA.

citrate A salt or ester of citric acid.

citric acid A white crystallinehydroxycarboxylic acid,HOOCCH2C(OH)(COOH)CH2COOH;r.d. 1.54; m.p. 153°C. It is present incitrus fruits and is an intermediate inthe *Krebs cycle in plant and animalcells.

citric-acid cycle See krebs cycle.

CL20 See hniw.

Claisen condensation A reactionof esters in which two molecules ofthe ester react to give a keto ester,e.g.

2CH3COOR → CH3COCH2COOR + ROH

The reaction is catalysed by sodiumethoxide, the mechanism being simi-lar to that of the *aldol reaction. It isnamed after Ludwig Claisen(1851–1930).

Clapeyron–Clausius equation Adifferential equation that describesthe relationship between variableswhen there is a change in the stateof a system. In a system that has two*phases of the same substance, forexample solid and liquid, heat isadded or taken away very slowly sothat one phase can change reversiblyinto the other while the system re-mains at equilibrium. If the twophases are denoted A and B, theClapeyron–Clausius equation is:

dp/dT = L/T(VB–VA),where p is the pressure, T is the ther-modynamic temperature, L is theheat absorbed per mole in the


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change from A to B, and VB and VAare the volumes of B and A respec-tively. In the case of a transition fromliquid to vapour, the volume of theliquid can be ignored. Taking thevapour to be an *ideal gas, theClapeyron–Clausius equation can bewritten:

dlogep/dT = L/RT2

The Clapeyron–Clausius equation isnamed after the French engineerBenoit-Pierre-Émile Clapeyron(1799–1864) and Rudolf *Clausius.

Clark cell A type of *voltaic cellconsisting of an anode made of zincamalgam and a cathode of mercuryboth immersed in a saturated solu-tion of zinc sulphate. The Clark cellwas formerly used as a standard ofe.m.f.; the e.m.f. at 15°C is 1.4345volts. It is named after the British sci-entist Hosiah Clark (d. 1898).

Clark process See hardness ofwater.

clathrate A solid mixture in whichsmall molecules of one compound orelement are trapped in holes in thecrystal lattice of another substance.Clathrates are sometimes calledenclosure compounds or cage com-pounds, but they are not true com-pounds (the molecules are not heldby chemical bonds). Quinol and iceboth form clathrates with such sub-stances as sulphur dioxide and xenon.

Claude process A process for liq-uefying air on a commercial basis.Air under pressure is used as theworking substance in a piston en-gine, where it does external workand cools adiabatically. This cool airis fed to a counter-current heat ex-changer, where it reduces the tem-perature of the next intake ofhigh-pressure air. The same air is re-compressed and used again, and afterseveral cycles eventually liqueÜes.

The process was perfected in 1902 bythe French scientist Georges Claude(1870–1960).

claudetite A mineral form of *ar-senic(III) oxide, As4O6.

Clausius, Rudolf Julius Em-manuel (1822–88) German physi-cist, who held teaching posts inBerlin and Zurich, before going toWürzburg in 1869. He is best knownfor formulating the second law of*thermodynamics in 1850, indepen-dently of William Thomson (Lord*Kelvin). In 1865 he introduced theconcept of *entropy, and later con-tributed to electrochemistry and elec-trodynamics (see clausius–mossottiequation).

Clausius inequality An inequalitythat relates the change in entropy dSin an irreversible process to the heatsupplied to the system dQ and thethermodynamic temperature T, i.e.dS ≥ dQ/T. In the case of an irre-versible adiabatic change, where dQ =0, the Clausius inequality has theform dS > 0, which means that forthis type of change the entropy ofthe system must increase. The in-equality is named after Rudolf JuliusEmmanuel *Clausius. The Clausiusinequality can be used to demon-strate that entropy increases in suchprocesses as the free expansion of agas and the cooling of an initially hotsubstance.

Clausius–Mossotti equation Arelation between the *polarizabilityα of a molecule and the dielectricconstant ε of a dielectric substancemade up of molecules with this po-larizability. The Clausius–Mossottiequation can be written in the form

α = (3/4πN)/[(ε – 1)/(ε – 2)],where N is the number of moleculesper unit volume. The equation pro-vides a link between a microscopic

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quantity (the polarizability) and amacroscopic quantity (the dielectricconstant); it was derived usingmacroscopic electrostatics by the Ital-ian physicist Ottaviano FabrizioMossotti (1791–1863) in 1850 and in-dependently by Rudolf *Clausius in1879. It works best for gases and isonly approximately true for liquidsor solids, particularly if the dielectricconstant is large. Compare lorentz–lorenz equation.

Claus process A process for obtain-ing sulphur from hydrogen sulphide(from natural gas or crude oil). It in-volves two stages. First, part of thehydrogen sulphide is oxidized to sul-phur dioxide:

2H2S + 3O2 → 2SO2 + 2H2OSubsequently, the sulphur dioxide re-acts with hydrogen sulphide to pro-duce sulphur:

SO2 + 2H2S → 3S + 2H2OThe second stage occurs at 300°C andneeds an iron or aluminium oxidecatalyst.

clay A Üne-grained deposit consist-ing chieÛy of *clay minerals. It ischaracteristically plastic and virtuallyimpermeable when wet and crackswhen it dries out. In geology the sizeof the constituent particles is usuallytaken to be less than 1/256 mm. Insoil science clay is regarded as a soilwith particles less than 0.002 mm insize.

clay minerals Very small particles,chieÛy hydrous silicates of alu-minium, sometimes with magne-sium and/or iron substituting for allor part of the aluminium, that arethe major constituents of clay ma-terials. The particles are essentiallycrystalline (either platy or Übrous)with a layered structure, but may beamorphous or metalloidal. The clayminerals are responsible for the plas-

tic properties of clay; the particleshave the property of being able tohold water. The chief groups of clayminerals are: kaolinite, Al4Si4O10(OH)8, the chief

constituent of *kaolin; halloysite, Al4Si4(OH)8O10.4H2O; illite, KAl4(Si,Al)8O18.2H2O; montmorillonite, (Na,Ca)0.33(Al,Mg)2-

Si4O10(OH)2.nH2O, formed chieÛythrough alteration of volcanic ash;and

vermiculite, (Mg,Fe,Al)3(Al,Si)4-O10(OH)2.4H2O, used as an insulat-ing material and potting soil.

cleavage The splitting of a crystalalong planes of atoms in the lattice.

Clemmensen reduction Amethod of reducing a *carbonylgroup (C=O) to CH2, using zinc amal-gam and concentrated hydrochloricacid. It is used as a method of ascend-ing a homologous series. The reac-tion is named after Erik Clemmensen(1876–1941).

clinal See torsion angle.

clock reaction See b–z reaction;oscillating reaction.

closed chain See chain; ring.

close packing The packing ofspheres so as to occupy the mini-mum amount of space. In a singleplane, each sphere is surrounded bysix close neighbours in a hexagonalarrangement. The spheres in the sec-ond plane Üt into depressions in theÜrst layer, and so on. Each sphere has12 other touching spheres. There aretwo types of close packing. In hexag-onal close packing the spheres in thethird layer are directly over those inthe Ürst, etc., and the arrangement ofplanes is ABAB…. In cubic close pack-ing the spheres in the third layer oc-cupy a different set of depressionsthan those in the Ürst. The arrange-

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ment is ABCABC…. See also cubiccrystal.A• An interactive version of cubic close-

packing• An interactive version of hexagonal

close packing

closo-structure See borane.

club drug A drug typically used byyoung people at clubs, parties, etc.Examples are ecstasy, gammahydrox-ybutyric acid (GHB), and metham-phetamines.

Clusius column A device for sepa-rating isotopes by thermal diffusion.One form consists of a vertical col-umn some 30 metres high with aheated electric wire running along itsaxis. The lighter isotopes in a gaseousmixture of isotopes diffuse fasterthan the heavier isotopes. Heated bythe axial wire, and assisted by nat-ural convection, the lighter atomsare carried to the top of the column,where a fraction rich in lighter iso-topes can be removed for further en-richment.

cluster compound A compoundin which groups of metal atoms arejoined together by metal–metalbonds. The formation of such com-pounds is a feature of the chemistryof certain elements, particularlymolybdenum and tungsten, but alsovanadium, tantalum, niobium, anduranium. Isopoly compounds areones in which the cluster containsatoms of the same element; het-eropoly compounds contain a mix-ture of different elements.

CML See chemical markup lan-guage.

coacervate An aggregate of macro-molecules, such as proteins, lipids,and nucleic acids, that form a stable*colloid unit with properties that re-semble living matter. Many are

coated with a lipid membrane andcontain enzymes that are capable ofconverting such substances as glu-cose into more complex molecules,such as starch. Coacervate dropletsarise spontaneously under appropri-ate conditions and may have beenthe prebiological systems fromwhich living organisms originated.

coagulation The process in whichcolloidal particles come together irre-versibly to form larger masses. Coag-ulation can be brought about byadding ions to change the ionicstrength of the solution and thusdestabilize the colloid (see floccula-tion). Ions with a high charge areparticularly effective (e.g. alum, con-taining Al3+, is used in styptics to co-agulate blood). Another example ofionic coagulation is in the formationof river deltas, which occurs whencolloidal silt particles in rivers are co-agulated by ions in sea water. Alumand iron (III) sulphate are also usedfor coagulation in sewage treatment.Heating is another way of coagulat-ing certain colloids (e.g. boiling anegg coagulates the albumin).

coal A brown or black carbonaceousdeposit derived from the accumula-tion and alteration of ancient vegeta-tion, which originated largely inswamps or other moist environ-ments. As the vegetation decom-posed it formed layers of peat, whichwere subsequently buried (for exam-ple, by marine sediments following arise in sea level or subsidence of theland). Under the increased pressureand resulting higher temperaturesthe peat was transformed into coal.Two types of coal are recognized:humic (or woody) coals, derived fromplant remains; and sapropelic coals,which are derived from algae, spores,and Ünely divided plant material.

As the processes of coaliÜcation (i.e.the transformation resulting from

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the high temperatures and pressures)continue, there is a progressive trans-formation of the deposit: the propor-tion of carbon relative to oxygenrises and volatile substances andwater are driven out. The variousstages in this process are referred toas the ranks of the coal. In ascendingorder, the main ranks of coal are: lig-nite (or brown coal), which is soft,brown, and has a high moisture con-tent; subbituminous coal, which isused chieÛy by generating stations;bituminous coal, which is the mostabundant rank of coal; semibitumi-nous coal; semianthracite coal, whichhas a Üxed carbon content of be-tween 86% and 92%; and anthracitecoal, which is hard and black with aÜxed carbon content of between 92%and 98%.

Most deposits of coal were formedduring the Carboniferous and Per-mian periods. More recent periods ofcoal formation occurred during theearly Jurassic and Tertiary periods.Coal deposits occur in all the majorcontinents; the leading producers in-clude the USA, China, Ukraine,Poland, UK, South Africa, India, Aus-tralia, and Germany. Coal is used as afuel and in the chemical industry; by-products include coke and coal tar.

coal gas A fuel gas produced by thedestructive distillation of coal. In thelate-19th and early-20th centuriescoal gas was a major source of energyand was made by heating coal in theabsence of air in local gas works. Typ-ically, it contained hydrogen (50%),methane (35%), and carbon monoxide(8%). By-products of the process were*coal tar and coke. The use of thistype of gas declined with the increas-ing availability of natural gas, al-though since the early 1970s interesthas developed in using coal in mak-ing *SNG.

coal tar A tar obtained from the de-

structive distillation of coal. For-merly, coal tar was obtained as a by-product in manufacturing *coal gas.Now it is produced in making cokefor steel making. The crude tar con-tains a large number of organiccompounds, such as benzene, naph-thalene, methylbenzene, phenols,etc., which can be obtained by distil-lation. The residue is pitch. At onetime coal tar was the major source oforganic chemicals, most of which arenow derived from petroleum andnatural gas.

cobalamin (vitamin B12) See vita-min b complex.

cobalt Symbol Co. A light-grey*transition element; a.n. 27; r.a.m.58.933; r.d. 8.9; m.p. 1495°C; b.p.2870°C. Cobalt is ferromagneticbelow its Curie point of 1150°C.Small amounts of metallic cobalt arepresent in meteorites but it is usuallyextracted from ore deposits workedin Canada, Morocco, and Zaïre. It ispresent in the minerals cobaltite,smaltite, and erythrite but also asso-ciated with copper and nickel as sul-phides and arsenides. Cobalt ores areusually roasted to the oxide and thenreduced with carbon or water gas.Cobalt is usually alloyed for use. Al-nico is a well-known magnetic alloyand cobalt is also used to make stain-less steels and in high-strength alloysthat are resistant to oxidation at hightemperatures (for turbine blades andcutting tools).

The metal is oxidized by hot airand also reacts with carbon, phos-phorus, sulphur, and dilute mineralacids. Cobalt salts, usual oxidationstates II and III, are used to give abrilliant blue colour in glass, tiles,and pottery. Anhydrous cobalt(II)chloride paper is used as a qualitativetest for water and as a heat-sensitiveink. Small amounts of cobalt salts areessential in a balanced diet for mam-

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mals (see essential element). ArtiÜ-cially produced cobalt–60 is an im-portant radioactive tracer andcancer-treatment agent. The elementwas discovered by Georg Brandt(1694–1768) in 1737.


cobalt(II) oxide A pink solid, CoO;cubic; r.d. 6.45; m.p. 1935°C. The ad-dition of potassium hydroxide to asolution of cobalt(II) nitrate gives abluish-violet precipitate, which onboiling is converted to pink impurecobalt(II) hydroxide. On heating thisin the absence of air, cobalt(II) oxideis formed. The compound is readilyoxidized in air to form tricobalttetroxide, Co3O4, and is readily re-duced by hydrogen to the metal.

cobalt(III) oxide (cobalt sesquiox-ide) A black grey insoluble solid,Co2O3; hexagonal or rhombic; r.d.5.18; decomposes at 895°C. It is pro-duced by the ignition of cobalt ni-trate; the product however never hasthe composition corresponding ex-actly to cobalt(III) oxide. On heatingit readily forms Co3O4, which con-tains both Co(II) and Co(III), and iseasily reduced to the metal by hydro-gen. Cobalt(III) oxide dissolves instrong acid to give unstable brownsolutions of trivalent cobalt salts.With dilute acids cobalt(II) salts areformed.

cobalt steel Any of a group ofalloy *steels containing 5–12% ofcobalt, 14–20% of tungsten, usuallywith 4% of chromium and 1–2% ofvanadium. They are very hard butsomewhat brittle. Their main use isin high-speed tools.

cobalt thiocyanate test Seescott’s test.

cocaine A powerful drug present inthe leaver of the coca plant (Eryth-

roxylon coca). It stimulates the centralnervous system and has effects simi-lar to the amphetamines. It was origi-nally used as a local anaesthetic. Theillegal drug is usually the soluble hy-drochloride. This can be convertedinto the free-base form (known ascrack cocaine) by dissolving in waterand heating with sodium bicarbon-ate. Cocaine is a class A drug in theUK. It can be detected by *Scott’stest.

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cocurrent Ûow Flow of two Ûuidsin the same direction with transfer ofmatter or heat between them. Com-pare countercurrent flow.

codeine An alkaloid C18H21NO3found in opium. It is structurally sim-ilar to morphine, from which it isproduced, and is used in the form ofthe sulphate or phosphate as apainkiller and cough medicine.Codeine is converted into morphinein the liver. It is used to some extentas a recreational drug. In the UK it isa class B drug but can be obtained incomposite over-the-counter prepara-tions in which it has a low concentra-tion and is combined with paraceta-mol or ibuprofen. See also opioids.

coenzyme An organic nonproteinmolecule that associates with anenzyme molecule in catalysing bio-chemical reactions. Coenzymes usu-ally participate in the substrate–enzyme interaction by donating oraccepting certain chemical groups.


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Many vitamins are precursors ofcoenzymes. See also cofactor.

coenzyme A (CoA) A complex or-ganic compound that acts in conjunc-tion with enzymes involved invarious biochemical reactions, no-tably the oxidation of pyruvate viathe *Krebs cycle and fatty-acid oxida-tion and synthesis. It comprises prin-cipally the B vitamin pantothenicacid, the nucleotide adenine, and aribose-phosphate group.

coenzyme Q See ubiquinone.

cofactor A nonprotein componentessential for the normal catalytic ac-tivity of an enzyme. Cofactors maybe organic molecules (*coenzymes)or inorganic ions. They may activatethe enzyme by altering its shape orthey may actually participate in thechemical reaction.

coherent anti-Stokes Ramanspectroscopy See cars.

coherent units A system of *unitsof measurement in which derivedunits are obtained by multiplying ordividing base units without the useof numerical factors. *SI units form acoherent system; for example theunit of force is the newton, which isequal to 1 kilogram metre per secondsquared (kg m s–2), the kilogram,metre, and second all being baseunits of the system.

cohesion The force of attractionbetween like molecules.

coinage metals A group of threemalleable ductile transition metalsforming group 11 (formerly IB) of the*periodic table: copper (Cu), silver(Ag), and gold (Au). Their outer elec-tronic conÜgurations have the formnd10(n+1)s1. Although this is similar tothat of alkali metals, the coinagemetals all have much higher ioniza-tion energies and higher (and posi-tive) standard electrode potentials.

Thus, they are much more difÜcult tooxidize and are more resistant to cor-rosion. In addition, the fact that theyhave d-electrons makes them showvariable valency (CuI, CuII, and CuIII;AgI and AgII; AuI and AuIII) and form awide range of coordination com-pounds. They are generally classiÜedwith the *transition elements.

coke A form of carbon made by thedestructive distillation of coal. Cokeis used for blast-furnaces and othermetallurgical and chemical processesrequiring a source of carbon. Lower-grade cokes, made by heating thecoal to a lower temperature, are usedas smokeless fuels for domestic heat-ing.

colchicine An *alkaloid derivedfrom the autumn crocus, Colchicumautumnale. It inhibits cell division.Colchicine is used in genetics, cytol-ogy, and plant breeding research andalso in cancer therapy to inhibit celldivision.

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collagen An insoluble Übrous pro-tein found extensively in the connec-tive tissue of skin, tendons, andbone. The polypeptide chains of col-lagen (containing the amino acidsglycine and proline predominantly)form triple-stranded helical coils that


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are bound together to form Übrils,which have great strength and lim-ited elasticity. Collagen accounts forover 30% of the total body protein ofmammals.

collective oscillation An oscilla-tion in a many-body system in whichall the particles in the system takepart in a cooperative. Plasma oscilla-tions provide an example of collec-tive oscillations. In systems describedby quantum mechanics, collective os-cillations are quantized to give collec-tive excitations.

colligation The combination oftwo free radicals to form a covalentbond, as in H3C• + Cl• → CH3Cl. It isthe reverse of *homolytic Üssion.

colligative properties Propertiesthat depend on the concentration ofparticles (molecules, ions, etc.) pre-sent in a solution, and not on the na-ture of the particles. Examples ofcolligative properties are osmoticpressure (see osmosis), *lowering ofvapour pressure, *depression offreezing point, and *elevation of boil-ing point.

collision density The number ofcollisions that occur in unit volumein unit time when a given particleÛux passes through matter.

collision quenching See externalconversion.

collodion A thin Ülm of cellulosenitrate made by dissolving the cellu-lose nitrate in ethanol or ethoxy-ethane, coating the surface, andevaporating the solvent.

colloid mills Machines used togrind aggregates into very Üne parti-cles or to apply very high shearingforces within a Ûuid to produce col-loid suspensions or emulsions inwhich the particle sizes are less than1 micrometer. One type of colloidmill is called a disc mill, in which a

mixture of a solid and liquid (or twoliquids) is passed between two discs asmall distance apart, which rotatevery rapidly relative to each other.Other types of colloid mills are thevalve and oriÜce types, in which themixture is forced through an oriÜceat a very high speed and then strikesa breaker ring. Applications of colloidmills occur in food processing, inpaint manufacture, and in the phar-maceutical industry.

colloids Colloids were originallydeÜned by Thomas *Grahamin 1861as substances, such as starch orgelatin, which will not diffusethrough a membrane. He distin-guished them from crystalloids (e.g.inorganic salts), which would passthrough membranes. Later it was rec-ognized that colloids were distin-guished from true solutions by thepresence of particles that were toosmall to be observed with a normalmicroscope yet were much largerthan normal molecules. Colloids arenow regarded as systems in whichthere are two or more phases, withone (the dispersed phase) distributedin the other (the continuous phase).Moreover, at least one of the phaseshas small dimensions (in the range10–9–10–6 m). Colloids are classiÜed invarious ways.

Sols are dispersions of small solidparticles in a liquid. The particlesmay be macromolecules or may beclusters of small molecules. Lyopho-bic sols are those in which there isno afÜnity between the dispersedphase and the liquid. An example issilver chloride dispersed in water. Insuch colloids the solid particles havea surface charge, which tends to stopthem coming together. Lyophobicsols are inherently unstable and intime the particles aggregate andform a precipitate. Lyophilic sols, onthe other hand, are more like true so-

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lutions in which the solute moleculesare large and have an afÜnity for thesolvent. Starch in water is an exam-ple of such a system. Association col-loids are systems in which thedispersed phase consists of clustersof molecules that have lyophobic andlyophilic parts. Soap in water is an as-sociation colloid (see micelle).

Emulsions are colloidal systems inwhich the dispersed and continuousphases are both liquids, e.g. oil-in-water or water-in-oil. Such systemsrequire an emulsifying agent to stabi-lize the dispersed particles.

Gels are colloids in which both dis-persed and continuous phases have athree-dimensional network through-out the material, so that it forms ajelly-like mass. Gelatin is a commonexample. One component may some-times be removed (e.g. by heating) toleave a rigid gel (e.g. silica gel).

Other types of colloid includeaerosols (dispersions of liquid or solid particles in a gas, as in a mist or smoke) and foams (dispersions ofgases in liquids or solids).

colophony See rosin

colorimetric analysis Quantita-tive analysis of solutions by estimat-ing their colour, e.g. by comparing itwith the colours of standard solu-tions.

colour centre A defect in a crystalthat changes the way in which it ab-sorbs light or other electromagneticradiation. Impurities in the crystal af-fect the bind structure and allowtransitions in different regions of thespectrums. Impurity colour centresare responsible for the characteristiccolours of many gemstones. A par-ticular type of colour centre is an F-centre. This is a missing negative ionin an ionic crystal, where the overallcharge neutrality occurs by trappingan electron in the vacancy. The elec-

tron has energy levels similar tothose of a particle in a box. F-centrescan be produced by chemical activityor by irradiation.

colour index (CI) A list, regarded asdeÜnitive, of dyes and pigments,which includes information on theircommercial names, method of appli-cation, colour fastness, etc.

colour photography Any of vari-ous methods of forming colouredimages on Ülm or paper by photo-graphic means. One common processis a subtractive reversal system thatutilizes a Ülm with three layers oflight-sensitive emulsion, one re-sponding to each of the three pri-mary colours. On development ablack image is formed where thescene is blue. The white areas aredyed yellow, the complementary col-our of blue, and the blackened areasare bleached clean. A yellow Ülterbetween this emulsion layer and the next keeps blue light from thesecond emulsion, which is green-sensitive. This is dyed magentawhere no green light has fallen. TheÜnal emulsion is red-sensitive and isgiven a cyan (blue-green) image onthe negative after dying. When whitelight shines through the three dyelayers the cyan dye subtracts redwhere it does not occur in the scene,the magenta subtracts green, and theyellow subtracts blue. The light pro-jected by the negative therefore re-constructs the original scene eitheras a transparency or for use withprinting paper.

columbium A former name for theelement *niobium.

column chromatography Seechromatography.

combinatorial chemistry A tech-nique in which very large numbersof related compounds are formed in

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small quantities in cells on a plate,and properties are investigated bysome automated technique. It is par-ticularly used in the development ofnew drugs.

combustion A chemical reactionin which a substance reacts rapidlywith oxygen with the production ofheat and light. Such reactions areoften free-radical chain reactions,which can usually be summarized asthe oxidation of carbon to form itsoxides and the oxidation of hydrogento form water. See also flame.

commensurate lattice A latticethat can be divided into two or moresublattices, with the basis vectors ofthe lattice being a rational multiple

of the basis vectors of the sublattice.The phase transition between a com-mensurate lattice and an incommen-surate lattice can be analysed usingthe Frenkel–Kontorowa model. Anion that is common to two or morecomponents in a mixture. In a solu-tion of XCl, for example, the additionof another chloride YCl, may precipi-tate XCl because the solubility prod-uct is exceeded as a result of theextra concentration of Cl

_ions. This

is an example of a common-ioneffect.

common salt See sodiumchloride.

competitive inhibition See inhi-bition.

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complementarity The conceptthat a single model may not be ade-quate to explain all the observationsmade of atomic or subatomic sys-tems in different experiments. Forexample, *electron diffraction is bestexplained by assuming that the elec-tron is a wave (see de broglie wave-length), whereas the *photoelectriceffect is described by assuming thatit is a particle. The idea of two differ-ent but complementary concepts totreat quantum phenomena was Ürstput forward by the Danish physicistNiels Bohr (1855–1962) in 1927.

complex A compound in whichmolecules or ions form coordinatebonds to a metal atom or ion (see il-lustration). Often complexes occur ascomplex ions, such as [Cu(H2O)6]2+ orFe[(CN)6]3–. A complex may also be aneutral molecule (e.g. PtCl2(NH3)2).The formation of such coordinationcomplexes is typical behaviour oftransition metals. The complexesformed are often coloured and haveunpaired electrons (i.e. are paramag-netic). See also ligand; chelate.

complex ion See complex.

complexometric analysis A typeof volumetric analysis in which thereaction involves the formation of aninorganic *complex.

component A distinct chemicalspecies in a mixture. If there are noreactions taking place, the number ofcomponents is the number of sepa-rate chemical species. A mixture ofwater and ethanol, for instance, hastwo components (but is a singlephase). A mixture of ice and waterhas two phases but one component(H2O). If an equilibrium reaction oc-curs, the number of components istaken to be the number of chemicalspecies minus the number of reac-tions. Thus, in

H2 + I2 ˆ 2HI

there are two components. See alsophase rule.

compound A substance formed bythe combination of elements in Üxedproportions. The formation of a com-pound involves a chemical reaction;i.e. there is a change in the conÜgura-tion of the valence electrons of theatoms. Compounds, unlike mixtures,cannot be separated by physicalmeans. See also molecule.

comproportionation A reactionin which an element in a higher oxi-dation state reacts with the same el-ement in a lower oxidation state togive the element in an intermediateoxidation state. For example

Ag2+(aq) + Ag(s) → 2Ag+(aq)It is the reverse of *disproportiona-tion.

Compton, Arthur Holly(1892–1962) US physicist, who be-came professor of physics at the Uni-versity of Chicago in 1923. He is bestknown for his discovery (1923) of the*Compton effect, for which heshared the 1927 Nobel Prize forphysics with C. T. R. Wilson.

Compton effect The reduction inthe energy of high-energy (X-ray orgamma-ray) photons when they arescattered by free electrons, whichthereby gain energy. The phenome-non, Ürst observed in 1923 by A. H.*Compton, occurs when the photoncollides with an electron; some of thephoton’s energy is transferred to theelectron and consequently the pho-ton loses energy h(ν1 – ν2), where h isthe *Planck constant and ν1 and ν2

are the frequencies before and aftercollision. As ν1 > ν2, the wavelengthof the radiation increases after thecollision. This type of inelastic scat-tering is known as Compton scatter-ing and is similar to the *Raman

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effect. See also inverse compton ef-fect.

computational chemistry Theuse of computers in chemical re-search. With the increase in process-ing power of computers, calculationson individual molecules and onchemical systems have become im-portant tools for research and indus-trial development. With simplemolecules, predictions can be madeabout electronic structure and prop-erties using *ab-initio calculations.For more complex molecules *semi-empirical calculations are used. The Üeld has been particularly ex-panded by the *density-functionalmethod of treating large moleculesand by the availability of software foranalysing molecular behaviour andstructure. See also molecular model-ling.

concentrated Describing a solu-tion that has a relatively high con-centration of solute.

concentration The quantity of dis-solved substance per unit quantity ofa solution. Concentration is meas-ured in various ways. The amount ofsubstance dissolved per unit volumeof the solution (symbol c) has units ofmol dm–3 or mol l–1. It is now calledamount concentration (formerly mo-larity). The mass concentration (sym-bol ρ) is the mass of solute per unitvolume of solution. It has units ofkg dm–3, g cm–3, etc. The molality isthe amount of substance per unitmass of solvent, commonly given inunits of mol kg–1. See also mole frac-tion.

concentration cell See cell.

concentration gradient (diffusiongradient) The difference in concen-tration between a region of a solu-tion or gas that has a high density ofparticles and a region that has a rela-

tively lower density of particles. Byrandom motion, particles will movefrom the area of high concentrationtowards the area of low concentra-tion, by the process of *diffusion,until the particles are evenly distrib-uted in the solution or gas.

concerted reaction A type of reac-tion in which there is only one stagerather than a series of steps. The SN2mechanism in *nucleophilic substitu-tions is an example. See also peri-cyclic reactions.

condensation The change of avapour or gas into a liquid. Thechange of phase is accompanied bythe evolution of heat (see latentheat).

condensation polymerizationSee polymer.

condensation pump See diffu-sion pump.

condensation reaction A chemi-cal reaction in which two moleculescombine to form a larger moleculewith elimination of a small mol-ecule (e.g. H2O). See aldehydes; ke-tones.

condenser A device used to cool avapour to cause it to condense to aliquid. See liebig condenser.

conducting polymer An organicpolymer that conducts electricity.Conducting polymers have a crys-talline structure in which chains ofconjugated unsaturated carbon–carbon bonds are aligned. Examplesare polyacetylene and polypyrrole.There has been considerable interestin the development of such materialsbecause they would be cheaper andlighter than metallic conductors.They do, however, tend to be chemi-cally unstable and, so far, no com-mercial conducting polymers havebeen developed.

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conductiometric titration A typeof titration in which the electricalconductivity of the reaction mixtureis continuously monitored as onereactant is added. The equivalencepoint is the point at which thisundergoes a sudden change. Themethod is used for titrating colouredsolutions, which cannot be used withnormal indicators.

conduction band See energybands.

conductivity water See distilledwater.

Condy’s Ûuid A mixture of cal-cium and potassium permanganates(manganate(VII)) used as an antisep-tic.

conÜguration 1. The arrangementof atoms or groups in a molecule. 2. The arrangement of electronsabout the nucleus of an *atom.

conÜguration space The n-dimen-sional space with coordinates(q1,q2,…,qn) associated with a system

that has n degrees of freedom, wherethe values q describe the degrees offreedom. For example, in a gas of Natoms each atom has three positionalcoordinates, so the conÜgurationspace is 3N-dimensional. If the parti-cles also have internal degrees offreedom, such as those caused by vi-bration and rotation in a molecule,then these must be included in theconÜguration space, which is conse-quently of a higher dimension. Seealso statistical mechanics.

conformation One of the verylarge number of possible spatialarrangements of atoms that can beinterconverted by rotation about asingle bond in a molecule. In the caseof ethane, H3C–CH3, one methylgroup can rotate relative to theother. There are two extreme cases.In one, the C–H bonds on one groupalign with the C–H bonds on theother (as viewed along the C–Cbond). This is an eclipsed conforma-tion (or eclipsing conformation) andcorresponds to a maximum in a

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graph of potential energy against ro-tation angle. In the other the C–Hbonds on one group bisect the anglebetween two C–H bonds on theother. This is a staggered conforma-tion (or bisecting conformation) andcorresponds to a minimum in the po-tential-energy curve. In the case ofethane, the energy difference be-tween the two conformations issmall (2.8 kcal/mole) and effectivefree rotation occurs under normalconditions.

In the case of butane, rotation canoccur about the bond between thesecond and third carbon atoms,MeH2C–CH2Me. The highest maxi-mum occurs when the methylgroups are eclipsed (they are at theirclosest). Maxima of lower energiesoccur when the methyl groups onone carbon atom eclipse the hydro-gen atoms on the other. The lowestpotential energy occurs when a C–Mebond bisects the angle between theC–H bonds (i.e. the two methylgroups are furthest apart). This iscalled the anti conformation. A lesserminimum occurs when the methylgroup bisects the angle between aC–H bond and a C–Me bond. Thisarrangement is called the gaucheconformation.

Similar rotational arrangementsoccur in other types of molecule. Forinstance, in a compound such asR3C–CHO, with a C=O double bond,one conformation has the C=O bondbisecting the angle between C–Rbonds. This is called the bisectingconformation (note that the C–Hbond eclipses one of the C–R bondsin this case). The other has the C=Obond eclipsing a C–R bond. This isthe eclipsed or eclipsing conforma-tion. (In this case the C–H bond bi-sects the angle between the C–Rbonds.)

See also torsion angle; ring con-formations.

conformational analysis The de-termination or estimation of therelative energies of possible confor-mations of a molecule and the effectof these on the molecule’s proper-ties.

conformational isomer (con-former) One of a set of stereoisomersthat differ from each other by tor-sion angle or angles (where onlystructures that are minima of poten-tial energy are considered).

congeners Elements that belong to the same group in the periodictable.

conjugate acids and bases Seeacid.

conjugated Describing double ortriple bonds in a molecule that areseparated by one single bond. For ex-ample, the organic compound buta-1,3-diene, H2C=CH–CH=CH2, hasconjugated double bonds. In suchmolecules, there is some delocaliza-tion of electrons in the pi orbitals be-tween the carbon atoms linked bythe single bond.

conjugate solutions Solutionsformed between two liquids that are partially miscible with one an-other; such solutions are in equilib-rium at a particular temperature.Examples of conjugate solutions are phenol in water and water inphenol.

conjugation Delocalization of pielectrons as occurs in *conjugatedsystems. Conjugation can also in-volve d orbitals and lone pairs ofelectrons.

connection table A way of repre-senting a molecule as a table show-ing the atoms, their coordinates, andthe links between them. The MDL*molÜle format uses a connectiontable in its representation of struc-

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ture. Connection tables are a usefulway of storing molecular data, bothfrom the point of view of graphicsprograms and also for databasesearches, in which it is possible touse the table to look for substruc-tures.

conservation The sensible use ofthe earth’s natural resources in orderto avoid excessive degradation andimpoverishment of the environment.It should include the search for alter-native food and fuel supplies whenthese are endangered (as by defor-estation and overÜshing); an aware-ness of the dangers of *pollution;and the maintenance and preserva-tion of natural habitats and the cre-ation of new ones, e.g. naturereserves, national parks, and sites ofspecial scientiÜc intrest (SSSIs).

conservation law A law statingthat the total magnitude of a certainphysical property of a system, suchas its mass, energy, or charge, re-main unchanged even though theremay be exchanges of that propertybetween components of the system.For example, imagine a table with abottle of salt solution (NaCl), a bottleof silver nitrate solution (AgNO3), anda beaker standing on it. The mass ofthis table and its contents will notchange even when some of the con-tents of the bottles are poured intothe beaker. As a result of the reactionbetween the chemicals two new sub-stances (silver chloride and sodiumnitrate) will appear in the beaker:

NaCl + AgNO3 → AgCl + NaNO3,but the total mass of the table and itscontents will not change. This con-servation of mass is a law of wideand general applicability, which istrue for the universe as a whole, pro-vided that the universe can be con-sidered a closed system (nothingescaping from it, nothing being

added to it). According to Einstein’smass–energy relationship, everyquantity of energy (E) has a mass (m),which is given by E/c2, where c is thespeed of light. Therefore if mass isconserved, the law of conservation ofenergy must be of equally wide appli-cation.

consolute temperature The tem-perature at which two partially mis-cible liquids become fully miscible asthe temperature is increased.

constantan An alloy having anelectrical resistance that varies onlyvery slightly with temperature (overa limited range around normal roomtemperatures). It consists of copper(50–60%) and nickel (40–50%) and isused in resistance wire, thermocou-ples, etc.

constant-boiling mixture Seeazeotrope.

constant proportions See chemi-cal combination.

constitutional isomerism See iso-merism.

contact insecticide Any insecti-cide (see pesticide) that kills its targetinsect by being absorbed through thecuticle or by blocking the spiracles,rather than by being ingested.

contact process A process formaking sulphuric acid from sulphurdioxide (SO2), which is made by burn-ing sulphur or by roasting sulphideores. A mixture of sulphur dioxideand air is passed over a hot catalyst

2SO2 + O2 → 2SO3

The reaction is exothermic and theconditions are controlled to keep thetemperature at an optimum 450°C.Formerly, platinum catalysts wereused but vanadium–vanadium oxidecatalysts are now mainly employed(although less efÜcient, they are lesssusceptible to poisoning). The sul-

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phur trioxide is dissolved in sul-phuric acid

H2SO4 + SO3 → H2S2O7

and the oleum is then diluted.

continuous phase See colloids.

continuous spectrum See spec-trum.

controlled substance In the UK,an illegal or restricted drug. Thereare three classes with different legalpenalties for possession and for deal-ing.Class A drugs (the most harmful) in-clude cocaine, ecstasy, heroin, LSD,and magic mushrooms. Ampheta-mines prepared for injection are alsoclass A drugs.Class B drugs include amphetamines,methylphenidate, and pholcodine.Class C drugs include cannabis,gammahydroxybutyric acid (GHB),ketamine, and some painkillers.Drug use is governed by the Misuseof Drugs Act, 1971 and 2005. Somesubstances fall under the MedicinesAct and are classiÜed as prescription-only, pharmacist without prescrip-tion, or generally available.

A• Information about the Misuse of Drugs

Act from the Home OfÜce website

convection A process by whichheat is transferred from one part of aÛuid to another by movement of theÛuid itself. In natural convection themovement occurs as a result of grav-ity; the hot part of the Ûuid expands,becomes less dense, and is displacedby the colder denser part of the Ûuidas this drops below it. This is theprocess that occurs in most domestichot-water systems between the boilerand the hot-water cylinder. A naturalconvection current is set up transfer-ring the hot water from the boiler upto the cylinder (always placed abovethe boiler) so that the cold water

from the cylinder can move downinto the boiler to be heated. In somemodern systems, where small-borepipes are used or it is inconvenient toplace the cylinder above the boiler,the circulation between boiler andhot-water cylinder relies upon apump. This is an example of forcedconvection, where hot Ûuid is trans-ferred from one region to another bya pump or fan.

converter The reaction vessel inthe *Bessemer process or some simi-lar steel-making process.

coomassie blue A biological dyeused for staining proteins.

coordinate bond See chemicalbond.

coordination compound A com-pound in which coordinate bonds areformed (see chemical bond). Theterm is used especially for inorganic*complexes.


coordination number The num-ber of groups, molecules, atoms, orions surrounding a given atom or ionin a complex or crystal. For instance,in a square-planar complex the cen-tral ion has a coordination number offour. In a close-packed crystal (seeclose packing) the coordinationnumber is twelve.

Cope rearrangement A type ofrearrangement of dienes in whichthe position of a substituent changes.For instance the substituted diene

CH2=CH–CH(R)– CH2–CH=CH2,when heated, can convert to

RCH=CH–CH2–CH2–CH=CH2.It involves the formation of a cyclictransition state, and is an example ofa *sigmatropic reaction.

copolymer See polymer.

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copper Symbol Cu. A red-brown*transition element; a.n. 29; r.a.m.63.546; r.d. 8.92; m.p. 1083.4°C; b.p.2567°C. Copper has been extractedfor thousands of years; it was knownto the Romans as cuprum, a namelinked with the island of Cyprus. Themetal is malleable and ductile and anexcellent conductor of heat and elec-tricity. Copper-containing mineralsinclude cuprite (Cu2O) as well asazurite (2CuCO3.Cu(OH)2), chalco-pyrite (CuFeS2), and malachite(CuCO3.Cu(OH)2). Native copper ap-pears in isolated pockets in someparts of the world. The large minesin the USA, Chile, Canada, Zambia,Democratic Republic of Congo, andPeru extract ores containing sul-phides, oxides, and carbonates. Theyare usually worked by smelting,leaching, and electrolysis. Coppermetal is used to make electric cablesand wires. Its alloys, brass (copper–zinc) and bronze (copper–tin), areused extensively.

Water does not attack copper butin moist atmospheres it slowly formsa characteristic green surface layer(patina). The metal will not reactwith dilute sulphuric or hydrochloricacids, but with nitric acid oxides ofnitrogen are formed. Copper com-pounds contain the element in the+1 and +2 oxidation states. Copper(I)compounds are mostly white (theoxide is red). Copper(II) salts are bluein solution. The metal also forms alarge number of coordination com-plexes.A• Information from the WebElements site

copperas See iron(ii) sulphate.

copper(I) chloride A white solidcompound, CuCl; cubic; r.d. 4.14;m.p. 430°C; b.p. 1490°C. It is ob-tained by boiling a solution contain-ing copper(II) chloride, excess copperturnings, and hydrochloric acid. Cop-

per(I) is present as the [CuCl2]– com-plex ion. On pouring the solutioninto air-free distilled water copper(I)chloride precipitates. It must be keptfree of air and moisture since it oxi-dizes to copper(II) chloride underthose conditions.

Copper(I) chloride is essentially co-valent and its structure is similar tothat of diamond; i.e. each copperatom is surrounded tetrahedrally byfour chlorine atoms and vice versa.In the vapour phase, dimeric andtrimeric species are present.Copper(I) chloride is used in conjunc-tion with ammonium chloride as acatalyst in the dimerization ofethyne to but-1-ene-3-yne (vinylacetylene), which is used in the pro-duction of synthetic rubber. In thelaboratory a mixture of copper(I)chloride and hydrochloric acid isused for converting benzene diazo-nium chloride to chlorobenzene –the Sandmeyer reaction.

copper(II) chloride A brown-yellow powder, CuCl2; r.d. 3.386;m.p. 620°C. It exists as a blue-greendihydrate (rhombic; r.d. 2.54; losesH2O at 100°C). The anhydrous solid isobtained by passing chlorine overheated copper. It is predominantlycovalent and adopts a layer structurein which each copper atom is sur-rounded by four chlorine atoms at adistance of 0.23 and two more at adistance of 0.295. A concentratedaqueous solution is dark brown incolour due to the presence of com-plex ions such as [CuCl4]2–. On dilu-tion the colour changes to green andthen blue because of successive re-placement of chloride ions by watermolecules, the Ünal colour being thatof the [Cu(H2O)6]2+ ion. The dihydratecan be obtained by crystallizing thesolution.

copper glance A mineral form ofcopper(I) sulphide, Cu2S.

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copper(II) nitrate A blue deliques-cent solid, Cu(NO3)2.3H2O; r.d. 2.32;m.p. 114.5°C. It may be obtained byreacting either copper(II) oxide orcopper(II) carbonate with dilute nitricacid and crystallizing the resultingsolution. Other hydrates containing 6or 9 molecules of water are known.On heating it readily decomposes togive copper(II) oxide, nitrogen diox-ide, and oxygen. The anhydrous formcan be obtained by reacting copperwith a solution of nitrogen dioxide inethyl ethanoate. It sublimes on heat-ing suggesting that it is appreciablycovalent.

copper(I) oxide A red insolublesolid, Cu2O; r.d. 6.0; m.p. 1235°C. It isobtained by reduction of an alkalinesolution of copper(II) sulphate. Sincethe addition of alkalis to a solution ofcopper(II) salt results in the precipita-tion of copper(II) hydroxide the cop-per(II) ions are complexed withtartrate ions; under such conditionsthe concentration of copper(II) ions isso low that the solubility product ofcopper(II) hydroxide is not exceeded.

When copper(I) oxide reacts withdilute sulphuric acid a solution ofcopper(II) sulphate and a deposit ofcopper results, i.e. disproportionationoccurs.

Cu2O + 2H+ → Cu2+ + Cu + H2OWhen dissolved in concentrated

hydrochloric acid the [CuCl2]– com-plex ion is formed. Copper(I) oxide isused in the manufacture of rectiÜersand the production of red glass.

copper(II) oxide A black insolublesolid, CuO; monoclinic; r.d. 6.3; m.p.1326°C. It is obtained by heating ei-ther copper(II) carbonate or copper(II)nitrate. It decomposes on heatingabove 800°C to copper(I) oxide andoxygen. Copper(II) oxide reacts read-ily with mineral acids on warming,with the formation of copper(II) salts;

it is also readily reduced to copper onheating in a stream of hydrogen.Copper(II) oxide is soluble in diluteacids forming blue solutions ofcupric salts.

copper pyrites See chalcopyrite.

copper(II) sulphate A blue crys-talline solid, CuSO4.5H2O; triclinic;r.d. 2.284. The pentahydrate loses4H2O at 110°C and the Üfth H2O at150°C to form the white anhydrouscompound (rhombic; r.d. 3.6; decom-poses above 200°C). The pentahy-drate is prepared either by reactingcopper(II) oxide or copper(II) carbon-ate with dilute sulphuric acid; the so-lution is heated to saturation and theblue pentahydrate crystallizes out oncooling (a few drops of dilute sul-phuric acid are generally added toprevent hydrolysis). It is obtained onan industrial scale by forcing airthrough a hot mixture of copper anddilute sulphuric acid. In the pentahy-drate each copper(II) ion is sur-rounded by four water molecules atthe corner of a square, the Üfth andsixth octahedral positions are occu-pied by oxygen atoms from the sul-phate anions, and the Üfth watermolecule is held in place by hydro-gen bonding. Copper(II) sulphate hasmany industrial uses, including thepreparation of the Bordeaux mixture(a fungicide) and the preparation ofother copper compounds. It is alsoused in electroplating and textiledying and as a timber preservative.The anhydrous form is used in thedetection of traces of moisture.

Copper(II) sulphate pentahydrate isalso known as blue vitriol.

coprecipitation The removal of asubstance from solution by its associ-ation with a precipitate of someother substance. For example, if Aand B are present in solution and areagent is added such that A forms

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an insoluble precipitate, then B maybe carried down with the precipitateof A, even though it is soluble underthe conditions. This can occur by oc-clusion or absorption.

cordite An explosive mixture of cel-lulose nitrate and nitroglycerin, withadded plasticizers and stabilizers,used as a propellant for guns.

core orbital An atomic orbital thatis part of the inner closed shell of anatom. Unlike the valence orbitals,which are the atomic orbitals of thevalence shell of an atom, core or-bitals on one atom have a small over-lap with core orbitals on anotheratom. They are therefore usually ig-nored in simple calculations of chem-ical bonding.

Corey–Pauling rules A set ofrules, formulated by Robert Coreyand Linus *Pauling, that govern thesecondary nature of proteins. TheCorey–Pauling rules are concernedwith the stability of structures pro-vided by *hydrogen bonds associatedwith the –CO–NH– peptide link. TheCorey–Pauling rules state that:(1) All the atoms in the peptide linklie in the same plane.(2) The N, H, and O atoms in a hydro-gen bond are approximately on astraight line.(3) All the CO and NH groups are in-volved in bonding.Two important structures in whichthe Corey–Pauling rules are obeyedare the alpha helix and the *betasheet.

CORN rule See absolute configu-ration.

correlation diagram A diagramthat relates the energy levels of sepa-rate atoms to the energy levels of di-atomic molecules and united atoms,the two limiting states being corre-lated with each other. Using this dia-

gram, the types of molecular orbitalpossible for the molecules and theirorder as a function of increasing en-ergy can be seen as a function of in-teratomic distance. Lines are drawnlinking each united-atom orbital to aseparated atom orbital. An importantrule for determining which energylevels correlate is the noncrossingrule. This states that two curves ofenergy plotted against interatomicdistance never cross if the invariantproperties (e.g. parity, spin, etc.) arethe same.

correlation functions Quantitiesused in condensed-matter physicsthat are *ensemble averages of prod-ucts of such quantities as *density atdifferent points in space. Each parti-cle affects the behaviour of its neigh-bouring particles, so creatingcorrelations, the range of which is atleast as long as the range of the inter-molecular forces. Thus, correlationfunctions contain a great deal of in-formation about systems in con-densed-matter physics. Correlationfunctions can be measured in consid-erable detail, frequently by experi-ments involving scattering of suchparticles as neutrons or of *electro-magnetic radiation (e.g. light or X-rays) and can be calculatedtheoretically using *statistical me-chanics.

correlation spectroscopy (COSY)A type of spectroscopy used in *nu-clear magnetic resonance (NMR) inwhich the pulse sequence used is:90°x – t1 – 90°x – acquire (t2). Thedelay t1 is variable and a series of ac-quisitions is made. Fourier trans-forms are then performed on boththe delay t1 and the real time t2. Theinformation thus gained can beshown on a diagram that plots con-tours of signal intensity. This repre-sentation of the information enablesNMR spectra to be interpreted more

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readily than in one-dimensionalNMR, particularly for complex spec-tra.

corrosion Chemical or electro-chemical attack on the surface of ametal. See also electrolytic corro-sion; rusting.

corundum A mineral form of alu-minium oxide, Al2O3. It crystallizesin the trigonal system and occurs aswell-developed hexagonal crystals. Itis colourless and transparent whenpure but the presence of other el-ements gives rise to a variety ofcolours. *Ruby is a red variety con-taining chromium; *sapphire is ablue variety containing iron andtitanium. Corundum occurs as arock-forming mineral in both meta-morphic and igneous rocks. It ischemically resistant to weatheringprocesses and so also occurs in allu-vial (placer) deposits. The secondhardest mineral after diamond (it hasa hardness of 9 on the Mohs’ scale), itis used as an abrasive.

COSY See correlation spec-troscopy.

COT See cyclo-octatetraene.

Cotton effect The wavelength de-pendence of the optical rotary disper-sion curve or the *circular dichroismcurve in the neighbourhood of an ab-sorption band, both having charac-teristic shapes. If the wavelength isdecreased, the rotation angle in-creases until it reaches a maximumand then decreases, passing throughzero at the wavelength at which themaximum of absorption occurs. Asthe wavelength is decreased furtherthe angle becomes negative, until itreaches a minimum after which itrises again. This pattern is called thepositive Cotton effect. A mirrorimage of this pattern can also occurabout the λ-axis, where λ is the wave-

length; this is called the negativeCotton effect. These effects occur forcoloured substances and for colour-less substances with bands in the ul-traviolet. It is named after the Frenchphysicist Aimé Cotton (1859–1951).

coulomb Symbol C. The *SI unit ofelectric charge. It is equal to thecharge transferred by a current ofone ampere in one second. The unitis named after Charles de Coulomb.

Coulomb explosion The suddendisruption of a molecule from whichthe electrons have been stripped toleave only the nuclei, which repeleach other because of their electriccharge. The technique of Coulombexplosion imaging uses this effect toinvestigate the shape of molecules. Abeam of high-energy neutral mol-ecules is produced by Ürst addingelectrons, accelerating the ions in anelectric Üeld, and then removing theelectrons. The beam collides with athin metal foil having a thickness ofabout 30 atoms. As the moleculespass through this foil their electronsare scattered and only the nuclei ofthe molecules emerge. The processoccurs within a very short period oftime, shorter than the time requiredfor a complete molecular vibration,and consequently the nuclei retainthe molecular shape until they aresuddenly repulsed by the likecharges. The nuclei then impinge ona detector that records their velocityand direction, thus enabling the spa-tial arrangement of the original mol-ecule to be derived.

coumarin (1,2-benzopyrone) Apleasant-smelling colourless crys-talline compound, C9H6O2, m.p. 70°C.It occurs naturally in tonka (or ton-quin) beans, and is synthesized fromsalicylaldehyde. It forms coumarinicacid on hydrolysis with sodium hy-droxide. Coumarin is used in making

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perfumes, to scent tobacco, and as ananticoagulant in medicine; *warfarinis derived from it.

coumarone See benzfuran.

countercurrent Ûow Flow of twoÛuids in opposite directions withtransfer of heat or matter betweenthem. Compare cocurrent flow.

counter ion An ion of oppositecharge to a given ion. For example,in a crystal of sodium chloride, thechloride ions can be regarded ascounter ions to the sodium ions. Incertain colloids, the charge on thesurface of colloidal particles is neu-tralized by oppositely chargedcounter ions in the surrounding solu-tion.

coupling 1. An interaction be-tween two different parts of a systemor between two or more systems.Examples of coupling in the *spectraof atoms and nuclei are *Russell–Saunders coupling, *j-j coupling, andspin–orbit coupling. In the spectra ofmolecules there are Üve idealizedways (called the Hund coupling cases)in which the different types of angu-lar momentum in a molecule (theelectron orbital angular momentumL, the electron spin angular momen-tum S, and the angular momentumof nuclear rotation N) couple to forma resultant angular momentum J. (Inpractice, the coupling for many mol-ecules is intermediate betweenHund’s cases due to interactions,which are ignored in the idealized

cases.) 2. A type of chemical reactionin which two molecules join to-gether; for example, the formation ofan *azo compound by coupling of adiazonium ion with a benzene ring.

covalent bond See chemicalbond.

covalent carbide See carbide.

covalent crystal A crystal inwhich the atoms are held togetherby covalent bonds. Covalent crystalsare sometimes called macromolecularor giant-molecular crystals. They arehard high-melting substances. Exam-ples are diamond and boron nitride.

covalent radius An effective ra-dius assigned to an atom in a cova-lent compound. In the case of asimple diatomic molecule, the cova-lent radius is half the distance be-tween the nuclei. Thus, in Cl2 theinternuclear distance is 0.198 nm sothe covalent radius is taken to be0.099 nm. Covalent radii can also becalculated for multiple bonds; for in-stance, in the case of carbon the val-ues are 0.077 nm for single bonds,0.0665 nm for double bonds, and0.0605 nm for triple bonds. The val-ues of different covalent radii cansometimes be added to give internu-clear distances. For example, thelength of the bond in interhalogens(e.g. ClBr) is nearly equal to the sumof the covalent radii of the halogensinvolved. This, however, is not al-ways true because of other effects

147 covalent radius

ccoumarin coumarinic acid

CHCHCH

COCOCO

OOO

H

HHH

CO2HCO2HCO2HOHOHOH

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(e.g. ionic contributions to the bond-ing).

CPMG sequence (Carr–Purcell–Meiboom–Gill sequence) A sequenceof pulses used for spin-echo experi-ments in nuclear magnetic resonance(NMR) in which the initial pulse is90° followed by a series of 180°pulses. A CPMG sequence is designedso that the spin echoes die away ex-ponentially with time. Spin–spin re-laxation occurs characterized by atime constant T2, which can be deter-mined from the decay signal.

crack cocaine See cocaine.

cracking The process of breakingdown chemical compounds by heat.The term is applied particularly tothe cracking of hydrocarbons in thekerosine fraction obtained from *pe-troleum reÜning to give smaller hy-drocarbon molecules and alkenes. Itis an important process, both as asource of branched-chain hydrocar-bons suitable for gasoline (for motorfuel) and as a source of ethene andother alkenes. Catalytic cracking is asimilar process in which a catalyst isused to lower the temperature re-quired and to modify the productsobtained.

cream of tartar See potassium hy-drogentartrate.

creosote 1. (wood creosote) An al-most colourless liquid mixture ofphenols obtained by distilling tar ob-tained by the destructive distillationof wood. It is used medically as anantiseptic and expectorant. 2. (coal-tar creosote) A dark liquid mixture ofphenols and cresols obtained by dis-tilling coal tar. It is used for preserv-ing timber.

cresols See methylphenols.

CRG process (catalytic rich gasprocess) An industrial process forproducing fuel gas from naphtha

and other hydrocarbon sources. It in-volves a nickel-based catalyst, pres-sures of up to 70 bar, andtemperatures between 250°C and650°C depending on the feedstock.The reactions are:

CnH2n+2 + nH2O → nCO + (2n +1)H2

CO + 3H2 → CH4 + H2O

CO + H2O → CO2 + H2

The result is a mixture of methane,carbon monoxide, carbon dioxide,and trace amounts of ethane andother hydrocarbons. With partial car-bon dioxide removal it is possible toproduce town gas with mediumcaloriÜc value containing about 30%CH4, 30% H2, and 2% CO. The processcan be used to produce SNG. In thiscase there are multiple methanationstages and complete removal of CO2to give a product containing about98.5% CH4, 0.9% H2, and 0.1% CO.

cristobalite A mineral form of *sil-icon(IV) oxide, SiO2.

critical pressure The pressure of aÛuid in its *critical state; i.e. when itis at its critical temperature and criti-cal volume.

critical state The state of a Ûuid inwhich the liquid and gas phases bothhave the same density. The Ûuid isthen at its critical temperature, criti-cal pressure, and critical volume.

critical temperature 1. The tem-perature above which a gas cannotbe liqueÜed by an increase of pres-sure. See also critical state. 2. Seetransition point.

critical volume The volume of aÜxed mass of a Ûuid in its *criticalstate; i.e. when it is at its critical tem-perature and critical pressure. Thecritical speciÜc volume is its volumeper unit mass in this state: in thepast this has often been called thecritical volume.

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Crookes, Sir William (1832–1919)British chemist and physicist, who in1861 used *spectroscopy to discover*thallium and in 1875 invented theradiometer. He also developed an im-proved vacuum tube (Crookes’ tube)for studying gas discharges. Crookeswas also involved in industrial chem-istry and realised the importance ofnitrogen Üxation for fertilizers.

crossed-beam reaction A chemi-cal reaction in which two molecularbeams are crossed; one beam is re-garded as the incident beam of gasand the other as the target gas. Thistechnique enables a great deal of in-formation to be gained about thechemical reaction since the states ofboth the target and projectile mol-ecules can be controlled. The inci-dent beam is characterized by itsincident beam Ûux, I, which is thenumber of particles per unit area perunit time. The scattered moleculescan be detected Ürst by ionizingthem and then detecting the ionselectrically or using spectroscopy ifchanges in the vibrational or rota-tional states of molecules in a reac-tion are of interest.

cross linkage A short side chain of

atoms linking two longer chains in apolymeric material.

crown See ring conformations.

crown ethers Organic compoundswith molecules containing largerings of carbon and oxygen atoms.The crown ethers are macrocyclicpolyethers. The Ürst to be synthe-sized was the compound 18-crown-6,which consists of a ring of six–CH2–CH2–O– units (i.e. C12H24O6).The general method of namingcrown ethers is to use the form n-crown-m, where n is the number ofatoms in the ring and m is the num-ber of oxygen atoms. Substitutedcrown ethers can also be made. Thecrown ethers are able to formstrongly bound complexes withmetal ions by coordination throughthe oxygen atoms. The stability ofthese complexes depends on the sizeof the ion relative to the cavity avail-able in the ring of the particularcrown ether. Crown ethers also formcomplexes with ammonium ions(NH4+) and alkyl ammonium ions(RNH3+). They can be used for in-creasing the solubility of ionic saltsin nonpolar solvents. For example,dicyclohexyl-18-crown-6 complexeswith the potassium ion of potassium

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18-crown-6 dicyclohexyl-18-crown-6 complex

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permanganate and allows it to dis-solve in benzene, giving a purpleneutral solution that can oxidizemany organic compounds. They alsoact as catalysts in certain reactionsinvolving organic salts by complex-ing with the positive metal cationand thereby increasing its separationfrom the organic anion, which showsa consequent increase in activity.Some of the uses of crown ethers de-pend on their selectivity for speciÜcsizes of anions. Thus they can beused to extract speciÜc ions frommixtures and enrich isotope mix-tures. Their selectivity also makesthem useful analytical reagents. Seealso cryptands.

crucible A dish or other vessel inwhich substances can be heated to ahigh temperature. See also goochcrucible.

crude oil See petroleum.

Crum Brown’s rule A rule thatpredicts how substituents will enterthe benzene ring. If C6H5X is a com-pound with one substituent in thebenzene ring, it will produce the 1,3(meta) disubstituted derivative if thesubstance HX can be directly oxi-dized to HOX. If not, a mixture of 1,2(ortho) and 1,4 (para) compounds willbe formed. The rule was proposed in1892 by the British chemist Alexan-der Crum Brown (1838–1922).

cryogenic pump A *vacuumpump in which pressure is reducedby condensing gases on surfacesmaintained at about 20 K by meansof liquid hydrogen or at 4 K bymeans of liquid helium. Pressuresdown to 10–8 mmHg (10–6 Pa) can bemaintained; if they are used in con-junction with a *diffusion pump,pressures as low as 10–15 mmHg(10–13 Pa) can be reached.

cryohydrate A eutectic mixture of

ice and some other substance (e.g. anionic salt) obtained by freezing a so-lution.

cryolite A rare mineral form ofsodium aluminoÛuoride, Na3AlF6,which crystallizes in the monoclinicsystem. It is usually white but mayalso be colourless. The only impor-tant occurrence of the mineral is inGreenland. It is used chieÛy to lowerthe melting point of alumina in theproduction of aluminium.

cryoscopic constant See depres-sion of freezing point.

cryoscopy The use of *depressionof freezing point to determine rela-tive molecular masses.

cryostat A vessel enabling a sampleto be maintained at a very low tem-perature. The *Dewar Ûask is themost satisfactory vessel for control-ling heat leaking in by radiation, con-duction, or convection. Cryostatsusually consist of two or more DewarÛasks nesting in each other. For ex-ample, a liquid nitrogen bath is oftenused to cool a Dewar Ûask containinga liquid helium bath.

cryptands Compounds with largethree-dimensional molecular struc-tures containing ether chains linkedby three-coordinate nitrogen atoms.Thus cryptands are macropolycyclicpolyaza-polyethers. For example, thecompound (2,2,2)-cryptand has threechains of the form

–CH2CH2OCH2CH2OCH2CH2–.These chains are linked at each endby a nitrogen atom. Cryptands, likethe *crown ethers, can form coordi-nation complexes with ions that canÜt into the cavity formed by the openthree-dimensional structure, i.e. theycan ‘cryptate’ the ion. Various typesof cryptand have been produced hav-ing both spherical and cylindricalcavities. The cryptands have the

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same kind of properties as the crownethers and the same uses. In general,they form much more stronglybound complexes and can be used tostabilize unusual ionic species. Forexample, it is possible to produce thenegative Na– ion in the compound[(2,2,2)-cryptand-Na]+Na–, which is agold-coloured crystalline substancestable at room temperature. Clusterions, such as Pb5

2–, can be similarlystabilized.

crystal A solid with a regular poly-hedral shape. All crystals of the samesubstance grow so that they have thesame angles between their faces.However, they may not have thesame external appearance becausedifferent faces can grow at differentrates, depending on the conditions.The external form of the crystal is re-ferred to as the crystal habit. Theatoms, ions, or molecules formingthe crystal have a regular arrange-ment and this is the crystal structure.

crystal defect An imperfection inthe regular lattice of a crystal. SeeFeature (pp 152–153).

crystal-Üeld theory A theory ofthe electronic structures of inorganic*complexes, in which the complex isassumed to consist of a central metalatom or ion surrounded by ligandsthat are ions. For example, the com-plex [PtCl4]2– is thought of as a Pt2+

ion surrounded by four Cl– ions at

the corners of a square. The presenceof these ions affects the energies ofthe d-orbitals, causing a splitting ofenergy levels. The theory can be usedto explain the spectra of complexesand their magnetic properties. Lig-and-Üeld theory is a development ofcrystal-Üeld theory in which the over-lap of orbitals is taken into account.Crystal-Üeld theory was initiated in1929 by the German-born US physi-cist Hans Albrecht Bethe (1906–2005)and extensively developed in the1930s.

crystal habit See crystal.

crystal lattice The regular patternof atoms, ions, or molecules in a crys-talline substance. A crystal lattice canbe regarded as produced by repeatedtranslations of a unit cell of the lat-tice. See also crystal system.

A• An interactive version of the structures

of a range of crystals from a US Navywebsite

• Crystal models from the University ofCalgary

crystalline Having the regular in-ternal arrangement of atoms, ions, ormolecules characteristic of crystals.Crystalline materials need not neces-sarily exist as crystals; all metals, forexample, are crystalline althoughthey are not usually seen as regulargeometric crystals.

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O O

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CRYSTAL DEFECTS

A crystal *lattice is formed by a repeated arrangement of atoms, ions, ormolecules. Within one cubic centimetre of material one can expect to find upto 1022 atoms and it is extremely unlikely that all of these will be arranged inperfect order. Some atoms will not be exactly in the right place with the resultthat the lattice will contain *defects. The presence of defects within the crystalstructure has profound consequences for certain bulk properties of the solid,such as the electrical resistance and the mechanical strength.

Point defectsLocal crystal defects called point defects, appear as either impurity atoms orgaps in the lattice. Impurity atoms can occur in the lattice either at interstitialsites (between atoms in a non-lattice site) or at substitutional sites (replacing anatom in the host lattice). Lattice gaps are called vacancies and arise when anatom is missing from its site in the lattice. Vacancies are sometimes calledSchottky defects. A vacancy in which the missing atom has moved to aninterstitial position is known as a Frenkel defect.

Colour centresIn ionic crystals, the ions and vacancies always arrange themselves so thatthere is no build-up of one type of charge in any small volume of the crystal. Ifions or charges are introduced into or removed from the lattice, there will, ingeneral, be an accompanying rearrangement of the ions and their outervalence electrons. This rearrangement is called charge compensation and ismost dramatically observed in colour centres. If certain crystals are irradiatedwith X-rays, gamma rays, neutrons, or electrons a colour change is observed.For example, diamond may be coloured blue by electron bombardment andquartz may be coloured brown by irradiation with neutrons. The high-energyradiation produces defects in the lattice and, in an attempt to maintain chargeneutrality, the crystal undergoes some measure of charge compensation. Justas electrons around an atom have a series of discrete permitted energy levels,so charges residing at point defects exhibit sets of discrete levels, which areseparated from one another by energies corresponding to wavelengths in thevisible region of the spectrum. Thus light of certain wavelengths can beabsorbed at the defect sites, and the material appears to be coloured. Heatingthe irradiated crystal can, in many cases, repair the irradiation damage and thecrystal loses its coloration.

DislocationsNon-local defects may involve entire planes of atoms. The most important of

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these is called a dislocation. Dislocations are essentially line-defects; that is,there is an incomplete plane of atoms in the crystal lattice. In 1934, Taylor,Orowan, and Polanyi independently proposed the concept of the dislocation toaccount for the mechanical strength of metal crystals. Their microscopicstudies revealed that when a metal crystal is plastically deformed, thedeformation does not occur by a separation of individual atoms but rather by aslip of one plane of atoms over another plane. Dislocations provide amechanism for this slipping of planes that does not require the bulkmovement of crystal material. The passage of a dislocation in a crystal issimilar to the movement of a ruck in a carpet. A relatively large force isrequired to slide the carpet as a whole. However, moving a ruck over thecarpet can inch it forward without needing such large forces. This movementof dislocations is called plastic flow.

Strength of materialsIn practice most metal samples are polycrystalline; that is they consist of manysmall crystals or grains at different angles to each other. The boundarybetween two such grains is called a grain boundary. The plastic flow ofdislocations may be hindered by the presence of grain boundaries, impurityatoms, and other dislocations. Pure metals produced commercially aregenerally too weak to be of much mechanical use. The weakness of thesesamples can be attributed to the ease with which the dislocations are able tomove within the sample. Slip, and therefore deformation, can then occurunder relatively low stresses. Impurity atoms, other dislocations, and grainboundaries can all act as obstructions to the slip of atomic planes.Traditionally, methods of making metals stronger involved introducing defectsthat provide regions of disorder in the material. For example, in an alloy, suchas steel, impurity atoms (e.g. carbon) are introduced into the lattice during theforging process. The perfection of the iron lattice structure is disturbed and theimpurities oppose the dislocation motion. This makes for greater strength andstiffness.

The complete elimination of dislocations may seem an obvious way tostrengthen materials. However, this has only proved possible for hair-likesingle crystal specimens called whiskers. These whiskers are only a fewmicrometers thick and are seldom more than a few millimetres long;nevertheless their strength approaches the theoretical value.

H A

B DG

CF

A E

G

F

B D

C

Dislocation in a two-dimensional crystal. The extra plane of atoms AB causes strain atbond CD. On breaking, the bond flips across to form CB. This incremental movement shifts thedislocation across so that the overall effect is to slide the two planes BDG and CF over eachother.


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crystallite A small crystal, e.g. oneof the small crystals forming part ofa microcrystalline substance.

crystallization The process offorming crystals from a liquid or gas.

crystallography The study of crys-tal form and structure. See also x-raycrystallography.

crystalloids See colloids.

crystal meth See amphetamines.

crystal structure See crystal.

crystal system A method of classi-fying crystalline substances on thebasis of their unit cell. There areseven crystal systems. If the cell is aparallelopiped with sides a, b, and cand if α is the angle between b and c,β the angle between a and c, and γthe angle between a and b, the sys-tems are:(1) cubic a=b=c and α=β=γ=90°(2) tetragonal a=b≠c and α=β=γ=90°(3) rhombic (or orthorhombic) a≠b≠cand α=β=γ=90°(4) hexagonal a=b≠c and α=β=γ=90°(5) trigonal a=b≠c and α=β=γ≠90°(6) monoclinic a≠b≠c and α=γ=90°≠β(7) triclinic a=b=c and α≠β≠γcrystal test A type of *presump-tive test in which a substance isidentiÜed by the formation of charac-teristic crystals when a certainreagent is added. Usually, such testsare conducted using a microscope(microcrystal test). An example is the*acetone–chlor–haemin test forblood.

CS gas The vapour from a whitesolid, C6H4(Cl)CH:C(CN)2, causingtears and choking, used in ‘crowdcontrol’.

cubane A crystalline hydrocarbon,C8H8; r.d. 1.29; m.p. 131°C. It has anovel structure with eight carbonatoms at the corners if a cube, eachattached to a hydrogen. Cubane was

crystallite 154

c

H H

HH

H H

H H

C

C C

C

C

C

C

C

Cubane

Ürst synthesized in 1964 by PhilipEaton. The C–C–C bond angle of 90°is highly strained and cubane and itsderivatives have been investigated ashigh-energy fuels and explosives. Inparticular, octanitrocubane, in whichthe hydrogen atoms are replaced by–NO2 groups is possibly the mostpowerful chemical explosive known,although, so far, only small amountshave been synthesized. It decom-poses to carbon dioxide and nitro-gen:

C8 (NO2)8 → 8CO2 + 4N2

cubic close packing See closepacking.

cubic crystal A crystal in whichthe unit cell is a cube (see crystalsystem). There are three possiblepackings for cubic crystals: simplecubic, face-centred cubic, and body-centred cubic. See illustration oppo-site.A• An interactive version of a simple cubic

crystal• An interactive version of a body-centred

cubic crystal• An interactive version of a face-centred

cubic crystal

cubic zircona (CZ) A crystal formor zircon(IV) oxide (zircon dioxide,


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ZrO2) made by fusing ZrO2 and allow-ing it to cool under controlled condi-tions. It is used as an inexpensivediamond substitute in jewellery.Often it is erroneously called *zircon.

cumene process An industrialprocess for making phenol from ben-zene. A mixture of benzene vapourand propene is passed over a phos-phoric acid catalyst at 250°C andhigh pressure

C6H6 + CH3CH:CH2 →C6H5CH(CH3)2

The product is called cumene, and itcan be oxidized in air to a peroxide,C6H5C(CH3)2O2H. This reacts with di-lute acid to give phenol (C6H5OH) andpropanone (acetone, CH3OCH3),which is a valuable by-product.

cupellation A method of separat-ing noble metals (e.g. gold or silver)from base metals (e.g. lead) by melt-ing the mixture with a blast of hotair in a shallow porous dish (thecupel). The base metals are oxidized,the oxide being carried away by theblast of air or absorbed by the porouscontainer.

cuprammonium ion The tetraam-minecopper(II) ion [Cu(NH3)4]2+. Seeammine.

cupric compounds Compoundscontaining copper in its higher (+2)

oxidation state; e.g. cupric chloride iscopper(II) chloride (CuCl2).

cuprite A red mineral cubic form ofcopper(I) oxide, Cu2O; an importantore of copper. It occurs where de-posits of copper have been subjectedto oxidation. The mineral has beenmined as a copper ore in Chile,Democratic Republic of Congo, Bo-livia, Australia, Russia, and the USA.

cupronickel A type of corrosion-re-sistant alloy of copper and nickelcontaining up to 45% nickel.

cuprous compounds Compoundscontaining copper in its lower (+1)oxidation state; e.g. cuprous chlorideis copper(I) chloride (CuCl).

curare A resin obtained from thebark of South American trees of thegenera Strychnos and Chondrodendronthat causes paralysis of voluntarymuscle. It acts by blocking the actionof the neurotransmitter *acetyl-choline at neuromuscular junctions.Curare is used as an arrow poison bySouth American Indians and was for-merly used as a muscle relaxant insurgery.

curie The former unit of *activity(see radiation units). It is namedafter Marie *Curie.

Curie, Marie (Marya Sklodowska;1867–1934) Polish-born Frenchchemist, who went to Paris in 1891.

155 Curie, Marie

c

body-centred simple cubic face-centred

Cubic crystal


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She married the physicist Pierre Curie(1859–1906) in 1895 and soon beganwork on seeking radioactive el-ements other than uranium in pitchblende (to account for its un-expectedly high radioactivity). By1898 she had discovered *radiumand *polonium, although it took herfour years to purify them. In 1903the Curies shared the Nobel Prize forphysics with Henri *Becquerel, whohad discovered radioactivity. In 1911Marie Curie was awarded the NobelPrize for chemistry.

Curie point (Curie temperature)The temperature at which a ferro-magnetic substance loses its ferro-magnetism and becomes onlyparamagnetic. For iron the Curiepoint is 760°C and for nickel 356°C.It is named after Pierre *Curie.

Curie’s law The susceptibility (χ) ofa paramagnetic substance is propor-tional to the thermodynamic temper-ature (T), i.e. χ = C/T, where C is theCurie constant. A modiÜcation of thislaw, the Curie–Weiss law, is moregenerally applicable. It states that χ = C/(T – θ), where θ is the Weissconstant, a characteristic of the ma-terial. The law was Ürst proposed byPierre *Curie and modiÜed by an-other French physicist, Pierre-ErnestWeiss (1865–1940).

curium Symbol Cm. A radioactivemetallic transuranic element belong-ing to the *actinoids; a.n. 96; massnumber of the most stable isotope247 (half-life 1.64 × 107 years); r.d.(calculated) 13.51; m.p. 1340±40°C.There are nine known isotopes. Theelement was Ürst identiÜed by GlennSeaborg (1912–99) and associates in1944 and Ürst produced by L. B.Werner and I. Perlman in 1947 bybombarding americium–241 withneutrons.

Curie point 156

c


cutting agent (adulterant) A sub-stance used to dilute illegal drugssuch as heroine and cocaine. Exam-ples include Ûour, starch, sugar, andcaffeine.

cyanamide 1. An inorganic saltcontaining the ion CN2

2–. See calciumcyanamide. 2. A colourless crys-talline solid, H2NCN, made by the ac-tion of carbon dioxide on hotsodamide. It is a weakly acidic com-pound (the parent acid of cyanamidesalts) that is soluble in water andethanol. It is hydrolysed to urea inacidic solutions.

cyanamide process See calciumcyanamide.

cyanate See cyanic acid.

cyanic acid An unstable explosiveacid, HOCN. The compound has thestructure H–O–C≡N, and is also calledfulminic acid. Its salts and esters arecyanates (or fulminates). The com-pound is a volatile liquid, whichreadily polymerizes. In water it hy-drolyses to ammonia and carbondioxide. It is isomeric with anotheracid, H–N=C=O, which is known asisocyanic acid. Its salts and esters areisocyanates.

cyanide 1. An inorganic salt con-taining the cyanide ion CN–. Cya-nides are extremely poisonousbecause of the ability of the CN– ionto coordinate with the iron inhaemoglobin, thereby blocking theuptake of oxygen by the blood. 2. Ametal coordination complex formedwith cyanide ions.

cyanide process A method of ex-tracting gold by dissolving it in potas-sium cyanide (to form the complexion [Au(CN)2]–). The ion can be re-duced back to gold with zinc.


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cyanine dyes A class of dyes thatcontain a –CH= group linking two ni-trogen-containing heterocyclic rings.They are used as sensitizers in pho-tography.

cyanoacrylate A compoundformed by the condensation of analkyl cyanoethanoate and methanal.Cyanoacrylates have the general for-mula CH2:C(CN)COOR. The methyland ethyl esters are the basis of vari-ous ‘superglues’, which rapidly poly-merize in air (under the action ofmoisture) to form strong adhesives.

cyanocobalamin See vitamin bcomplex.

cyanogen A colourless gas, (CN)2,with a pungent odour; soluble inwater, ethanol, and ether; d. 2.335g dm–3; m.p. –27.9°C; b.p. –20.7°C.The compound is very toxic. It maybe prepared in the laboratory byheating mercury(II) cyanide; industri-ally it is made by gas-phase oxidationof hydrogen cyanide using air over asilver catalyst, chlorine over acti-vated silicon(IV) oxide, or nitrogendioxide over a copper(II) salt.Cyanogen is an important intermedi-ate in the preparation of various fer-tilizers and is also used as a stabilizerin making nitrocellulose. It is an ex-ample of a *pseudohalogen.

cyano group The group –CN in achemical compound. See nitriles.

cyanohydrins Organic compounds

formed by the addition of hydrogencyanide to aldehydes or ketones (inthe presence of a base). The Ürst stepis attack by a CN– ion on the car-bonyl carbon atom. The Ünal productis a compound in which a –CN and–OH group are attached to the samecarbon atom. For example, ethanalreacts as follows

CH3CHO + HCN → CH3CH(OH)(CN)The product is 2-hydroxypropanoni-trile. Cyanohydrins of this type canbe oxidized to α-hydroxy carboxylicacids.

cyanuric acid A white crystallinewater-soluble trimer of cyanic acid,(HNCO)3. It is a cyclic compound hav-ing a six-membered ring made of al-ternating imide (NH) and carbonyl(CO) groups (i.e. three –NH–C(O)–units). It can also exist in a phenolicform (three –N=C(OH)– units).

cyclamates Salts of the acid,C6H11.NH.SO3H, where C6H11– is a cy-clohexyl group. Sodium and calciumcyclamates were formerly used assweetening agents in soft drinks, etc,until their use was banned whenthey were suspected of causing can-cer.

cyclic Describing a compound thathas a ring of atoms in its molecules.In homocyclic compounds all theatoms in the ring are the same type,e.g. benzene (C6H6) and cyclohexane(C6H12). These two examples are alsoexamples of carbocyclic compounds;

157 cyclic

cN N

N

H

H

OO

O

HN N

N OHOH

OH

Cyanuric acid


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i.e. the rings are of carbon atoms. Ifdifferent atoms occur in the ring, asin pyridine (C5H5N), the compound issaid to be heterocyclic.

cyclic AMP A derivative of *ATPthat is widespread in animal cells asa second messenger in many bio-chemical reactions induced by hor-mones. Upon reaching their targetcells, the hormones activate adeny-late cyclase, the enzyme that cataly-ses cyclic AMP production. CyclicAMP ultimately activates the en-zymes of the reaction induced by thehormone concerned. Cyclic AMP isalso involved in controlling gene ex-pression and cell division, in im-mune responses, and in nervoustransmission.

cyclization The formation of acyclic compound from an open-chaincompound. See ring.

cyclo- PreÜx designating a cycliccompound, e.g. a cycloalkane or acyclosilicate.

cycloaddition A reaction in whichtwo or more unsaturated compoundsform a cyclic adduct or in which acyclic compound is formed by addi-tion between unsaturated parts ofthe same molecule. In cycloaddition,there is no net reduction in bondmultiplicity. The *Diels–Alder reac-tion is an example. Cycloadditionsmay be stepwise reactions or may be*pericyclic reactions.

cycloalkanes Cyclic saturated hy-drocarbons containing a ring of car-bon atoms joined by single bonds.They have the general formula CnH2n, for example cyclohexane,C6H12, etc. In general they behavelike the *alkanes but are rather lessreactive.A• Information about IUPAC nomenclature

cyclobutadiene A short-lived

cyclic diene hydrocarbon, C4H4, withits four carbon atoms in a squarering. It is made by degradation of itsmetal complexes, and has a shortlifetime of a few seconds. It under-goes addition reactions with alkynes.

cyclohexadiene-1,4-dione(benzoquinone; quinone) A yellowsolid, C6H4O2; r.d. 1.3; m.p. 116°C. Ithas a six-membered ring of carbonatoms with two opposite carbonatoms linked to oxygen atoms (C=O)and the other two pairs of carbonatoms linked by double bonds(HC=CH). The compound is used inmaking dyes. See also quinhydroneelectrode.

cyclohexane A colourless liquid*cycloalkane, C6H12; r.d. 0.78; m.p.6.5°C; b.p. 81°C. It occurs in petro-leum and is made by passing ben-zene and hydrogen under pressureover a heated Raney nickel catalyst at150°C, or by the reduction of cyclo-hexanone. It is used as a solvent andpaint remover and can be oxidizedusing hot concentrated nitric acid tohexanedioic acid (adipic acid). The cy-clohexane ring is not planar and canadopt boat and chair *conforma-tions; in formulae it is representedby a single hexagon.

cyclonite A highly explosive nitrocompound, (CH2N.NO2)3. It has acyclic structure with a six-memberedring of alternating CH2 groups andnitrogen atoms, with each nitrogenbeing attached to a NO2 group. It ismade by nitrating hexamine,

cyclic AMP 158

c

CH2

NCH2

N

CH2

N

NO2

NO2

O2N

Cyclonite


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C6H12N4, which is obtained from am-monia and methanal. Cyclonite is avery powerful explosive used mainlyfor military purposes. It is also calledRDX. The abbreviation is for ‘Re-search Department composition X’,used at the Chemical Research andDevelopment Department, Wool-wich.

cyclo-octatetraene (COT) A yellowliquid cyclic compound, C8H8, witheight carbon atoms and four doublebonds in its ring; b.p. 142°C. It ismade by polymerizing ethyne (acety-lene) in the presence of nickel salts.It forms addition compounds and cer-tain metal complexes, such as ura-nocene, (C8H8)2U.

cyclopentadiene A colourless liq-uid cyclic *alkene, C5H6; r.d. 0.8021;m.p. –97.2°C; b.p. 40.0°C. It is pre-pared as a by-product during the frac-tional distillation of crude benzenefrom coal tar. It undergoes condensa-tion reactions with ketones to givehighly coloured compounds (ful-venes) and readily undergoes poly-merization at room temperature togive the dimer, dicyclopentadiene.Cyclopentadiene itself is not an aro-matic compound because it does nothave the required number of pi elec-trons. However, removal of a hydro-gen atom produces the stablecyclopentadienyl ion, C5H5

–, whichdoes have aromatic properties. Inparticular, the ring can coordinate topositive ions in such compounds as*ferrocene.

159 cystine

c

CH2

CH

CH

CH

CH

Cyclopentadiene

Cyclophane

cyclopentadienyl ion See cy-clopentadiene.

cyclophane A compound consist-ing of one or more aromatic ringsforming part of a larger ring systemin which aliphatic chains of the CH2groups link the aromatic rings. Com-pounds of this type have the sufÜxphane in their names. Depending onthe sizes of the (CH2)n chains, the aro-matic rings may not be planar.


cyclopropane A colourless gas,C3H6, b.p. –34.5°C, whose moleculescontain a triangular ring of carbonatoms. It is made by treating 1,3-di-bromopropane with zinc metal, andis used as a general anaesthetic.

cyclosarin A highly toxic colourlessliquid, C7H14FO2P; r.d. 1.13; m.p.–30°C; b.p. 239°C. it is a Ûuorinatedorganophosphorus compound,(Ûuoromethylphosphoryl)oxycyclo-hexane. Cyclosarin was discovered in1949 and belongs to the G-series of*nerve agents (GF).

cyclotetramethylenetetran-itramine See hmx.

cylinder gas See bottled gas.

cysteine See amino acid.

cystine A molecule resulting fromthe oxidation reaction between thesulphydryl (–SH) groups of two cys-


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teine molecules (see amino acid).This often occurs between adjacentcysteine residues in polypeptides.The resultant disulphide bridges(–S–S–) are important in stabilizingthe structure of protein molecules.

cytidine A nucleoside comprisingone cytosine molecule linked to a d-ribose sugar molecule. The derivednucleotides, cytidine mono-, di-, andtriphosphate (CMP, CDP, and CTP re-spectively) participate in various bio-chemical reactions, notably inphospholipid synthesis.

cytochrome Any of a group ofproteins, each with an iron-contain-ing *haem group, that form part ofthe *electron transport chain in mi-tochondria and chloroplasts. Elec-trons are transferred by reversiblechanges in the iron atom betweenthe reduced Fe(II) and oxidized Fe(III)states.

cytosine A *pyrimidine derivative.It is one of the principal componentbases of *nucleotides and the nucleicacids *DNA and *RNA.

cytidine 160

c

Cytidine

Cytosine

CZ See cubic zircona.

CZE Capillary zone electro-phoresis. See capillary elec-trophoresis.


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D

2,4-D 2,4-dichlorophenoxyaceticacid (2,4-dichlorophenoxyethanoicacid): a synthetic auxin frequentlyused as a weedkiller of broad-leavedweeds. See also pesticide.

Dakin reaction See baeyer–villiger reaction.

dalton See atomic mass unit.

Dalton, John (1766–1844) Britishchemist and physicist. In 1801 he for-mulated his law of partial pressures(see dalton’s law), but he is best re-membered for *Dalton’s atomictheory, which he announced in 1803. Dalton also studied colourblindness (a condition, once calledDaltonism, that he shared with hisbrother).

Dalton’s atomic theory A theoryof *chemical combination, Ürststated by John *Dalton in 1803. It in-volves the following postulates:(1) Elements consist of indivisiblesmall particles (atoms).(2) All atoms of the same element areidentical; different elements have dif-ferent types of atom.(3) Atoms can neither be created nordestroyed.(4) ‘Compound elements’ (i.e. com-pounds) are formed when atoms ofdifferent elements join in simple ra-tios to form ‘compound atoms’ (i.e.molecules).

Dalton also proposed symbols foratoms of different elements (later re-placed by the present notation usingletters).

Dalton’s law The total pressure ofa mixture of gases or vapours isequal to the sum of the partial pres-

sures of its components, i.e. the sumof the pressures that each compo-nent would exert if it were presentalone and occupied the same volumeas the mixture of gases. Strictlyspeaking, the principle is true onlyfor ideal gases. The law was discov-ered by John Dalton.

Daniell cell A type of primary*voltaic cell with a copper positiveelectrode and a negative electrode ofa zinc amalgam. The zinc-amalgamelectrode is placed in an electrolyteof dilute sulphuric acid or zinc sul-phate solution in a porous pot, which stands in a solution of coppersulphate in which the copper elec-trode is immersed. While the reac-tion takes place ions move throughthe porous pot, but when it is not inuse the cell should be dismantled toprevent the diffusion of one elec-trolyte into the other. The e.m.f. ofthe cell is 1.08 volts with sulphuricacid and 1.10 volts with zinc sul-phate. It was invented in 1836 by theBritish chemist John Daniell (1790–1845).

dark reaction See photosynthesis.

darmstadtium Symbol Ds. A radio-active transactinide; a.n. 110. It hasseveral isotopes; the most stablebeing 281Ds, with a half-life of about1.6 minutes. It can be produced bybombarding a plutonium target withsulphur nuclei or by bombarding alead target with nickel nuclei. Itschemical properties probably resem-ble those of platinum. Darmstadtiumwas named after the German city ofDarmstadt, the location of the Insti-


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tute for Heavy Ion Research where itwas Ürst produced.A• Information from the WebElements site

date-rape drugs Drugs that areused to render the victim unable toresist a sexual assault. Particulardrugs used for this purpose are Ûuni-trazepam and gammahydroxybutyricacid (GHB).

dating techniques Methods of es-timating the age of rocks, palaeonto-logical specimens, archaeologicalsites, etc. Relative dating techniquesdate specimens in relation to one an-other; for example, stratigraphy isused to establish the succession offossils. Absolute (or chronometric)techniques give an absolute estimateof the age and fall into two maingroups. The Ürst depends on the exis-tence of something that develops at aseasonally varying rate, as in den-drochronology and varve dating. Theother uses some measurable changethat occurs at a known rate, as in*chemical dating, radioactive (or ra-diometric) dating (see carbon dating;fission-track dating; potassium–argon dating; rubidium–strontiumdating; uranium–lead dating),*amino acid racemization, and *ther-moluminescence.

dative bond See chemical bond.

daughter 1. A nuclide produced byradioactive *decay of some other nu-clide (the parent). 2. An ion or freeradical produced by dissociation orreaction of a parent ion or radical.

Davisson–Germer experimentSee electron diffraction.

Davy, Sir Humphry (1778–1829)British chemist, who studied gases atthe Pneumatic Institute in Bristol,where he discovered the anaestheticproperties of *dinitrogen oxide (ni-trous oxide). He moved to the Royal

Institution, London, in 1801 and Üveyears later isolated potassium andsodium by electrolysis. He also pre-pared barium, boron, calcium, andstrontium as well as proving thatchlorine and iodine are elements. In1816 he invented the *Davy lamp.

Davy lamp An oil-burning miner’ssafety lamp invented by Sir HumphryDavy in 1816 when investigatingÜredamp (methane) explosions incoal mines. The lamp has a metalgauze surrounding the Ûame, whichcools the hot gases by conductionand prevents ignition of gas outsidethe gauze. If Üredamp is present itburns within the gauze cage, andlamps of this type are still used fortesting for gas.

d-block elements The block of el-ements in the *periodic table consist-ing of scandium, yttrium, andlanthanum together with the threeperiods of transition elements: tita-nium to zinc, zirconium to cadmium,and hafnium to mercury. These el-ements all have two outer s-electronsand have d-electrons in their penulti-mate shell; i.e. an outer electronconÜguration of the form (n–1)dxns2,where x is 1 to 10. See also transitionelements.

DDT Dichlorodiphenyltrichloro-ethane; a colourless organic crys-talline compound, (ClC6H4)2CH(CCl3),made by the reaction of trichloro-methanal with chlorobenzene. DDTis the best known of a number ofchlorine-containing *pesticides usedextensively in agriculture in the1940s and 1950s. The compound isstable, accumulates in the soil, andconcentrates in fatty tissue, reachingdangerous levels in carnivores highin the food chain. Restrictions arenow placed on the use of DDT andsimilar pesticides.

deacetylation The removal of an

date-rape drugs 162

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acetyl group (–COCH3) from a mol-ecule. Deacetylation is an importantreaction in several biochemical path-ways, including the *Krebs cycle.

Deacon process A former processfor making chlorine by oxidizing hy-drogen chloride in air at 450°C usinga copper chloride catalyst. It waspatented in 1870 by Henry Deacon(1822–76).

deactivation A partial or completereduction in the reactivity of a sub-stance, as in the poisoning of a cata-lyst.

deamination The removal of anamino group (–NH2) from a com-pound. Enzymatic deamination oc-curs in the liver and is important inamino-acid metabolism, especially intheir degradation and subsequent ox-idation. The amino group is removedas ammonia and excreted, either un-changed or as urea or uric acid.

de Broglie, Louis-Victor PierreRaymond (1892–1987) Frenchphysicist, who taught at the Sor-bonne in Paris for 34 years. He is bestknown for his 1924 theory ofwave–particle duality (see de brogliewavelength), which reconciled thecorpuscular and wave theories oflight and proved important in quan-tum theory. For this work he wasawarded the 1929 Nobel Prize.

de Broglie wavelength Thewavelength of the wave associatedwith a moving particle. The wave-length (λ) is given by λ = h/mv, whereh is the Planck constant, m is themass of the particle, and v its veloc-ity. The de Broglie wave was Ürstsuggested by Louis de Broglie in 1924on the grounds that electromagneticwaves can be treated as particles(photons) and one could thereforeexpect particles to behave in somecircumstances like waves. The subse-quent observation of *electron dif-

fraction substantiated this argumentand the de Broglie wave became thebasis of *wave mechanics.

debye A unit of electric dipole mo-ment in the electrostatic system,used to express dipole moments ofmolecules. It is the dipole momentproduced by two charges of oppositesign, each of 1 statcoulomb andplaced 10–18 cm apart, and has thevalue 3.335 64 × 10–30 coulombmetre. It is named after Peter Debye(1884–1966).

Debye, Peter Joseph William(1884–1966) Dutch-born physicalchemist who worked on a number oftopics. He introduced the idea of elec-tric dipole moments in moleculesand, in 1923, working with ErichHückel, he published the *Debye-Hückel theory of electrolytes. Debyewas awarded the 1936 Nobel Prizefor chemistry.

Debye–Hückel–Onsager theoryA theory providing quantitative re-sults for the conductivity of ions indilute solutions of strong elec-trolytes, which enables the*Kohlrausch equation to be derived.This theory can be stated as: K = A +BΛ0

m, where Λ0m is the limiting molar

conductivity.A = z2eF2/3πη(2/εRT)½,

B = qz3eF/24πεRT(2/εRT)½,

where z is the charge of an ion, e isthe charge of an electron, F is Fara-day’s constant, η is the viscosity ofthe liquid, R is the gas constant, T isthe thermodynamic temperature,and q = 0.586 in the case of a 1,1 elec-trolyte. The Debye–Hückel–Onsagertheory uses the same assumptionsand approximations as the*Debye–Hückel theory and is alsolimited to very dilute solutions (usu-ally less than 10–3 M) for which thereis good agreement between theoryand experiment. The modiÜcations

163 Debye–Hückel–Onsager theory

d


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were made by the Norwegian-bornUS chemist Lars Onsager (1903–76).

Debye–Hückel theory A theory toexplain the nonideal behaviour ofelectrolytes, published in 1923 byPeter *Debye and Erich Hückel(1896–1980). It assumes that elec-trolytes in solution are fully dissoci-ated and that nonideal behaviourarises because of electrostatic interac-tions between the ions. The theoryshows how to calculate the extra freeenergy per ion resulting from suchinteractions, and consequently theactivity coefÜcient. It gives a good de-scription of nonideal electrolyte be-haviour for very dilute solutions, butcannot be used for more concen-trated electrolytes.

Debye–Scherrer method A tech-nique used in *X-ray diffraction inwhich a crystal in powder form isÜxed to a thin Übre or thin silicatube, which is then rotated in thepath of a beam of monochromatic X-rays. A circular diffraction ring,called the Debye–Scherrer ring, con-centric with the undeÛected beam is formed. The diffraction pattern isrecorded on a cylindrical Ülm, whichhas its axis parallel to the axis of ro-tation of the material. The Debye–Scherrer method is used to obtain in-formation about the material. Thegrains of the powdered crystal mustbe much larger than the atomic di-mensions in order for them to dif-fract X-rays.

Debye temperature See debyetheory of specific heat.

Debye theory of speciÜc heat Atheory of the speciÜc heat capacity ofsolids put forward by Peter *Debye in1912, in which it was assumed thatthe speciÜc heat is a consequence ofthe vibrations of the atoms of the lat-tice of the solid. In contrast to the*Einstein theory of speciÜc heat,

which assumes that each atom hasthe same vibrational frequency,Debye postulated that there is a con-tinuous range of frequencies thatcuts off at a maximum frequency νD,which is characteristic of a particularsolid. The theory leads to the conclu-sion that the speciÜc heat capacity ofsolids is proportional to T3, where Tis the thermodynamic temperature.This result is in very good agreementwith experiment at low tempera-tures.

A key quantity in this theory is theDebye temperature, θD, deÜned by θD =hνDk, where h is the Planck constantand k is the Boltzmann constant. TheDebye temperature is characteristicof a particular solid. For example, theDebye temperature of sodium is 150K and the Debye temperature of cop-per is 315 K.

Debye–Waller factor A quantitythat characterizes the effect of latticevibrations on the scattering intensityin X-ray diffraction by crystals. Theexistence of the Debye–Waller factorwas pointed out and calculated byPeter *Debye in 1913–1914 and IvarWaller in 1923–1925. Because theamplitude of lattice vibrations in-creases with temperature, it wasthought in the very early days of X-ray diffraction studies that the dif-fraction pattern would disappear athigh temperatures. The work ofDebye and Waller showed that thelattice vibrations at higher tempera-tures reduce the intensities of the dif-fracted radiation but do not destroythe diffraction pattern altogether.

deca- Symbol da. A preÜx used inthe metric system to denote tentimes. For example, 10 coulombs = 1decacoulomb (daC).

decahydrate A crystalline hydratecontaining ten moles of water permole of compound.

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decalin (decahydronaphthalene) Aliquid bicyclic hydrocarbon, C10H18,used as a solvent. There are twostereoisomers, cis (b.p. 198°C) andtrans (b.p. 185°C), made by the cat-alytic hydrogenation of naphthaleneat high temperatures and pressures.

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decanedioic acid (sebacic acid) Awhite crystalline dicarboxylic acid,HOOC(CH2)8COOH; r.d. 1.12; m.p.131–134.5°C; b.p. 294.4°C (100mmHG). Obtained from castor oil, itis used in plasticizers, lubricants, andcosmetics and in the production ofother organic chemicals.

decanoic acid (capric acid) A whitecrystalline straight-chain saturated*carboxylic acid, CH3(CH2)8COOH;m.p. 31.5°C. Its esters are used in per-fumes and Ûavourings.

decantation The process of sepa-rating a liquid from a settled solidsuspension or from a heavier immis-cible liquid by carefully pouring itinto a different container.

decarboxylation The removal ofcarbon dioxide from a molecule. De-carboxylation is an important reac-tion in many biochemical processes,such as the *Krebs cycle and the syn-thesis of fatty acids.

decay 1. The spontaneous transfor-mation of one radioactive nuclideinto a daughter nuclide, which maybe radioactive or may not, with theemission of one or more particles orphotons. The decay of N0 nuclides togive N nuclides after time t is givenby N = N0exp(–γt), where γ is called

the decay constant or the disintegra-tion constant. The reciprocal of thedecay constant is the mean life. Thetime required for half the originalnuclides to decay (i.e. N = ½N0) iscalled the half-life of the nuclide. Thesame terms are applied to elemen-tary particles that spontaneouslytransform into other particles. Forexample, a free neutron decays into aproton and an electron. 2. The rever-sion of excited states of atoms ormolecules to the ground state.

deci- Symbol d. A preÜx used in themetric system to denote one tenth.For example, 0.1 coulomb = 1 deci-coulomb (dC); 0.1 metre = 1 decime-tre (dm).

decoction A solution made by boil-ing material (e.g. plant substances) inwater, followed by Ültration.

decomposition 1. The chemicalbreakdown of organic matter into itsconstituents by the action of bacteriaand other organisms. 2. A chemicalreaction in which a compoundbreaks down into simpler com-pounds or into elements.

decrepitation A crackling noiseproduced when certain crystals areheated, caused by changes in struc-ture resulting from loss of water ofcrystallization.

defect See crystal defect.

defect state A quantum mechani-cal state that exists due to the pres-ence of a *crystal defect.

deÜnite proportions See chemi-cal combination.

deÛagration A type of explosionin which the shock wave arrives be-fore the reaction is complete (be-cause the reaction front moves moreslowly than the speed of sound in themedium).

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solved, absorbed, or adsorbed gasesfrom a liquid or solid. Degassing isimportant in vacuum systems, wheregas absorbed in the walls of the vac-uum vessel starts to desorb as thepressure is lowered.

degeneracy The state of being *de-generate.

degenerate Having quantumstates with the same energy. For ex-ample, the Üve d-orbitals in an iso-lated transition-metal atom have thesame energy (although they have dif-ferent spatial arrangements) and arethus degenerate. The application of amagnetic or electric Üeld may causethe quantum states to have differentenergies (see crystal-field theory).In this case, the degeneracy is said tobe ‘lifted’.

degenerate rearrangement Arearrangement of a molecule inwhich the product is chemically in-distinguishable from the reactant.Degenerate rearrangements can bedetected by using isotopic labelling.

degradation A type of organicchemical reaction in which a com-pound is converted into a simplercompound. An example is the *Bar-bier–Wieland degradation.

degree A division on a *tempera-ture scale.

degrees absolute See absolute.

degrees of freedom 1. The num-ber of independent parameters re-quired to specify the conÜguration ofa system. This concept is applied inthe *kinetic theory to specify thenumber of independent ways inwhich an atom or molecule can takeup energy. There are however vari-ous sets of parameters that may bechosen, and the details of the conse-quent theory vary with the choice.For example, in a monatomic gaseach atom may be allotted three de-

grees of freedom, corresponding tothe three coordinates in space re-quired to specify its position. Themean energy per atom for eachdegree of freedom is the same, ac-cording to the principle of the *equi-partition of energy, and is equal tokT/2 for each degree of freedom(where k is the *Boltzmann constantand T is the thermodynamic temper-ature). Thus for a monatomic gas thetotal molar energy is 3LkT/2, where Lis the Avogadro constant (the num-ber of atoms per mole). As k = R/L,where R is the molar gas constant,the total molar energy is 3RT/2.

In a diatomic gas the two atoms re-quire six coordinates between them,giving six degrees of freedom. Com-monly these are interpreted as six in-dependent ways of storing energy: onthis basis the molecule has three de-grees of freedom for different direc-tions of translational motion, and inaddition there are two degrees offreedom for rotation of the molecu-lar axis and one vibrational degree offreedom along the bond between theatoms. The rotational degrees of free-dom each contribute their share,kT/2, to the total energy; similarly thevibrational degree of freedom has anequal share of kinetic energy andmust on average have as much po-tential energy. The total energy permolecule for a diatomic gas is there-fore 3kT/2 (for translational energy ofthe whole molecule) plus 2kT/2 (forrotational energy) plus 2kT/2 (for vi-brational energy), i.e. a total of 7kT/2.2. The least number of independentvariables required to deÜne the stateof a system in the *phase rule. In thissense a gas has two degrees of free-dom (e.g. temperature and pressure).

dehydration 1. Removal of waterfrom a substance. 2. A chemical re-action in which a compound loseshydrogen and oxygen in the ratio

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2:1. For instance, ethanol passed overhot pumice undergoes dehydrationto ethene:

C2H5OH – H2O → CH2:CH2

Substances such as concentrated sul-phuric acid, which can remove H2Oin this way, are known as dehydrat-ing agents. For example, with sul-phuric acid, methanoic acid givescarbon monoxide:

HCOOH – H2O → CO

dehydrogenase Any enzyme thatcatalyses the removal of hydrogenatoms (*dehydrogenation) in biologi-cal reactions. Dehydrogenases occurin many biochemical pathways butare particularly important in drivingthe *electron-transport-chain reac-tions of cell respiration. They workin conjunction with the hydrogen-accepting coenzymes *NAD and*FAD.

dehydrogenation A chemical re-action in which hydrogen is removedfrom a compound. Dehydrogenationof organic compounds converts sin-gle carbon–carbon bonds into doublebonds. It is usually effected by meansof a metal catalyst or – in biologicalsystems – by *dehydrogenases.

dehydrohalogenation A type ofchemical reaction in which a hydro-gen halide is removed from a mol-ecule with formation of a doublebond. A simple example is the forma-tion of ethene from chloroethaneusing alcoholic potassium hydroxide:

CH3CH2Cl + KOH → CH2 = CH2 +KCl +H2O.

deionized water Water fromwhich ionic salts have been removedby ion-exchange. It is used for manypurposes as an alternative to distilledwater.

deliquescence The absorption ofwater from the atmosphere by a hy-

groscopic solid to such an extent thata concentrated solution of the solideventually forms.

delocalization The spreading ofvalence electrons over two or morebonds in a chemical compound. Incertain compounds, the valence elec-trons cannot be regarded as re-stricted to deÜnite bonds betweenthe atoms but move over severalatoms in the molecule. Such elec-trons are said to be delocalized. Delo-calization occurs particularly whenthe compound contains alternating(conjugated) double or triple bonds,the delocalized electrons being thosein the pi *orbitals. The molecule isthen more stable than it would be ifthe electrons were localized, an ef-fect accounting for the properties ofbenzene and other aromatic com-pounds. The energy difference be-tween the actual delocalized stateand a localized state is the delocaliza-tion energy. Another example is inthe ions of carboxylic acids, contain-ing the carboxylate group –COO–. Interms of a simple model of chemicalbonding, this group would have thecarbon joined to one oxygen by adouble bond (i.e. C=O) and the otherjoined to O– by a single bond (C–O–).In fact, the two C–O bonds are identi-cal because the extra electron on theO– and the electrons in the pi bondof C=O are delocalized over the threeatoms. Delocalization of electrons isa feature of metallic bonding. The de-localization energy of molecules canbe calculated approximately usingthe *Hückel approximation, as wasdone originally by Hückel. However,modern computing power enablesdelocalization energy to be calculatedusing *ab-initio calculations, even forlarge molecules. See also localization.

delta bonding Chemical bondinginvolving delta (δ) orbitals. A δ orbitalis so called because it resembles a

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d-orbital when viewed along the axisof a molecule and has two units oforbital angular momentum aroundthe internuclear axis. The formationof δ bonds originates in the overlapof d-orbitals on different atoms. Deltaorbitals contribute to the bonding ofcluster compounds of transition met-als.

delta-brass A strong hard type of*brass that contains, in addition tocopper and zinc, a small percentageof iron. It is mainly used for makingcartridge cases.

deltahedron A polyhedron thathas triangular faces. See wade’srules.

delta-iron See iron.

delta orbital See delta bonding.

delta value A quantity that meas-ures the shift in *nuclear magneticresonance (NMR).

denature 1. To add a poisonous orunpleasant substance to ethanol tomake it unsuitable for human con-sumption (see methylated spirits).2. To produce a structural change ina protein or nucleic acid that resultsin the reduction or loss of its biologi-cal properties. Denaturation is causedby heat, chemicals, and extremes ofpH. The differences between raw andboiled eggs are largely a result of de-naturation. 3. To add another iso-tope to a Üssile material to make itunsuitable for use in a nuclearweapon.

dendrimer (dendritic polymer) Atype of macromolecule in which anumber of chains radiate out from acentral atom or cluster of atoms.Dendritic polymers have a number of possible applications. See alsosupramolecular chemistry.

dendrite A crystal that hasbranched in growth into two parts.

Crystals that grow in this way (den-dritic growth) have a branching tree-like appearance.

dendrochronology An absolute*dating technique using the growthrings of trees. It depends on the factthat trees in the same locality show acharacteristic pattern of growth ringsresulting from climatic conditions.Thus it is possible to assign a deÜnitedate for each growth ring in livingtrees, and to use the ring patterns todate fossil trees or specimens ofwood (e.g. used for buildings or ob-jects on archaeological sites) withlifespans that overlap those of livingtrees. The bristlecone pine (Pinus aris-tata), which lives for up to 5000years, has been used to date speci-mens over 8000 years old. Fossilspecimens accurately dated bydendrochronology have been used to make corrections to the *carbon-dating technique. Dendrochronologyis also helpful in studying past cli-matic conditions. Analysis of trace el-ements in sections of rings can alsoprovide information on past atmos-pheric pollution.

denitriÜcation A chemical processin which nitrates in the soil are re-duced to molecular nitrogen, whichis released into the atmosphere. Thisprocess is effected by the bacteriumPseudomonas denitriÜcans, which usesnitrates as a source of energy forother chemical reactions in a mannersimilar to respiration in other organ-isms. Compare nitrification. See ni-trogen cycle.

de novo pathway Any metabolicpathway in which a *biomolecule issynthesized from simple precursormolecules. Nucleotide synthesis is anexample.

density The mass of a substanceper unit of volume. In *SI units it is

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measured in kg m–3. See also relativedensity; vapour density.

density-function theory Amethod for the theoretical treatmentof molecules, in which the electrondensity is considered rather than theinteractions of individual electrons. Itis successful for large molecules.

deoxyribonucleic acid See dna.

depolarization The prevention of*polarization in a *primary cell. Forexample, maganese(IV) oxide (the de-polarizer) is placed around the posi-tive electrode of a *Leclanché cell tooxidize the hydrogen released at thiselectrode.

depression of freezing pointThe reduction in the freezing pointof a pure liquid when another sub-stance is dissolved in it. It is a *col-ligative property – i.e. the loweringof the freezing point is proportionalto the number of dissolved particles(molecules or ions), and does not de-pend on their nature. It is given by ∆t= KfCm, where Cm is the molar con-centration of dissolved solute and Kfis a constant (the cryoscopic constant)for the solvent used. Measurementsof freezing-point depression (using aBeckmann thermometer) can be usedfor Ünding relative molecular massesof unknown substances.

A• Raoult’s original paper

depsides A class of compoundsformed by condensation of a pheno-lic carboxylic acid (such as gallic acid)with a similar compound, the reac-tion being between the carboxylicacid group on one molecule and aphenolic OH group on the other.Depsides are similar to esters, exceptthat the OH group is linked directlyto an aromatic ring. In such com-pounds, the –O–CO– group is called adepside linkage. Depsides are found

in tannins and other natural prod-ucts.

derivative A compound that is de-rived from some other compoundand usually maintains its generalstructure, e.g. trichloromethane(chloroform) is a derivative ofmethane.

derived unit See base unit.

desalination The removal of saltfrom sea water for irrigation of theland or to provide drinking water.The process is normally only eco-nomic if a cheap source of energy,such as the waste heat from a nu-clear power station, can be used. De-salination using solar energy has thegreatest economic potential sinceshortage of fresh water is most acutein hot regions. The methods em-ployed include evaporation, oftenunder reduced pressure (Ûash evapo-ration); freezing (pure ice forms fromfreezing brine); *reverse osmosis;*electrodialysis; and *ion exchange.

dessicant A drying agent. Thereare many types, including anhydrouscalcium chloride, anhydrous calciumsulphate, concentrated sulphuricacid, phosphorus (V) oxide, solidsodium hydroxide, lime, and *silicagel.

desiccation A method of preserv-ing organic material by the removalof its water content. Cells and tissuescan be preserved by desiccation afterlowering the samples to freezingtemperatures; thereafter they can bestored at room temperature.

desiccator A container for dryingsubstances or for keeping them freefrom moisture. Simple laboratorydesiccators are glass vessels contain-ing a drying agent, such as silica gel.They can be evacuated through a tapin the lid.

designer drug A synthetic drug

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with a chemical structure that is sim-ilar to that of an existing drug. De-signer drugs are produced either toincrease potency or to avoid detec-tion. An example is ecstasy (MDMA),which is a variant of MDA.

desorption The removal of ad-sorbed atoms, molecules, or ionsfrom a surface.

destructive distillation Theprocess of heating complex organicsubstances in the absence of air sothat they break down into a mixtureof volatile products, which are con-densed and collected. At one timethe destructive distillation of coal (togive coke, coal tar, and coal gas) wasthe principal source of industrial or-ganic chemicals.

desulphuration The removal ofsulphur from a compound. Desulphu-ration has been implicated in thetoxicity of a number of sulphur-containing organic compounds: it ispostulated that the atomic sulphurreleased into a cell by desulphura-tion, which is highly electrophilic,can bind to proteins and therebyalter their function.

detailed balance The cancellationof the effect of one process by an-other process that operates at thesame time with the opposite effect.An example of detailed balance isprovided by a chemical reaction be-tween two molecular species A andB, which results in the formation ofthe molecular species C and D. De-tailed balance for this chemical reac-tion occurs if the rate at which thereaction A + B → C + D occurs isequal to the rate at which the reac-tion C + D → A + B occurs. The equi-librium state in thermodynamics ischaracterized by detailed balance.

detergent A substance added towater to improve its cleaning proper-ties. Although water is a powerful

solvent for many compounds, it willnot dissolve grease and natural oils.Detergents are compounds that causesuch nonpolar substances to go intosolution in water. *Soap is the origi-nal example, owing its action to the presence of ions formed fromlong-chain fatty acids (e.g. theoctadecanoate (stearate) ion,CH3(CH2)16COO–). These have twoparts: a nonpolar part (the hydro-carbon chain), which attaches to thegrease; and a polar part (the –COO–

group), which is attracted to thewater. A disadvantage of soap is thatit forms a scum with hard water (seehardness of water) and is relativelyexpensive to make. Various synthetic(‘soapless’) detergents have been de-veloped from petrochemicals. Thecommonest, used in washing pow-ders, is sodium dodecylbenzene-sulphonate, which containsCH3(CH2)11C6H4SO2O– ions. This, likesoap, is an example of an anionic de-tergent, i.e. one in which the activepart is a negative ion. Cationic deter-gents have a long hydrocarbon chain connected to a positive ion.Usually they are amine salts, as inCH3(CH2)15N(CH3)3+Br–, in which thepolar part is the –N(CH3)3+ group.Nonionic detergents have non-ionic polar groups of the type–C2H4–O–C2H4–OH, which formhydrogen bonds with the water.Synthetic detergents are also used aswetting agents, emulsiÜers, and sta-bilizers for foam.

detonating gas See electrolyticgas.

deuterated compound A com-pound in which some or all of thehydrogen–1 atoms have been re-placed by deuterium atoms.

deuterium (heavy hydrogen) Sym-bol D. The isotope of hydrogen thathas a mass number 2 (r.a.m. 2.0144).Its nucleus contains one proton and

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one neutron. The abundance of deu-terium in natural hydrogen is about0.015%. It is present in water as theoxide HDO (see also heavy water),from which it is usually obtained byelectrolysis or fractional distillation.Its chemical behaviour is almostidentical to hydrogen although deu-terium compounds tend to reactrather more slowly than the corre-sponding hydrogen compounds. Itsphysical properties are slightly differ-ent from those of hydrogen, e.g. b.p.23.6 K (hydrogen 20.4 K).

deuterium oxide See heavywater.

deuteron A nucleus of a deuteriumatom, consisting of a proton and aneutron bound together; the ion D+

formed by ionization of a deuteriumatom. See also hydron.

Devarda’s alloy An alloy of copper(50%), aluminium (45%) and zinc (5%),used in chemical tests for the nitrateion (in alkaline solutions it reduces anitrate to ammonia).

devitriÜcation Loss of the amor-phous nature of glass as a result ofcrystallization.

Dewar, Sir James (1842–1923)British chemist and physicist, born inScotland. In 1875 he became a pro-fessor at Cambridge University, whilecarrying out much of his experimen-tal work at the Royal Institution inLondon. He began studying gases atlow temperatures and in 1872 in-vented the *Dewar Ûask. In 1891, to-gether with Frederick Abel (1827–1902), he developed the smokelesspropellant explosive *cordite, and in1898 was the Ürst to liquefy hydro-gen.

Dewar benzene An isomer of ben-zene, C6H6. Dewar benzene has thesystematic name bicyclo [2,2,0] hexa-2,5-diene, and is a nonplanar bicyclic

molecule. It has a high bond strainand will spontaneously slowly con-vert to benzene. The unsubstitutedcompound was Ürst synthesized in1963.

Dewar Ûask A vessel for storinghot or cold liquids so that they main-tain their temperature independentlyof the surroundings. Heat transfer tothe surroundings is reduced to a min-imum: the walls of the vessel consistof two thin layers of glass (or, inlarge vessels, steel) separated by avacuum to reduce conduction andconvection; the inner surface of aglass vessel is silvered to reduce radi-ation; and the vessel is stoppered toprevent evaporation. It was devisedaround 1872 by Sir James *Dewarand is also known by its Ürst tradename Thermos Ûask. See also cryo-stat.

Dewar structure A structure of*benzene proposed by Sir James*Dewar, having a hexagonal ring ofsix carbon atoms with two oppositeatoms joined by a long single bondacross the ring and with two doubleC–C bonds, one on each side of thehexagon. Dewar structures con-tribute to the resonance hybrid ofbenzene.

dextran A glutinous glucose poly-mer produced by certain bacteria. Itcan be made by fermenting sucrose(cane sugar) and is used as a thicken-ing agent, as a stabilizer in ice cream,and as a substitute for plasma inblood transfusions. Esters with sul-phuric acid yield sodium salts thatare employed as anticoagulant drugs.

dextrin An intermediate polysac-charide compound resulting fromthe hydrolysis of starch to maltose byamylase enzymes.

dextro- form See optical activity.

dextrorotatory Denoting a chemi-

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cal compound that rotates the planeof polarization of plane-polarizedlight to the right (clockwise as ob-served by someone facing the on-coming radiation). See opticalactivity.

dextrose See glucose.

diagonal relationship A relation-ship within the periodic table bywhich certain elements in the secondperiod have a close chemical similar-ity to their diagonal neighbours inthe next group of the third period.This is particularly noticeable withthe following pairs.

Lithium and magnesium:(1) both form chlorides and bromidesthat hydrolyse slowly and are solublein ethanol;(2) both form colourless or slightlycoloured crystalline nitrides by directreaction with nitrogen at high tem-peratures;(3) both burn in air to give the nor-mal oxide only;(4) both form carbonates that decom-pose on heating.

Beryllium and aluminium:(1) both form highly refractory ox-ides with polymorphs;(2) both form crystalline nitrides thatare hydrolysed in water;(3) addition of hydroxide ion to solu-tions of the salts gives an amphoterichydroxide, which is soluble in excesshydroxide giving beryllate or alumi-nate ions [Be(OH)4]2– and [Al(OH)4]–;(4) both form covalent halides and co-valent alkyl compounds that displaybridging structures;(5) both metals dissolve in alkalis.

Boron and silicon:(1) both display semiconductor prop-erties;(2) both form hydrides that are unsta-ble in air and chlorides that hydrol-yse in moist air;(3) both form acidic oxides with cova-lent crystal structures, which are

readily incorporated along withother oxides into a wide range ofglassy materials.

The reason for this relationship is acombination of the trends to increasesize down a group and to decreasesize along a period, and a similar, butreversed, effect in electronegativity,i.e. decrease down a group and in-crease along a period.

dialysis A method by which largemolecules (such as starch or protein)and small molecules (such as glucoseor amino acids) in solution may beseparated by selective diffusionthrough a semipermeable mem-brane. For example, if a mixed solu-tion of starch and glucose is placed ina closed container made of a semi-permeable substance (such as Cello-phane), which is then immersed in abeaker of water, the smaller glucosemolecules will pass through themembrane into the water while thestarch molecules remain behind. Thecell membranes of living organismsare semipermeable, and dialysistakes place naturally in the kidneysfor the excretion of nitrogenouswaste. An artiÜcial kidney (dialyser)utilizes the principle of dialysis bytaking over the functions of diseasedkidneys.

diamagnetism See magnetism.

diaminobenzene (phenylenedi-amine) Any one of three isomericaromatic compounds, C6H4(NH2)2,with strong basic properties. 1,2-Diaminobenzene is a yellow-browncrystalline solid, m.p. 104°C, used asa photographic developer and inmaking dyes. 1,3-Diaminobenzene isa colourless crystalline solid, m.p.63°C, which turns brown on standingin air. It is made by reducing 1,3-dinitrobenzene with iron and hy-drochloric acid, and is used for mak-ing dyes. 1,4-Diaminobenzene is awhite crystalline solid, m.p. 147°C,

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which rapidly darkens on standing inair. It is made by the reduction of 1,4-nitrophenylamine and is used as aphotographic develop and hair dye.

1,2-diaminoethane (ethylenedi-amine) A colourless fuming liquidthat smells of ammonia,H2NCH2CH2NH2; b.p. 116°C. It ismade from ammonia and 1,2-dichloroethane, which are heatedunder pressure with a copper(I) chlo-ride catalyst. In air it absorbs waterand carbon dioxide to form di-aminoethane carbamate. With fattyacids diaminoethane produces soaps,employed as emulsifying agents anddetergents. It is also used as a solvent(for resins) and in making coatings,paper and textiles. Crystals of its tar-trate derivative are employed inpiezoelectric devices. It is a bidentateligand, given the symbol en in formu-lae.

1,6-diaminohexane (hexamethyl-enediamine) A solid colourlessamine, H2N(CH2)6NH2; m.p. 41°C; b.p.204°C. It is made by oxidizing cyclo-hexane to hexanedioic acid, reactingthis with ammonia to give the am-monium salt, and dehydrating thesalt to give hexanedionitrile(NC(CH2)6CN). This is reduced withhydrogen to the diamine. The com-pound is used, with hexanedioic acid,for producing *nylon 6,6.

diamond The hardest known min-eral (with a hardness of 10 on Mohs’scale). It is an allotropic form of pure*carbon that has crystallized in thecubic system, usually as octahedra orcubes, under great pressure. Dia-mond crystals may be colourless andtransparent or yellow, brown, orblack. They are highly prized as gem-stones but also have extensive usesin industry, mainly for cutting andgrinding tools. Diamonds occur in an-cient volcanic pipes of kimberlite;the most important deposits are in

South Africa but others are found inTanzania, the USA, Russia, and Aus-tralia. Diamonds also occur in riverdeposits that have been derived fromweathered kimberlite, notably inBrazil, Zaïre, Sierra Leone, and India.Industrial diamonds are increasinglybeing produced synthetically.

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diamond-anvil cell A device forproducing very high pressures. Asample of material to be subject tothe high pressure is placed in a cav-ity between two high-quality dia-monds. The diamond-anvil celloperates like a nutcracker, with pres-sures up to 1 megabar (1011 Pa) beingexerted by turning a screw. The pres-sure exerted can be determined byspectroscopy for small samples ofruby in the material being com-pressed, while the sample itself is ob-served optically. A use of thediamond-anvil cell is to study the in-sulator–metal transition in such sub-stances as iodine as the pressure isincreased. This type of study is thenearest laboratory approach to thestructure of matter in the conditionsobtaining in the interior of the earth.

diaspore A mineral form of amixed aluminium oxide and hydrox-ide, AlO.OH. See aluminium hydrox-ide.

diastase See amylase.

diastereoisomers Stereoisomersthat are not identical and yet notmirror images. For instance, the d-form of tartaric acid and the meso


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form constitute a pair of diastereoiso-mers. See optical activity.

diatomaceous earth See kiesel-guhr.

diatomic molecule A moleculeformed from two atoms (e.g. H2 orHCl).

diatomite See kieselguhr.

diazepam A *benzodiazepine usedmedically to treat anxiety, convul-sions, insomnia, and alcohol with-drawal. It is widely used, and oftenknown under its tradename Valium.

diazine See azine.

diazo compounds Organic com-pounds containing two linked nitro-gen compounds. The term includes*azo compounds, diazonium com-pounds, and also such compounds asdiazomethane, CH2N2.A• Information about IUPAC nomenclature

diazonium salts Unstable saltscontaining the ion C6H5N2

+ (the dia-zonium ion: see formula). They areformed by *diazotization reactions.

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Diazonium salt

diazotization The formation of a*diazonium salt by reaction of anaromatic amine with nitrous acid at

low temperature (below 5°C). The ni-trous acid is produced in the reactionmixture from sodium nitrite and hy-drochloric acid:

ArNH2 + NaNO2 + HCl → ArN+N +Cl– + Na+ + OH– + H2O

dibasic acid An *acid that has twoacidic hydrogen atoms in its mol-ecules. Sulphuric (H2SO4) and car-bonic (H2CO3) acids are commonexamples.

diboron trioxide See boron(iii)oxide.

1,2-dibromoethane A colourlessliquid *haloalkane, BrCH2CH2Br; r.d.2.2; m.p. 9.79°C; b.p. 131.36°C. It ismade by addition of bromine toethene and used as an additive inpetrol to remove lead during com-bustion as the volatile lead bromide.

dicarbide See carbide.

dicarboxylic acid A *carboxylicacid having two carboxyl groups inits molecules. In systematic chemicalnomenclature, dicarboxylic acids aredenoted by the sufÜx -dioic; e.g. hexa-nedioic acid, HOOC(CH2)4COOH.

dichlorine oxide (chlorine monox-ide) A strongly oxidizing orange gas,Cl2O, made by oxidation of chlorineusing mercury(II) oxide. It is the acidanhydride of chloric(I) acid.

dichlorobenzene Any one of threeisomeric liquid aromatic compounds,C6H4Cl2. 1,2-Dichlorobenzene (b.p.179°C) and 1,4-dichlorobenzene (b.p.174°C) are made by chlorinatingbenzene with an iron catalyst andseparating the mixed isomers byfractional distillation; 1,3-dichloro-benzene (b.p. 172°C) is made fromone of the other two by catalytic iso-merization. The 1,2 isomer is used asan insecticide and in making dyes;the 1,4 compound is employed as adeodorant and moth repellent.


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dichloroethanoic acid Seechloroethanoic acids.

dichloromethane (methylene chlo-ride) A colourless, slightly toxic liq-uid, CH2Cl2, b.p. 41°C. It has acharacteristic odour similar to that oftrichloromethane (chloroform), fromwhich it is made by heating withzinc and hydrochloric acid. It is usedas a refrigerant and solvent (for paintstripping and degreasing).

2,4-dichlorophenoxyacetic acidSee 2,4-d.

dichroism The property of somecrystals, such as tourmaline, of selec-tively absorbing light vibrations inone plane while allowing light vibra-tions at right angles to this plane topass through. Polaroid is a syntheticdichroic material. See polarization.

dichromate(VI) A salt containingthe ion Cr2O7

–. Solutions containingdichromate(VI) ions are strongly oxi-dizing.

1,2-didehydrobenzene See ben-zyne.

dielectric constant See permittiv-ity.

Diels, Otto Paul Hermann(1876–1954) German organic chemistwho worked mostly at the Universityof Kiel. In 1906 he discovered tricar-bon dioxide (C3O2). Diels also didimportant work on steroids but is re-membered for his discovery in 1928of the *Diels–Alder reaction, whichhe made with his assistant Kurt Alder

(1902–58). Diels and Alder shared the1950 Nobel Prize for chemistry.

Diels–Alder reaction A type ofchemical reaction in which a com-pound containing two double bondsseparated by a single bond (i.e. a con-jugated *diene) adds to a suitablecompound containing one doublebond (known as the dienophile) togive a ring compound. In thedienophile, the double bond musthave a carbonyl group on each side.It is named after the Germanchemists Otto *Diels and Kurt Alder.

diene An *alkene that has two dou-ble bonds in its molecule. If the twobonds are separated by one singlebond, as in buta-1,3-dieneCH2:CHCH:CH2, the compound is aconjugated diene.

dienophile See diels–alder reac-tion.

Dieterici equation An *equationof state for a gas of the form

P(V – b)[exp (a/VRT)] = RT,where P is the pressure, V is the vol-ume, T is the thermodynamic tem-perature, R is the gas constant, and aand b are constants characteristic ofthe gas. The Dieterici equation is amodiÜcation of van der Waals’ equa-tion, which takes account of the pres-sure gradient at the boundary of thegas. At low pressures the Dietericiequation becomes identical to vander Waals’ equation.

175 Dieterici equation

dCH2

CH

CHCH2

CH

CH

COOH

COOH

CH

CHCH2

CH

CHC

H2

COOH

COOH

+

Diels-Alder reaction


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diethanolamine See ethanol-amine.

diethyl ether See ethoxyethane.

differential scanning calorime-try (DSC) See thermal analysis.

differential thermal analysis(DTA) See thermal analysis.

diffusion 1. The process by whichdifferent substances mix as a resultof the random motions of their com-ponent atoms, molecules, and ions.In gases, all the components are per-fectly miscible with each other andmixing ultimately becomes nearlyuniform, though slightly affected bygravity (see also graham’s law). Thediffusion of a solute through a sol-vent to produce a solution of uni-form concentration is slower, butotherwise very similar to the processof gaseous diffusion. In solids, diffu-sion occurs very slowly at normaltemperatures. See also fick’s law. 2. The passage of elementary parti-cles through matter when there is ahigh probability of scattering and alow probability of capture.

diffusion constant See fick’s law.

diffusion gradient See concen-tration gradient.

diffusion limited aggregation(DLA) A process of aggregation domi-nated by particles diffusing and hav-ing a nonzero probability of stickingtogether irreversibly when theytouch. The clusters formed by DLAare *fractal in type.

diffusion pump (condensationpump) A vacuum pump in which oilor mercury vapour is diffusedthrough a jet, which entrains the gasmolecules from the container inwhich the pressure is to be reduced.The diffused vapour and entrainedgas molecules are condensed on thecooled walls of the pump. Pressures

down to 10–7 Pa can be reached by so-phisticated forms of the diffusionpump.

diffusive Ûux See fick’s law.

dihedral (dihedron) An angleformed by the intersection of twoplanes (e.g. two faces of a polyhe-dron). The dihedral angle is the angleformed by taking a point on the lineof intersection and drawing two linesfrom this point, one in each plane,perpendicular to the line of intersec-tion.

dihydrate A crystalline hydratecontaining two moles of water permole of compound.

dihydric alcohol See diol.

dihydrogen The normal form ofmolecular hydrogen, H2, used to dis-tinguish it from hydrogen atoms.

1,2-dihydroxybenzene (catechol)A colourless crystalline phenol,C6H4(OH)2; r.d. 1.15; m.p. 105°C; b.p.245°C. It is used as a photographicdeveloper.

1,3-dihydroxybenzene (resorcinol)A colourless crystalline aromaticcompound, C6H4(OH)2; m.p. 111°C. Itis made by the fusion of benzene-disulphonic acid with sodium hy-droxide and used mainly in thedyestuffs industry. On fusing withphthalic anhydride it forms Ûuores-cein dyes. It is also used to makecold-setting adhesives (withmethanal), plasticizers, and resins.

2,3-dihydroxybutanedioic acidSee tartaric acid.

diketones Organic compounds thathave two carbonyl (>C=O) groups (seeketones). There are three kinds, de-pending on the location of the car-bonyl groups. 1,2-Diketones (alsocalled α-diketones), R.CO.CO.R′, havethe carbonyl groups on adjacentcarbon atoms. The aliphatic 1,2-

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diketones are pungent-smelling yel-low oils, whereas the aromatic com-pounds are crystalline solids. 1,3-Diketones (or β-diketones), R.CO.CH2.CO.R′, are more acidic and exist inboth keto and enol forms (see keto–enol tautomerism); they form stablecompounds with metals. 1,4-Dike-tones (or γ-diketones), R.CO.CH2.CH2.CO.R′, also exist and readily re-arrange to form cyclic compounds.

dilatancy See newtonian fluid.

dilation (dilatation) An increase involume.

dilead(II) lead(IV) oxide A redpowder, Pb3O4; r.d. 9.1; decomposesat 500°C to lead(II) oxide. It is pre-pared by heating lead(II) oxide to400°C and has the unusual propertyof being black when hot and red-orange when cold. The compound isnonstoichiometric, generally contain-ing less oxygen than implied by theformula. It is largely covalent and hasPb(IV)O6 octahedral groups linked to-gether by Pb(II) atoms, each joined tothree oxygen atoms. It is used inglass making but its use in the paintindustry has largely been discontin-ued because of the toxicity of lead.Dilead(II) lead(IV) oxide is commonlycalled red lead or, more accurately,red lead oxide.

Dillie–Koppanyi test A presump-tive test for barbituates. The Dillie–Koppanyi test reagent has two solu-tions: a 1% solution of colbalt acetatein methanol, follow by a 5% solutionof isopropylamine (CH3CH(CH3)NH2)in methanol. Barbiturates give a red-dish-violet colour.

diluent A substance added to dilutea solution or mixture (e.g. a *Üller).

dilute Describing a solution thathas a relatively low concentration ofsolute.

dilute spin species A type of nu-

cleus in which it is statistically un-likely that there will be more thanone such nucleus in a molecule (un-less the substance has been artiÜ-cially enriched with isotopes havingthat nucleus). An example of a dilutespin species is 13C since it has a nat-ural abundance of 1.1%. This meansthat for dilute spin systems it is usu-ally not necessary to consider spin–spin interactions between two nucleiof the dilute spin species, e.g. 13C–13Cin the same molecule. The oppositeof a dilute spin species is an abun-dant spin species, an example beingthe proton.

dilution The volume of solvent inwhich a given amount of solute isdissolved.

dilution law See ostwald’s dilu-tion law.

dimer An association of two identi-cal molecules linked together. Themolecules may react to form a largermolecule, as in the formation of dini-trogen tetroxide (N2O4) from nitro-gen dioxide (NO2), or the formationof an *aluminium chloride dimer(Al2Cl6) in the vapour. Alternatively,they may be held by hydrogen bonds.For example, carboxylic acids formdimers in organic solvents, in whichhydrogen bonds exist between the Oof the C=O group and the H of the–O–H group.

p-dimethylaminobenzaldehyde

177 p-dimethylaminobenzaldehyde

d

O H

NCH3CH3

p-dimethylaminobenzaldehyde


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(p-DMAB) A substituted aldehydeused in a presumptive test for LSDand for psilocybin. A 1% solution of p-BMAB is used in 10% mineral acid (ei-ther sulphuric acid or hydrochloricacid). This reagent is called Ehrlick’sreagent or Van Urk’s reagent. LSD isindicated by a purple colouration;psilocybin by a red-brown colour.

dimethylbenzenes (xylenes)Three compounds with the formula(CH3)2C6H4, each having two methylgroups substituted on the benzenering. 1,2-dimethylbenzene is ortho-xylene, etc. A mixture of the isomers(b.p. 135–145°C) is obtained from pe-troleum and is used as a clearingagent in preparing specimens for op-tical microscopy.

dimethylformamide (DMF) Acolourless liquid compound,(CH3)2NCHO; r.d. 0.944; m.p. –61°C;b.p. 153°C. The systematic name isN,N-dimethylmethanamide. It can bemade from methanoic acid (formicacid) and dimethylamine, and iswidely used as a solvent.

1,3-dimethylxanthine See theo-phylline.

3,7-dimethylxanthine See theo-bromine.

dimethylglyoxime (DMG) Acolourless solid, (CH3CNOH)2, m.p.234°C. It sublimes at 215°C andslowly polymerizes if left to stand. Itis used in chemical tests for nickel,with which it forms a dark-red com-plex.

dimethyl sulphoxide (DMSO) Acolourless solid, (CH3)2SO; m.p. 18°C;b.p. 189°C. It is used as a solvent andas a reagent in organic synthesis.

dimorphism See polymorphism.

dinitrogen The normal form ofmolecular nitrogen, N2 used to distin-guish it from nitrogen atoms.

dinitrogen oxide (nitrous oxide) Acolourless gas, N2O, d. 1.97 g dm–3;m.p. –90.8°C; b.p. –88.5°C. It is solu-ble in water, ethanol, and sulphuricacid. It may be prepared by the con-trolled heating of ammonium nitrate(chloride free) to 250°C and passingthe gas produced through solutionsof iron(II) sulphate to remove impuri-ties of nitrogen monoxide. It is rela-tively unreactive, being inert tohalogens, alkali metals, and ozone atnormal temperatures. It is decom-posed on heating above 520°C to ni-trogen and oxygen and will supportthe combustion of many compounds.Dinitrogen oxide is used as an anaes-thetic gas (‘laughing gas’) and as anaerosol propellant.

dinitrogen tetroxide A colourlessto pale yellow liquid or a brown gas,N2O4; r.d. 1.45 (liquid); m.p. –11.2°C;b.p. 21.2°C. It dissolves in water withreaction to give a mixture of nitricacid and nitrous acid. It may be read-ily prepared in the laboratory by thereaction of copper with concentratednitric acid; mixed nitrogen oxidescontaining dinitrogen oxide may alsobe produced by heating metal ni-trates. The solid compound is whollyN2O4 and the liquid is about 99%N2O4 at the boiling point; N2O4 is dia-magnetic. In the gas phase it dissoci-ates to give nitrogen dioxide

N2O4 ˆ 2NO2

Because of the unpaired electron thisis paramagnetic and brown. LiquidN2O4 has been widely studied as anonaqueous solvent system (self-ionizes to NO+ and NO3

–). Dinitrogentetroxide, along with other nitrogenoxides, is a product of combustionengines and is thought to be involvedin the depletion of stratosphericozone.

dinucleotide A compound consist-ing of two *nucleotides.

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diol (dihydric alcohol) An *alcoholcontaining two hydroxyl groups permolecule.

dioxan A colourless toxic liquid,C4H8O2; r.d. 1.03; m.p. 11°C; b.p.101.5°C. The molecule has a six-membered ring containing four CH2groups and two oxygen atoms at op-posite corners. It can be made fromethane-1,2-diol and is used as a sol-vent.

dioxin (2,4,7,8-tetrachlorodibenzo-p-dioxin) A toxic solid, formed in themanufacture of the herbicide *2,4,5-T and present as an impurity inAgent Orange. It can cause skin dis-Ügurement (chloracne) and severefetal defects.

dioxonitric(III) acid See nitrousacid.

dioxygen The normal form of mo-lecular oxygen, O2, used to distin-guish it from oxygen atoms or fromozone (O3).

dioxygenyl compounds Com-pounds containing the positive ionO2

+, as in dioxygenyl hexaÛuoro-platinate O2PtF6 – an orange solidthat sublimes in vacuum at 100°C.Other ionic compounds of the typeO2

+[MF6]– can be prepared, where Mis P, As, or Sb.

dipeptide A compound consistingof two amino acid units joined at theamino (–NH2) end of one and the car-boxyl (–COOH) end of the other. Thispeptide bond (see peptide) is formedby a condensation reaction that in-volves the removal of one moleculeof water.

diphenylamine A colourless crys-talline aromatic compound,(C6H5)2NH; m.p. 54°C. It is made byheating phenylamine (aniline) withphenylamine hydrochloride. It is asecondary amine and is both slightlyacidic (forming an N-potassium salt)

and slightly basic (forming salts withmineral acids). Its derivatives are em-ployed as stabilizers for syntheticrubber and rocket fuels.

diphenylamine test A presump-tive test for nitrates. The reagent is asolution of diphenylamine((C6H5)2NH) in sulphuric acid. A posi-tive result is indicated by a blue col-our. It was once used in testing forgunshot residue, but is not particu-larly reliable.

diphenylmethanone (benzo-phenone) A colourless solid,C6H5COC6H5, m.p. 49°C. It has a char-acteristic smell and is used in makingperfumes. It is made from benzeneand benzoyl chloride using the*Friedel–Crafts reaction with alu-minium chloride as catalyst.

diphosgene A colourless liquid,ClCO.O.CCl3, originally used in 1916by Germany in World War I as achemical warfare agent. It is nowused as a reagent in organic synthe-sis. See also carbonyl chloride.

diphosphane (diphosphine) A yel-low liquid, P2H4, which is sponta-neously Ûammable in air. It isobtained by hydrolysis of calciumphosphide. Many of the references tothe spontaneous Ûammability ofphosphine (PH3) are in fact due totraces of P2H4 as impurities.

diphosphine See diphosphane.

diphosphonates See bisphospho-nates.

dipolar bond See chemical bond.

dipole A pair of separated oppositeelectric charges. The dipole moment(symbol µ) is the product of the posi-tive charge and the distance betweenthe charges. Dipole moments areoften stated in *debyes; the SI unit isthe coulomb metre. In a diatomicmolecule, such as HCl, the dipole

179 dipole

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moment is a measure of the polar na-ture of the bond (see polar mol-ecule); i.e. the extent to which theaverage electron charge is displacedtowards one atom (in the case of HCl,the electrons are attracted towardsthe more electronegative chlorineatom). In a polyatomic molecule, thedipole moment is the vector sum ofthe dipole moments of the individualbonds. In a symmetrical molecule,such as tetrachloromethane (CCl4),there is no overall dipole moment, al-though the individual C–Cl bonds arepolar.

dipole–dipole interaction Theinteraction of two systems, such asatoms or molecules, by their *dipolemoments. The energy of dipole–dipole interaction depends on therelative orientation and the strengthof the dipoles and how far apart theyare. A water molecule has a perma-nent dipole moment, thus causing adipole–dipole interaction if twowater molecules are near each other.Although isolated atoms do not havepermanent dipole moments, a dipolemoment can be induced by the pres-ence of another atom near it, thusleading to induced dipole–dipole in-teractions. Dipole–dipole interactionsare responsible for *van der Waals’forces and *surface tension in liq-uids.

dipole radiation See forbiddentransitions.

dipyridyl (bipyridyl) A compoundformed by linking two pyridine ringsby a single C–C bond, (C5H4N)2. Vari-ous isomers are possible dependingon the relative positions of the nitro-gen atoms. A mixture of isomers canbe made by reacting pyridine withsodium metal and oxidizing the re-sulting sodium salt. The 22′-isomer isa powerful chelating agent denotedbipy in chemical formulae. Both the22′- and 44′-isomers can form quater-

nary compounds that are used in her-bicides such as Paraquat (44′) and Di-quat (22′).

dipole–dipole interaction 180

d N N

N N

Dipyridyl

Diquat Tradename for a herbicide.See dipyridyl.

Dirac, Paul Adrien Maurice(1902–84) British physicist, whoshared the 1933 Nobel Prize forphysics with Erwin *Schrödinger for developing Schrödinger’s non-relativistic wave equations to takeaccount of relativity. Dirac also in-vented, independently of EnricoFermi, the form of *quantum statis-tics known as Fermi–Dirac statistics.

Dirac constant See planck con-stant.

Dirac equation A version of thenonrelativistic *Schrödinger equa-tion taking special relativity theoryinto account. The Dirac equation isneeded to discuss the quantum me-chanics of electrons in heavy atomsand, more generally, to discuss Üne-structure features of atomic spectra,such as *spin–orbit coupling. Theequation was put forward by Paul*Dirac in 1928. It can be solved ex-actly in the case of the hydrogenatom but can only be solved usingapproximation techniques for morecomplicated atoms.

diradical See biradical.

direct dye See dyes.

disaccharide A sugar consisting of


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two linked *monosaccharide mol-ecules. For example, sucrose com-prises one glucose molecule and onefructose molecule bonded together.

discharge 1. The conversion of thechemical energy stored in a *sec-ondary cell into electrical energy. 2. The release of electric charge froma capacitor in an external circuit. 3. The passage of charge carriersthrough a gas at low pressure in adischarge tube. A potential differenceapplied between cathode and anodecreates an electric Üeld that acceler-ates any free electrons and ions totheir appropriate electrodes. Colli-sions between electrons and gas mol-ecules create more ions. Collisionsalso produce excited ions and mol-ecules (see excitation), which decaywith emission of light in certainparts of the tube.

disconnection See retrosyntheticanalysis.

disilane See silane.

dislocation See crystal defect.

disodium hydrogenphos-phate(V) (disodium orthophos-phate) A colourless crystalline solid,Na2HPO4, soluble in water and insol-uble in ethanol. It is known as the di-hydrate (r.d. 2.066), heptahydrate(r.d. 1.68), and dodecahydrate (r.d.1.52). It may be prepared by titratingphosphoric acid with sodium hydrox-ide to an alkaline end point (phe-nolphthalein) and is used in treatingboiler feed water and in the textileindustry.

disodium orthophosphate Seedisodium hydrogenphosphate(v).

disodium tetraborate-10-waterSee borax.

d-isomer See optical activity.

d-isomer See absolute configura-tion.

disordered solid A material thatneither has the structure of a perfect*crystal lattice nor of a crystal latticewith isolated *crystal defects. In arandom alloy, one type of disorderedsolid, the order of the different typesof atom occurs at random. Anothertype of disordered solid is formed byintroducing a high concentration ofdefects, with the defects distributedrandomly throughout the solid. In an*amorphous solid, such as glass,there is a random network of atomswith no lattice.

disperse dye See dyes.

disperse phase See colloids.

dispersion forces See van derwaals’ force.

displacement reaction See substi-tution reaction.

disproportionation A type ofchemical reaction in which the samecompound is simultaneously reducedand oxidized. For example, copper(I)chloride disproportionates thus:

2CuCl → Cu + CuCl2The reaction involves oxidation ofone molecule

CuI → CuII + eand reduction of the other

CuI + e → CuThe reaction of halogens with hy-droxide ions is another example of adisproportionation reaction, for ex-ample

Cl2(g) + 2OH–(aq) ˆ Cl–(aq) +ClO–(aq) + H2O(l)

The reverse process is *compropor-tionation.

dissipative system A system thatinvolves *irreversible processes. Allreal systems are dissipative (in con-trast to such idealized systems as thefrictionless pendulum, which is in-variant under time reversal). In a dis-

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sipative system the system is movingtowards a state of equilibrium, whichcan be regarded as moving toward apoint attractor in phase space; this isequivalent to moving towards theminimum of the free energy, F.

dissociation The breakdown of amolecule, ion, etc., into smaller mol-ecules, ions, etc. An example of disso-ciation is the reversible reaction ofhydrogen iodide at high tempera-tures

2HI(g) ˆ H2(g) + I2(g)The *equilibrium constant of a re-versible dissociation is called thedissociation constant. The term ‘dis-sociation’ is also applied to ionizationreactions of *acids and *bases inwater; for example

HCN + H2O ˆ H3O+ + CN–

which is often regarded as a straight-forward dissociation into ions

HCN ˆ H+ + CN–

The equilibrium constant of such adissociation is called the acid dissoci-ation constant or acidity constant,given by

Ka = [H+][A–]/[HA]for an acid HA (the concentration ofwater [H2O] can be taken as con-stant). Ka is a measure of the strengthof the acid. Similarly, for a nitroge-nous base B, the equilibrium

B + H2O ˆ BH+ + OH–

is also a dissociation; with the basedissociation constant, or basicity con-stant, given by

Kb = [BH+][OH–]/[B]For a hydroxide MOH,

Kb = [M+][OH–]/[MOH]

dissociation pressure When asolid compound dissociates to giveone or more gaseous products, thedissociation pressure is the pressureof gas in equilibrium with the solidat a given temperature. For example,

when calcium carbonate is main-tained at a constant high tempera-ture in a closed container, thedissociation pressure at that tempera-ture is the pressure of carbon dioxidefrom the equilibrium

CaCO3(s) ˆ CaO(s) + CO2(g)

distillation The process of boilinga liquid and condensing and collect-ing the vapour. The liquid collectedis the distillate. It is used to purifyliquids and to separate liquid mix-tures (see fractional distillation;steam distillation). See also de-structive distillation; extractivedistillation.

distilled water Water puriÜed bydistillation so as to free it from dis-solved salts and other compounds.Distilled water in equilibrium withthe carbon dioxide in the air has aconductivity of about 0.8 × 10–6

siemens cm–1. Repeated distillation ina vacuum can bring the conductivitydown to 0.043 × 10–6 siemens cm–1 at18°C (sometimes called conductivitywater). The limiting conductivity isdue to self ionization: H2O ˆ H+ +OH–. See also deionized water.

disulphur dichloride (sulphurmonochloride) An orange–red liquid,S2Cl2, which is readily hydrolysed bywater and is soluble in benzene andether; r.d. 1.678; m.p. –80°C; b.p.136°C. It may be prepared by passingchlorine over molten sulphur; in thepresence of iodine or metal chloridessulphur dichloride, SCl2, is alsoformed. In the vapour phase S2Cl2molecules have Cl–S–S–Cl chains.The compound is used as a solventfor sulphur and can form higherchlorosulphanes of the typeCl–(S)n–Cl (n < 100), which are ofgreat value in *vulcanizationprocesses.

disulphuric(VI) acid (pyrosulphuricacid) A colourless hygroscopic crys-

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talline solid, H2S2O7; r.d. 1.9; m.p.35°C. It is commonly encounteredmixed with sulphuric acid as it isformed by dissolving sulphur triox-ide in concentrated sulphuric acid.The resulting fuming liquid, calledoleum or Nordhausen sulphuric acid,is produced during the *contactprocess and is also widely used in the*sulphonation of organic com-pounds. See also sulphuric acid.

dithionate A salt of dithionic acid,containing the ion S2O6

2–, usuallyformed by the oxidation of a sulphiteusing manganese(IV) oxide. The ionhas neither pronounced oxidizingnor reducing properties.

dithionic acid An acid, H2S2O6,known in the form of its salts(dithionates).

dithionite See sulphinate.

dithionous acid See sulphinicacid.

divalent (bivalent) Having a va-lency of two.

DLA See diffusion limited aggre-gation.

D-lines Two close lines in the yel-low region of the visible spectrum ofsodium, having wavelengths 589.0and 589.6 nm. As they are prominentand easily recognized they are usedas a standard in spectroscopy.

dl-isomer See optical activity;racemic mixture.

DLVO theory A theory of colloidstability proposed in the 1940s by theSoviet scientists Boris Derjaguin andLev Landau and independently by theDutch scientists Evert Verwey andTheo Overbeek. The DLVO theorytakes account of two types of force ina stable colloid: the van der Waals’force, which is attractive and bindsparticles together, and electrostaticrepulsion. The total interaction po-

tential can be calculated as a func-tion of distance, with colloid stabilitybeing attained when the two forcesbalance each other. The DLVO theoryis the basis for understanding colloidstability and has a considerableamount of experimental support.However, it is inadequate for theproperties of colloids in the aggre-gated state, which depend on short-range interactions taking intoaccount the speciÜc properties ofions, rather than regarding them aspoint particles.

p-DMAB See p-dimethylaminoben-zadehyde.

DMF See dimethylformamide.

DMG See dimethylglyoxime.

DMSO See dimethyl sulphoxide.

DNA (deoxyribonucleic acid) The ge-netic material of most living organ-isms, which is a major constituent ofthe chromosomes within the cell nu-cleus and plays a central role in thedetermination of hereditary charac-teristics by controlling protein syn-thesis in cells. DNA is a nucleic acidcomposed of two chains of *nu-cleotides in which the sugar is de-oxyribose and the bases are*adenine, *cytosine, *guanine, and*thymine (compare rna). The twochains are wound round each otherand linked together by hydrogenbonds between speciÜc complemen-tary bases to form a spiral ladder-shaped molecule (double helix: seeillustration).

When the cell divides, its DNA alsoreplicates in such a way that each ofthe two daughter molecules is identi-cal to the parent molecule. Thehydrogen bonds between the com-plementary bases on the two strandsof the parent molecule break and thestrands unwind. Using as buildingbricks nucleotides present in thenucleus, each strand directs the

183 DNA

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synthesis of a new one complemen-tary to itself. Replication is initiated,controlled, and stopped by means ofpolymerase enzymes.

Döbereiner’s triads A set of triadsof chemically similar elements notedby Johann Döbereiner (1780–1849) in1817. Even with the inaccurateatomic mass data of the day it wasobserved that when each triad wasarranged in order of increasingatomic mass, then the mass of thecentral member was approximatelythe average of the values for theother two. The chemical and physicalproperties were similarly related. The

triads are now recognized as consecu-tive members of the groups of theperiodic table. Examples are: lithium,sodium, and potassium; calcium,strontium, and barium; and chlorine,bromine, and iodine.

A• Döbereiner’s original paper

dodecanoic acid (lauric acid) Awhite crystalline *fatty acid,CH3(CH2)10COOH; r.d. 0.8; m.p. 44°C;b.p. 225°C. Glycerides of the acid arepresent in natural fats and oils (e.g.coconut and palm-kernel oil).

dodecene A straight-chain alkene,

Döbereiner’s triads 184

d

hydrogen bondthymine

(T)adenine

(A)

O–

O–

PO

OCH 2 O

OH

P

O–O

OCH 2 O

O

P

O–O

OCH 2

HO

O

P

O–

OCH 2

H

O

O

OH H

A T

T A

G C

C G

T A

G C

A T

A T

C G

base

sugar–phosphatebackbone

hydrogenbond

3.4 nm

Double helical structure of DNA

Detail of molecular structure of sugar–phosphatebackbone. Each deoxyribose unit is attached to aphosphate group and a base, forming a nucleotide

CH3 O

NN

OHN H O N

NN H NN

O H NN

H

H NH

NHN

N

N

cytosine(C)

guanine(G)

sugar–phosphatebackbone

The four bases of DNA, showing thehydrogen bonding between base pairs

nucleotide

deoxyribose

bases

DNA


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CH3(CH2)9CH:CH2, obtained from pe-troleum and used in making *dode-cylbenzene.

dodecylbenzene A hydrocarbon,CH3(CH2)11C6H5, manufactured by aFriedel–Crafts reaction between do-decene (CH3(CH2)9CH:CH2) and ben-zene. It can be sulphonated, and thesodium salt of the sulphonic acid isthe basis of common *detergents.

dolomite A carbonate mineral con-sisting of a mixed calcium–magne-sium carbonate, CaCO3.MgCO3,crystallizing in the rhombohedralsystem. It is usually white or colour-less. The term is also used to denotea rock with a high ratio of magne-sium to calcium carbonate. See lime-stone.

domain A functional unit of thetertiary structure of a *protein. Itconsists of chains of amino acidsfolded into alpha helices and *betasheets to form a globular structure.Different domains are linked to-gether by relatively straight sectionsof polypeptide chain to form the pro-tein molecule. Domains allow a de-gree of movement in the proteinstructure.

Donnan equilibrium The equilib-rium set up when two solutions areseparated by a membrane permeableto some but not all of the ions in thesolutions. In practice, the membraneis often permeable to the solvent andsmall ions but not to charged entitiesof colloidal size or such polyelec-trolytes as proteins. An electrical po-tential develops between the twosides of the membrane with the twosolutions having varying osmoticpressure. Donnan equilibrium isnamed after the British chemist Fred-erick George Donnan (1870–1956),who developed the theory of mem-brane equilibrium. Donnan equilib-rium is important in biology.

donor An atom, ion, or moleculethat provides a pair of electrons informing a coordinate bond.

dopa (dihydroxyphenylalanine) Aderivative of the amino acid tyrosine.It is found in particularly high levelsin the adrenal glands and is a precur-sor in the synthesis of *dopamine,*noradrenaline, and *adrenaline.The laevorotatory form, L-dopa, is ad-ministered in the treatment ofParkinson’s disease, in which brainlevels of dopamine are reduced.

185 dose

d

CH2

NH2

CH2

OHOH

OH

Dopamine

OH

OH

CH2

CH2

NH2

Dopa

dopamine A *catecholamine thatis a precursor in the synthesis of *no-radrenaline and *adrenaline. It alsofunctions as a neurotransmitter inthe brain.

d-orbital See orbital.

dose A measure of the extent towhich matter has been exposed to*ionizing radiation. The absorbeddose is the energy per unit mass ab-sorbed by matter as a result of suchexposure. The SI unit is the gray, al-though it is often measured in rads (1rad = 0.01 gray; see radiation units).The maximum permissible dose is therecommended upper limit of ab-sorbed dose that a person or organshould receive in a speciÜed period


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according to the International Com-mission on Radiological Protection.

dosimeter Any device used tomeasure absorbed *dose of ionizingradiation. Methods used include theionization chamber, photographicÜlm, or the rate at which certainchemical reactions occur in the pres-ence of ionizing radiation.

double bond See chemical bond.

double decomposition Seemetathesis.

double layer See electrical dou-ble layer.

double refraction The property,possessed by certain crystals (notablycalcite), of forming two refracted raysfrom a single incident ray. The ordi-nary ray obeys the normal laws of re-fraction. The other refracted ray,called the extraordinary ray, followsdifferent laws. The light in the ordi-nary ray is polarized at right anglesto the light in the extraordinary ray.Along an optic axis the ordinary andextraordinary rays travel with thesame speed. Some crystals, such ascalcite, quartz, and tourmaline, haveonly one optic axis; they are uniaxialcrystals. Others, such as mica and se-lenite, have two optic axes; they arebiaxial crystals. The phenomenon isalso known as birefringence and thedouble-refracting crystal as a birefrin-gent crystal. See also polarization.

double salt A crystalline salt inwhich there are two different anionsand/or cations. An example is themineral dolomite, CaCO3.MgCO3,which contains a regular arrange-ment of Ca2+ and Mg2+ ions in itscrystal lattice. *Alums are double sul-phates. Double salts only exist in thesolid; when dissolved they act as amixture of the two separate salts.Double oxides are similar.

doublet A pair of associated lines

in certain spectra, e.g. the two linesthat make up the sodium D-lines.

Downs process A process for ex-tracting sodium by the electrolysis ofmolten sodium chloride. The Downscell has a central graphite anode sur-rounded by a cylindrical steel cath-ode. Chlorine released is led awaythrough a hood over the anode.Molten sodium is formed at the cath-ode and collected through anotherhood around the top of the cathodecylinder (it is less dense than thesodium chloride). The two hoods andelectrodes are separated by a coaxialcylindrical steel gauze. A smallamount of calcium chloride is addedto the sodium chloride to lower itsmelting point. The sodium chlorideis melted electrically and keptmolten by the current through thecell. More sodium chloride is addedas the electrolysis proceeds.

Dow process A method of extract-ing magnesium from sea water byadding calcium hydroxide to precipi-tate magnesium hydroxide.

Dragendorff test A *presumptivetest for alkaloids. The Dragendorffreagent has two solutions. One is bis-muth nitrate in acetic acid. This isfollowed by a sodium nitrate solu-tion. Alkaloids are indicated by a red-dish-brown deposit.

dropping-mercury electrode Seepolarography.

dry cell A primary or secondary cellin which the electrolytes are re-strained from Ûowing in some way.Many torch, radio, and calculator bat-teries are *Leclanché cells in whichthe electrolyte is an ammonium chlo-ride paste and the container is thenegative zinc electrode (with anouter plastic wrapping). VariousmodiÜcations of the Leclanché cellare used in dry cells. In the zinc chlo-ride cell, the electrolyte is a paste of

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zinc chloride rather than ammoniumchloride. The electrical characteris-tics are similar to those of theLeclanché cell but the cell works bet-ter at low temperatures and hasmore efÜcient depolarization charac-teristics. A number of alkaline sec-ondary cells can be designed for useas dry cells. In these, the electrolyteis a liquid (sodium or potassium hy-droxide) held in a porous material orin a gel. Alkaline dry cells typicallyhave zinc–manganese dioxide, silveroxide–zinc, nickel–cadmium, ornickel–iron electrode systems (seenickel–iron accumulator). For spe-cialized purposes, dry cells and bat-teries have been produced with solidelectrolytes. These may contain asolid crystalline salt, such as silver io-dide, an ion-exchange membrane, oran organic wax with a small amountof dissolved ionic material. Such cellsdeliver low currents. They are usedin miniature cells for use in elec-tronic equipment.

dry ice Solid carbon dioxide used asa refrigerant. It is convenient becauseit sublimes at –78°C (195 K) at stan-dard pressure rather than melting.

drying oil A natural oil, such as lin-seed oil, that hardens on exposure tothe air. Drying oils contain unsatu-rated fatty acids, such as linoleic andlinolenic acids, which polymerize onoxidation. They are used in paints,varnishes, etc.

DSC Differential scanning calorime-try. See thermal analysis.

D-series See absolute configura-tion.

DTA Differential thermal analysis.See thermal analysis.

dubnium Symbol Db. A radioactive*transactinide element; a.n. 105. Itwas Ürst reported in 1967 by a groupat Dubna near Moscow and was

conÜrmed in 1970 at Dubna and atBerkeley, California. It can be madeby bombarding californium–249 nu-clei with nitrogen–15 nuclei. Only afew atoms have ever been made.


Dulong and Petit’s law For asolid element the product of the rela-tive atomic mass and the speciÜcheat capacity is a constant equal toabout 25 J mol–1 K–1. Formulated inthese terms in 1819 by the Frenchscientists Pierre Dulong (1785–1838)and Alexis Petit (1791–1820), the lawin modern terms states: the molarheat capacity of a solid element is ap-proximately equal to 3R, where R isthe *gas constant. The law is only ap-proximate but applies with fair accu-racy at normal temperatures toelements with a simple crystal struc-ture.

A• Original paper

Dumas, Jean Baptiste André(1800–84) French chemist, who be-came an apothecary in Geneva,where in 1818 he investigated theuse of iodine to treat goitre. He thentook up chemistry and moved toParis. In 1826 he devised a method ofmeasuring *vapour density. He wenton to discover various organic com-pounds, including anthracene (1832),urethane (1833), and methanol(1834), which led him in 1840 to pro-pose the theory of types (functionalgroups).

Dumas’ method 1. A method ofÜnding the amount of nitrogen in anorganic compound. The sample isweighed, mixed with copper(II)oxide, and heated in a tube. Any ni-trogen present in the compound isconverted into oxides of nitrogen,which are led over hot copper to re-duce them to nitrogen gas. This is

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collected and the volume measured,from which the mass of nitrogen in aknown mass of sample can be found.2. A method of Ünding the relativemolecular masses of volatile liquidsby weighing. A thin-glass bulb with along narrow neck is used. This isweighed full of air at known temper-ature, then a small amount of sampleis introduced and the bulb heated (ina bath) so that the liquid is vaporizedand the air is driven out. The tip ofthe neck is sealed and the bulbcooled and weighed at known (room)temperature. The volume of the bulbis found by Ülling it with water andweighing again. If the density of airis known, the mass of vapour in aknown volume can be calculated.

The techniques are named afterJean Baptiste André *Dumas.

duplet A pair of electrons in a cova-lent chemical bond.

Duquenois–Levine test A widelyused presumptive test for tetrahydro-cannabinol and other cannabinoids.The Duquenois–Levine reagent has asolution containing 2% vanillin and1% ethanal. Concentrated hydrochlo-ric acid is added and then chloro-form. A purple colour in thechloroform layer indicates a positiveresult.

Duralumin Tradename for a classof strong lightweight aluminium al-loys containing copper, magnesium,manganese, and sometimes silicon.Duralumin alloys combine strengthwith lightness and are extensivelyused in aircraft, racing cars, etc.

Dutch metal An alloy of copperand zinc, which can be produced invery thin sheets and used as imita-tion gold leaf. It spontaneously in-Ûames in chlorine.

dye laser A type of laser in whichthe active material is a dye dissolvedin a suitable solvent (e.g. Rhodanine

G in methanol). The dye is excited byan external source. The solventbroadens the states into bands andconsequently laser action can be ob-tained over a range of wavelengths.This allows one to select a speciÜcwavelength (using a grating) and tochange the wavelength of the laser.Such a device is called a tuneablelaser. Dye lasers are also used in pro-ducing very short pulses of radiation.The technique is to use a dye thatstops absorbing radiation when ahigh proportion of its molecules be-come excited. The cavity then be-comes resonant and a pulse ofradiation is produced. This techniquecan give pulses of about 10 nanosec-onds duration and is used in *femto-chemistry.

dyes Substances used to impart col-our to textiles, leather, paper, etc.Compounds used for dyeing(dyestuffs) are generally organic com-pounds containing conjugated dou-ble bonds. The group producing thecolour is the *chromophore; othernoncoloured groups that inÛuence orintensify the colour are called *aux-ochromes. Dyes can be classiÜed ac-cording to the chemical structure ofthe dye molecule. For example, azodyes contain the –N=N– group (seeazo compounds). In practice, theyare classiÜed according to the way inwhich the dye is applied or is held onthe substrate.

Acid dyes are compounds in which the chromophore is part of a negative ion (usually an organicsulphonate RSO2O–). They can beused for protein Übres (e.g. wool andsilk) and for polyamide and acrylicÜbres. Originally, they were appliedfrom an acidic bath. Metallized dyesare forms of acid dyes in which thenegative ion contains a chelatedmetal atom. Basic dyes have chro-mophores that are part of a positive

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ion (usually an amine salt or ionizedimino group). They are used foracrylic Übres and also for wool andsilk, although they have only moder-ate fastness with these materials.

Direct dyes are dyes that have ahigh afÜnity for cotton, rayon, andother cellulose Übres. They are ap-plied directly from a neutral bathcontaining sodium chloride orsodium sulphate. Like acid dyes, theyare usually sulphonic acid salts butare distinguished by their greatersubstantivity (afÜnity for the sub-strate), hence the alternative namesubstantive dyes.

Vat dyes are insoluble substancesused for cotton dyeing. They usuallycontain keto groups, C=O, which arereduced to C–OH groups, renderingthe dye soluble (the leuco form of thedye). The dye is applied in this form,then oxidized by air or oxidizingagents to precipitate the pigment inthe Übres. Indigo and anthroquinonedyes are examples of vat dyes. Sul-phur dyes are dyes applied by thistechnique using sodium sulphide so-lution to reduce and dissolve the dye.Sulphur dyes are used for celluloseÜbres.

Disperse dyes are insoluble dyesapplied in the form of a Üne disper-sion in water. They are used for cellu-lose acetate and other syntheticÜbres.

Reactive dyes are compounds thatcontain groups capable of reactingwith the substrate to form covalentbonds. They have high substantivityand are used particularly for cellu-lose Übres.

dynamical system A system gov-erned by dynamics (either classicalmechanics or quantum mechanics).

The evolution of dynamical systemscan be very complex, even for sys-tems with only a few degrees of free-dom, sometimes involving suchconsiderations as ergodicity, whichoriginated in *statistical mechanics.The evolution of dynamical systemscan be studied using the phase spacefor the system. *Chaos is an exampleof the complex behaviour that canoccur in a dynamical system.

dynamic equilibrium See equilib-rium.

dynamite Any of a class of high ex-plosives based on nitroglycerin. Theoriginal form, invented in 1867 byAlfred Nobel, consisted of nitroglyc-erin absorbed in kieselguhr. Moderndynamites, which are used for blast-ing, contain sodium or ammoniumnitrate sensitized with nitroglycerinand use other absorbers (e.g. woodpulp).

dysprosium Symbol Dy. A soft sil-very metallic element belonging tothe *lanthanoids; a.n. 66; r.a.m.162.50; r.d. 8.551 (20°C); m.p. 1412°C;b.p. 2562°C. It occurs in apatite,gadolinite, and xenotime, fromwhich it is extracted by an ion-exchange process. There are sevennatural isotopes and twelve artiÜcialisotopes have been identiÜed. It Ündslimited use in some alloys as a neu-tron absorber, particularly in nucleartechnology. It was discovered by PaulLecoq de Boisbaudran (1838–1912) in1886.


dystectic mixture A mixture ofsubstances that has a constant maxi-mum melting point.

189 dystectic mixture

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E

EAC Emergency action code. Seehazchem code.

Earnshaw’s theorem The princi-ple that a system of particles interact-ing via an inverse square law, such asCoulomb’s law of electrostatics, can-not exist in a state of static equilib-rium. The Reverend SamuelEarnshaw (1805–88) proved this re-sult in 1842. The theorem is of funda-mental importance in chemistrysince it shows that it is not possibleto construct a correct model of atomsand molecules if the electrons aretaken to be stationary.

earth The planet that orbits the sunbetween the planets Venus and Mars.The earth consists of three layers: thegaseous atmosphere (see earth’s at-mosphere), the liquid hydrosphere,and the solid lithosphere. The solidpart of the earth also consists ofthree layers: the crust with a meanthickness of about 32 km under theland and 10 km under the seas; themantle, which extends some2900 km below the crust; and thecore, part of which is believed to beliquid. The composition of the crustis: oxygen 47%, silicon 28%, alu-minium 8%, iron 4.5%, calcium 3.5%,sodium and potassium 2.5% each,and magnesium 2.2%. Hydrogen,carbon, phosphorus, and sulphur areall present to an extent of less than1%.

earth’s atmosphere The gas thatsurrounds the earth. The composi-tion of dry air at sea level is: nitrogen78.08%, oxygen 20.95%, argon 0.93%,carbon dioxide 0.03%, neon 0.0018%,helium 0.0005%, krypton 0.0001%,

and xenon 0.00001%. In addition towater vapour, air in some localitiescontains sulphur compounds, hydro-gen peroxide, hydrocarbons, anddust particles.

ebonite See vulcanite.

ebullioscopic constant See eleva-tion of boiling point.

ebullioscopy The use of *elevationof boiling point to determine relativemolecular masses.

echelon A form of interferometerconsisting of a stack of glass platesarranged stepwise with a constantoffset. It gives a high resolution andis used in spectroscopy to studyhyperÜne line structure.

eclipsed See conformation.

eclipsed conformation See con-formation.

eclipsing See conformation.

ecstasy (methylenedioxymetham-phetamine; MDMA) A designer drugbased on methamphetamine (see am-phetamines). Originally intended asan appetite suppressant, it producesa feeling of euphoria and is widelyused as a club drug. It is a class Adrug in the UK.

O

O

CH2

NH

CH3

CH3

Ecstasy

Edison cell See nickel–iron accu-mulator.


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EDTA Ethylenediaminetetraaceticacid,

(HOOCCH2)2N(CH2)2N(CH2COOH)2, a compound that acts as a chelatingagent, reversibly binding with iron,magnesium, and other metal ions. Itis used in certain culture mediabound with iron, which it slowly re-leases into the medium, and also insome forms of quantitative analysis.

EELS (electron energy loss spec-troscopy) A technique for studyingadsorbates and their dissociation. Abeam of electrons is reÛected from asurface and the energy loss they suf-fer upon reÛection is measured. Thisloss can be used to interpret the vi-brational spectrum of the adsorbate.Very small amounts of adsorbate canbe detected using EELS (as few as 50atoms in a sample); this is particu-larly useful for light elements thatare not readily detected using othertechniques.

effervescence The formation ofgas bubbles in a liquid by chemicalreaction.

efÜciency A measure of the perfor-mance of a machine, engine, etc.,being the ratio of the energy orpower it delivers to the energy orpower fed to it. In general, the efÜ-ciency of a machine varies with theconditions under which it operatesand there is usually a load at which itoperates with the highest efÜciency.The thermal efÜciency of a heat en-gine is the ratio of the work done bythe engine to the heat supplied bythe fuel. For a reversible heat enginethis efÜciency equals (T1 – T2)/T1,where T1 is the thermodynamic tem-perature at which all the heat istaken up and T2 is the thermody-namic temperature at which it isgiven out (see carnot cycle). For realengines it is always less than this.

efÛorescence The process in

which a crystalline hydrate loseswater, forming a powdery deposit onthe crystals.

effusion The Ûow of a gas througha small aperture. The relative rates atwhich gases effuse, under the sameconditions, is approximately in-versely proportional to the squareroots of their densities.

Ehrenfest classiÜcation AclassiÜcation of phase transitions interms of their thermodynamic prop-erties put forward by the Dutchphysicist Paul Ehrenfest (1880–1933).A Ürst-order phase transition is aphase transition in which the Ürst de-rivative of the chemical potential isdiscontinuous. In a Ürst-order phasetransition there is a nonzero changein the value of the enthalpy, entropy,and volume at the transition temper-ature. Melting and boiling are exam-ples of Ürst-order phase transitions.In a second-order phase transition,the Ürst derivative of the chemicalpotential is continuous but its secondderivative is not continuous. In a sec-ond-order phase transition there isno jump in the value of the enthalpy,entropy, and volume at the transitiontemperature. Examples of second-order phase transitions include thetransition to ferromagnetism andorder–disorder transitions in alloys.

Ehrlich’s reagent See p-dimethyl-aminobenzadehyde.

eigenfunction In general, a solu-tion of an *eigenvalue equation. In*quantum mechanics eigenfunctionsoccur as the allowed wave functionsof a system and so satisfy the*Schrödinger equation.

eigenvalues The allowed set of val-ues of the constants in an eigenvalueequation. In an eigenvalue equationthe left-hand side consists of an oper-ator and an *eigenfunction, uponwhich the operator operates; the

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right-hand side of the equation con-sists of the product of the sameeigenfunction and a constant. Eigen-value equations occur in quantummechanics, with the eigenvaluesbeing the values of the quantizedquantities. In particular, theSchrödinger equation is an eigen-value equation with the allowedquantized energy levels being theeigenvalues of this equation.

Einstein, Albert (1879–1955) Ger-man-born US physicist, who tookSwiss nationality in 1901. A yearlater he went to work in the Bernpatent ofÜce. In 1905 he publishedÜve enormously inÛuential papers,one on *Brownian movement, oneon the *photoelectric effect, one onthe special theory of relativity, andone on energy and inertia (which in-cluded the famous expression E =mc2). In 1915 he published the gen-eral theory of relativity, concernedmainly with gravitation. In 1921 hewas awarded the Nobel Prize forphysics. In 1933, as a Jew, Einsteindecided to remain in the USA (wherehe was lecturing), as Hitler had cometo power. For the remainder of hislife he sought a uniÜed Üeld theory.In 1939 he informed President Roo-sevelt that an atom bomb was feasi-ble and that Germany might be ableto make one.

Einstein coefÜcients CoefÜcientsused in the quantum theory of radia-tion, related to the probability of atransition occurring between theground state and an excited state (orvice versa) in the processes of *in-duced emission and *spontaneousemission. For an atom exposed toelectromagnetic radiation, the rate ofabsorption Ra is given by Ra = Bρ,where ρ is the density of electromag-netic radiation and B is the Einstein BcoefÜcient associated with absorp-tion. The rate of induced emission is

also given by Bρ, with the coefÜcientB of induced emission being equal tothe coefÜcient of absorption. The rateof spontaneous emission is given byA, where A is the Einstein A coefÜ-cient of spontaneous emission. The Aand B coefÜcients are related by A =8πhν3B/c3, where h is the Planck con-stant, ν is the frequency of electro-magnetic radiation, and c is the speedof light. The coefÜcients were put for-ward by Albert *Einstein in 1916–17in his analysis of the quantum theoryof radiation.

Einstein equation 1. The mass–energy relationship announced by Al-bert *Einstein in 1905 in the form E= mc2, where E is a quantity of en-ergy, m its mass, and c is the speed oflight. It presents the concept that en-ergy possesses mass. 2. The relation-ship Emax = hf – W, where Emax is themaximum kinetic energy of elec-trons emitted in the photoemissiveeffect, h is the Planck constant, f thefrequency of the incident radiation,and W the *work function of theemitter. This is also written Emax = hf– φe, where e is the electronic chargeand φ a potential difference, alsocalled the work function. (SometimesW and φ are distinguished as workfunction energy and work functionpotential.) The equation can also beapplied to photoemission from gases,when it has the form: E = hf – I,where I is the ionization potential ofthe gas.

einsteinium Symbol Es. A radio-active metallic transuranic elementbelonging to the *actinoids; a.n. 99;mass number of the most stable iso-tope 254 (half-life 270 days). Elevenisotopes are known. The element wasÜrst identiÜed by A. Ghiorso and as-sociates in debris from the Ürst hy-drogen bomb explosion in 1952.Microgram quantities of the element

Einstein, Albert 192

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did not become available until 1961.It is named after Albert *Einstein.


Einstein–Smoluchowski equa-tion A relation between the diffu-sion coefÜcient D and the distance λ that a particle can jump when dif-fusing in a time τ. The Einstein–Smoluchowski equation, which is D = λ2/2τ, gives a connection betweenthe microscopic details of particle dif-fusion and the macroscopic quanti-ties associated with the diffusion,such as the viscosity. The equation isderived by assuming that the parti-cles undergo a random walk. Thequantities in the equation can be re-lated to quantities in the *kinetictheory of gases, with λ/τ taken to bethe mean speed of the particles and λ their mean free path. The Einstein–Smoluchowski equation was derivedby Albert Einstein and the Polishphysicist Marian Ritter von Smolan-Smoluchowski.

Einstein theory of speciÜc heatA theory of the speciÜc heat capacityof solids put forward by Albert *Ein-stein in 1906, in which it was as-sumed that the speciÜc heat capacityis a consequence of the vibrations ofthe atoms of the lattice of the solid.Einstein assumed that each atom hasthe same frequency ν. The theoryleads to the correct conclusion thatthe speciÜc heat of solids tends tozero as the temperature goes to ab-solute zero, but does not give a cor-rect quantitative description of thelow-temperature behaviour of thespeciÜc heat capacity. In the *Debyetheory of speciÜc heat, and in otheranalyses of this problem, Einstein’ssimplifying approximation was im-proved on by taking account of thefact that the frequencies of lattice vi-brations can have a range of values.

E-isomer See e–z convention.

elastic collision A collision inwhich the total kinetic energy of thecolliding bodies after collision isequal to their total kinetic energy be-fore collision. Elastic collisions occuronly if there is no conversion of ki-netic energy into other forms, as inthe collision of atoms. In the case ofmacroscopic bodies this will not bethe case as some of the energy willbecome heat. In a collision betweenpolyatomic molecules, some kineticenergy may be converted into vibra-tional and rotational energy of themolecules.

elastin A Übrous protein that is themajor constituent of the yellow elas-tic Übres of connective tissue. It isrich in glycine, alanine, proline, andother nonpolar amino acids that arecross-linked, making the protein rela-tively insoluble. Elastic Übres canstretch to several times their lengthand then return to their original size.Elastin is particularly abundant inelastic cartilage, blood-vessel walls,ligaments, and the heart.

elastomer A natural or syntheticrubber or rubberoid material, whichhas the ability to undergo deforma-tion under the inÛuence of a forceand regain its original shape once theforce has been removed.

electret A permanently electriÜedsubstance or body that has oppositecharges at its extremities. Electretsresemble permanent magnets inmany ways. An electret can be madeby cooling certain waxes in a strongelectric Üeld.

electrical double layer A modelof the interface between an electrodeand the solution close to it. In thismodel a sheet of one type of elec-trical charge surrounds the surface of the electrode and a sheet of the opposite charge surrounds the

193 electrical double layer

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Ürst sheet in the solution. In theHelmholtz model the double layer isregarded as consisting of two planesof charge, with the inner plane ofions from the solution being causedby the charge on the electrode andthe outer plane being caused by op-positely charged ions in the solutionresponding to the Ürst layer of ions.In the *Gouy–Chapman model (dif-fuse double layer) thermal motion ofions is taken into account. Neithermodel is completely successful sincethe Helmholtz model exaggerates therigidity of the structure of thecharges and the Gouy–Chapmanmodel underestimates the rigidity ofthe structure. The Stern model im-proves on both models by assumingthat the ions next to the electrodehave a rigid structure, while takingthe second layer to be as describedby the Gouy–Chapman model.

electric-arc furnace A furnaceused in melting metals to make al-loys, especially in steel manufacture,in which the heat source is an elec-tric arc. In the direct-arc furnace,such as the Héroult furnace, an arc isformed between the metal and anelectrode. In the indirect-arc furnace,such as the Stassano furnace, the arcis formed between two electrodesand the heat is radiated onto themetal.

electrochemical cell See cell.

electrochemical equivalentSymbol z. The mass of a given el-ement liberated from a solution of itsions in electrolysis by one coulombof charge. See faraday’s laws.

electrochemical series See elec-tromotive series.

electrochemistry The study ofchemical properties and reactions in-volving ions in solution, includingelectrolysis and electric cells.

electrochromatography See elec-trophoresis.

electrocyclic reaction A type ofcyclic rearrangement in which asigma bond is formed between twoterminal carbon atoms of a conju-gated molecule, resulting in a de-crease of one in the number of pibonds present.

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Electrocyclic reaction

electrode 1. A conductor thatemits or collects electrons in a cell,thermionic valve, semiconductor de-vice, etc. The anode is the positiveelectrode and the cathode is the neg-ative electrode. 2. See half cell.

electrodeposition The process ofdepositing one metal on another byelectrolysis, as in *electroformingand *electroplating.

electrode potential The potentialdifference produced between theelectrode and the solution in a *halfcell. It is not possible to measure thisdirectly since any measurement in-volves completing the circuit withthe electrolyte, thereby introducinganother half cell. Standard electrodepotentials EŠ are deÜned by measur-ing the potential relative to a stan-dard *hydrogen half cell using 1.0molar solution at 25°C. The conven-tion is to designate the cell so thatthe oxidized form is written Ürst. Forexample,

Pt(s)|H2(g)H+(aq)|Zn2+(aq)|Zn(s)The e.m.f. of this cell is –0.76 volt (i.e.the zinc electrode is negative). Thusthe standard electrode potential ofthe Zn2+|Zn half cell is –0.76 V. Elec-trode potentials are also called reduc-


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tion potentials. See also electromo-tive series.

electrodialysis A method of ob-taining pure water from water con-taining a salt, as in *desalination.The water to be puriÜed is fed into acell containing two electrodes. Be-tween the electrodes is placed anarray of *semipermeable membranesalternately semipermeable to posi-tive ions and negative ions. The ionstend to segregate between alternatepairs of membranes, leaving purewater in the other gaps betweenmembranes. In this way, the feedwater is separated into two streams:one of pure water and the other ofmore concentrated solution.

electroendosmosis See electro-osmosis.

electroforming A method of form-ing intricate metal articles or partsby *electrodeposition of the metal ona removable conductive mould.

electrokinetic potential (zeta po-tential) Symbol ζ. The electric poten-tial associated with an *electricaldouble layer around a colloid at theradius of shear, relative to the valueof the potential in the bulk of the so-lution far from the colloid, where theradius of shear is the radius of theentity made up of the colloid and therigid layer of ions at the surface ofthe colloid.

electroluminescence See lumines-cence.

electrolysis The production of achemical reaction by passing an elec-tric current through an electrolyte.In electrolysis, positive ions migrateto the cathode and negative ions tothe anode. The reactions occurringdepend on electron transfer at theelectrodes and are therefore redox re-actions. At the anode, negative ionsin solution may lose electrons to

form neutral species. Alternatively,atoms of the electrode can lose elec-trons and go into solution as positiveions. In either case the reaction is anoxidation. At the cathode, positiveions in solution can gain electrons toform neutral species. Thus cathodereactions are reductions.

electrolyte A liquid that conductselectricity as a result of the presenceof positive or negative ions. Elec-trolytes are molten ionic compoundsor solutions containing ions, i.e. solu-tions of ionic salts or of compoundsthat ionize in solution. Liquid metals,in which the conduction is by freeelectrons, are not usually regarded aselectrolytes. Solid conductors of ions,as in the sodium–sulphur cell, arealso known as electrolytes.

electrolytic cell A cell in whichelectrolysis occurs; i.e. one in whichcurrent is passed through the elec-trolyte from an external source.

electrolytic corrosion Corrosionthat occurs through an electrochemi-cal reaction. See rusting.

electrolytic gas (detonating gas)The highly explosive gas formed bythe electrolysis of water. It consists oftwo parts hydrogen and one part oxy-gen by volume.

electrolytic reÜning The puriÜca-tion of metals by electrolysis. It iscommonly applied to copper. A largepiece of impure copper is used as theanode with a thin strip of pure cop-per as the cathode. Copper(II) sul-phate solution is the electrolyte.Copper dissolves at the anode: Cu →Cu2+ + 2e, and is deposited at thecathode. The net result is transfer ofpure copper from anode to cathode.Gold and silver in the impure copperform a so-called anode sludge at thebottom of the cell, which is recov-ered.

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electrolytic separation A methodof separating isotopes by exploitingthe different rates at which they arereleased in electrolysis. It was for-merly used for separating deuteriumand hydrogen. On electrolysis ofwater, hydrogen is formed at thecathode more readily than deu-terium, thus the water becomes en-riched with deuterium oxide.

electromagnetic radiation En-ergy resulting from the accelerationof electric charge and the associatedelectric Üelds and magnetic Üelds.The energy can be regarded as wavespropagated through space (requiringno supporting medium) involving os-cillating electric and magnetic Üeldsat right angles to each other and tothe direction of propagation. In a vac-uum the waves travel with a con-stant speed (the speed of light) of2.9979 × 108 metres per second; ifmaterial is present they are slower.Alternatively, the energy can be re-garded as a stream of *photons trav-elling at the speed of light, eachphoton having an energy hc/λ, whereh is the Planck constant, c is thespeed of light, and λ is the wave-length of the associated wave. A fu-sion of these apparently conÛictingconcepts is possible using the meth-ods of *quantum mechanics. Thecharacteristics of the radiation de-pend on its wavelength. See electro-magnetic spectrum.

electromagnetic spectrum Therange of wavelengths over whichelectromagnetic radiation extends.The longest waves (105–10–3 metres)are radio waves, the next longest(10–3–10–6 m) are infrared waves,then comes the narrow band (4–7 ×10–7 m) of visible radiation, followedby ultraviolet waves (10–7–10–9 m)and *X-rays and gamma radiation(10–9–10–14 m).

electromeric effect See elec-tronic effects.

electrometallurgy The uses ofelectrical processes in the separationof metals from their ores, the reÜn-ing of metals, or the forming or plat-ing of metals.

electromotive force (e.m.f.) Thegreatest potential difference that canbe generated by a particular sourceof electric current. In practice thismay be observable only when thesource is not supplying current, be-cause of its internal resistance.

electromotive series (electro-chemical series) A series of chemicalelements arranged in order of their*electrode potentials. The hydrogenelectrode (H+ + e → ½H2) is taken ashaving zero electrode potential. El-ements that have a greater tendencythan hydrogen to lose electrons totheir solution are taken as elec-tropositive; those that gain electronsfrom their solution are below hydro-gen in the series and are called elec-tronegative. The series shows theorder in which metals replace oneanother from their salts; electroposi-tive metals will replace hydrogenfrom acids. The chief metals and hy-drogen, placed in order in the series,are: potassium, calcium, sodium,magnesium, aluminium, zinc,cadmium, iron, nickel, tin, lead,hydrogen, copper, mercury, silver,platinum, gold. This type of series issometimes referred to as an activityseries.

electron An elementary particlewith a rest mass of 9.109 3897(54) ×10–31 kg and a negative charge of1.602 177 33(49) × 10–19 coulomb.Electrons are present in all atoms ingroupings called shells around thenucleus; when they are detachedfrom the atom they are called free

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electrons. The antiparticle of the elec-tron is the positron.

electron afÜnity Symbol A. Theenergy change occurring when anatom or molecule gains an electronto form a negative ion. For an atomor molecule X, it is the energy re-leased for the electron-attachment re-action

X(g) + e → X–(g)Often this is measured in electron-volts. Alternatively, the molar en-thalpy change, ∆H, can be used.

electron capture 1. The formationof a negative ion by an atom or mol-ecule when it acquires an extra freeelectron. 2. A radioactive transfor-mation in which a nucleus acquiresan electron from an inner orbit ofthe atom, thereby transforming, ini-tially, into a nucleus with the samemass number but an atomic numberone less than that of the original nu-cleus (capture of the electron trans-forms a proton into a neutron). Thistype of capture is accompanied byemission of an X-ray photon or Augerelectron as the vacancy in the innerorbit is Ülled by an outer electron.

electron conÜguration See con-figuration.

electron-deÜcient compound Acompound in which there are fewerelectrons forming the chemicalbonds than required in normal elec-tron-pair bonds. Such compounds use*multicentre bonds. See borane.

electron diffraction Diffraction ofa beam of electrons by atoms or mol-ecules. The fact that electrons can bediffracted in a similar way to lightand X-rays shows that particles canact as waves (see de broglie wave-length). An electron (mass m, chargee) accelerated through a potential dif-ference V acquires a kinetic energymv2/2 = eV, where v is the velocity of

the electron (nonrelativistic). Thus,the momentum (p) of the electron is√(2eVm). As the de Broglie wave-length (λ) of an electron is given byh/p, where h is the Planck constant,then λ = h/√(2eVm). For an accelerat-ing voltage of 3600 V, the wave-length of the electron beam is 0.02nanometre, some 3 × 104 timesshorter than visible radiation.

Electrons then, like X-rays, showdiffraction effects with moleculesand crystals in which the interatomicspacing is comparable to the wave-length of the beam. They have theadvantage that their wavelength canbe set by adjusting the voltage. Un-like X-rays they have very low pene-trating power. The Ürst observationof electron diffraction was by GeorgePaget *Thomson in 1927, in an ex-periment in which he passed a beamof electrons in a vacuum through avery thin gold foil onto a photo-graphic plate. Concentric circleswere produced by diffraction of elec-trons by the lattice. The same yearClinton J. Davisson (1881–1958) andLester Germer (1896–1971) per-formed a classic experiment inwhich they obtained diffraction pat-terns by glancing an electron beamoff the surface of a nickel crystal.Both experiments were importantveriÜcations of de Broglie’s theoryand the new quantum theory.

Electron diffraction, because of thelow penetration, cannot easily beused to investigate crystal structure.It is, however, employed to measurebond lengths and angles of moleculesin gases. Moreover, it is extensivelyused in the study of solid surfacesand absorption. The main techniquesare low-energy electron diffraction(LEED) in which the electron beam isreÛected onto a Ûuorescent screen,and high-energy electron diffraction(HEED) used either with reÛection or

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transmission in investigating thinÜlms.

electronegative Describing el-ements that tend to gain electronsand form negative ions. The halogensare typical electronegative elements.For example, in hydrogen chloride,the chlorine atom is more elec-tronegative than the hydrogen andthe molecule is polar, with negativecharge on the chlorine atom. Thereare various ways of assigning valuesfor the electronegativity of an el-ement. Mulliken electronegativitiesare calculated from E = (I + A)/2,where I is ionization potential and Ais electron afÜnity. More commonly,Pauling electronegativities are used.These are based on bond dissociationenergies using a scale in whichÛuorine, the most electronegative el-ement, has a value 4. Some other val-ues on this scale are B 2, C 2.5, N 3.0,O 3.5, Si 1.8, P 2.1, S 2.5, Cl 3.0, Br2.8.

electron energy loss spectros-copy See eels.

electron Ûow The transfer of elec-trons along a series of carrier mol-ecules in the *electron transportchain.

electron gas A model of the elec-trons in a metal or a plasma in whichthey are regarded as forming a gasthat interacts with a uniformly dis-tributed background of positivecharge to ensure that the system iselectrically neutral. The electron gasis analysed theoretically using eitherclassical or quantum statistical me-chanics and the kinetic theory ofgases. The electron-gas model ac-counts for many properties of metalsand plasmas in a qualitative and ap-proximately quantitative way butcannot give an accurate quantitativeaccount of these systems, as thiswould require the motions of the

positive ions to be taken into ac-count.

electronic effects Effects bywhich the reactivity at one part of amolecule is affected by electron at-traction or repulsion originating inanother part of a molecule. Oftenthis is called an *inductive effect (orresonance effect), although some-times the term ‘inductive effect’ is re-served for an inÛuence transmittedthrough chemical bonds and is dis-tinguished from a Üeld effect, whichis transmitted through space. An in-ductive effect through chemicalbonds was formerly called a me-someric effect (or mesomerism) or anelectromeric effect. It is common torefer to all effects (through bonds orthrough space) as resonance effects.

electronic spectra of moleculesThe spectra associated with transi-tions between the electronic states ofmolecules. These transitions corre-spond to the visible or ultravioletregions of the electromagnetic spec-trum. There are changes in vibra-tional and rotational energy whenelectronic transitions occur. Conse-quently there are spectral bands asso-ciated with changes in vibrationalmotion, with these bands having Ünestructure due to changes in rota-tional motion. Because electronictransitions are associated withchanges in vibrational motion thecorresponding spectra are sometimescalled vibrational spectra. The elec-tronic spectra of molecules are usedto obtain information about energylevels in molecules, interatomic dis-tances, dissociation energies of mol-ecules, and force constants ofchemical bonds.

electron microscope A form ofmicroscope that uses a beam of elec-trons instead of a beam of light (as inthe optical microscope) to form alarge image of a very small object. In

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optical microscopes the resolution islimited by the wavelength of thelight. High-energy electrons, how-ever, can be associated with a consid-erably shorter wavelength than light;for example, electrons accelerated toan energy of 105 electronvolts have awavelength of 0.004 nanometre (seede broglie wavelength) enabling aresolution of 0.2–0.5 nm to beachieved. The transmission electronmicroscope has an electron beam,sharply focused by electron lenses,passing through a very thin metal-lized specimen (less than 50 nanome-tres thick) onto a Ûuorescent screen,where a visual image is formed. Thisimage can be photographed. Thescanning electron microscope can beused with thicker specimens andforms a perspective image, althoughthe resolution and magniÜcation arelower. In this type of instrument abeam of primary electrons scans the specimen and those that arereÛected, together with any sec-ondary electrons emitted, are col-lected. This current is used tomodulate a separate electron beamin a TV monitor, which scans thescreen at the same frequency, conse-quently building up a picture of thespecimen. The resolution is limitedto about 10–20 nm.

electron-nuclear double reso-nance See endor.

electron paramagnetic reso-nance (EPR) A spectroscopicmethod of locating electrons withinthe molecules of a paramagnetic sub-stance (see magnetism) in order toprovide information regarding itsbonds and structure. The spin of anunpaired electron is associated witha magnetic moment that is able toalign itself in one of two ways withan applied external magnetic Üeld.These two alignments correspond todifferent energy levels, with a statis-

tical probability, at normal tempera-tures, that there will be slightly morein the lower state than in the higher.By applying microwave radiation tothe sample a transition to the higherstate can be achieved. The precise en-ergy difference between the twostates of an electron depends on thesurrounding electrons in the atom ormolecule. In this way the position ofunpaired electrons can be investi-gated. The technique is used particu-larly in studying free radicals andparamagnetic substances such as in-organic complexes. It is also calledelectron-spin resonance (ESR). See alsonuclear magnetic resonance;endor.

electron probe microanalysis(EPM) A method of analysing a verysmall quantity of a substance (as lit-tle as 10–13 gram). The method con-sists of directing a very Ünely focusedbeam of electrons on to the sampleto produce the characteristic X-rayspectrum of the elements present. Itcan be used quantitatively for el-ements with atomic numbers in ex-cess of 11.

electron-spin resonance See elec-tron paramagnetic resonance.

electron-transfer reaction Achemical reaction that involves thetransfer, addition, or removal of elec-trons. Electron-transfer reactionsoften involve complexes of transitionmetals. In such complexes one gen-eral mechanism for electron transferis the inner-sphere mechanism, inwhich two complexes form an inter-mediate, with ligand bridges en-abling electrons to be transferredfrom one complex to another com-plex. The other main mechanism isthe outer-sphere mechanism, inwhich two complexes retain all theirligands, with electrons passing fromone complex to the other.

The rates of electron-transfer reac-

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tions vary enormously. These ratescan be explained in terms of the wayin which molecules of the solventsolvating the reactants rearrange soas to solvate the products in the caseof the outer-sphere mechanism. Inthe case of the inner-sphere (ligand-bridged) reactions the rate of the re-action depends on the intermediateand the way in which the electron istransferred.

electron transport chain (elec-tron transport system) A sequence ofbiochemical reduction–oxidation re-actions that effects the transfer ofelectrons through a series of carriers.An electron transport chain, alsoknown as the respiratory chain,forms the Ünal stage of aerobic respi-ration. It results in the transfer ofelectrons or hydrogen atoms derivedfrom the *Krebs cycle to molecularoxygen, with the formation of water.At the same time it conserves energyfrom food or light in the form of*ATP. The chain comprises a series ofcarrier molecules that undergo re-versible reduction–oxidation reac-tions, accepting electrons and thendonating them to the next carrier inthe chain – a process known as elec-tron Ûow. In the mitochondria,NADH and FADH2, generated by theKrebs cycle, transfer their electronsto a chain including coenzyme Q (seeubiquinone) and a series of *cy-tochromes. This process is coupled tothe formation of ATP at three sites

along the chain. The ATP is then car-ried across the mitochondrial mem-brane in exchange for ADP. Anelectron transport chain also occursin *photosynthesis.

electronvolt Symbol eV. A unit ofenergy equal to the work done on anelectron in moving it through a po-tential difference of one volt. It isused as a measure of particle ener-gies although it is not an *SI unit. 1eV = 1.602 × 10–19 joule.

electroorganic reaction An or-ganic reaction produced in an elec-trolytic cell. Electroorganic reactionsare used to synthesize compoundsthat are difÜcult to produce by con-ventional techniques. An example ofan electroorganic reaction is *Kolbe’smethod of synthesizing alkanes.

electroosmosis The movement ofa polar liquid through a membraneunder the inÛuence of an appliedelectric Üeld. The linear velocity ofÛow divided by the Üeld strength isthe electroosmotic mobility. Elelec-troosmosis was formerly called elec-troendosmosis.

electroosmotic mobility See elec-troosmosis.

electropherogram See capillaryelectrophoresis.

electrophile An ion or moleculethat is electron deÜcient and can ac-cept electrons. Electrophiles areoften reducing agents and Lewis

electron transport chain 200

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inner mitochondrialmembrane

H+

NADHdehydrogenase

NADH+H+

NAD+

2e– 2e–2e–

H+H+

H2O

2H+ + ½O2

ubiquinonecytochromeb-c1 complex

cytochromeoxidase

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*acids. They are either positive ions(e.g. NO2

+) or molecules that have apositive charge on a particular atom(e.g. SO3, which has an electron-deÜcient sulphur atom). In organicreactions they tend to attack nega-tively charged parts of a molecule.Compare nucleophile.

electrophilic addition An *addi-tion reaction in which the Ürst step isattack by an electrophile (e.g. a posi-tive ion) on an electron-rich part ofthe molecule. An example is additionto the double bonds in alkenes.

electrophilic substitution A*substitution reaction in which theÜrst step is attack by an electrophile.Electrophilic substitution is a featureof reactions of benzene (and its com-pounds) in which a positive ion ap-proaches the delocalized pi electronson the benzene ring.

electrophoresis (cataphoresis) Atechnique for the analysis and sepa-ration of colloids, based on the move-ment of charged colloidal particles inan electric Üeld. There are various ex-perimental methods. In one the sam-ple is placed in a U-tube and a buffersolution added to each arm, so thatthere are sharp boundaries betweenbuffer and sample. An electrode isplaced in each arm, a voltage ap-plied, and the motion of the bound-aries under the inÛuence of the Üeldis observed. The rate of migration ofthe particles depends on the Üeld,the charge on the particles, and onother factors, such as the size andshape of the particles. More simply,electrophoresis can be carried outusing an adsorbent, such as a strip ofÜlter paper, soaked in a buffer withtwo electrodes making contact. Thesample is placed between the elec-trodes and a voltage applied. Differ-ent components of the mixturemigrate at different rates, so the sam-ple separates into zones. The compo-

nents can be identiÜed by the rate at which they move. In gel elec-trophoresis the medium is a gel, typi-cally made of polyacrylamide (seepage), agarose, or starch.

Electrophoresis, which has alsobeen called electrochromatography,is used extensively in studying mix-tures of proteins, nucleic acids, car-bohydrates, enzymes, etc. In clinicalmedicine it is used for determiningthe protein content of body Ûuids.

electrophoretic deposition Atechnique for coating a materialmaking it an electrode in a bath con-taining a colloidal suspension ofcharged particles. Under suitable con-ditions, the particles are attracted to,and deposited on, the electrode. Elec-trophoretic deposition is used exten-sively in industry; for example, inapplying paint to metal components.

electrophoretic effect The effectin which the mobility of ions in solu-tion moving under the inÛuence ofan applied electric Üeld is affected bythe Ûow of ions of opposite charge inthe opposite direction.

electroplating A method of plat-ing one metal with another by *elec-trodeposition. The articles to beplated are made the cathode of anelectrolytic cell and a rod or bar ofthe plating metal is made the anode.Electroplating is used for coveringmetal with a decorative, more expen-sive, or corrosion-resistant layer ofanother metal.

electropositive Describing el-ements that tend to lose electronsand form positive ions. The alkalimetals are typical electropositive el-ements.

electrospray ionization (ESI) Atechnique for producing ions formass spectrometry, used especiallyfor obtaining ions from large mol-ecules, which would be likely to pro-

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duce fragment ions in electron-impact impact ionization sources.The sample is dissolved in a volatilesolvent, which may also containvolatile acids or bases so that thesample exists in an ionic form. Thesolution is forced through a chargedmetal capillary tube and forms anaerosol. Evaporation of the solventresults in single ions of the sample,which are analyzed by the mass spec-trometer. Electrospray ionization isthe ionization technique often usedin chromatography–mass spectrome-try.

electrovalent bond See chemicalbond.

electrum 1. An alloy of gold andsilver containing 55–88% of gold. 2. A *German silver alloy containing52% copper, 26% nickel, and 22%zinc.

element A substance that cannotbe decomposed into simpler sub-stances. In an element, all the atomshave the same number of protons orelectrons, although the number ofneutrons may vary. There are 92naturally occurring elements. See also periodic table; transuranic el-ements; transactinide elements.

elementary particle One of thefundamental particles of which mat-ter is composed, such as the electron,proton, or neutron.

elementary reaction A reactionwith no intermediates; i.e. one thattakes place in a single step with a sin-gle transition state.

elevation of boiling point An in-crease in the boiling point of a liquidwhen a solid is dissolved in it. The el-evation is proportional to the num-ber of particles dissolved (moleculesor ions) and is given by ∆t = kBC,where C is the molal concentrationof solute. The constant kB is the ebul-

lioscopic constant of the solvent andif this is known, the molecularweight of the solute can be calcu-lated from the measured value of ∆t.The elevation is measured by a Beck-mann thermometer. See also colliga-tive properties.

elimination reaction A reactionin which one molecule decomposesinto two, one much smaller than theother.

Elinvar Trade name for a nickel–chromium steel containing about36% nickel, 12% chromium, andsmaller proportions of tungsten andmanganese. Its elasticity does notvary with temperature and it istherefore used to make hairspringsfor watches.

Ellingham diagram A diagramused to show the conditions underwhich a metal oxide can be reducedto a metal. The standard Gibbs freeenergy of formation of the oxide isconsidered, for example,

M + ½O2 → MOThis value, ∆GŠ, is plotted againsttemperature. In general, the result isa straight line. In some cases, there isan abrupt change in the line’s slopeat a point because of a phase change.The value of ∆GŠ for the reducingagent is also plotted. For example, ifthe reducing agent is carbon formingcarbon dioxide, it is ∆GŠ for the re-action

C + O2 → CO2

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temperature

C + O2 = CO2

2M + O2 = 2MO

CO + O2 = 2CO2

2C + O2 = 2CO

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Reduction can occur in the range oftemperatures in which the carboncurve is lower than the metal curve.The diagram was devised by thephysical chemist H. J. T. Ellingham.

elliptical polarization See polar-ization of light.

elongation (in protein synthesis)The phase in which amino acids arelinked together by sequentiallyformed peptide bonds to form apolypeptide chain. Elongation factorsare proteins that – by binding to atRNA–amino-acid complex – enablethe correct positioning of this com-plex on the ribosome, so that transla-tion can proceed.

eluate See chromatography; elu-tion.

eluent See chromatography; elu-tion.

elution The process of removing anadsorbed material (adsorbate) froman adsorbent by washing it in a liq-uid (eluent). The solution consistingof the adsorbate dissolved in the elu-ent is the eluate. Elution is theprocess used to wash components ofa mixture through a *chromatogra-phy column.

elutriation The process of suspend-ing Ünely divided particles in an up-ward Ûowing stream of air or waterto wash and separate them into sizedfractions.

emanation The former name forthe gas radon, of which there arethree isotopes: Rn–222 (radium ema-nation), Rn–220 (thoron emanation),and Rn–219 (actinium emanation).

emerald The green gem variety of*beryl: one of the most highly prizedgemstones. The Ünest specimensoccur in the Muzo mines, Colombia.Other occurrences include the UralMountains, the Transvaal in South

Africa, and Kaligunan in India. Emer-alds can also be successfully synthe-sized.

emergency action code Seehazchem code.

emery A rock composed of corun-dum (natural aluminium oxide,Al2O3) with magnetite, haematite, orspinel. It occurs on the island ofNaxos (Greece) and in Turkey. Emeryis used as an abrasive and polishingmaterial and in the manufacture ofcertain concrete Ûoors.

e.m.f. See electromotive force.

emission spectrum See spectrum.

empirical Denoting a result that isobtained by experiment or observa-tion rather than from theory.

empirical formula See formula.

emulsiÜcation (in digestion) Thebreakdown of fat globules in the duo-denum into tiny droplets, which pro-vides a larger surface area on whichthe enzyme pancreatic *lipase canact to digest the fats into fatty acidsand glycerol. EmulsiÜcation is as-sisted by the action of the bile saltsin bile.

emulsion A *colloid in which smallparticles of one liquid are dispersedin another liquid. Usually emulsionsinvolve a dispersion of water in anoil or a dispersion of oil in water, andare stabilized by an emulsiÜer. Com-monly emulsiÜers are substances,such as *detergents, that havelyophobic and lyophilic parts in theirmolecules.

en The symbol for ethylene diamine(1,2-diaminoethane) functioning as abidentate ligand, used in formulae.

enamine A type of compound withthe general formula

R1R2C = C(R3)–NR4R5,

203 enamine

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where R is a hydrocarbon group orhydrogen. Enamines can be producedby condensation of an aldehyde orketone with a secondary amine.

enantiomeric pair A pair of mol-ecules consisting of one chiral mol-ecule and the mirror image of thismolecule. The molecules making upan enantiomeric pair rotate the planeof polarized light in equal, but oppo-site, directions.

enantiomers See optical activity.

enantiomorph See optical activ-ity.

enantiomorphism See optical ac-tivity.

enantiotopic Denoting a ligand a,attached to a *prochiral centre, inwhich the replacement of this ligandwith a ligand d (which is differentfrom the ligands a, b, and c attachedto the prochiral centre) gives rise to apair of enantiomers Cabcd.

enantiotropy See allotropy.

endo- PreÜx used to designate abridged ring molecule with a sub-stituent on the ring that is on thesame side as the bridge. If the sub-stituent is on the opposite side thecompound is designated exo-.

endocannabinoids See cannabi-noids.

ENDOR Electron-nuclear double res-onance. A magnetic resonance tech-nique involving exitation of bothelectron spins and nuclear spins. Twosources of radiation are used. One isa Üxed source at microwave fre-quency, which partially saturates theelectron spins. The other is a variableradiofrequency source, which excitesthe atomic nuclear spins. Excitationof the nuclear spins affects the elec-tron spins by hyperÜre coupling, in-creasing the relaxation time of theexcited electron spins and increasing

the signal strength. The technique,which is usually done at low temper-atures, is used to investigate para-magnetic molecules. See alsoelectron paramagnetic resonance.

endothermic Denoting a chemicalreaction that takes heat from its sur-roundings. Compare exothermic.

end point The point in a titrationat which reaction is complete asshown by the *indicator.

energy A measure of a system’sability to do work. Like work itself, itis measured in joules. Energy is con-veniently classiÜed into two forms:potential energy is the energy storedin a body or system as a consequenceof its position, shape, or state (this in-cludes gravitational energy, electricalenergy, nuclear energy, and chemicalenergy); kinetic energy is energy ofmotion and is usually deÜned as thework that will be done by the bodypossessing the energy when it isbrought to rest. For a body of mass mhaving a speed v, the kinetic energyis mv2/2 (classical) or (m – m0)c2 (rela-tivistic). The rotational kinetic energyof a body having an angular velocityω is Iω2/2, where I is its moment ofinertia.

The *internal energy of a body isthe sum of the potential energy andthe kinetic energy of its componentatoms and molecules.

energy bands A range of energiesthat electrons can have in a solid. Ina single atom, electrons exist in dis-crete *energy levels. In a crystal, inwhich large numbers of atoms areheld closely together in a lattice,electrons are inÛuenced by a numberof adjacent nuclei and the sharplydeÜned levels of the atoms becomebands of allowed energy; this ap-proach to energy levels in solids isoften known as the band theory.Each band represents a large number

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of allowed quantum states. Betweenthe bands are forbidden bands. Theoutermost electrons of the atoms (i.e.the ones responsible for chemicalbonding) form the valence band ofthe solid. This is the band, of thoseoccupied, that has the highest en-ergy.

The band structure of solids ac-counts for their electrical properties.In order to move through the solid,the electrons have to change fromone quantum state to another. Thiscan only occur if there are emptyquantum states with the same en-ergy. In general, if the valence bandis full, electrons cannot change tonew quantum states in the sameband. For conduction to occur, theelectrons have to be in an unÜlledband – the conduction band. Metalsare good conductors either becausethe valence band and the conductionband are only half-Ülled or becausethe conduction band overlaps withthe valence band; in either case va-cant states are available. In insulatorsthe conduction band and valenceband are separated by a wide forbid-den band and electrons do not haveenough energy to ‘jump’ from one tothe other. In intrinsic semiconduc-tors the forbidden gap is narrow and,at normal temperatures, electrons at

the top of the valence band can moveby thermal agitation into the conduc-tion band (at absolute zero, a semi-conductor would act as an insulator).Doped semiconductors have extrabands in the forbidden gap. See illus-tration.

energy level A deÜnite Üxed en-ergy that a molecule, atom, electron,or nucleus can have. In an atom, forexample, the atom has a Üxed energycorresponding to the *orbitals inwhich its electrons move around thenucleus. The atom can accept a quan-tum of energy to become an excitedatom (see excitation) if that extra en-ergy will raise an electron to a per-mitted orbital. Between the groundstate, which is the lowest possibleenergy level for a particular system,and the Ürst excited state there areno permissible energy levels. Accord-ing to the *quantum theory, onlycertain energy levels are possible. Anatom passes from one energy level tothe next without passing throughfractions of that energy transition.These levels are usually described bythe energies associated with the indi-vidual electrons in the atoms, whichare always lower than an arbitrarylevel for a free electron. The energylevels of molecules also involve quan-

205 energy level

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E E E

electrondistribution

forbidden band

conductionband

valenceband

conductionband

valenceband

valence band

conduction band

Insulator Conductor Semiconductor

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tized vibrational and rotational mo-tion.

Engel’s salt See potassium carbon-ate.

enolate ion A negative ion ob-tained from an enol, by removal of ahydrogen atom. Enolate ions canhave two forms: one with a singleC–C bond and the negative charge onthe beta carbon atom and the otherwith a double C–C bond and the neg-ative charge on the oxygen atom.

enols Compounds containing thegroup –CH=C(OH)– in their mol-ecules. See also keto–enol tau-tomerism.

enrichment The process of increas-ing the abundance of a speciÜed iso-tope in a mixture of isotopes. It isusually applied to an increase in theproportion of U–235, or the additionof Pu–239 to natural uranium for usein a nuclear reactor or weapon.

ensemble A set of systems of parti-cles used in *statistical mechanics todescribe a single system. The conceptof an ensemble was put forward bythe US scientist Josiah Willard Gibbs(1839–1903) in 1902 as a way of cal-culating the time average of the sin-gle system, by averaging over thesystems in the ensemble at a Üxedtime. An ensemble of systems is con-structed from knowledge of the sin-gle system and can be represented asa set of points in phase space witheach system of the ensemble repre-sented by a point. Ensembles can beconstructed both for isolated systemsand for open systems.

enthalpy Symbol H. A thermody-namic property of a system deÜnedby H = U + pV, where H is the en-thalpy, U is the internal energy of thesystem, p its pressure, and V its vol-ume. In a chemical reaction carriedout in the atmosphere the pressure

remains constant and the enthalpy ofreaction, ∆H, is equal to ∆U + p∆V.For an exothermic reaction ∆H istaken to be negative.

entropy Symbol S. A measure ofthe unavailability of a system’s en-ergy to do work; in a closed system,an increase in entropy is accompa-nied by a decrease in energy avail-ability. When a system undergoes areversible change the entropy (S)changes by an amount equal to theenergy (Q) transferred to the systemby heat divided by the thermody-namic temperature (T) at which thisoccurs, i.e. ∆S = ∆Q/T. However, allreal processes are to a certain extentirreversible changes and in anyclosed system an irreversible changeis always accompanied by an increasein entropy.

In a wider sense entropy can be in-terpreted as a measure of disorder;the higher the entropy the greaterthe disorder (see boltzmannformula). As any real change to aclosed system tends towards higherentropy, and therefore higher disor-der, it follows that the entropy of theuniverse (if it can be considered aclosed system) is increasing and itsavailable energy is decreasing. Thisincrease in the entropy of the uni-verse is one way of stating the sec-ond law of *thermodynamics.

envelope See ring conformations.

enyl complex A type of complexin which there is a link between themetal atom or ion and the pi elec-trons of a double bond. *Zeise’s saltwas the Ürst known example.

enzyme A protein that acts as a*catalyst in biochemical reactions.Each enzyme is speciÜc to a particu-lar reaction or group of similar reac-tions. Many require the association ofcertain nonprotein *cofactors inorder to function. The molecule un-

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dergoing reaction (the substrate)binds to a speciÜc *active site on theenzyme molecule to form a short-lived intermediate (see enzyme–substrate complex): this greatly in-creases (by a factor of up to 1020) therate at which the reaction proceedsto form the product. Enzyme activityis inÛuenced by substrate concentra-tion and by temperature and pH,which must lie within a certainrange. Other molecules may competefor the active site, causing *inhibi-tion of the enzyme or even irre-versible destruction of its catalyticproperties.

Enzyme production is governed bya cell’s genes. Enzyme activity is fur-ther controlled by pH changes, alter-ations in the concentrations ofessential cofactors, feedback inhibi-tion by the products of the reaction,and activation by another enzyme,either from a less active form or aninactive precursor (zymogen). Suchchanges may themselves be underthe control of hormones or thenervous system. See also enzyme ki-netics.

Enzymes are classiÜed into sixmajor groups, according to the typeof reaction they catalyse: (1) oxidore-ductases; (2) transferases; (3) hydro-lases; (4) lyases; (5) isomerases; (6)ligases. The names of most individualenzymes also end in -ase, which isadded to the names of the substrateson which they act. Thus lactase is theenzyme that acts to break down lac-tose; it is classiÜed as a hydrolase.A• Information about IUPAC nomenclature

enzyme inhibition See inhibition.

enzyme kinetics The study of therates of enzyme-catalysed reactions.Rates of reaction are usually meas-ured by using the puriÜed enzyme invitro with the substrate and then ob-serving the formation of the product

or disappearance of the substrate. Asthe concentration of the substrate isincreased the rate of reaction in-creases proportionally up to a certainpoint, after which any further in-crease in substrate concentration nolonger increases the reaction rate (seemichaelis–menten curve). At thispoint, all active sites of the enzymeare saturated with substrate; any fur-ther increase in the rate of reactionwill occur only if more enzyme isadded. Reaction rates are also af-fected by the presence of inhibitors(see inhibition), temperature, and pH(see enzyme).

enzyme–substrate complex Theintermediate formed when a sub-strate molecule interacts with the*active site of an enzyme. Followingthe formation of an enzyme–substrate complex, the substratemolecule undergoes a chemical reac-tion and is converted into a newproduct. Various mechanisms for theformation of enzyme–substrate com-plexes have been suggested, includ-ing the *induced-Üt model and the*lock-and-key mechanism.

ephedrine An alkaloid,C6H5CH(OH)CH(CH3)NHCH3 found inplants of the genus Ephedra, onceused as a bronchodilator in the treat-ment of asthma. It is also used as astimulant and appetite suppressant.Structurally, it is a phenylethylamineand is similar to amphetamines, al-though less active. It is, however,widely used in the illegal synthesis ofmethamphetamine. The moleculehas two chiral centres. If the stereo-

207 ephedrine

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chemical conformations are opposite(i.e. 1R,2S or 1S,1R) the nameephedrine is used. If the conforma-tions are the same (1R,2R or 1S,2S)then the compound is called pseu-doephedrine.

epimerism A type of optical iso-merism in which a molecule has twochiral centres; two optical isomers(epimers) differ in the arrangementabout one of these centres. See alsooptical activity.

epinephrine See adrenaline.

epitaxy (epitaxial growth) Growthof a layer of one substance on a sin-gle crystal of another, such that thecrystal structure in the layer is thesame as that in the substrate. It isused in making semiconductor de-vices.

EPM See electron probe micro-analysis.

epoietin (EPO) A hormone that reg-ulates the production of red bloodcells in the body. The drug is usedmedically to treat anemia, and is alsoused illegally by athletes participat-ing in endurance events.

epoxides Compounds that containoxygen atoms in their molecules aspart of a three-membered ring (seeformula). Epoxides are thus cyclicethers.

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CH3

CH3CH3

CH3

Epoxides

epoxyethane (ethylene oxide) Acolourless Ûammable gas, C2H4O;m.p. –111°C; b.p. 13.5°C. It is a cyclicether (see epoxides), made by the cat-alytic oxidation of ethene. It can behydrolysed to ethane-1,2-diol andalso polymerizes to …-OC2H4-O-C2H4-…, which is used for lowering the

viscosity of water (e.g. in Üre Üght-ing).

epoxy resins Synthetic resins pro-duced by copolymerizing epoxidecompounds with phenols. They con-tain –O– linkages and epoxide groupsand are usually viscous liquids. Theycan be hardened by addition ofagents, such as polyamines, thatform cross-linkages. Alternatively,catalysts may be used to induce fur-ther polymerization of the resin.Epoxy resins are used in electricalequipment and in the chemical in-dustry (because of resistance tochemical attack). They are also usedas adhesives.

EPR See electron paramagneticresonance.

epsomite A mineral form of *mag-nesium sulphate heptahydrate,MgSO4.7H2O.

Epsom salt See magnesium sul-phate.

equation of state An equationthat relates the pressure p, volume V,and thermodynamic temperature Tof an amount of substance n. Thesimplest is the ideal *gas law:

pV = nRT,where R is the universal gas constant.Applying only to ideal gases, thisequation takes no account of the vol-ume occupied by the gas molecules(according to this law if the pressureis inÜnitely great the volume be-comes zero), nor does it take into ac-count any forces between molecules.A more accurate equation of statewould therefore be

(p + k)(V – nb) = nRT,where k is a factor that reÛects thedecreased pressure on the walls ofthe container as a result of the attrac-tive forces between particles, and nbis the volume occupied by the parti-cles themselves when the pressure is


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inÜnitely high. In van der Waals’sequation, proposed by the Dutchphysicist J. D. van der Waals (1837–1923),

k = n2a/V2,where a is a constant. This equationmore accurately reÛects the behav-iour of real gases; several others havedone better but are more complicated.

equatorial 1. See ring conforma-tions. 2. See apical.

equilibrium A state in which a sys-tem has its energy distributed in thestatistically most probable manner; astate of a system in which forces,inÛuences, reactions, etc., balanceeach other out so that there is no netchange. A body is said to be in ther-mal equilibrium if no net heat ex-change is taking place within it orbetween it and its surroundings. Asystem is in *chemical equilibriumwhen a reaction and its reverse areproceeding at equal rates (see alsoequilibrium constant). These areexamples of dynamic equilibrium, inwhich activity in one sense or direc-tion is in aggregate balanced by com-parable reverse activity.

equilibrium constant For a re-versible reaction of the type

xA + yB ˆ zC + wDchemical equilibrium occurs whenthe rate of the forward reactionequals the rate of the back reaction,so that the concentrations of prod-ucts and reactants reach steady-statevalues. It can be shown that at equi-librium the ratio of concentrations

[C]z[D]w/[A]x[B]y

is a constant for a given reaction andÜxed temperature, called the equilib-rium constant Kc (where the c indi-cates concentrations have been used).Note that, by convention, the prod-ucts on the right-hand side of the re-action are used on the top line of the

expression for equilibrium constant.This form of the equilibrium con-stant was originally introduced in1863 by C. M. Guldberg and P. Waageusing the law of *mass action. Theyderived the expression by taking therate of the forward reaction

kf[A]x[B]y

and that of the back reactionkb[C]z[D]w

Since the two rates are equal at equi-librium, the equilibrium constant Kcis the ratio of the rate constants kf/kb.The principle that the expression is aconstant is known as the equilibriumlaw or law of chemical equilibrium.

The equilibrium constant showsthe position of equilibrium. A lowvalue of Kc indicates that [C] and [D]are small compared to [A] and [B]; i.e.that the back reaction predominates.It also indicates how the equilibriumshifts if concentration changes. Forexample, if [A] is increased (byadding A) the equilibrium shifts to-wards the right so that [C] and [D] in-crease, and Kc remains constant.

For gas reactions, partial pressuresare used rather than concentrations.The symbol Kp is then used. Thus, inthe example above

Kp = pCzpD

w/pAxpB

y

It can be shown that, for a given re-action Kp = Kc(RT)∆ν, where ∆ν is thedifference in stoichiometric coefÜ-cients for the reaction (i.e. z + w – x –y). Note that the units of Kp and Kc de-pend on the numbers of moleculesappearing in the stoichiometric equa-tion. The value of the equilibriumconstant depends on the tempera-ture. If the forward reaction isexothermic, the equilibrium constantdecreases as the temperature rises; ifendothermic it increases (see alsovan’t hoff’s isochore).

The expression for the equilibriumconstant can also be obtained by

209 equilibrium constant

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thermodynamics; it can be shownthat the standard equilibrium con-stant KŠ is given by exp(–∆GŠ/RT),where ∆GŠ is the standard Gibbsfree energy change for the completereaction. Strictly, the expressionsabove for equilibrium constants aretrue only for ideal gases (pressure) orinÜnite dilution (concentration). Foraccurate work *activities are used.

equilibrium law See equilibriumconstant.

equipartition of energy Thetheory, proposed by Ludwig *Boltz-mann and given some theoreticalsupport by James Clerk *Maxwell,that the energy of gas molecules in alarge sample under thermal *equilib-rium is equally divided among theiravailable *degrees of freedom, theaverage energy for each degree offreedom being kT/2, where k is the*Boltzmann constant and T is thethermodynamic temperature. Theproposition is not generally true ifquantum considerations are impor-tant, but is frequently a good approx-imation.

equivalence point The point in atitration at which reaction is com-plete. See indicator.

equivalent proportions Seechemical combination.

equivalent weight The mass ofan element or compound that couldcombine with or displace one gramof hydrogen (or eight grams of oxy-gen or 35.5 grams of chlorine) in achemical reaction. The equivalentweight represents the ‘combiningpower’ of the substance. For an el-ement it is the relative atomic massdivided by the valency. For a com-pound it depends on the reactionconsidered.

erbium Symbol Er. A soft silverymetallic element belonging to the

*lanthanoids; a.n. 68; r.a.m. 167.26;r.d. 9.006 (20°C); m.p. 1529°C; b.p.2863°C. It occurs in apatite, gadolin-ite, and xenotine from certainsources. There are six natural iso-topes, which are stable, and twelveartiÜcial isotopes are known. It hasbeen used in alloys for nuclear tech-nology as it is a neutron absorber; itis being investigated for other poten-tial uses. It was discovered by CarlMosander (1797–1858) in 1843.


ergocalciferol See vitamin d.

ergodic hypothesis A hypothesisin *statistical mechanics concerningphase space. If a system of N atomsor molecules is enclosed in a Üxedvolume, the state of this system isgiven by a point in 6N-dimensionalphase space with qi representing co-ordinates and pi representing mo-menta. Taking the energy E to beconstant, a representative point inphase space describes an orbit on thesurface E(qi,pi) = c, where c is a con-stant. The ergodic hypothesis statesthat the orbit of the representativepoint in phase space eventually goesthrough all points on the surface.The quasi-ergodic hypothesis statesthat the orbit of the representativepoint in phase space eventuallycomes close to all points on the sur-face. In general, it is very difÜcult toprove the ergodic or quasi-ergodichypotheses for a given system. Seealso ergodicity.

ergodicity A property of a systemthat obeys the *ergodic hypothesis.The ergodicity of systems has beendiscussed extensively in the founda-tions of *statistical mechanics, al-though it is now thought by manyphysicists to be irrelevant to theproblem. Considerations of ergodic-ity occur in dynamics, since the be-

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haviour can be very complex evenfor simple *dynamical systems (seeattractor). Systems, such as *spinglasses, in which ergodicity isthought not to hold are described ashaving broken ergodicity.

ergosterol A *sterol occurring infungi, bacteria, algae, and plants. It isconverted into vitamin D2 by the ac-tion of ultraviolet light.

ESCA See photoelectron spec-troscopy.

eserine (physostigmine) An alka-loid, derived from the calabar beanplant, that inhibits cholinesterase bycovalently binding with it (see inhibi-tion). Eserine is used to treat the eyecondition glaucoma.

211 esterification

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N

N

O NH

CH3

OCH3

CH3

CH3

Eserine

ESI See electrospray ionization.

ESR See electron paramagneticresonance.

essential amino acid An *aminoacid that an organism is unable tosynthesize in sufÜcient quantities. Itmust therefore be present in the diet.In man the essential amino acids arearginine, histidine, lysine, threonine,methionine, isoleucine, leucine, va-line, phenylalanine, and tryptophan.These are required for protein syn-thesis and deÜciency leads to re-tarded growth and other symptoms.Most of the amino acids required byman are also essential for all othermulticellular animals and for mostprotozoans.

essential element Any of a num-

ber of elements required by living or-ganisms to ensure normal growth,development, and maintenance.Apart from the elements found in or-ganic compounds (i.e. carbon, hydro-gen, oxygen, and nitrogen), plants,animals, and microorganisms all re-quire a range of elements in inor-ganic forms in varying amounts,depending on the type of organism.The major elements, present in tis-sues in relatively large amounts(greater than 0.005%), are calcium,phosphorus, potassium, sodium,chlorine, sulphur, and magnesium.The trace elements occur at muchlower concentrations and thus re-quirements are much less. The mostimportant are iron, manganese, zinc,copper, iodine, cobalt, selenium,molybdenum, chromium, and sili-con. Each element may fulÜl one ormore of a variety of metabolic roles.

essential fatty acids *Fatty acidsthat must normally be present in thediet of certain animals, includingman. Essential fatty acids, which in-clude *linoleic and *linolenic acids,all possess double bonds at the sametwo positions along their hydrocar-bon chain and so can act as precur-sors of *prostaglandins. DeÜciency ofessential fatty acids can cause der-matosis, weight loss, irregularoestrus, etc. An adult human re-quires 2–10 g linoleic acid or itsequivalent per day.

essential oil A natural oil with adistinctive scent secreted by theglands of certain aromatic plants.*Terpenes are the main constituents.Essential oils are extracted fromplants by steam distillation, extrac-tion with cold neutral fats or solvents(e.g. alcohol), or pressing and used inperfumes, Ûavourings, and medicine.Examples are citrus oils, Ûower oils(e.g. rose, jasmine), and oil of cloves.

esteriÜcation A reaction of an al-


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cohol with an acid to produce anester and water; e.g.

CH3OH + C6H5COOH ˆCH3OOCC6H5 + H2O

The reaction is an equilibrium and isslow under normal conditions, butcan be speeded up by addition of astrong acid catalyst. The ester canoften be distilled off so that the reac-tion can proceed to completion. Thereverse reaction is ester hydrolysis or*saponiÜcation. See also labelling.

esters Organic compounds formedby reaction between alcohols andacids (see illustration). Esters formedfrom carboxylic acids have the gen-eral formula RCOOR′. Examples areethyl ethanoate, CH3COOC2H5, andmethyl propanoate, C2H5COOCH3. Es-ters containing simple hydrocarbongroups are volatile fragrant sub-stances used as Ûavourings in thefood industry. Triesters, moleculescontaining three ester groups, occurin nature as oils and fats. See also dep-sides; glycerides.


ethanal (acetaldehyde) A colourlesshighly Ûammable liquid aldehyde,CH3CHO; r.d. 0.78; m.p. –121°C; b.p.20.8°C. It is made from ethene by the*Wacker process and used as a start-ing material for making many or-ganic compounds. The compoundpolymerizes if dilute acid is added togive ethanal trimer (or paraldehyde),which contains a six-membered ringof alternating carbon and oxygenatoms with a hydrogen atom and a

methyl group attached to each car-bon atom. It is used as a drug for in-ducing sleep. Addition of dilute acidbelow 0°C gives ethanal tetramer (ormetaldehyde), which has a similarstructure to the trimer but with aneight-membered ring. It is used as asolid fuel in portable stoves and inslug pellets.

ethanamide (acetamide) A colour-less solid crystallizing in the form oflong white crystals with a character-istic smell of mice, CH3CONH2; r.d.1.159; m.p. 82.3°C; b.p. 221.25°C. It ismade by the dehydration of ammo-nium ethanoate or by the action ofammonia on ethanoyl chloride,ethanoic anhydride, or ethylethanoate.

ethane A colourless Ûammablegaseous hydrocarbon, C2H6; m.p.–183°C; b.p. –89°C. It is the secondmember of the *alkane series of hy-drocarbons and occurs in natural gas.

ethanedial See glyoxal.

ethanedioic acid See oxalic acid.

ethane-1,2-diol (ethylene glycol;glycol) A colourless viscous hygro-scopic liquid, CH2OHCH2OH; m.p.–11.5°C; b.p. 198°C. It is made by hy-drolysis of epoxyethane (fromethene) and used as an antifreezeand a raw material for making *poly-esters (e.g. Terylene).

ethanenitrile (acetonitrile, methylcyanide) A poisonous liquid, CH3CN;b.p. 82°C. It is made by dehydratingethanamide or from ammonia andethyne. It is a good polar solvent and

esters 212

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CH3 O H H O

OC C2H5

CH3 OC C2H5

O+ H2O

methanol propanoic acidmethyl propanoate water

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is employed for dissolving ionic com-pounds when water cannot be used.

ethanoate (acetate) A salt or esterof ethanoic acid (acetic acid).

ethanoic acid (acetic acid) A clearviscous liquid or glassy solid *car-boxylic acid, CH3COOH, with a char-acteristically sharp odour of vinegar;r.d. 1.049; m.p. 16.6°C; b.p. 117.9°C.The pure compound is called glacialethanoic acid. It is manufactured bythe oxidation of ethanol or by the ox-idation of butane in the presence ofdissolved manganese(II) or cobalt(II)ethanoates at 200°C, and is used inmaking ethanoic anhydride for pro-ducing cellulose ethanoates. It is alsoused in making ethenyl ethanoate(for polyvinylacetate). The compoundis formed by the fermentation of al-cohol and is present in vinegar,which is made by fermenting beer orwine. ‘Vinegar’ made from ethanoicacid with added colouring matter iscalled ‘nonbrewed condiment’. In liv-ing organisms it combines with *co-enzyme A to form acetyl coenzymeA, which plays a crucial role in en-ergy metabolism.

ethanoic anhydride (acetic anhy-dride) A pungent-smelling colourlessliquid, (CH3CO)2O, b.p. 139.5°C. It isused in organic synthesis as an*ethanoylating agent (attacking an–OH or –NH group) and in the manu-facture of aspirin and cellulose plas-tics. It hydrolyses in water to giveethanoic acid.

ethanol (ethyl alcohol) A colourlesswater-soluble *alcohol, C2H5OH; r.d.0.789 (0°C); m.p. –117.3°C; b.p.–78.3°C. It is the active principle inintoxicating drinks, in which it isproduced by fermentation of sugarusing yeast

C6H12O6 → 2C2H5OH + 2CO2

The ethanol produced kills the yeastand fermentation alone cannot pro-

duce ethanol solutions containingmore than 15% ethanol by volume.Distillation can produce a constant-boiling mixture containing 95.6%ethanol and 4.4% water. Pure ethanol(absolute alcohol) is made by remov-ing this water by means of dryingagents.

The main industrial use of ethanolis as a solvent although at one time itwas a major starting point for mak-ing other chemicals. For this it wasproduced by fermentation of mo-lasses. Now ethene has replacedethanol as a raw material and indus-trial ethanol is made by hydrolysis ofethene.

ethanolamine Any of three low-melting hygroscopic colourlesssolids. They are strong bases, smell ofammonia, and absorb water readilyto form viscous liquids. Mono-ethanolamine, HOCH2CH2NH2, is aprimary *amine, m.p. 10.5°C; di-ethanolamine, (HOCH2CH2)2NH, is asecondary amine, m.p. 28°C; and tri-ethanolamine, (HOCH2CH2)3N, is atertiary amine, m.p. 21°C. All aremade by heating ethylene oxide withconcentrated aqueous ammoniaunder pressure and separating theproducts by fractional distillation.With fatty acids they form neutralsoaps, used as emulsifying agentsand detergents, and in bactericidesand cosmetics.

ethanoylating agent (acetylatingagent) A chemical reagent used tointroduce an ethanoyl group(–COCH3) instead of hydrogen in anorganic compound. Examples include*ethanoic anhydride and ethanoylchloride (acetyl chloride, CH3COCl).

ethanoyl chloride (acetyl chloride)A colourless liquid acyl chloride (seeacyl halides), CH3COCl, with a pun-gent smell; r.d. 1.105; m.p. –112.15°C;b.p. 50.9°C. It is made by reactingethanoic acid with a halogenating

213 ethanoyl chloride

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agent such as phosphorus(III) chlo-ride, phosphorus(V) chloride, or sul-phur dichloride oxide and is used tointroduce ethanoyl groups into or-ganic compounds containing –OH,–NH2, and –SH groups. See acylation.

ethanoyl group (acetyl group) Theorganic group CH3CO–.

ethene (ethylene) A colourlessÛammable gaseous hydrocarbon,C2H4; m.p. –169°C; b.p. –103.7°C. It isthe Ürst member of the *alkene se-ries of hydrocarbons. It is made bycracking hydrocarbons from petro-leum and is now a major raw ma-terial for making other organicchemicals (e.g. ethanal, ethanol,ethane-1,2-diol). It can be polymer-ized to *polyethene. It occurs natu-rally in plants, in which it acts as agrowth substance promoting theripening of fruits.

ethenone See ketene.

ethenyl ethanoate (vinyl acetate)An unsaturated organic ester, CH2:CHOOCCH3; r.d. 0.9; m.p. –100°C;b.p. 73°C. It is made by catalytic reac-tion of ethanoic acid and ethene andused to make polyvinylacetate.

ether See ethoxyethane; ethers.

ethers Organic compounds contain-ing the group –O– in their molecules.Examples are dimethyl ether,CH3OCH3, and diethyl ether,C2H5OC2H5 (see ethoxyethane). Theyare volatile highly Ûammable com-pounds made by dehydrating alco-hols using sulphuric acid.


ethoxyethane (diethyl ether;ether) A colourless Ûammablevolatile ether, C2H5OC2H5; r.d. 0.71;m.p. –116°C; b.p. 34.5°C. It can bemade by *Williamson’s synthesis. It

is an anaesthetic and useful organicsolvent. See ethers.

ethyl 3-oxobutanoate (ethylacetoacetonate) A colourless liquidester with a pleasant odour,CH3COCH2COOC2H5; r.d. 1.03; m.p.<–80°C; b.p. 180.4°C. It can be pre-pared by reacting ethyl ethanoate(CH3COOC2H5) with sodium orsodium ethoxide. The compoundshows keto–enol *tautomerism andcontains about 7% of the enol form,CH3C(OH):CHCOOC2H5, under nor-mal conditions. Sometimes known asacetoacetic ester, it is used in organicsynthesis.

ethyl acetate See ethyl ethanoate.

ethyl acetoacetonate See ethyl 3-oxobutanoate.

ethyl alcohol See ethanol.

ethylamine A colourless Ûamma-ble volatile liquid, C2H5NH2; r.d. 0.69;m.p. –81°C; b.p. 16.6°C. It is a pri-mary amine made by reactingchloroethane with ammonia andused in making dyes.

ethylbenzene A colourless Ûam-mable liquid, C6H5C2H5; r.d. 0.867;m.p. –95°C; b.p. 136°C. It is madefrom ethene and ethybenzene by a*Friedel–Crafts reaction and is usedin making phenylethene (for poly-styrene).

ethyl bromide See bromoethane.

ethylene See ethene.

ethylenediamine See 1,2-diamino-ethane.

ethylene glycol See ethane-1,2-diol.

ethylene oxide See epoxyethane.

ethyl ethanoate (ethyl acetate) Acolourless Ûammable liquid ester,C2H5OOCCH3; r.d. 0.9; m.p. –83.6°C;

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b.p. 77.06°C. It is used as a solventand in Ûavourings and perfumery.

ethyl group The organic groupCH3CH2–.

ethyl iodide See iodoethane.

ethyne (acetylene) A colourless un-stable gas, C2H2, with a characteristicsweet odour; r.d. 0.618; m.p. –80.8°C;b.p. –84.0°C. It is the simplest mem-ber of the *alkyne series of unsatu-rated hydrocarbons, and is preparedby the action of water on calcium di-carbide or by adding alcoholic potas-sium hydroxide to 1,2-dibromoethane.It can be manufactured by heatingmethane to 1500°C in the presenceof a catalyst. It is used in oxyacety-lene welding and in the manufactureof ethanal and ethanoic acid. Ethynecan be polymerized easily at hightemperatures to give a range of prod-ucts. The inorganic saltlike dicar-bides contain the ion C2

2–, althoughethyne itself is a neutral compound(i.e. not a protonic acid).

eudiometer An apparatus formeasuring changes in volume ofgases during chemical reactions. Asimple example is a graduated glasstube sealed at one end and invertedin mercury. Wires passing into thetube allow the gas mixture to besparked to initiate the reaction be-tween gases in the tube.

europium Symbol Eu. A soft silverymetallic element belonging to the*lanthanoids; a.n. 63; r.a.m. 151.96;r.d. 5.245 (20°C); m.p. 822°C; b.p.1597°C. It occurs in small quantitiesin bastanite and monazite. Two sta-ble isotopes occur naturally: eu-ropium–151 and europium–153, bothof which are neutron absorbers. Ex-perimental europium alloys havebeen tried for nuclear-reactor partsbut until recently the metal has notbeen available in sufÜcient quanti-ties. It is widely used in the form of

the oxide in phosphors for televisionscreens. It was discovered by SirWilliam Crookes in 1889.


eutectic mixture A solid solutionconsisting of two or more substancesand having the lowest freezing pointof any possible mixture of these com-ponents. The minimum freezingpoint for a set of components iscalled the eutectic point. Low-melting-point alloys are usually eutectic mixtures.

Evans balance See gouy balance.

evaporation The change of stateof a liquid into a vapour at a temper-ature below the boiling point of theliquid. Evaporation occurs at the sur-face of a liquid, some of those mol-ecules with the highest kineticenergies escaping into the gas phase.The result is a fall in the average ki-netic energy of the molecules of theliquid and consequently a fall in itstemperature.

exa- Symbol E. A preÜx used in themetric system to denote 1018 times.For example, 1018 metres = 1 exame-tre (Em).

EXAFS (extended X-ray absorptionÜne structure) Oscillations of the X-ray absorption coefÜcient beyond anabsorption edge. The physical causeof EXAFS is the modiÜcation of theÜnal state of a photoelectron *wavefunction caused by back-scatteringfrom atoms surrounding the excitedatom. EXAFS is used to determinestructure in chemical, solid state, orbiological systems; it is especiallyuseful in those systems in which it isnot possible to use diffraction tech-niques. EXAFS experiments are usu-ally performed using synchrotronradiation. It is possible to interpretEXAFS experiments using single-

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scattering theory for short-rangeorder.

excimer See exciplex.

exciplex A combination of two dif-ferent atoms that exists only in anexcited state. When an exciplexemits a photon of electromagnetic ra-diation, it immediately dissociatesinto the atoms, rather than revertingto the ground state. A similar tran-sient excited association of twoatoms of the same kind is anexcimer. An example of an exciplexis the species XeCl* (the asterisk indi-cates an excited state), which can beformed by an electric discharge inxenon and chlorine. This is used inthe exciplex laser, in which a popula-tion inversion is produced by an elec-trical discharge.

excitation A process in which a nu-cleus, electron, atom, ion, or mol-ecule acquires energy that raises it toa quantum state (excited state)higher than that of its *ground state.The difference between the energy inthe ground state and that in the ex-cited state is called the excitation en-ergy. See energy level.

exclusion principle See pauli ex-clusion principle.

exo- See endo-.

exothermic Denoting a chemicalreaction that releases heat into itssurroundings. Compare endothermic.

exotic atom A species in whichsome other charged particle replacesthe electron or nucleon. An exampleis an atom in which an electron hasbeen replaced by another negativelycharged particle, such as a muon ormeson. In this case the negative par-ticle eventually collides with the nu-cleus with the emission of X-rayphotons. Another system is one inwhich the nucleus of an atom hasbeen replaced by a positively charged

meson. An association of an electronand a positron is also regarded as anexotic atom (known as positronium).This has a mean life of about 10–75,decaying to give three photons.

expanded plastic (cellular plastic)A plastic in the form of a rigid solidfoam. Polystyrene, used as a packingmaterial, is a common example.

explosive A compound or mixturethat, when ignited or detonated, un-dergoes a rapid violent chemical re-action that produces large amountsof gas and heat, accompanied bylight, sound and a high-pressureshock wave. Low explosives burncomparatively slowly when ignited,and are employed as propellants inÜrearms and guns; they are also usedin blasting. Examples include *gun-powder and various smokeless pro-pellants, such as *cordite. Highexplosives decompose very rapidly to produce an uncontrollable blast.Examples of this type include *dyna-mite, *nitroglycerine, and *trinitro-toluene (TNT); they are explodedusing a detonator. Other high-powerexplosives include pentaerythritoltetranitrate (PETN) and ammoniumnitride/fuel oil mixture (ANFO). Cy-clonite (RDX) is a military high explo-sive; mixed with oils and waxes, itforms a plastic explosive (such asSemtex). See also Chronology.

extended X-ray absorption Ünestructure See exafs.

extender An inert substance addedto a product (paint, rubber, washingpowder, etc.) to dilute it (for econ-omy) or to modify its physical prop-erties.

extensive variable A quantity ina *macroscopic system that is pro-portional to the size of the system.Examples of extensive variables in-clude the volume, mass, and total en-ergy. If an extensive variable is

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divided by an arbitrary extensivevariable, such as the volume, an *in-tensive variable results. A macro-scopic system can be described byone extensive variable and a set of in-tensive variables.

external conversion A process inwhich molecules in electronically ex-cited states pass to a lower electronicstate (which is frequently the groundstate) by colliding with other mol-ecules. In this process the electronic

217 external conversion

eEXPLOSIVES

900–1000 Gunpowder developed in China.

1242 English monk Roger Bacon (1220–92) describes the preparation ofgunpowder.

c.1250 German alchemist Berthold Schwarz claims to have reinventedgunpowder.

1771 French chemist Pierre Woulfe discovers picric acid (originally used as ayellow dye).

1807 Scottish cleric Alexander Forsyth (1767–1843) discovers mercuryfulminate.

1833 French chemist Henri Braconnot (1781–1855) nitrates starch, making ahighly flammable compound (crude nitrocellulose).

1838 French chemist Théophile Pelouze (1807–67) nitrates paper, makingcrude nitrocellulose.

1845 German chemist Christian Schönbein (1799–1868) nitrates cotton,making nitrocellulose.

1846 Italian chemist Ascania Sobrero (1812–88) discovers nitroglycerine.

1863 Swedish chemist J. Wilbrand discovers trinitrotoluene (TNT). Swedish chemist Alfred Nobel (1833–96) invents a detonating cap based on mercury fulminate.

1867 Alfred Nobel invents dynamite by mixing nitroglycerine and kieselguhr.

1871 German chemist Hermann Sprengel shows that picric acid can be usedas an explosive.

1875 Alfred Nobel invents blasting gelatin (nitroglycerine mixed withnitrocellulose).

1885 French chemist Eugène Turpin discovers ammonium picrate (Mélinite).

1888 Alfred Nobel invents a propellant from nitroglycerine and nitrocellulose(Ballistite).

1889 British scientists Frederick Abel (1826–1902) and James Dewar invent apropellant (Cordite) similar to Ballistite.

1891 German chemist Bernhard Tollens (1841–1918) discovers pentaerythritoltetranitrate (PETN).

1899 Henning discovers cyclotrimethylenetrinitramine (RDX or cyclonite).

1905 US army officer B. W. Dunn (1860–1936) invents ammonium picrateexplosive (Dunnite).

1915 British scientists invent amatol (TNT + ammonium nitrate).

1955 US scientists develop ammonium nitrate–fuel oil mixtures (ANFO) asindustrial explosives.


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energy is eventually converted intoheat. Since this process involves colli-sions, the rate at which it occurs de-pends on how frequently collisonsoccur. As a result, this process occursmuch faster in liquids than in gases.It is sometimes called collisionquenching.

extraction The separation of acomponent from its mixture by selec-tive solubility. See partition.

extractive distillation A distilla-tion technique in which a solvent isadded to the mixture in order to sep-arate two closely boiling compo-nents. The added solvent is usuallynonvolatile and is selected for itsability to have different effects onthe volatilities of the components.

extraordinary ray See double re-fraction.

extrinction coefÜcient See ab-sorption coefficient.

extrusion reaction See insertionreaction.

Eyring, Henry (1901–81) USchemist who worked at Princetonand Utah. His main work was onchemical kinetics and he is noted forthe *Eyring equation for absolute re-action rates.

Eyring equation An equation usedextensively to describe chemical re-actions. For a rate constant κ, it isgiven by

κ = K(kT/h)exp(–∆G‡/kT),where k is the *Boltzmann constant,T is the thermodynamic temperature,h is the *Planck constant, ∆G‡ is thefree energy of activation, and K is aconstant called the transmissioncoefÜcient, which is the probabilitythat a chemical reaction takes placeonce the system has reached the acti-

vated state. A similar equation (with-out the K) has been used to describetransport processes, such as diffu-sion, thermal conductivity, and vis-cosity in dense gases and liquids. Inthese cases it is assumed that themain kinetic process is the motion ofa molecule to a vacant site near it.The equation is derived by assumingthat the reactants are in equilibriumwith the excited state. This assump-tion of equilibrium is not necessarilycorrect for small activation energies.The Eyring equation is named afterHenry *Eyring, who derived it andapplied it widely in the theory ofchemical reactions and transportprocesses.

E–Z convention A convention forthe description of a molecule show-ing cis-trans isomerism (see iso-merism). In a molecule ABC=CDE,where A,B,D, and E are substituentgroups, the sequence rule (see cip sys-tem) is applied to the pair A and B toÜnd which has priority and similarlyto the pair C and D. If the two groupsof highest priority are on the sameside of the bond then the isomer isdesignated Z (from Germanzusammen, together). If they are onopposite sides the isomer is desig-nated E (German entgegen, opposite).The letters are used in the names of compounds; for example (E)-butenedioic acid (fumaric acid) and(Z)-butenedioic acid (maleic acid). Incompounds containing two (or more)double bonds numbers are used todesignate the bonds (e.g. (2E, 4Z)-2,4-hexadienoic acid). The system is lessambiguous than the cis/trans systemof describing isomers.

A• Information about the convention from

IUPAC

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F

FAB mass spectroscopy See fast-atom bombardment mass spec-troscopy.

face-centred cubic (f.c.c.) Seecubic crystal.

fac-isomer See isomerism.

FAD (Ûavin adenine dinucleotide) A*coenzyme important in various bio-chemical reactions. It comprises aphosphorylated vitamin B2 (ri-boÛavin) molecule linked to the nu-cleotide adenine monophosphate(AMP). FAD is usually tightly boundto the enzyme forming a Ûavopro-tein. It functions as a hydrogen ac-ceptor in dehydrogenation reactions,being reduced to FADH2. This in turnis oxidized to FAD by the *electrontransport chain, thereby generatingATP (two molecules of ATP per mol-ecule of FADH2).

Fahrenheit, Gabriel Daniel(1686–1736) German physicist, whobecame an instrument maker in Am-sterdam. In 1714 he developed themercury-in-glass thermometer, anddevised a temperature scale to gowith it (see fahrenheit scale).

Fahrenheit scale A temperaturescale in which (by modern deÜnition)the temperature of boiling water istaken as 212 degrees and the temper-ature of melting ice as 32 degrees. Itwas invented in 1714 by G. D.*Fahrenheit, who set the zero at thelowest temperature he knew how toobtain in the laboratory (by mixingice and common salt) and took hisown body temperature as 96°F. Thescale is no longer in scientiÜc use. To

convert to the *Celsius scale the for-mula is C =5(F – 32)/9.

Fajan’s method See adsorptionindicator.

Fajans’ rules Rules indicating theextent to which an ionic bond has co-valent character caused by polariza-tion of the ions. Covalent character ismore likely if:(1) the charge of the ions is high;(2) the positive ion is small or thenegative ion is large;(3) the positive ion has an outer elec-tron conÜguration that is not a noble-gas conÜguration.

The rules were introduced by thePolish-born US chemist Kasimir Fa-jans (1887–1975).

fall-out 1. (radioactive fall-out)Radioactive particles deposited fromthe atmosphere either from a nu-clear explosion or from a nuclear ac-cident. Local fall-out, within 250 kmof an explosion, falls within a fewhours of the explosion. Troposphericfall-out consists of Üne particles de-posited all round the earth in the ap-proximate latitude of the explosionwithin about one week. Stratosphericfall-out may fall anywhere on earthover a period of years. The most dan-gerous radioactive isotopes in fall-outare the Üssion fragments iodine–131and strontium–90. Both can be takenup by grazing animals and passed onto human populations in milk, milkproducts, and meat. Iodine–131 accu-mulates in the thyroid gland andstrontium–90 accumulates in bones.2. (chemical fall-out) Hazardouschemicals discharged into and subse-quently released from the atmos-


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phere, especially by factory chim-neys.

farad Symbol F. The SI unit of ca-pacitance, being the capacitance of acapacitor that, if charged with onecoulomb, has a potential differenceof one volt between its plates. 1 F = 1C V–1. The farad itself is too large formost applications; the practical unitis the microfarad (10–6 F). The unit isnamed after Michael *Faraday.

Faraday, Michael (1791–1867)British chemist and physicist, who re-ceived little formal education. Hestarted to experiment on electricityand in 1812 attended lectures by SirHumphry *Davy at the Royal Institu-tion; a year later he became Davy’sassistant. He remained at the Institu-tion until 1861. Faraday’s chemicaldiscoveries include the liquefactionof chlorine (1823) and benzene (1825)as well as the laws of electrolysis (seefaraday’s laws). He is also remem-bered for his work in physics: in1821 he demonstrated electromag-netic rotation (the principle of theelectric motor) and in 1832 discov-ered electromagnetic induction (theprinciple of the dynamo).

Faraday constant Symbol F. Theelectric charge carried by one moleof electrons or singly ionized ions,i.e. the product of the *Avogadroconstant and the charge on an elec-tron (disregarding sign). It has thevalue 9.648 5309(29) × 104 coulombsper mole. This number of coulombsis sometimes treated as a unit of elec-tric charge called the faraday.

Faraday’s laws Two laws describ-ing electrolysis:(1) The amount of chemical changeduring electrolysis is proportional tothe charge passed.(2) The charge required to deposit orliberate a mass m is given by Q =Fmz/M, where F is the Faraday con-

stant, z the charge of the ion, and Mthe relative ionic mass.

These are the modern forms of thelaws. Originally, they were stated byMichael *Faraday in a different form:(1) The amount of chemical changeproduced is proportional to the quan-tity of electricity passed.(2) The amount of chemical changeproduced in different substances by aÜxed quantity of electricity is propor-tional to the electrochemical equiva-lent of the substance.

fast-atom bombardment massspectroscopy (FAB mass spec-troscopy) A technique in *mass spec-troscopy in which ions are producedby bombardment with high-energyneutral atoms or molecules. It is usedfor samples that are nonvolatile orare thermally unstable.

fat A mixture of lipids, chieÛy*triglycerides, that is solid at normalbody temperatures. Fats occur widelyin plants and animals as a means ofstoring food energy, having twice thecaloriÜc value of carbohydrates. Inmammals, fat is deposited in a layerbeneath the skin (subcutaneous fat)and deep within the body as a spe-cialized adipose tissue.

Fats derived from plants and Üshgenerally have a greater proportionof unsaturated *fatty acids thanthose from mammals. Their meltingpoints thus tend to be lower, causinga softer consistency at room tempera-tures. Highly unsaturated fats are liq-uid at room temperatures and aretherefore more properly called *oils.

fatty acid An organic compoundconsisting of a hydrocarbon chainand a terminal carboxyl group (seecarboxylic acids). Chain lengthranges from one hydrogen atom(methanoic, or formic, acid, HCOOH)to nearly 30 carbon atoms. Ethanoic(acetic), propanoic (propionic), andbutanoic (butyric) acids are impor-

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tant in metabolism. Long-chain fattyacids (more than 8–10 carbon atoms)most commonly occur as con-stituents of certain lipids, notablyglycerides, phospholipids, sterols,and waxes, in which they areesteriÜed with alcohols. These long-chain fatty acids generally have aneven number of carbon atoms; un-branched chains predominate overbranched chains. They may be satu-rated (e.g. *palmitic (hexadecanoic)acid and *stearic (octadecanoic) acid)or unsaturated, with one doublebond (e.g. *oleic (cis-octodec-9-enoic)acid) or two or more double bonds,in which case they are called polyun-saturated fatty acids (e.g. *linoleicacid and *linolenic acid). See also es-sential fatty acids.

The physical properties of fattyacids are determined by chainlength, degree of unsaturation, andchain branching. Short-chain acidsare pungent liquids, soluble in water.As chain length increases, meltingpoints are raised and water-solubilitydecreases. Unsaturation and chainbranching tend to lower meltingpoints.

fatty-acid oxidation (β-oxidation)The metabolic pathway in which fatsare metabolized to release energy.Fatty-acid oxidation occurs continu-ally but does not become a majorsource of energy until the animal’scarbohydrate resources are ex-hausted, for example during starva-tion. Fatty-acid oxidation occurschieÛy in the mitochondria. A seriesof reactions cleave off two carbonatoms at a time from the hydrocar-bon chain of the fatty acid. Thesetwo-carbon fragments are combinedwith *coenzyme A to form acetylcoenzyme A (acetyl CoA), which thenenters the *Krebs cycle. The forma-tion of acetyl CoA occurs repeatedly

until all the hydrocarbon chain hasbeen used up.

f-block elements The block of el-ements in the *periodic table consist-ing of the lanthanoid series (fromcerium to lutetium) and the actinoidseries (from thorium to lawrencium).They are characterized by having twos-electrons in their outer shell (n) andf-electrons in their inner (n–1) shell.

f.c.c. Face-centred cubic. See cubiccrystal.

F-centre See colour centre.

Fehling’s test A chemical test todetect reducing sugars and aldehydesin solution, devised by the Germanchemist H. C. von Fehling (1812–85).Fehling’s solution consists of FehlingsA (copper(II) sulphate solution) andFehling’s B (alkaline 2,3-dihydroxybu-tanedioate (sodium tartrate) solu-tion), equal amounts of which areadded to the test solution. After boil-ing, a positive result is indicated bythe formation of a brick-red precipi-tate of copper(I) oxide. Methanal,being a strong reducing agent, alsoproduces copper metal; ketones donot react. The test is now little used,having been replaced by *Benedict’stest.

feldspars A group of silicate miner-als, the most abundant minerals inthe earth’s crust. They have a struc-ture in which (Si,Al)O4 tetrahedra arelinked together with potassium,sodium, and calcium and very occa-sionally barium ions occupying thelarge spaces in the framework. Thechemical composition of feldsparsmay be expressed as combinations ofthe four components: anorthite (An),CaAl2Si2O8; albite (Ab), NaAlSi3O8; or-thoclase (Or), KAlSi3O8; celsian (Ce),BaAl2Si2O8. The feldspars are subdi-vided into two groups: the alkalifeldspars (including microcline, or-thoclase, and sanidine), in which

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potassium is dominant with asmaller proportion of sodium andnegligible calcium; and the plagio-clase feldspars, which vary in compo-sition in a series that ranges frompure sodium feldspar (albite) throughto pure calcium feldspar (anorthite)with negligible potassium. Feldsparsform colourless, white, or pink crys-tals with a hardness of 6 on theMohs’ scale.

feldspathoids A group of alkalialuminosilicate minerals that aresimilar in chemical composition tothe *feldspars but are relatively deÜ-cient in silica and richer in alkalis.The structure consists of a frame-work of (Si,Al)O4 tetrahedra with alu-minium and silicon atoms at theircentres. The feldspathoids occurchieÛy with feldspars but do not co-exist with free quartz (SiO2) as theyreact with silica to yield feldspars.The chief varieties of feldspathoidsare: nepheline, KNa3(AlSiO4)4; leucite,KAlSi2O6; analcime, NaAlSi2O6.H2O;cancrinite, Na8(AlSiO4)6(HCO3)2; andthe sodalite subgroup comprising:sodalite, 3(NaAlSiO4).NaCl; nosean,3(NaAlSiO4).Na2SO4; haüyne, 3(NaAlSiO4).CaSO4; lazurite(Na,Ca)8(Al,Si)12O24(S,SO4) (see lapis lazuli).

FEM See field-emissionmicroscope.

femto- Symbol f. A preÜx used inthe metric system to denote 10–15.For example, 10–15 second = 1 femto-second (fs).

femtochemistry The investigationof chemical processes that occur onthe timescale of a femtosecond (10–15

s). Femtochemistry has become possi-ble as a result of the development of*lasers capable of being pulsed infemtoseconds. This has enabled ob-servations to be made on very short-lived species, such as activated

complexes, which only exist forabout a picosecond (10–12 s). In a fem-tochemical experiment a femtosec-ond pulse causes dissociation of amolecule. A series of femtosecondpulses is then released, the frequencyof the pulses being that of an absorp-tion of one of the products of the dis-sociation. The absorption can be usedas a measure of the abundance of theproduct of the dissociation. This typeof study enables the course of themechanism of a chemical reaction tobe studied in detail.

Fenton’s reagent A mixture of hy-drogen peroxide and iron(II) sulphateused to produce free radicals by reac-tions of the type

Fe2+ + H2O2 → Fe3+ + .OH + OH-

Fe3+ + H2O2 → Fe2+ + .OOH + H+

It is used in water treatment and as areagent in organic synthesis to intro-duce a OH group into an aromaticdrug.

fermentation A form of anaerobicrespiration occurring in certain mi-croorganisms, e.g. yeasts. Alcoholicfermentation comprises a series ofbiochemical reactions by which pyru-vate (the end product of *glycolysis)is converted to ethanol and carbondioxide. Fermentation is the basis ofthe baking, wine, and beer indus-tries.

fermi A unit of length formerlyused in nuclear physics. It is equal to10–15 metre. In SI units this is equalto 1 femtometre (fm). It was namedafter Enrico *Fermi.

Fermi–Dirac statistics See quan-tum statistics.

Fermi level The energy in a solidat which the average number of par-ticles per quantum state is ½; i.e. onehalf of the quantum states are occu-pied. The Fermi level in conductorslies in the conduction band (see en-

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ergy bands), in insulators it lies inthe valence band, and in semicon-ductors it falls in the gap betweenthe conduction band and the valenceband. At absolute zero all the elec-trons would occupy energy levels upto the Fermi level and no higher lev-els would be occupied. It is namedafter the Italian-born US physicist En-rico Fermi (1901–54).

fermion An *elementary particle(or bound state of an elementary par-ticle, e.g. an atomic nucleus or anatom) with half-integral spin; i.e. a particle that conforms to Fermi–Dirac statistics (see quantum statis-tics). Compare boson.

fermium Symbol Fm. A radioactivemetallic transuranic element belong-ing to the *actinoids; a.n. 100; massnumber of the most stable isotope257 (half-life 10 days). Ten isotopesare known. The element was ÜrstidentiÜed by A. Ghiorso and associ-ates in debris from the Ürst hydro-gen-bomb explosion in 1952. It isnamed after Enrico Fermi.


ferrate An iron-containing anion,FeO4

2–. It exists only in strong alka-line solutions, in which it forms pur-ple solutions.

ferric alum One of the *alums,K2SO4.Fe2(SO4)3.24H2O, in which thealuminium ion Al3+ is replaced bythe iron(III) (ferric) ion Fe3+.

ferric chloride test A *presump-tive test for morphine. The reagent isa 10% solution of ferric chloride(iron(III) chloride, Fe Cl3) in water.With morphine a blue-green col-oration occurs, changing to green.

ferric compounds Compounds ofiron in its +3 oxidation state; e.g. fer-ric chloride is iron(III) chloride, FeCl3.

ferricyanide A compound contain-ing the complex ion [Fe(CN)6]3–, i.e.the hexacyanoferrate(III) ion.

ferrimagnetism See magnetism.

ferrite 1. A member of a class ofmixed oxides MO.Fe2O3, where M is ametal such as cobalt, manganese,nickel, or zinc. The ferrites are ce-ramic materials that show either fer-rimagnetism or ferromagnetism, butare not electrical conductors. For this reason they are used in high-frequency circuits as magnetic cores.2. See steel.

ferroalloys Alloys of iron withother elements made by smeltingmixtures of iron ore and the metalore; e.g. ferrochromium, ferrovana-dium, ferromanganese, ferrosilicon,etc. They are used in making alloy*steels.

ferrocene An orange-red crys-talline solid, Fe(C5H5)2; m.p. 173°C. Itcan be made by adding the ioniccompound Na+C5H5

– (cyclopentadi-enyl sodium, made from sodium andcyclopentadiene) to iron(III) chloride.In ferrocene, the two rings are paral-lel, with the iron ion sandwiched be-tween them (hence the namesandwich compound: see formula).The bonding is between pi orbitalson the rings and d-orbitals on theFe2+ ion. The compound can undergoelectrophilic substitution on the C5H5rings (they have some aromatic char-acter). It can also be oxidized to theblue ion (C5H5)2Fe+. Ferrocene is the

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Ürst of a class of similar complexescalled *sandwich compounds. Its sys-tematic name is di-π-cyclopentadi-enyl iron(II).

ferrocyanide A compound con-taining the complex ion [Fe(CN)6]4–,i.e. the hexacyanoferrate(II) ion.

ferroelectric materials Ceramicdielectrics, such as Rochelle salt andbarium titanate, that have a domainstructure making them analogous toferromagnetic materials. They ex-hibit hysteresis and usually thepiezoelectric effect.

ferromagnetism See magnetism.

ferrosoferric oxide See triirontetroxide.

ferrous compounds Compoundsof iron in its +2 oxidation state; e.g.ferrous chloride is iron(II) chloride,FeCl2.

fertilizer Any substance that isadded to soil in order to increase itsproductivity. Fertilizers can be of nat-ural origin, such as composts, or theycan be made up of synthetic chemi-cals, particularly nitrates and phos-phates. Synthetic fertilizers canincrease crop yields dramatically, butwhen leached from the soil by rain,which runs into lakes, they also in-

crease the process of eutrophication.Bacteria that can Üx nitrogen aresometimes added to the soil to in-crease its fertility; for example, intropical countries the cyanobac-terium Anabaena is added to rice pad-dies to increase soil fertility.

Übrous protein See protein.

Fick’s law A law describing the dif-fusion that occurs when solutions ofdifferent concentrations come intocontact, with molecules moving fromregions of higher concentration to re-gions of lower concentration. Fick’slaw states that the rate of diffusiondn/dt, called the diffusive flux and de-noted J, across an area A is given by:dn/dt = J = –DA∂c/∂x, where D is a con-stant called the diffusion constant,∂c/∂x is the concentration gradient ofthe solute, and dn/dt is the amount ofsolute crossing the area A per unittime. D is constant for a speciÜcsolute and solvent at a speciÜc tem-perature. Fick’s law was formulatedby the German physiologist AdolfEugen Fick (1829–1901) in 1855.

Üeld effect See electroniceffects.

Üeld-emission microscope (FEM)A type of electron microscope in

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metaltip

liquidhelium

electronselectronselectrons

to vacuumpump

fluorescentscreen

highnegativevoltage

Field-emission microscope


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which a high negative voltage is ap-plied to a metal tip placed in an evac-uated vessel some distance from aglass screen with a Ûuorescent coat-ing. The tip produces electrons byÜeld emission, i.e. the emission ofelectrons from an unheated sharpmetal part as a result of a high elec-tric Üeld. The emitted electrons forman enlarged pattern on the Ûuores-cent screen, related to the individualexposed planes of atoms. As the reso-lution of the instrument is limited bythe vibrations of the metal atoms, itis helpful to cool the tip in liquid he-lium. Although the individual atomsforming the point are not displayed,individual adsorbed atoms of othersubstances can be, and their activityis observable.

Üeld-ionization microscope(Üeld-ion microscope; FIM) A type ofelectron microscope that is similar inprinciple to the *Üeld-emissionmicroscope, except that a high posi-tive voltage is applied to the metaltip, which is surrounded by low-pressure gas (usually helium) ratherthan a vacuum. The image is formedin this case by Üeld ionization: ion-ization at the surface of an unheatedsolid as a result of a strong electricÜeld creating positive ions by elec-tron transfer from surroundingatoms or molecules. The image isformed by ions striking the Ûuores-cent screen. Individual atoms on thesurface of the tip can be resolvedand, in certain cases, adsorbed atomsmay be detected. See also atom-probefield-ion microscopy.

Üller A solid inert material added toa synthetic resin or rubber, either tochange its physical properties or sim-ply to dilute it for economy.

Ülm badge A lapel badge contain-ing masked photographic Ülm wornby personnel who could be exposedto ionizing radiation. The Ülm is de-

veloped to indicate the extent thatthe wearer has been exposed toharmful radiation.

Ülter A device for separating solidparticles from a liquid or gas. Thesimplest laboratory Ülter for liquidsis a funnel in which a cone of paper(Ülter paper) is placed. Special con-tainers with a porous base of sinteredglass are also used. See also goochcrucible.

Ülter pump A simple laboratoryvacuum pump in which air is re-moved from a system by a jet ofwater forced through a narrow noz-zle. The lowest pressure possible isthe vapour pressure of water.

Ültrate The clear liquid obtained byÜltration.

Ültration The process of separatingsolid particles using a Ülter. In vac-uum Ültration, the liquid is drawnthrough the Ülter by a vacuumpump. UltraÜltration is Ültrationunder pressure.

FIM See field-ionization micro-scope.

Üne chemicals Chemicals pro-duced industrially in relatively smallquantities and with a high purity;e.g. dyes and drugs.

Üneness of gold A measure of thepurity of a gold alloy, deÜned as theparts of gold in 1000 parts of thealloy by mass. Gold with a Üneness of750 contains 75% gold, i.e. 18 *caratgold.

Üne structure Closely spaced spec-tral lines arising from transitions be-tween energy levels that are split bythe vibrational or rotational motionof a molecule or by electron spin.They are visible only at high resolu-tion. HyperÜne structure, visible onlyat very high resolution, results fromthe inÛuence of the atomic nucleus

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on the allowed energy levels of theatom.

Ünger domain A Ünger-shapedstructure produced in a protein whena series of the constituent aminoacids combines with a metal atom.

Üredamp Methane formed in coalmines.

Üre extinguisher A substance thatsmothers Ûames or prevents theirspreading. Many substances can beused. Liquids include water, sodiumhydrogencarbonate solution, chlori-nated organic compounds such astetrachloromethane and carbondioxide-containing foams. Solidextinguishers, such as sodium orpotassium hydrogencarbonate, pro-duce carbon dioxide gas whenstrongly heated; solid carbon dioxide(dry ice) may also be used.

Ürst-order reaction See order.

Fischer, Emil Hermann (1852–1919) German organic chemist whostudied under *Kekulé in Bonn andlater under *Baeyer in Strasbourg. He moved to Munich in 1875 andWürzburg in 1885. Fischer is notedfor his extensive pioneering work on natural products and especiallyhis work on sugar chemistry. In 1899he began work on synthesizing pep-tides and proteins. Fischer wasawarded the 1902 Nobel Prize forchemistry.

Fischer, Hans (1881–1945) Germanorganic chemist who worked mainlyin Munich. He worked on por-phyrins, synthesizing haemin in1927. Fischer worked also on chloro-phyll, showing that it was a por-phyrin containing magnesium. In1944 he synthesized bilirubin. Hewas awarded the 1930 Nobel Prizefor chemistry.

Fischer projection A type of *pro-jection in which a molecule is drawn

with horizontal bonds representingbonds coming out of the page andvertical bonds representing bonds inthe plane of the page or behind thepage. It is named after Emil Fischer.See absolute configuration.

Fischer–Tropsch process An in-dustrial method of making hydrocar-bon fuels from carbon monoxide andhydrogen. The process was inventedin 1933 and used by Germany inWorld War II to produce motor fuel.Hydrogen and carbon monoxide aremixed in the ratio 2:1 (water gas wasused with added hydrogen) andpassed at 200°C over a nickel orcobalt catalyst. The resulting hydro-carbon mixture can be separated intoa higher-boiling fraction for Dieselengines and a lower-boiling gasolinefraction. The gasoline fraction con-tains a high proportion of straight-chain hydrocarbons and has to bereformed for use in motor fuel. Alco-hols, aldehydes, and ketones are alsopresent. The process is also used inthe manufacture of SNG from coal. Itis named after the German chemistFranz Fischer (1852–1932) and theCzech Hans Tropsch (1839–1935).

Üssion-track dating A method ofestimating the age of glass and othermineral objects by observing thetracks made in them by the Üssionfragments of the uranium nuclei thatthey contain. By irradiating the ob-jects with neutrons to induce Üssionand comparing the density and num-ber of the tracks before and after ir-radiation it is possible to estimate thetime that has elapsed since the ob-ject solidiÜed.

Fittig reaction See wurtz reac-tion.

Üxation See nitrogen fixation.

Üxed point A temperature thatcan be accurately reproduced to en-

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able it to be used as the basis of a*temperature scale.

Ûagpole See ring conformations.

Ûame A hot luminous mixture ofgases undergoing combustion. Thechemical reactions in a Ûame aremainly free-radical chain reactionsand the light comes from Ûuores-cence of excited molecules or ions orfrom incandescence of small solidparticles (e.g. carbon).

Ûame test A simple test for met-als, in which a small amount of thesample (usually moistened with hy-drochloric acid) is placed on the endof a platinum wire and held in a Bun-sen Ûame. Certain metals can be de-tected by the colour produced:barium (green), calcium (brick red),lithium (crimson), potassium (lilac),sodium (yellow), strontium (red).

Ûash photolysis A technique forstudying free-radical reactions ingases. The apparatus used typicallyconsists of a long glass or quartz tubeholding the gas, with a lamp outsidethe tube suitable for producing an in-tense Ûash of light. This dissociatesmolecules in the sample creating freeradicals, which can be detected spec-troscopically by a beam of lightpassed down the axis of the tube. It ispossible to focus the spectrometer onan absorption line for a particularproduct and measure its change inintensity with time using an oscillo-scope. In this way the kinetics of veryfast free-radical gas reactions can bestudied.

Ûash point The temperature atwhich the vapour above a volatile liq-uid forms a combustible mixturewith air. At the Ûash point the appli-cation of a naked Ûame gives a mo-mentary Ûash rather than sustainedcombustion, for which the tempera-ture is too low.

Ûavin adenine dinucleotide Seefad.

Ûavones A group of flavanoid com-pounds found in many plants.

227 flip-flop

f

O

O

Flavone structure

Ûavonoids A group of naturally oc-curring phenolic compounds manyof which are plant pigments. They in-clude the anthocyanins, Ûavonols,and Ûavones.

Ûavoprotein See fad.

Fleming, Sir Alexander(1881–1955) British bacteriologist,born in Scotland. He studied medi-cine at St Mary’s Hospital, London,where he remained all his life. In1922 he identiÜed lysozyme, an en-zyme that destroys bacteria, and in1928 discovered the antibiotic *peni-cillin. He shared the 1945 Nobel Prizefor physiology or medicine with *Flo-rey and *Chain, who Ürst isolated thedrug.

Ûint (chert) Very hard dense nod-ules of microcrystalline quartz andchalcedony found in chalk and lime-stone.

Ûip-Ûop The movement (transversediffusion) of a lipid molecule fromone surface of a *lipid bilayer mem-brane to the other, which occurs at avery slow rate. This contrasts withthe much faster rate at which lipidmolecules exchange places withneighbouring molecules on the samesurface of the membrane (lateral dif-fusion).


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Ûocculation The process in whichparticles in a colloid aggregate intolarger clumps. Often, the term isused for a reversible aggregation ofparticles in which the forces holdingthe particles together are weak andthe colloid can be redispersed by agi-tation. The stability of a lyophobiccolloidal dispersion depends on theexistence of a layer of electric chargeon the surface of the particles.Around this are attracted electrolyteions of opposite charge, which forma mobile ionic ‘atmosphere’. The re-sult is an electrical double layer onthe particle, consisting of an innershell of Üxed charges with an outermobile atmosphere. The potential en-ergy between two particles dependson a repulsive interaction betweendouble layers on adjacent particlesand an attractive interaction due to*van der Waals’ forces between theparticles.

At large separations, the repulsiveforces dominate, and this accountsfor the overall stability of the colloid.As the particles become closer to-gether, the potential energy in-creases to a maximum and then fallssharply at very close separations,where the van der Waals’ forcesdominate. This potential-energy min-imum corresponds to *coagulationand is irreversible. If the *ionicstrength of the solution is high, theionic atmosphere around the parti-cles is dense and the potential-energycurve shows a shallow minimum atlarger separation of particles. Thiscorresponds to Ûocculation of theparticles. Ions with a high charge areparticularly effective for causing Ûoc-culation and coagulation.

Ûocculent Aggregated in woollymasses; used to describe precipitates.

Florey, Howard Walter, Baron(1898–1968) Australian pathologist,who moved to Oxford in 1922. After

working in Cambridge and ShefÜeld,he returned to Oxford in 1935. Therehe teamed up with Ernst *Chain andby 1939 they succeeded in isolatingand purifying *penicillin. They alsodeveloped a method of producing thedrug in large quantities and carriedout its Ürst clinical trials. The twomen shared the 1945 Nobel Prize forphysiology or medicine with peni-cillin’s discoverer, Alexander *Flem-ing.

Flory temperature (theta tempera-ture) Symbol θ. The unique tempera-ture at which the attractions andrepulsions of a polymer in a solutioncancel each other. It is analogous tothe Boyle temperature of a nonidealgas. A polymer solution at the Florytemperature is called a theta (θ) solu-tion. At the Flory temperature thevirial coefÜcient B, asociated with theexcluded volume of the polymer, iszero, which results in the polymerchain behaving almost ideally. Thisenables the theory of polymer solu-tions at the Flory temperature to pro-vide a more accurate description ofevents than for polymer solutions atother temperatures, even if the poly-mer solution is concentrated. It is notalways possible to attain the Florytemperature experimentally. TheFlory temperature is named after theUS physicist Paul Flory (1910–85).

Ûotation See froth flotation.

Ûuctuation–dissipation theoremA theory relating quantities in equi-librium and *nonequilibrium statisti-cal mechanics and microscopic andmacroscopic quantities. The Ûuctua-tion–dissipation theorem was Ürst de-rived for electrical circuits with noisein 1928 by H. Nyquist; a general theo-rem in statistical mechanics was de-rived by H. B. Callen and T. A.Welton in 1951. The underlying prin-ciple of the Ûuctuation–dissipationtheorem is that a nonequilibrium

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state may have been reached eitheras a result of a random Ûuctuation oran external force (such as an electricor magnetic Üeld) and that the evolu-tion towards equilibrium is the samein both cases (for a sufÜciently small Ûuctuation). The Ûuctuation–dissipation theorem enables *trans-port coefÜcients to be calculated interms of response to external Üelds.

Ûuidization A technique used insome industrial processes in whichsolid particles suspended in a streamof gas are treated as if they were inthe liquid state. Fluidization is usefulfor transporting powders, such ascoal dust. Fluidized beds, in whichsolid particles are suspended in anupward stream, are extensively usedin the chemical industry, particularlyin catalytic reactions where the pow-dered catalyst has a high surfacearea. They are also used in furnaces,being formed by burning coal in ahot turbulent bed of sand or ashthrough which air is passed. The bedbehaves like a Ûuid, enabling thecombustion temperature to be re-duced so that the production ofpolluting oxides of nitrogen is dimin-ished. By adding limestone to the bed with the fuel, the emission ofsulphur dioxide is reduced.

High-pressure Ûuidized beds arealso used in power-station furnacesin a combined cycle in which theproducts of combustion from theÛuidized bed are used to drive a gasturbine, while a steam-tube boiler inthe Ûuid bed raises steam to drive asteam turbine. This system both in-creases the efÜciency of the combus-tion process and reduces pollution.

Ûunitrazepam A *benzodiazepineused medically in some countries asa powerful hypnotic, sedative, andmuscle relaxant. It was marketed inthe US under the tradename Rohyp-nol. Flunitrazepam has become noto-

rious as a so-called ‘date rape’ drug. Itis quickly eliminated from the body,difÜcult to detect, and causes amne-sia, so that victims cannot rememberevents that occur when under theinÛuence of the drug.

229 fluorine

fN

NNH2

CH3O

F

Flunitrazepam

Ûuorescein A yellowish-red dyethat produces yellow solutions with agreen Ûuorescence. It is used in trac-ing water Ûow and as an *adsorptionindicator.

Ûuorescence See luminescence.

Ûuoridation The process of addingvery small amounts of Ûuorine salts(e.g. sodium Ûuoride, NaF) to drink-ing water to prevent tooth decay. TheÛuoride becomes incorporated intothe Ûuoroapatite (see apatite) of thegrowing teeth and reduces the inci-dence of dental caries.

Ûuoride See halide.

Ûuorination A chemical reactionin which a Ûuorine atom is intro-duced into a molecule. See halogena-tion.

Ûuorine Symbol F. A poisonouspale yellow gaseous element belong-ing to group 17 (formerly VIIB) of theperiodic table (the *halogens); a.n. 9;r.a.m. 18.9984; d. 1.7 g dm–3; m.p.–219.62°C; b.p. –188.1°C. The mainmineral sources are *Ûuorite (CaF2)


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and *cryolite (Na3AlF). The element isobtained by electrolysis of a moltenmixture of potassium Ûuoride andhydrogen Ûuoride. It is used in thesynthesis of organic Ûuorine com-pounds. Chemically, it is the most re-active and electronegative of allelements. It is a highly dangerous el-ement, causing severe chemicalburns on contact with the skin. Theelement was identiÜed by Scheele in1771 and Ürst isolated by Moissan in1886.


Ûuorite (Ûuorspar) A mineral formof calcium Ûuoride, CaF2, crystalliz-ing in the cubic system. It is variablein colour; the most common Ûuoritesare green and purple (blue john), butother forms are white, yellow, orbrown. Fluorite is used chieÛy as aÛux material in the smelting of ironand steel; it is also used as a source ofÛuorine and hydroÛuoric acid and inthe ceramic and optical-glass indus-tries.

Ûuorite structure A type of ioniccrystal structure in which the cationshave an expanded face-centred cubicarrangement with the anions occupy-ing both types of tetrahedral hole.The cations have a coordinationnumber of 8 and the anions have acoordination number of 4. Examplesof compounds with this structure areCaF2, BaCl2, and PbO2.

The antiÛuorite structure is the op-posite arrangement, with anions inthe fcc array with coordination num-ber 8 and cations in the tetrahedralholes with coordination number 4.Examples of the antiÛurite structureare K2O, Li2O, Na2O, K2S, and Na2S.

A• An interactive version of the structure

Ûuorocarbons Compounds ob-tained by replacing the hydrogen

atoms of hydrocarbons by Ûuorineatoms. Their inertness and high sta-bility to temperature make themsuitable for a variety of uses as oils,polymers, etc. See also chlorofluoro-carbon; halon.

5-Ûuorouracil (5-FU) A Ûuorine de-rivative of the pyrimidine uracil. It isused in chemotherapy where it in-hibits the cell’s ability to synthesizeDNA. It is often used in a treatmentregime along with cisplatin.

Ûux 1. A substance applied to thesurfaces of metals to be soldered toinhibit oxidation. 2. A substanceused in the smelting of metals to as-sist in the removal of impurities asslag.

Ûuxional molecule A moleculethat undergoes alternate very rapidrearrangements of its atoms and thusonly has a speciÜc structure for avery short period of time. For exam-ple, the molecule ClF3 has a T-shapeat low temperatures (–60°C); at roomtemperature the Ûuorine atomschange position very rapidly and ap-pear to have identical positions.

foam A dispersion of bubbles in aliquid. Foams can be stabilized by*surfactants. Solid foams (e.g. ex-panded polystyrene or foam rubber)are made by foaming the liquid andallowing it to set. See also colloids.

foaming agent (blowing agent) Asubstance used to produce a liquid orsolid foam (e.g. an expanded plastic).Physical agents are compressedgases; chemical foaming agents aresubstances that release gas under cer-tain conditions (e.g. sodium hydro-gencarbonate).

Fokker–Planck equation Anequation in *nonequilibrium statisti-cal mechanics that describes a super-position of a dynamic friction(slowing-down) process and a diffu-

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sion process for the evolution of vari-ables in a system. The Fokker–Planckequation, which can be used toanalyse such problems as *Brownianmovement, is soluble using statisticalmethods and the theory of probabil-ity. It is named after the Dutch physi-cist Adriaan Fokker (1887–1968) andMax *Planck.

folacin See folic acid.

folic acid (folacin) A vitamin of the*vitamin B complex. In its activeform, tetrahydrofolic acid, it is a*coenzyme in various reactions in-volved in the metabolism of aminoacids, purines, and pyrimidines. It issynthesized by intestinal bacteriaand is widespread in food, especiallygreen leafy vegetables. DeÜciencycauses poor growth and nutritionalanaemia.

food additive A substance addedto a food during its manufacture orprocessing in order to improve itskeeping qualities, texture, appear-ance, or stability or to enhance itstaste or colour. Additives are usuallypresent in minute quantities; they in-clude colouring materials, sweeten-ers, preservatives, *antioxidants,emulsiÜers, and stabilizers. In mostcountries the additives used must beselected from an approved list ofsuch compounds, which have beentested for safety, and they must belisted on the food labels of individualproducts.

fool’s gold See pyrite.

forbidden band See energybands.

forbidden transitions Transitionsbetween energy levels in a quantum-mechanical system that are not al-lowed to take place because of*selection rules. In practice, forbid-den transitions can occur, but theydo so with much lower probability

than allowed transitions. There arethree reasons why forbidden transi-tions may occur:(1) the selection rule that is violatedis only an approximate rule. An ex-ample is provided by those selectionrules that are only exact in the ab-sence of *spin–orbit coupling. Whenspin–orbit coupling is taken into ac-count, the forbidden transitions be-come allowed – their strengthincreasing with the size of thespin–orbit coupling;(2) the selection rule is valid for di-pole radiation, i.e. in the interactionbetween a quantum-mechanical sys-tem, such as an atom, and an electro-magnetic Üeld, only the (variable)electric dipole moment is considered.Actual transitions may involve mag-netic dipole radiation or quadrupoleradiation;(3) the selection rule only applies foran atom, molecule, etc., in isolationand does not necessarily apply if ex-ternal Üelds, collisions, etc., are takeninto account.

force constant A constant charac-terizing the strength of the bond in adiatomic molecule. Near the equilib-rium position, Re, of the potential en-ergy curve of a diatomic molecule,the potential energy V is accuratelyrepresented by a parabola of theform V = k/2(R – Re)2, where R is theinternuclear distance and k is theforce constant. The greater the forceconstant, the stronger is the bond be-tween the atoms, since the walls ofthe potential curve become steeper.Regarding the molecular vibrationsas simple harmonic motion, theforce constant occurs in the analysisof the vibrational energy levels.

forced convection See convec-tion.

formaldehyde See methanal.

formalin A colourless solution of

231 formalin

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methanal (formaldehyde) in waterwith methanol as a stabilizer; r.d.1.075–1.085. When kept at tempera-tures below 25°C a white polymer ofmethanal separates out. It is used asa disinfectant and preservative forbiological specimens.

formate See methanoate.

formic acid See methanoic acid.

formula A way of representing achemical compound using symbolsfor the atoms present. Subscripts areused for the numbers of atoms. Themolecular formula simply gives thetypes and numbers of atoms present.For example, the molecular formulaof ethanoic acid is C2H4O2. The em-pirical formula gives the atoms intheir simplest ratio; for ethanoic acidit is CH2O. The structural formulagives an indication of the way theatoms are arranged. Commonly, thisis done by dividing the formula intogroups; ethanoic acid can be writtenCH3.CO.OH (or more usually simplyCH3COOH). Structural formulae canalso show the arrangement of atomsor groups in space.

formula weight The relative mo-lecular mass of a compound as calcu-lated from its molecular formula.

formylation A chemical reactionthat introduces a formyl group(methanoyl, –CHO) into an organicmolecule.

formyl group The group HCO–.

fossil fuel Coal, oil, and naturalgas, the fuels used by man as asource of energy. They are formedfrom the remains of living organismsand all have a high carbon or hydro-gen content. Their value as fuels re-lies on the exothermic oxidation ofcarbon to form carbon dioxide (C +O2 → CO2) and the oxidation of hy-drogen to form water (H2 + ½O2 →H2O).

four-circle diffractometer An in-strument used in X-ray crystallogra-phy to automatically determine theshape and symmetry of the unit cellof a crystal. The crystal is placed inthe goniometer head with an arbi-trary orientation. If the dimensionsof the unit cell have been deter-mined, they can be used to calculatethe settings of the four angles of thediffractometer needed to observe aspeciÜc (h k l) reÛection, where (h k l)are Miller indices. A computer con-trols the settings so that each (h k l)is examined in turn and the diffrac-tion intensity measured. This infor-mation enables the electron densityto be calculated, as the intensity ofthe reÛection for a set of (h k l)planes is proportional to the squareof the modulus of a function calledthe structure factor Fhkl of the set,which is related to the electron den-sity.

Fourier analysis The representa-tion of a function f(x), which is peri-odic in x, as an inÜnite series of sineand cosine functions:

f(x) = a0/2 + ∑n=1

∞(ancosnx + bnsinnx)

A series of this type is called a*Fourier series. If the function is pe-riodic with a period 2π, the coefÜ-cients a0, an, bn are:

a0 = ∫+π–π f(x)dx,

an = ∫+π–π f(x) cosnxdx, (n = 1,2,3,…),

bn = ∫+π–π 1/π f(x)sinnxdx, (n = 1,2,3,…).

Fourier analysis and Fourier seriesare named after the French mathe-matician and engineer Joseph Fourier(1768–1830). Fourier series havemany important applications inmathematics, science, and engineer-ing, having been invented by Fourierin the Ürst quarter of the 19th cen-tury in his analysis of the problem ofheat conduction.

Fourier series An expansion of a

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periodic function as a series oftrigonometric functions. Thus,

f(x) = a0 + (a1cosx + b1sinx) + (a2cos2x + b2sin2x)+…,

where a0, a1, b1, b2, etc., are con-stants, called Fourier coefÜcients. Theseries was Ürst formulated by JosephFourier and is used in *Fourier analy-sis.

Fourier transform An integraltransform of the type:

F(y) = ∫ ∞–∞ f(x)e–xydy.

The inverse is:f(x) = (1/2π) ∫ ∞

–∞ F(y)eixydy.Fourier transform techniques areused in obtaining information fromspectra, especially in NHR and in-frared spectroscopy (see fourier-transform infrared).

Fourier-transform infrared (FT-IR)Infrared spectroscopy in which com-puters are part of the spectroscopicapparatus and use *Fourier trans-forms to enable the curve of inten-sity against wave number to beplotted with very high sensitivity.This has allowed spectra to be ob-tained in the far infrared region; pre-viously it was difÜcult to attainspectra in this region as the resolu-tion was obscured by the signal-to-noise ratio being too high to resolvethe vibrational and/or rotationalspectra of small molecules in theirgas phase. FT-IR has been used in re-search on the atmosphere. Anotherapplication of this technique is thedetection of impurities in samples ofcondensed matter.

four-level laser A laser in whichfour energy levels are involved. Thedisadvantage of a three-level laser isthat it is difÜcult to attain populationinversion because many moleculeshave to be raised from their groundstate to an excited state by pumping.In a four-level laser, the laser transi-

tion Ünishes in an initially unoccu-pied state F, having started in a stateI, which is not the ground state. Asthe state F is initially unoccupied,any population in I constitutes popu-lation inversion. Thus laser action ispossible if I is sufÜciently metastable.If transitions from F to the groundstate G are rapid, population inver-sion is maintained since this lowersthe population in F caused by thetransition in the laser action.

fractal A curve or surface generatedby a process involving successive sub-division. For example, a snowÛakecurve can be produced by startingwith an equilateral triangle and di-viding each side into three segments.The middle segments are then re-placed by two equal segments, whichwould form the sides of a smallerequilateral triangle. This gives a 12-sided star-shaped Ügure. The nextstage is to subdivide each of the sidesof this Ügure in the same way, and soon. The result is a developing Ügurethat resembles a snowÛake. In thelimit, this Ügure has ‘fractional di-mension’ – i.e. a dimension betweenthat of a line (1) and a surface (2); thedimension of the snowÛake curve is1.26. The study of this type of ‘self-similar’ Ügure is used in certainbranches of chemistry – for example,crystal growth. Fractals are also im-portant in *chaos theory and in com-puter graphics.

fraction See fractional distilla-tion.

fractional crystallization Amethod of separating a mixture ofsoluble solids by dissolving them in asuitable hot solvent and then lower-ing the temperature slowly. The leastsoluble component will crystallizeout Ürst, leaving the other compo-nents in solution. By controlling thetemperature, it is sometimes possibleto remove each component in turn.

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fractional distillation (fractiona-tion) The separation of a mixture ofliquids by distillation. Effective sepa-ration can be achieved by using along vertical column (fractionatingcolumn) attached to the distillationvessel and Ülled with glass beads.Vapour from the liquid rises up thecolumn until it condenses and runsback into the vessel. The risingvapour in the column Ûows over thedescending liquid, and eventually asteady state is reached in whichthere is a decreasing temperaturegradient up the column. The vapourin the column has more volatile com-ponents towards the top and lessvolatile components at the bottom.Various fractions of the mixture canbe drawn off at points on the col-umn. Industrially, fractional distilla-tion is performed in large towerscontaining many perforated trays. Itis used extensively in petroleumreÜning.

fractionating column See frac-tional distillation.

fractionation See fractional dis-tillation.

francium Symbol Fr. A radioactiveelement belonging to *group 1 (for-merly IA) of the periodic table; a.n.87; r.d. 2.4; m.p. 27±1°C; b.p.677±1°C. The element is found inuranium and thorium ores. All 22known isotopes are radioactive, themost stable being francium–223. Theexistence of francium was conÜrmedby Marguerite Perey in 1939.A• Information from the WebElements site

Franck–Condon principle A prin-ciple governing the intensity of tran-sitions in the vibrational structureduring an electronic transition in amolecule. The principle states thatsince nuclei are much heavier andmove much more slowly than elec-

trons (see born–oppenheimer approx-imation), an electronic transition oc-curs much more rapidly than thetime required for the nuclei to re-spond to it. Therefore, in a diagramshowing the electronic states of themolecule as a function of internu-clear distance, the most intense elec-tronic transition is represented by avertical line. For this reason a transi-tion obeying the Franck–Condonprinciple is called a verticaltransition; when it occurs the relativepositions of the nuclei remain un-changed. The Franck–Condon princi-ple is named after James Franck(1882–1964), who stated it in 1925,and Edward Condon, who formu-lated it mathematically in terms ofquantum mechanics in 1928.

Frankland, Sir Edward (1825–99)British organic chemist who was theÜrst to produce organometallic com-pounds (zinc dialkyls). He is remem-bered as the originator of the theoryof valency and introduced a methodof writing structural formulas.

Frasch process A method of ob-taining sulphur from undergrounddeposits using a tube consisting ofthree concentric pipes. Superheatedsteam is passed down the outer pipeto melt the sulphur, which is forcedup through the middle pipe by com-pressed air fed through the innertube. The steam in the outer casingkeeps the sulphur molten in thepipe. It was named after the German-born US chemist Hermann Frasch(1851–1914).

Fraunhofer, Josef von(1787–1826) German physicist, whotrained as an optician. In 1814 he ob-served dark lines in the spectrum ofthe sun (see fraunhofer lines). Healso studied diffraction.

Fraunhofer lines Dark lines in thesolar spectrum, discovered by Josef

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von Fraunhofer, that result from theabsorption by elements in the solarchromosphere of some of the wave-lengths of the visible radiation emit-ted by the hot interior of the sun.

free electron See electron.

free-electron approximationThe approximation resulting fromthe assumption that electrons in*metals can be analysed using the*kinetic theory of gases, without tak-ing the periodic potential of themetal into account. This approxima-tion gives a good qualitative accountof some properties of metals, such astheir electrical conductivity. At verylow temperatures it is necessary touse quantum statistical mechanicsrather than classical statistical me-chanics. The free-electron approxi-mation does not, however, give anadequate quantitative description ofthe properties of metals. It can be im-proved by the nearly free electron ap-proximation, in which the periodicpotential is treated as a perturbationon the free electrons.

free energy A measure of a sys-tem’s ability to do work. The Gibbsfree energy (or Gibbs function), G, isdeÜned by G = H – TS, where G is theenergy liberated or absorbed in a re-versible process at constant pressureand constant temperature (T), H isthe *enthalpy and S the *entropy ofthe system. Changes in Gibbs free en-ergy, ∆G, are useful in indicating theconditions under which a chemicalreaction will occur. If ∆G is positivethe reaction will only occur if energyis supplied to force it away from theequilibrium position (i.e. when ∆G =0). If ∆G is negative the reaction willproceed spontaneously to equilib-rium.

The Helmholtz free energy (orHelmholtz function), F, is deÜned by F= U – TS, where U is the *internal en-ergy. For a reversible isothermal

process, ∆F represents the usefulwork available.

free radical An atom or group ofatoms with an unpaired valence elec-tron. Free radicals can be producedby photolysis or pyrolysis in which abond is broken without forming ions(see homolytic fission). Because oftheir unpaired valence electron, mostfree radicals are extremely reactive.See also chain reaction.

freeze drying A process used indehydrating food, blood plasma, andother heat-sensitive substances. Theproduct is deep-frozen and the icetrapped in it is removed by reducingthe pressure and causing it to sub-lime. The water vapour is then re-moved, leaving an undamaged dryproduct.

freezing mixture A mixture ofcomponents that produces a lowtemperature. For example, a mixtureof ice and sodium chloride gives atemperature of –20°C.

freezing-point depression Seedepression of freezing point.

Frenkel defect See crystal defect.

Frenkel–Kontorowa model Aone-dimensional model of atoms,such as xenon, adsorbed on a peri-odic substrate, such as graphite. Thismodel, which can be used to investi-gate the nature of the lattice formedby the adsorbed gas, was invented in1938 by Y. I. Frenkel and T. Kon-torowa and independently in 1949 byF. C. Frank and J. H. Van der Merwe.The Frenkel–Kontorowa model canbe used to investigate the phase tran-sition between a *commensurate lat-tice and an *incommensurate lattice.

freon See chlorofluorocarbon.

Freundlich isotherm An isothermfor adsorption with the form θ =c1P1/c2, where the fractional coverage

235 Freundlich isotherm

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θ is the ratio of adsorption sites occu-pied to the number of adsorptionsites available, c1, and c2 are con-stants, and P is the pressure of gas.The Freundlich isotherm is fre-quently used with adsorption fromliquid solutions, when it has theform: w = c1 × c1/c2, where w is themass of solute adsorbed per unitmass of adsorbent (a quantity calledthe mass fraction) and c is the con-centration of the solution. Like otherisotherms the Freundlich isothermagrees with experiment for limitedranges of parameter. If the limits ofapplicability have been establishedthese isotherms, such as the Fre-undlich isotherm, are useful in con-sidering heterogeneous catalysis.

Friedel–Crafts reaction A type ofreaction in which an alkyl group(from a haloalkane) or an acyl group(from an acyl halide) is substituted ona benzene ring (see illustration). Theproduct is an alkylbenzene (for alkylsubstitution) or an alkyl aryl ketone

(for acyl substitution). The reactionsoccur at high temperature (about100°C) with an aluminium chloridecatalyst. The catalyst acts as an elec-tron acceptor for a lone pair on thehalide atom. This polarizes thehaloalkane or acyl halide, producinga positive charge on the alkyl or acylgroup. The mechanism is then elec-trophilic substitution. Alcohols andalkenes can also undergo Friedel–Crafts reactions. The reaction isnamed after the French chemistCharles Friedel (1832–99) and the USchemist James M. Craft (1839–1917).

Froehde’s test A *presumptivetest for opioids. Froehde’s reagentconsists of 0.5 gram of sodiummolybdate (Na2MoO4) dissolved in100 ml of concentrated sulphuricacid. LSD gives a blue-green colour,heroin gives purple to olive green,and mescaline gives a greenish col-our.

frontier orbital One of two or-

Friedel–Crafts reaction 236

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5 (

5 (

6 "678

6 " " "9

* %("

* %(

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bitals in a molecule: the *highest oc-cupied molecular orbital (HOMO) andthe *lowest unoccupied molecularorbital (LUMO). These two molecularorbitals are usually the most impor-tant ones in determining chemicaland spectroscopic properties of themolecule.

frontier-orbital theory A theoryof the reactions of molecules thatemphasizes the energies and symme-tries of *frontier orbitals. Frontier or-bital theory was developed by theJapanese chemist Kenichi Fukui(1919–98) in the 1950s and is an al-ternative approach to the *Wood-ward–Hoffmann rules. It has beenvery successful in explaining such re-actions as the *Diels–Alder reaction.

Frost diagram A graph showinghow standard electrode potentialsvary with oxidation state for differ-ent oxidation states of an element.Frost diagrams can be constructedfrom *Latimer diagrams. For an el-ement M, the standard electrode po-tential E Š is calculated for thereaction.

M(N) + Ne– → M(0)where M(N) indicates the species inoxidation state N and M(0) indicatesthe zero oxidation state. The Frost di-agram is then obtained by plottingNEŠ against N for the differentspecies. NEŠ is proportional to thestandard Gibbs free energy of theparticular half reaction.

In a Frost diagram, the lowestpoint corresponds to the most stableoxidation state of the element. Also,the slope of a line between twopoints is the standard potential ofthe couple represented by the points.Like Latimer diagrams, Frost dia-grams depend on pH.

froth Ûotation A method of sepa-rating mixtures of solids, used indus-trially for separating ores from the

unwanted gangue. The mixture isground to a powder and water and afrothing agent added. Air is blownthrough the water. With a suitablefrothing agent, the bubbles adhereonly to particles of ore and carrythem to the surface, leaving thegangue particles at the bottom.

fructose (fruit sugar; laevulose) Asimple sugar, C6H12O6, stereoiso-meric with glucose (see monosaccha-ride). (Although natural fructose isthe d-form, it is in fact laevorotatory.)Fructose occurs in green plants,fruits, and honey and tastes sweeterthan sucrose (cane sugar), of which itis a constituent. Derivatives of fruc-tose are important in the energy me-tabolism of living organisms. Somepolysaccharide derivatives (fructans)are carbohydrate energy stores incertain plants.

fructose 1,6-bisphosphate An in-termediate formed in the initial stageof *glycolysis by the phosphorylationof glucose using ATP.

fruit sugar See fructose.

FT-IR See fourier-transform in-frared.

5-FU See 5-fluorouracil.

fuel A substance that is oxidized orotherwise changed in a furnace orheat engine to release useful heat orenergy. For this purpose wood, veg-etable oil, and animal products havelargely been replaced by *fossil fuelssince the 18th century.

The limited supply of fossil fuelsand the expense of extracting themfrom the earth has encouraged thedevelopment of nuclear fuels to pro-duce electricity.

fuel cell A cell in which the chemi-cal energy of a fuel is converted di-rectly into electrical energy. Thesimplest fuel cell is one in which hy-drogen is oxidized to form water

237 fuel cell

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over porous sintered nickel elec-trodes. A supply of gaseous hydrogenis fed to a compartment containingthe porous anode and a supply ofoxygen is fed to a compartment con-taining the porous cathode; the elec-trodes are separated by a thirdcompartment containing a hot alka-line electrolyte, such as potassiumhydroxide. The electrodes are porousto enable the gases to react with theelectrolyte, with the nickel in theelectrodes acting as a catalyst. At theanode the hydrogen reacts with thehydroxide ions in the electrolyte toform water, with the release of twoelectrons per hydrogen molecule:

H2 + 2OH– → 2H2O + 2e–

At the cathode, the oxygen reactswith the water, taking up electrons,to form hydroxide ions:

½O2 + H2O + 2e– → 2OH–

The electrons Ûow from the anode tothe cathode through an external cir-cuit as an electric current. The deviceis a more efÜcient converter of elec-tric energy than a heat engine, but itis bulky and requires a continuoussupply of gaseous fuels. Their use topower electric vehicles is being ac-tively explored.

fugacity Symbol f. A thermody-namic function used in place of par-tial pressure in reactions involvingreal gases and mixtures. For a compo-nent of a mixture, it is deÜned by dµ= RTd(lnf), where µ is the chemicalpotential. It has the same units aspressure and the fugacity of a gas isequal to the pressure if the gas isideal. The fugacity of a liquid or solidis the fugacity of the vapour withwhich it is in equilibrium. The ratioof the fugacity to the fugacity insome standard state is the *activity.For a gas, the standard state is cho-sen to be the state at which the fu-

gacity is 1. The activity then equalsthe fugacity.

fullerene See buckminster-fullerene.

fullerite See buckminster-fullerene.

fuller’s earth A naturally occur-ring clay material (chieÛy montmoril-lonite) that has the property ofdecolorizing oil and grease. In thepast raw wool was cleaned of greaseand whitened by kneading it inwater with fuller’s earth; a processknown as fulling. Fuller’s earth isnow widely used to decolorize fatsand oils and also as an insecticidecarrier and drilling mud. The largestdeposits occur in the USA, UK, andJapan.

fulminate See cyanic acid.

fulminic acid See cyanic acid.

fumaric acid See butenedioicacid.

functional group The group ofatoms responsible for the characteris-tic reactions of a compound. Thefunctional group is –OH for alcohols,–CHO for aldehydes, –COOH for car-boxylic acids, etc.

fundamental See harmonic.

fundamental constants (universalconstants) Those parameters that donot change throughout the universe.The charge on an electron, the speedof light in free space, the Planck con-stant, the gravitational constant, theelectric constant, and the magneticconstant are all thought to be exam-ples.

A• fundamental physical constants from

NIST

fundamental units A set of inde-pendently deÜned *units of measure-ment that forms the basis of a

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system of units. Such a set requiresthree mechanical units (usually oflength, mass, and time) and one elec-trical unit; it has also been foundconvenient to treat certain otherquantities as fundamental, eventhough they are not strictly inde-pendent. In the metric system thecentimetre–gram–second (c.g.s.)system was replaced by the metre–kilogram–second (m.k.s.) system; thelatter has now been adapted to pro-vide the basis for *SI units. In BritishImperial units the foot–pound–second (f.p.s.) system was formerlyused.

fungicide See pesticide.

furan A colourless liquid com-pound, C4H4O; r.d. 0.94; m.p. –86°C;b.p. 31.4°C. It has a Üve-memberedring consisting of four CH2 groupsand one oxygen atom.

239 fusion

f

O

Furan

furanose A *sugar having a Üve-membered ring containing four car-bon atoms and one oxygen atom.

furfural A colourless liquid,C5H4O2, b.p. 162°C, which darkens

on standing in air. It is the aldehydederivative of *furan and occurs invarious essential oils and in *fuseloil. It is used as a solvent for extract-ing mineral oils and natural resinsand itself forms resins with somearomatic compounds.

fused ring See ring.


fusel oil A mixture of high-molecular weight *alcohols contain-ing also esters and fatty acids, some-times formed as a toxic impurity inthe distillation products of alcoholicfermentation. It is used as a source ofhigher alcohols and in making paintsand plastics.

fusible alloys Alloys that melt atlow temperature (around 100°C).They have a number of uses, includ-ing constant-temperature baths, pipebending, and automatic sprinklers toprovide a spray of water to preventÜres from spreading. Fusible alloysare usually *eutectic mixtures of bis-muth, lead, tin, and cadmium.*Wood’s metal, *Rose’s metal, andLipowitz’s alloy are examples of al-loys that melt in the range 70–100°C.

fusion Melting.


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G

GABA See gamma-aminobutyricacid.

Gabriel reaction A method ofmaking a primary *amine (free fromany secondary or tertiary amine im-purities) from a haloalkane (alkylhalide) using potassium phthalimide.It is named after Siegmund Gabriel(1851–1924).

gadolinium Symbol Gd. A soft sil-very metallic element belonging tothe *lanthanoids; a.n. 64; r.a.m.157.25; r.d. 7.901 (20°C); m.p. 1313°C;b.p. 3266°C. It occurs in gadolinite,xenotime, monazite, and residuesfrom uranium ores. There are sevenstable natural isotopes and elevenartiÜcial isotopes are known. Two ofthe natural isotopes, gadolinium–155and gadolinium–157, are the bestneutron absorbers of all the el-ements. The metal has found limitedapplications in nuclear technologyand in ferromagnetic alloys (withcobalt, copper, iron, and cerium).Gadolinium compounds are used inelectronic components. The elementwas discovered by Jean de Marignac(1817–94) in 1880.


galactose A simple sugar, C6H12O6,stereoisomeric with glucose, that oc-curs naturally as one of the productsof the enzymic digestion of milksugar (lactose) and as a constituent ofgum arabic.

galena A mineral form of lead(II)sulphide, PbS, crystallizing in thecubic system; the chief ore of lead. Itusually occurs as grey metallic cubes,

frequently in association with silver,arsenic, copper, zinc, and antimony.Important deposits occur in Australia(at Broken Hill), Germany, the USA(especially in Missouri, Kansas, andOklahoma), and the UK.

gallic acid (3,4,5-trihydroxybenzoicacid) A colourless crystalline aro-matic compound, C6H2(OH)3COOH;m.p. 253°C. It occurs in wood, oakgalls, and tea, and is a component oftannins. It can be made from tanninby acid hydrolysis or fermentation.Gallic acid is used to make ink andvarious dyes. On heating it yields*pyrogallol.

OH

OH

OH

OH O

Gallic acid

gallium Symbol Ga. A soft silverymetallic element belonging to group13 (formerly IIIB) of the periodictable; a.n. 31; r.a.m. 69.72; r.d. 5.90(20°C); m.p. 29.78°C; b.p. 2403°C. Itoccurs in zinc blende, bauxite, andkaolin, from which it can be ex-tracted by fractional electrolysis. Italso occurs in gallite, CuGaS2, to anextent of 1%; although bauxite onlycontains 0.01% this is the only com-mercial source. The two stable iso-topes are gallium–69 and gallium–71;there are eight radioactive isotopes,all with short half-lives. The metal


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has only a few minor uses (e.g. as anactivator in luminous paints), but gal-lium arsenide is extensively used as asemiconductor in many applications.Gallium corrodes most other metalsbecause it rapidly diffuses into theirlattices. Most gallium(I) and some gal-lium(II) compounds are unstable. Theelement was Ürst identiÜed by PaulLecoq de Boisbaudran (1838–1912) in1875.A• Information from the WebElements site

GALP See glyceraldehyde 3-phos-phate.

galvanic cell See voltaic cell.

galvanized iron Iron or steel thathas been coated with a layer of zincto protect it from corrosion in aprocess invented by Luigi Galvani.Corrugated mild-steel sheets forrooÜng and mild-steel sheets for dust-bins, etc., are usually galvanized bydipping them in molten zinc. Theformation of a brittle zinc–iron alloyis prevented by the addition of smallquantities of aluminium or magne-sium. Wire is often galvanized by acold electrolytic process as no alloyforms in this process. Galvanizing isan effective method of protectingsteel because even if the surface isscratched, the zinc still protects theunderlying metal. See sacrificialprotection.

gamma-aminobutyric acid(GABA) An inhibitory neurotransmit-ter in the central nervous system(principally the brain) that is capableof increasing the permeability ofpostsynaptic membranes. GABA is

synthesized by *decarboxylation ofthe amino acid glutamate.

gammahydroxybutyric acid See4-hydroxybutanoic acid.

gamma-iron See iron.

gamma radiation Electromag-netic radiation emitted by excitedatomic nuclei during the process ofpassing to a lower excitation state.Gamma radiation ranges in energyfrom about 10–15 to 10–10 joule (10keV to 10 MeV) corresponding to awavelength range of about 10–10 to10–14 metre. A common source ofgamma radiation is cobalt–60, thedecay process of which is:

62

07Co β

→62

08Ni γ

→62

08Ni

The de-excitation of nickel–60 is ac-companied by the emission ofgamma-ray photons having energies1.17 MeV and 1.33 MeV.

gangue Rock and other waste ma-terial present in an ore.

garnet Any of a group of silicateminerals that conform to the generalformula A3B2(SiO4)3. The elementsrepresenting A may include magne-sium, calcium, manganese, andiron(II); those representing B mayinclude aluminium, iron(III),chromium, or titanium. Six varietiesof garnet are generally recognized:pyrope, Mg3Al2Si3O12;almandine, Fe3

2+Al2Si3O12;spessartite, Mn3Al2Si3O12;grossularite, Ca3Al2Si3O12;andradite, Ca3(Fe3+,Ti)2Si3O12;uvarovite, Ca3Cr2Si3O12.Varieties of garnet are used as gem-stones and abrasives.

gas A state of matter in which thematter concerned occupies the wholeof its container irrespective of itsquantity. In an *ideal gas, whichobeys the *gas laws exactly, the mol-ecules themselves would have a neg-ligible volume and negligible forces

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CH2

CH2

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between them, and collisions be-tween molecules would be perfectlyelastic. In practice, however, the be-haviour of real gases deviates fromthe gas laws because their moleculesoccupy a Ünite volume, there aresmall forces between molecules, andin polyatomic gases collisions are to acertain extent inelastic (see equationof state).

gas chromatography A techniquefor separating or analysing mixturesof gases by *chromatography. Theapparatus consists of a very long tubecontaining the stationary phase. Thismay be a solid, such as kieselguhr(gas–solid chromatography, or GSC),or a nonvolatile liquid, such as a hy-drocarbon oil coated on a solid sup-port (gas–liquid chromatography, orGLC). The sample is often a volatileliquid mixture, which is vaporizedand swept through the column by acarrier gas (e.g. hydrogen). The com-ponents of the mixture pass throughthe column at different rates becausethey adsorb to different extents onthe stationary phase. They are de-tected as they leave, either by meas-uring the thermal conductivity of thegas or by a Ûame detector.

Gas chromatography is usuallyused for analysis; components can beidentiÜed by the time they take topass through the column. It is some-times also used for separating mix-tures.

Gas chromatography is often usedto separate a mixture into its compo-nents, which are then directly in-jected into a mass spectrometer. Thistechnique is known as gas chro-matography–mass spectroscopy orGCMS.

gas chromatography infrared(GC-IR) A form of *Fourier-transforminfrared (FT-IR) used to identify smallamounts of gas obtained by gas chro-matography. Since functional groups

in molecules are characteristic in in-frared spectra this produces informa-tion to supplement that obtained bymass spectrometry. It is easier to per-form FT-IR for gas chromatographythan for liquid chromatography be-cause carrier solvents in liquidchromatography absorb infraredradiation.

gas constant (universal molar gasconstant) Symbol R. The constantthat appears in the universal gasequation (see gas laws). It has thevalue 8.314 510(70) J K–1 mol–1.

gas equation See gas laws.

gasiÜcation The conversion ofsolid or liquid hydrocarbons to fuelgas. Solid fuels such as coal or cokeare converted into producer gas (car-bon monoxide) or water gas (carbonmonoxide and hydrogen) by the ac-tion of air (or oxygen) and steam.Solid fuels may also be hydrogenatedto produce methane. Liquid fuels,from petroleum, are gasiÜed to pro-duce synthesis gas (carbon monoxideand hydrogen) or town gas (mostlyhydrogen and methane), usually by*cracking or *hydrogenation.

gas laws Laws relating the temper-ature, pressure, and volume of an*ideal gas. *Boyle’s law states thatthe pressure (p) of a specimen is in-versely proportional to the volume(V) at constant temperature (pV = con-stant). The modern equivalent of*Charles’ law states that the volumeis directly proportional to the ther-modynamic temperature (T) at con-stant pressure (V/T = constant);originally this law stated the con-stant expansivity of a gas kept at con-stant pressure. The pressure lawstates that the pressure is directlyproportional to the thermodynamictemperature for a specimen kept atconstant volume. The three laws canbe combined in the universal gas

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equation, pV = nRT, where n is theamount of gas in the specimen and Ris the *gas constant. The gas lawswere Ürst established experimentallyfor real gases, although they areobeyed by real gases to only a limitedextent; they are obeyed best at hightemperatures and low pressures. Seealso equation of state.

gasohol A mixture of petrol (gaso-line) and alcohol (i.e. typicallyethanol at 10%, or methanol at 3%),used as an alternative fuel for carsand other vehicles in many coun-tries. The ethanol is obtained as a*biofuel by fermentation of agricul-tural crops or crop residues, for ex-ample sugar cane waste. Many carscan also use a mixture of 85% ethanoland 15% petrol, called E85. Ethanol-based gasohol has a higher octanerating and burns more completelythan conventional petrol, thus lower-ing some emissions. However, theethanol can damage certain enginecomponents, such as rubber seals.Methanol-based gasohol is more toxicand corrosive, and its emissions in-clude formaldehyde, a known car-cinogen.

gas oil A high-density petroleumfraction (between kerosene and lubri-cating oil), whose molecules have upto 25 carbon atoms. It is used as a do-mestic and industrial heating fuel.

gasoline See petroleum.

gas-phase electrophoresis Seeion-mobility spectrometry.

gas thermometer A device formeasuring temperature in which theworking Ûuid is a gas. It provides themost accurate method of measuringtemperatures in the range 2.5 to1337 K. Using a Üxed mass of gas aconstant-volume thermometer meas-ures the pressure of a Üxed volumeof gas at relevant temperatures, usu-

ally by means of a mercury manome-ter and a barometer.

Gattermann–Koch reaction A re-action of the type C6H6 → C6H5CHO,used to introduce aldehyde groupsonto benzene rings. It is used in theindustrial manufacture of benzalde-hyde. A mixture of hydrogen chlo-ride and carbon monoxide is passedthrough benzene using a Lewis acidsuch as aluminium chloride as a cata-lyst. An intermediate H–COC+ isformed and electrophilic substitutiontakes place on the benzene ring.There is a variation of the reaction,sometimes simply called the Gatter-mann reaction, in which the –CHOgroup is substituted onto the ben-zene ring of a phenol. In this reac-tion hydrogen cyanide is used ratherthan carbon monoxide. A mixture ofzinc cyanide and hydrochloric acidproduces the hydrogen cyanide alongwith zinc chloride to act as the cata-lyst. The electrophile is HCNH+

which produces an imine intermedi-ate

C6H5OH → HOC6H4CH=NHThis then hydrolyses to the aldehydeHOC6H4CHO. If an organic cyanide(nitrite) RCN is used, a ketone is pro-duced. The reaction was discoveredby Ludwig Gattermann and J.C. Kochin 1897. The use of hydrogen cyanidewas reported by Gattermann in 1907.

Gattermann reaction A variationof the *Sandmeyer reaction forpreparing chloro- or bromoarenes byreaction of the diazonium com-pound. In the Gattermann reactionthe aromatic amine is added tosodium nitrite and the halogen acid(10°C), then fresh copper powder(e.g. from Zn + CuSO4) is added andthe solution warmed. The diazoniumsalt then forms the haloarene, e.g.

C6H5N2+Cl– → C6H5Cl + N2

The copper acts as a catalyst. The re-

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action is easier to perform than theSandmeyer reaction and takes placeat lower temperature, but generallygives lower yields. It was discoveredin 1890 by the German chemist Lud-wig Gattermann (1860–1920). See alsogattermann–koch reaction.

gauche See conformation; tor-sion angle.

Gay-Lussac, Joseph (1778–1850)French chemist and physicist whosediscovery of the laws of chemicalcombination in gases helped to estab-lish the atomic theory. It also led to*Avogadro’s law. See also charles’law.

Gay Lussac’s law 1. When gasescombine chemically the volumes ofthe reactants and the volume of theproduct, if it is gaseous, bear simplerelationships to each other whenmeasured under the same conditionsof temperature and pressure. The lawwas Ürst stated in 1808 by J. L. Gay-Lussac and led to *Avogadro’s law. 2. See charles’ law.

gaylussite A mineral consisting of a hydrated mixed carbonate ofsodium and calcium, Na2CO3.CaCO3.5H2O.

GC-IR See gas chromatography in-frared.

GCMS See gas chromatography.

Geiger counter (Geiger–Müllercounter) A device used to detect andmeasure ionizing radiation. It con-sists of a tube containing a low-pressure gas (usually argon or neonwith methane) and a cylindrical hol-low cathode through the centre ofwhich runs a Üne-wire anode. A po-tential difference of about 1000 voltsis maintained between the elec-trodes. An ionizing particle or pho-ton passing through a window intothe tube will cause an ion to be pro-duced and the high p.d. will acceler-

ate it towards its appropriate elec-trode, causing an avalanche of fur-ther ionizations by collision. Theconsequent current pulses can becounted in electronic circuits or sim-ply ampliÜed to work a small loud-speaker in the instrument. It wasÜrst devised in 1908 by the Germanphysicist Hans Geiger (1882–1945).Geiger and W. Müller produced animproved design in 1928.

gel A lyophilic *colloid that has co-agulated to a rigid or jelly-like solid.In a gel, the disperse medium hasformed a loosely-held network oflinked molecules through the disper-sion medium. Examples of gels aresilica gel and gelatin.

gelatin(e) A colourless or pale yel-low water-soluble protein obtainedby boiling collagen with water andevaporating the solution. It swellswhen water is added and dissolves inhot water to form a solution that setsto a gel on cooling. It is used in pho-tographic emulsions and adhesives,and in jellies and other foodstuffs.

gel electrophoresis See elec-trophoresis.

gel Ültration A type of column*chromatography in which a mix-ture of liquids is passed down a col-umn containing a gel. Smallmolecules in the mixture can enterpores in the gel and move slowlydown the column; large molecules,which cannot enter the pores, movemore quickly. Thus, mixtures of mol-ecules can be separated on the basisof their size. The technique is usedparticularly for separating proteinsbut it can also be applied to otherpolymers and to cell nuclei, viruses,etc.

gelignite A high explosive madefrom nitroglycerin, cellulose nitrate,sodium nitrate, and wood pulp.

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gem Designating molecules inwhich two functional groups areattached to the same atom in amolecule. For example, 1,1-dichloroethane (CH3CHCl2) is a gemdihalide and can be named gem-dichloroethane. Compare vicinal.

geminate pair A pair of mol-ecules, ions, etc., in close proximitysurrounded by a solvent cage (seecage effect).

geochemistry The scientiÜc studyof the chemical composition of theearth. It includes the study of theabundance of the earth’s elementsand their isotopes and the distribu-tion of the elements in environmentsof the earth (lithosphere, atmos-phere, biosphere, and hydrosphere).

geometrical isomerism See iso-merism.

gerade Symbol g. Describing a mo-lecular orbital of a homonuclear di-atomic molecule with even parity(gerade is the German word for even).This means that during the processof inversion, the sign of the orbital isunchanged upon going from anypoint in the molecule through thecentre of inversion to the corre-sponding point on the other side.The symbol g is written as a sub-script. The opposite of gerade isungerade, symbol u. The symbols gand u are used to determine selectionrules for diatomic molecules. Thesesymbols are only applicable tohomonuclear diatomic molecules, asheteronuclear diatomic molecules,such as CO, do not have a centre ofinversion.

geraniol An alcohol, C9H15CH2OH,present in a number of essential oils.

germanium Symbol Ge. A lustroushard metalloid element belonging togroup 14 (formerly IVB) of the peri-odic table; a.n. 32; r.a.m. 72.59; r.d.

5.36; m.p. 937°C; b.p. 2830°C. It isfound in zinc sulphide and in certainother sulphide ores, and is mainlyobtained as a by-product of zincsmelting. It is also present in somecoal (up to 1.6%). Small amounts areused in specialized alloys but themain use depends on its semiconduc-tor properties. Chemically, it formscompounds in the +2 and +4 oxida-tion states, the germanium(IV) com-pounds being the more stable. Theelement also forms a large numberof organometallic compounds. Pre-dicted in 1871 by *Mendeleev (eka-silicon), it was discovered by Winklerin 1886.


German silver (nickel silver) Analloy of copper, zinc, and nickel,often in the proportions 5:2:2. It re-sembles silver in appearance and isused in cheap jewellery and cutleryand as a base for silver-plated wire.See also electrum.

getter A substance used to removesmall amounts of other substancesfrom a system by chemical combina-tion. For example, a metal such asmagnesium may be used to removethe last traces of air when achievinga high vacuum. Various getters arealso employed to remove impuritiesfrom semiconductors.

GHB Gammahydroxybutyric acid.See 4-hydroxybutanoic acid.

gibberellic acid (GA3) A plantgrowth substance, abundant inyoung actively growing areas of theplant, that is involved in stem elon-gation. A *terpene, it was discoveredin 1954. Gibberellic acid and relatedgrowth substances are called gib-berellins.

Gibbs, Josiah Willard (1839–1903)US mathematician and physicist,

245 Gibbs, Josiah Willard

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who spent his entire academic careerat Yale University. During the 1870she developed the theory of chemicalthermodynamics, devising functionssuch as Gibbs *free energy; he alsoderived the *phase rule. In mathe-matics he introduced vector notation.

Gibbs–Duhem equation An equa-tion describing the relation betweenthe chemical potentials of species ina mixture. If ni is the amount ofspecies i and µi is the chemical poten-tial of species i, the Gibbs–Duhemequation states that

∑i

nidµi = 0

This equation implies that the chemi-cal potentials for the species in amixture do not change indepen-dently. Thus, in the case of a binarymixture, if the chemical potential ofone species increases, the chemicalpotential of the other species mustdecrease. The equation was derivedindependently by J. W. *Gibbs andthe French physicist P. Duhem(1861–1916).

Gibbs free energy (Gibbs function)See free energy.

Gibbs–Helmholtz equation Anequation used in thermodynamics toshow the temperature dependence ofthe *Gibbs free energy. It has theform:

(∂G/∂T)p = (G – H)/T,where G is the Gibbs free energy, H isthe enthalpy, T is the thermody-namic temperature, and p is the pres-sure (which is held constant). TheGibbs–Helmholtz equation can be de-rived from (∂G/∂T)p = – S and S = (H –G)T using the rules of differentiation.The equation was derived by J. W.*Gibbs and H. L. F. von *Helmholtz.

gibbsite A mineral form of hy-drated *aluminium hydroxide(Al(OH)3). It is named after the USmineralogist George Gibbs (d. 1833).

giga- Symbol G. A preÜx used in themetric system to denote one thou-sand million times. For example, 109

joules = 1 gigajoule (GJ).

gilbert Symbol Gb. The c.g.s. unitof magnetomotive force equal to10/4π (= 0.795 77) ampere-turn. It isnamed after the English physicianand physicist William Gilbert(1544–1603), who studied magnet-ism.

glacial ethanoic acid Seeethanoic acid.

glass Any noncrystalline solid; i.e. asolid in which the atoms are randomand have no long-range ordered pat-tern. Glasses are often regarded as su-percooled liquids. Characteristicallythey have no deÜnite melting point,but soften over a range of tempera-tures.

The common glass used in win-dows, bottles, etc., is soda glass,which is made by heating a mixtureof lime (calcium oxide), soda (sodiumcarbonate), and sand (silicon(IV)oxide). It is a form of calcium silicate.Borosilicate glasses (e.g. Pyrex) aremade by incorporating some boronoxide, so that silicon atoms are re-placed by boron atoms. They aretougher than soda glass and more re-sistant to temperature changes,hence their use in cooking utensilsand laboratory apparatus. Glasses forspecial purposes (e.g. optical glass)have other elements added (e.g. bar-ium, lead). See also spin glass.

glass electrode A type of *halfcell having a glass bulb containing anacidic solution of Üxed pH, intowhich dips a platinum wire. Theglass bulb is thin enough for hydro-gen ions to diffuse through. If thebulb is placed in a solution contain-ing hydrogen ions, the electrode po-tential depends on the hydrogen-ion

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concentration. Glass electrodes areused in pH measurement.

glass Übres Melted glass drawninto thin Übres some 0.005 mm–0.01mm in diameter. The Übres may bespun into threads and woven intofabrics, which are then impregnatedwith resins to give a material that isboth strong and corrosion resistant.It is used in car bodies, boat building,and similar applications.

glauberite A mineral consisting ofa mixed sulphate of sodium and cal-cium, Na2SO4.CaSO4.

Glauber’s salt *Sodium sulphatedecahydrate, Na2SO4.10H2O, used asa laxative. It is named after JohannGlauber (1604–68).

GLC (gas–liquid chromatography)See gas chromatography.

global warming See greenhouseeffect.

globin See haemoglobin.

globular protein See protein.

globulin Any of a group of globularproteins that are generally insolublein water and present in blood, eggs,milk, and as a reserve protein inseeds. Blood serum globulins com-prise four types: α1-, α2-, and β-globulins, which serve as carrier pro-teins; and γ-globulins, which includethe immunoglobulins responsible forimmune responses.

glove box A metal box that hasgloves Ütted to ports in its walls. It isused to manipulate mildly radio-active materials and in laboratorytechniques in which an inert, sterile,dry, or dust-free atmosphere has tobe maintained.

glow discharge An electrical dis-charge that passes through a gas atlow pressure and causes the gas tobecome luminous. The glow is pro-

duced by the decay of excited atomsand molecules.

gluconic acid An optically active hydroxycarboxylic acid,CH2(OH)(CHOH)4COOH. It is the car-boxylic acid corresponding to the al-dose sugar glucose, and can be madeby the action of certain moulds.

glucosan Any one of a class of*polysaccharide compounds that canbe converted to glucose by hydroly-sis. Glucosans include dextrin, starch,and cellulose.

glucose (dextrose; grape sugar) Awhite crystalline sugar, C6H12O6, oc-curring widely in nature. Like other*monosaccharides, glucose is opti-cally active: most naturally occurringglucose is dextrorotatory. Glucoseand its derivatives are crucially im-portant in the energy metabolism ofliving organisms. Glucose is also aconstituent of many polysaccharides,most notably starch and cellulose.These yield glucose when brokendown, for example by enzymes dur-ing digestion.

glucuronic acid A compound,OC6H9O6, derived from the oxidationof glucose. It is an important con-stituent of gums and mucilages. Glu-curonic acid can combine withhydroxyl (–OH), carboxyl (–COOH), oramino (–NH2) groups to form a glu-curonide. The addition of a glu-curonide group to a molecule(glucuronidation) generally increasesthe solubility of a compound; henceglucuronidation plays an importantrole in the excretion of foreign sub-stances.

glucuronide See glucuronic acid.

glutamic acid See amino acid.

glutamine See amino acid.

glutaric acid See pentanedioicacid.

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glyceraldehyde 3-phosphate(GALP) A triose phosphate,CHOCH(OH)CH2OPO3H2, that is an in-termediate in the *Calvin cycle (seealso photosynthesis) and glycolysis.

glycerate 3-phosphate A phos-phorylated three-carbon monosac-charide that is an intermediate in the*Calvin cycle of photosynthesis andalso in *glycolysis. It was formerlyknown as 3-phosphoglycerate or phos-phoglyceric acid (PGA).

glycerides (acylglycerols) Fatty-acidesters of glycerol. EsteriÜcation canoccur at one, two, or all three hy-droxyl groups of the glycerol mol-ecule producing mono-, di-, andtriglycerides respectively. *Triglyc-erides are the major constituent offats and oils found in living organ-isms. Alternatively, one of the hy-droxyl groups may be esteriÜed witha phosphate group forming a phos-phoglyceride (see phospholipid) or toa sugar forming a *glycolipid.

glycerine See glycerol.

glycerol (glycerine; propane-1,2,3,-triol) A trihydric alcohol,HOCH2CH(OH)CH2OH. Glycerol is acolourless sweet-tasting viscous liq-uid, miscible with water but insolu-ble in ether. It is widely distributedin all living organisms as a con-stituent of the *glycerides, whichyield glycerol when hydrolysed.

glycerophospholipids See phos-pholipids.

glycine See amino acid.

glycobiology The study of carbo-hydrates and carbohydrate com-plexes, especially *glycoproteins.

glycogen (animal starch) A *poly-saccharide consisting of a highlybranched polymer of glucose occur-ring in animal tissues, especially inliver and muscle cells. It is the major

store of carbohydrate energy in ani-mal cells and is present as granularclusters of minute particles.

glycogenesis The conversion ofglucose to glycogen, which is stimu-lated by insulin from the pancreas.Glycogenesis occurs in skeletal mus-cles and to a lesser extent in theliver. Glucose that is taken up bycells is phosphorylated to glucose 6-phosphate; this is converted succes-sively to glucose 1-phosphate, uridinediphosphate glucose, and Ünally toglycogen. Compare glycogenolysis.

glycogenolysis The conversion ofglycogen to glucose, which occurs inthe liver and is stimulated byglucagon from the pancreas andadrenaline from the adrenal medulla.These hormones activate an enzymethat phosphorylates glucose mol-ecules in the glycogen chain to formglucose 1-phosphate, which is con-verted to glucose 6-phosphate. This isthen converted to glucose by a phos-phatase enzyme. In skeletal muscleglycogen is degraded to glucose 6-phosphate, which is then convertedinto pyruvate and used in ATP pro-duction during glycolysis and theKrebs cycle. However, pyruvate canalso be converted, in the liver, to glu-cose; thus muscle glycogen is indi-rectly a source of blood glucose.Compare glycogenesis.

glycol See ethane-1,2-diol.

glycolic acid (hydroxyethanoicacid) A colourless crystalline com-pound, CH2(OH)COOH; m.p. 80°C. Itoccurs in sugar cane and sugar beet,and is made by the electrolytic re-duction of oxalic acid or by boiling asolution of sodium monochloro-ethanoate. Glycolic acid is used inmaking textiles and leather and forcleaning metals.

glycolipid Any of a group of sugar-containing lipids, in which the lipid

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portion of the molecule is usuallybased on glycerol (see glyceride) orsphingosine and the sugar is typicallygalactose, glucose, or inositol. Glycol-ipids are components of biologicalmembranes. In animal plasma mem-branes they are found in the outerlayer of the lipid bilayer; the simplestanimal glycolipids are the *cerebro-sides. Plant glycolipids are glyceridesin which the sugar group is mostcommonly galactose. They are theprincipal lipid constituents of chloro-plasts.

glycolysis (Embden–Meyerhof path-way) The series of biochemical reac-tions in which glucose is brokendown to pyruvate with the release ofusable energy in the form of *ATP.One molecule of glucose undergoestwo phosphorylation reactions and is then split to form two triose-phosphate molecules. Each of theseis converted to pyruvate. The net en-ergy yield is two ATP molecules perglucose molecule. In aerobic respira-tion pyruvate then enters the *Krebscycle. Alternatively, when oxygen isin short supply or absent, the pyru-vate is converted to various productsby anaerobic respiration. Other sim-ple sugars, e.g. fructose and galac-tose, and glycerol (from fats) enterthe glycolysis pathway at intermedi-ate stages.

glycoprotein A carbohydratelinked covalently to a protein.Formed in the Golgi apparatus in theprocess of *glycosylation, glycopro-teins are important components ofcell membranes. They are also con-stituents of body Ûuids, such asmucus, that are involved in lubrica-tion. Many of the hormone receptorson the surfaces of cells have beenidentiÜed as glycoproteins. Glycopro-teins produced by viruses attachthemselves to the surface of the hostcell, where they act as markers for

the receptors of leucocytes. Viral gly-coproteins can also act as target mol-ecules and help viruses to detectcertain types of host cell; for exam-ple, a glycoprotein on the surface ofHIV (the AIDS virus) enables the virusto Ünd and infect white blood cells.

glycosaminoglycan Any one of agroup of polysaccharides that containamino sugars (such as glucosamine).Formerly known as mucopolysaccha-rides, they include *hyaluronic acidand chondroitin, which provide lu-brication in joints and form part ofthe matrix of cartilage. The three-dimensional structure of these mol-ecules enables them to trap water,which forms a gel and gives gly-cosaminoglycans their elastic proper-ties.

glycoside Any one of a group ofcompounds consisting of a pyranosesugar residue, such as glucose, linkedto a noncarbohydrate residue (R) by a*glycosidic bond: the hydroxyl group(–OH) on carbon-1 of the sugar is re-placed by –OR. Glycosides are widelydistributed in plants; examples arethe *anthocyanin pigments and thecardiac glycosides, such as digoxinand ouabain, which are used medici-nally for their stimulant effects onthe heart.

glycosidic bond (glycosidic link)The type of chemical linkage be-tween the monosaccharide units ofdisaccharides, oligosaccharides, andpolysaccharides, which is formed bythe removal of a molecule of water(i.e. a *condensation reaction). Thebond is normally formed betweenthe carbon-1 on one sugar and thecarbon-4 on the other. An α-glyco-sidic bond is formed when the –OHgroup on carbon-1 is below the planeof the glucose ring and a β-glycosidicbond is formed when it is above theplane. Cellulose is formed of glucosemolecules linked by 1-4 β-glycosidic

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bonds, whereas starch is composedof 1-4 α-glycosidic bonds.

glycosidic link See glycoside.

glycosylation The process inwhich a carbohydrate is joined to an-other molecule, such as a protein toform a *glycoprotein or to a lipid toform a glycolipid (see glyceride). Gly-cosylation occurs in the rough endo-plasmic reticulum and the Golgiapparatus of cells.

glyoxal (ethanedial) A low melting-point yellow crystalline solid, CHO-CHO, m.p. 15°C. It is a dialdehyde. Itgives off a green vapour when heatedand burns with a violet-colouredÛame. Glyoxal is made by the cat-alytic oxidation of ethanediol and isused to harden the gelatin employedfor photographic emulsions.

goethite A yellow-brown mineral,FeO.OH, crystallizing in the or-thorhombic system. It is formed as aresult of the oxidation and hydrationof iron minerals or as a direct precip-itate from marine or fresh water (e.g.in swamps and bogs). Most *limoniteis composed largely of cryptocrys-talline goethite. Goethite is mined asan ore of iron.

Golay cell A transparent cell con-taining gas, used to detect *infraredradiation. Incident radiation is ab-sorbed within the cell, causing a risein the gas temperature and pressure.The amount of incident radiation can

be measured from the pressure risein the tube.

gold Symbol Au. A soft yellow mal-leable metallic *transition element;a.n. 79; r.a.m. 196.967; r.d. 19.32;m.p. 1064.43°C; b.p. 2807±2°C. Goldhas a face-centred-cubic crystal struc-ture. It is found as the free metal ingravel or in quartz veins, and is alsopresent in some lead and copper sul-phide ores. It also occurs combinedwith silver in the telluride sylvanite,(Ag,Au)Te2. It is used in jewellery,dentistry, and electronic devices.Chemically, it is unreactive, beingunaffected by oxygen. It reacts withchlorine at 200°C to form gold(III)chloride. It forms a number of com-plexes with gold in the +1 and +3 ox-idation states.


Goldschmidt process A methodof extracting metals by reducing theoxide with aluminium powder, e.g.

Cr2O3 + 2Al → 2Cr + Al2O3

The reaction can also be used to pro-duce molten iron (see thermite). Itwas discovered by the Germanchemist Hans Goldschmidt(1861–1923).

Gooch crucible A porcelain dishwith a perforated base over which alayer of asbestos is placed, used forÜltration in gravimetric analysis. It is

glycosidic link 250

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CH2OH CH2OH CH2OH CH2OH

O

OH

OHC1

OH

OH OH

C4OH

OH

OH

O

OH

OH

OH

condensation O H

C1

O

H

C4 OH

OH

OH

O

+ H2O

1–4 α-glycosidicbond

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named after the US chemist FrankGooch (1852–1929).

Gouy balance A method of meas-uring magnetic susceptibility. Thesample is suspended from a balance,with the bottom part of the samplebetween the poles of an electromag-net. When the magnetic Üeld isswitched on, the sample experiencesa Üeld gradient which causes an ap-parent change in weight. In particu-lar, paramagnetic substances showan increase in weight, which, aftercorrection for a smaller diamagneticcontribution, can be used to calculatethe paramagnetic part of the suscep-tibility. This can be used to calculatethe number of unimpaired electronsin the sample. Magnetic measure-ments of this type are widely used toinvestigate the electronic structuresof metal complexes. The Evans bal-ance is a portable version of theGouy balance using permanent mag-nets and giving a direct readout.Other methods of measuring mag-netic susceptibility include magneticresonance techniques and the use ofa SQID (superconducting quantuminterference device).

Gouy–Chapman model A modelof the *electrical double layer inwhich thermal motion causing disor-dering of the layer is taken into ac-count. This is very similar to the*Debye–Hückel theory of the ionicatmosphere around an ion, exceptthat the concept of a central singleion is replaced by that of an inÜniteplane electrode. The Gouy–Chapmanmodel understates the structure in adouble layer, but can be improved bythe Stern model, in which the ionsclosest to the electrode are orderedand the ions are described by theGouy–Chapman model outside theÜrst layer.

graft copolymer See polymer.

Graham, Thomas (1805–69) Scot-tish chemist, who became professorof chemistry at Glasgow University in1830, moving to University College,London, in 1837. His 1829 paper ongaseous diffusion introduced *Gra-ham’s law. He went on to study diffu-sion in liquids, leading in 1861 to thedeÜnition of *colloids.

Graham’s law The rates at whichgases diffuse is inversely propor-tional to the square roots of theirdensities. This principle is made useof in the diffusion method of separat-ing isotopes. The law was formulatedin 1829 by Thomas *Graham.

gram Symbol g. One thousandth ofa kilogram. The gram is the funda-mental unit of mass in *c.g.s. unitsand was formerly used in such unitsas the gram-atom, gram-molecule,and gram-equivalent, which havenow been replaced by the *mole.

grape sugar See glucose.

graphite See carbon.

graphitic compounds Substancesin which atoms or molecules aretrapped between the layers ingraphite. See lamellar solids.

gravimetric analysis A type ofquantitative analysis that depends onweighing. For instance, the amountof silver in a solution of silver saltscould be measured by adding excesshydrochloric acid to precipitate silverchloride, Ültering the precipitate,washing, drying, and weighing.

gray Symbol Gy. The derived SI unitof absorbed dose of ionizing radia-tion (see radiation units). It isnamed after the British radiobiologistL. H. Gray (1905–65).

greenhouse effect An effect oc-curring in the atmosphere because ofthe presence of certain gases (green-house gases) that absorb infrared ra-

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diation. Light and ultraviolet radia-tion from the sun are able to pen-etrate the atmosphere and warm theearth’s surface. This energy is re-radi-ated as infrared radiation, which, be-cause of its longer wavelength, isabsorbed by such substances as car-bon dioxide. Emissions of carbondioxide from human activities haveincreased markedly in the last 150years or so. The overall effect is thatthe average temperature of the earthand its atmosphere is increasing (so-called global warming). The effect issimilar to that occurring in a green-house, where light and long-wave-length ultraviolet radiation can passthrough the glass into the green-house but the infrared radiation isabsorbed by the glass and part of it isre-radiated into the greenhouse.

The greenhouse effect is seen as amajor environmental hazard. Aver-age increases in temperature arelikely to change weather patternsand agricultural output. It is alreadycausing the polar ice caps to melt,with a corresponding rise in sealevel. Carbon dioxide, from fossil-fuelpower stations and car exhausts, isthe main greenhouse gas. Other con-tributory pollutants are nitrogenoxides, ozone, methane, andchloroÛuorocarbons.

greenhouse gas See greenhouseeffect.

greenockite A mineral form ofcadmium sulphide, CdS.

green vitriol See iron(ii) sulphate.

Griess test A test for nitrates or ni-trites, once widely used to detect thepossible existence of gunshotresidue.

Grignard reagents A class oforganometallic compounds of mag-nesium, with the general formulaRMgX, where R is an organic groupand X a halogen atom (e.g. CH3MgCl,

C2H5MgBr, etc.). They actually havethe structure R2Mg.MgCl2, and can bemade by reacting a haloalkane withmagnesium in ether; they are rarelyisolated but are extensively used inorganic synthesis, when they aremade in one reaction mixture. Grig-nard reagents have a number of reac-tions that make them useful inorganic synthesis. With methanalthey give a primary alcohol

CH3MgCl + HCHO → CH3CH2OHOther aldehydes give a secondary al-cohol

CH3CHO + CH3MgCl →(CH3)2CHOH

With alcohols, hydrocarbons areformed

CH3MgCl + C2H5OH → C2H5CH3

Water also gives a hydrocarbonCH3MgCl + H2O → CH4

The compounds are named aftertheir discoverer, the French chemistVictor Grignard (1871–1935).

Grotrian diagram A diagram thatsummarizes the energy levels andthe *allowed transitions betweenthese energy levels in an atom. Theenergy is plotted vertically with ahorizontal line for each energy level.The intensity of the transition can berepresented on a Grotrian diagramby allowing the thickness of the lineto represent the transition propor-tional to the intensity. Grotrian dia-grams are named after the Germanspectroscopist W. Grotrian, who in-vented them in 1928.

Grottius–Draper law A law inphotochemistry stating that only thelight absorbed by a substance or sub-stances is effective in bringing aboutchemical change. Not all the lightfalling on the substances will neces-sarily bring about chemical change,since some of it can be re-emitted inthe form of heat or light. The light

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does not need to be absorbed directlyby the reacting substances; it is possi-ble, in photosensitization for exam-ple, that light can be absorbed by aninert substance, which subsequentlytransfers the absorbed energy (asthermal energy) to the reactants.

ground state The lowest stable en-ergy state of a system, such as a mol-ecule, atom, or nucleus. See energylevel.

group 1. See periodic table. 2. Amathematical structure consisting ofa set of elements A, B, C, etc., forwhich there exists a law of composi-tion, referred to as ‘multiplication’.Any two elements can be combinedto give a ‘product’ AB.(1) Every product of two elements isan element of the set.(2) The operation is associative, i.e.A(BC) = (AB)C.(3) The set has an element I, calledthe identity element, such that IA =AI = A for all A in the set.(4) Each element of the set has an in-verse A–1 belonging to the set suchthat AA–1 = A–1A = I.

Although the law of combination iscalled ‘multiplication’ this does notnecessarily have its usual meaning.For example, the set of integersforms a group if the law of composi-tion is addition.

Two elements A, B of a group com-mute if AB = BA. If all the elements ofa group commute with each otherthe group is said to be Abelian. If thisis not the case the group is said to benon-Abelian.

The interest of group theory inphysics and chemistry is in analysingsymmetry. Discrete groups have aÜnite number of elements, such asthe symmetries involved in rotationsand reÛections of molecules, whichgive rise to point groups. Continuousgroups have an inÜnite number of el-ements where the elements are con-

tinuous. An example of a continuousgroup is the set of rotations about aÜxed axis. The rotation group thusformed underlies the quantumtheory of angular momentum, whichhas many applications to atoms andnuclei.

group 0 elements See noblegases.

group 1 elements A group of el-ements in the *periodic table:lithium (Li), sodium (Na), potassium(K), rubidium (Rb), caesium (Cs), andFrancium (Fr). They are known as the*alkali metals. Formerly, they wereclassiÜed in group I, which consistedof two subgroups: group IA (the maingroup) and group IB. Group IB con-sisted of the *coinage metals, copper,silver, and gold, which comprisegroup 11 and are usually consideredwith the *transition elements.

group 2 elements A group of el-ements in the *periodic table: beryl-lium (Be), magnesium (Hg), calcium(Ca), strontium (Sr), barium (Ba), andradium (Ra). They are known as the*alkaline-earth metals. Formerly,they were classiÜed in group II,which consisted of two subgroups:group IIA (the main group, see alka-line-earth metals) and group IIB.Group IIB consisted of the three met-als zinc (Zn), cadmium (Cd), and mer-cury (Hg), which have two s-electronsoutside Ülled d-subshells. Moreover,none of their compounds haveunÜlled d-levels, and the metals areregarded as nontransition elements.They now form group 12 and aresometimes called the zinc group. Zincand cadmium are relatively elec-tropositive metals, forming com-pounds containing divalent ions Zn2+

or Cd2+. Mercury is more unreactiveand also unusual in forming mer-cury(I) compounds, which containthe ion Hg2

2+.

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groups 3–12 See transition el-ements.

group 13 elements A group of el-ements in the *periodic table: boron(B), aluminium (Al), gallium (Ga), in-dium (In), and thallium (Tl), which allhave outer electronic conÜgurationsns2np1 with no partly Ülled inner lev-els. They are the Ürst members of thep-block. The group differs from thealkali metals and alkaline-earth met-als in displaying a considerable varia-tion in properties as the group isdescended. Formerly, they wereclassiÜed in group III, which con-sisted of two subgroups: group IIIB(the main group) and group IIIA.Group IIIA consisted of scandium(Sc), yttrium (Yt), and lanthanum (La),which are generally considered withthe *lanthanoids, and actinium (Ac),which is classiÜed with the *acti-noids. Scandium and yttrium now be-long to group 3 (along with lutetiumand lawrencium).

Boron has a small atomic radiusand a relatively high ionization en-ergy. In consequence its chemistry islargely covalent and it is generallyclassed as a metalloid. It forms alarge number of volatile hydrides,some of which have the uncommonbonding characteristic of *electron-deÜcient compounds. It also forms aweakly acidic oxide. In some ways,boron resembles silicon (see diago-nal relationship).

As the group is descended, atomicradii increase and ionization energiesare all lower than for boron. There isan increase in polar interactions andthe formation of distinct M3+ ions.This increase in metallic character isclearly illustrated by the increasingbasic character of the hydroxides:boron hydroxide is acidic, aluminiumand gallium hydroxides are ampho-teric, indium hydroxide is basic, andthallium forms only the oxide. As the

elements of group 13 have a vacantp-orbital they display many electron-acceptor properties. For example,many boron compounds formadducts with donors such as ammo-nia and organic amines (acting asLewis acids). A large number of com-plexes of the type [BF4]–, [AlCl4]–,[InCl4]–, [TlI4]– are known and theheavier members can expand theircoordination numbers to six as in[AlF6]3– and [TlCl6]3–. This acceptorproperty is also seen in bridgeddimers of the type Al2Cl6. Anotherfeature of group 13 is the increasingstability of the monovalent statedown the group. The electronconÜguration ns2np1 suggests thatonly one electron could be lost orshared in forming compounds. Infact, for the lighter members of thegroup the energy required to pro-mote an electron from the s-subshellto a vacant p-subshell is small. It ismore than compensated for by theresulting energy gain in formingthree bonds rather than one. This en-ergy gain is less important for theheavier members of the group. Thus,aluminium forms compounds of thetype AlCl in the gas phase at hightemperatures. Gallium similarlyforms such compounds andgallium(I) oxide (Ga2O) can be iso-lated. Indium has a number ofknown indium(I) compounds (e.g.InCl, In2O, In3

I[InIIICl6]). Thallium hasstable monovalent compounds. Inaqueous solution, thallium(I) com-pounds are more stable than the cor-responding thallium(III) compounds.See inert-pair effect.

group 14 elements A group of el-ements in the *periodic table: carbon(C), silicon (Si), germanium (Ge), tin(Sn), and lead (Pb), which all haveouter electronic conÜgurations ns2np2

with no partly Ülled inner levels. For-merly, they were classiÜed in group

groups 3–12 254

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IV, which consisted of two sub-groups: IVB (the main group) andgroup IVA. Group IVA consisted of ti-tanium (Ti), zirconium (Zr), andhafnium (Hf), which now form group4 and are generally considered withthe *transition elements.

The main valency of the elementsis 4, and the members of the groupshow a variation from nonmetallic tometallic behaviour in moving downthe group. Thus, carbon is a non-metal and forms an acidic oxide(CO2) and a neutral oxide. Carboncompounds are mostly covalent. Oneallotrope (diamond) is an insulator,although graphite is a fairly goodconductor. Silicon and germaniumare metalloids, having semiconduc-tor properties. Tin is a metal, butdoes have a nonmetallic allotrope(grey tin). Lead is deÜnitely a metal.Another feature of the group is thetendency to form divalent com-pounds as the size of the atom in-creases. Thus carbon has only thehighly reactive carbenes. Siliconforms analogous silylenes. Germa-nium has an unstable hydroxide(Ge(OH)2), a sulphide (GeS), andhalides. The sulphide and halides dis-proportionate to germanium and thegermanium(IV) compound. Tin has anumber of tin(II) compounds, whichare moderately reducing, being oxi-dized to the tin(IV) compound. Leadhas a stable lead(II) state. See inert-pair effect.

In general, the reactivity of the el-ements increases down the groupfrom carbon to lead. All react withoxygen on heating. The Ürst fourform the dioxide; lead forms themonoxide (i.e. lead(II) oxide, PbO).Similarly, all will react with chlorineto form the tetrachloride (in the caseof the Ürst four) or the dichloride (forlead). Carbon is the only one capableof reacting directly with hydrogen.The hydrides all exist from the stable

methane (CH4) to the unstableplumbane (PbH4).

group 15 elements A group of el-ements in the *periodic table: nitro-gen (N), phosphorus (P), arsenic (As),antimony (Sb), and bismuth (Bi),which all have outer electronicconÜgurations ns2np3 with no partlyÜlled inner levels. Formerly, theywere classiÜed in group V, whichconsisted of two subgroups: group VB(the main group) and group VA.Group VA consisted of vanadium (V),niobium (Nb), and tantalum (Ta),which are generally considered withthe *transition elements:

The lighter elements (N and P) arenonmetals; the heavier elements aremetalloids. The lighter elements areelectronegative in character and havefairly large ionization energies. Nitro-gen has a valency of 3 and tends toform covalent compounds. The otherelements have available d-sublevelsand can promote an s-electron intoone of these to form compoundswith the V oxidation state. Thus,they have two oxides P2O3, P2O5,Sb2O3, Sb2O5, etc. In the case of bis-muth, the pentoxide Bi2O5 is difÜcultto prepare and unstable – an exam-ple of the increasing stability of theIII oxidation state in going fromphosphorus to bismuth. The oxidesalso show how there is increasingmetallic (electropositive) characterdown the group. Nitrogen and phos-phorus have oxides that are eitherneutral (N2O, NO) or acidic. Bismuthtrioxide (Bi2O3) is basic. Bismuth isthe only member of the group thatforms a well-characterized positiveion Bi3+.

group 16 elements A group of el-ements in the *periodic table: oxy-gen (O), sulphur (S), selenium (Se),tellurium (Te), and polonium (Po),which all have outer electronicconÜgurations ns2np4 with no partly

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Ülled inner levels. They are alsocalled the chalcogens. Formerly, theywere classiÜed in group VI, whichconsisted of two subgroups: groupVIB (the main group) and group VIA.Group VIA consisted of chromium(Cr), molybdenum (Mo), and tungsten(W), which now form group 6 aregenerally classiÜed with the *transi-tion elements.

The conÜgurations are just twoelectrons short of the conÜgurationof a noble gas and the elements arecharacteristically electronegative andalmost entirely nonmetallic. Ioniza-tion energies are high, (O 1314 to Po813 kJ mol–1) and monatomic cationsare not known. Polyatomic cationsdo exist, e.g. O2

+, S82+, Se8

2+, Te42+.

Electronegativity decreases down thegroup but the nearest approach tometallic character is the occurrenceof ‘metallic’ allotropes of selenium,tellurium, and polonium along withsome metalloid properties, in partic-ular, marked photoconductivity. Theelements of group 16 combine with awide range of other elements and thebonding is largely covalent. The el-ements all form hydrides of the typeXH2. Apart from water, these ma-terials are all toxic foul-smellinggases; they show decreasing thermalstability with increasing relativeatomic mass of X. The hydrides dis-solve in water to give very weakacids (acidity increases down thegroup). Oxygen forms the additionalhydride H2O2 (hydrogen peroxide),but sulphur forms a range of sul-phanes, such as H2S2, H2S4, H2S6.

Oxygen forms the Ûuorides O2F2and OF2, both powerful Ûuorinatingagents; sulphur forms analogousÛuorides along with some higherÛuorides, S2F2, SF2, SF4, SF6, S2F10. Se-lenium and tellurium form only thehigher Ûuorides MF4 and MF6; this isin contrast to the formation of lowervalence states by heavier elements

observed in groups 13, 14, and 15.The chlorides are limited to M2Cl2and MCl4; the bromides are similarexcept that sulphur only forms S2Br2.All metallic elements form oxidesand sulphides and many form se-lenides.

group 17 elements A group of el-ements in the *periodic table:Ûuorine (F), chlorine (Cl), bromine(Br), iodine (I), and astatine (At). Theyare known as the *halogens. For-merly, they were classiÜed in groupVII, which consisted of two sub-groups: group VIIB (the main group)and group VIIA. Group VIIA consistedof the elements manganese (Mn),technetium (Te), and rhenium (Re),which now form group 7 and areusually considered with the transi-tion elements.

group 18 elements A group of el-ements in the *periodic table: he-lium (He), neon (Ne), argon (Ar),krypton (Kr), xenon (Xe), and radon(Rn). Formerly classiÜed as group 0elements, they are usually referred toas the *noble gases.

group representation A group ofmathematical objects that is homo-morphic to (i.e. has the same mathe-matical structure as) the originalgroup. In particular, group represen-tations made up of square matricesare of interest. The dimension of therepresentation is the number of rowsor group representation columns ofthe matrix. The *irreducible repre-sentations of a group, i.e. the repre-sentations that cannot be expressedin terms of lower-dimensional repre-sentations, are of great importancein quantum mechanics since the en-ergy levels of a quantum mechanicalsystem are labelled by the irreduciblerepresentations of the symmetrygroup of the system. This enables*selection rules for the system to bederived.

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GSC (gas–solid chromatography) Seegas chromatography.

G-series See nerve agents.

guanidine A crystalline basic com-pound HN:C(NH2)2, related to urea.

guanine A *purine derivative. It isone of the major component bases of*nucleotides and the nucleic acids*DNA and *RNA.

gum Any of a variety of substancesobtained from plants. Typically theyare insoluble in organic solvents butform gelatinous or sticky solutionswith water. Gum resins are mixturesof gums and natural resins. Gums areproduced by the young xylem vesselsof some plants (mainly trees) in re-sponse to wounding or pruning. Theexudate hardens when it reaches theplant surface and thus provides atemporary protective seal while thecells below divide to form a perma-nent repair. Excessive gum formationis a symptom of some plant diseases.

guncotton See cellulose nitrate.

gun metal A type of bronze usually

containing 88–90% copper, 8–10% tin,and 2–4% zinc. Formerly used for can-nons, it is still used for bearings andother parts that require high resis-tance to wear and corrosion.

gunpowder An explosive consist-ing of a mixture of potassium nitrate,sulphur, and charcoal.

gypsum A monoclinic mineralform of hydrated *calcium sulphate,CaSO4.2H2O. It occurs in Üve vari-eties: rock gypsum, which is oftenred stained and granular; gypsite, animpure earthy form occurring as asurface deposit; alabaster, a pureÜne-grained translucent form; satinspar, which is Übrous and silky; andselenite, which occurs as transparentcrystals in muds and clays. It is usedin the building industry and in themanufacture of cement, rubber,paper, and plaster of Paris.

gyromagnetic ratio Symbol γ.The ratio of the angular momentumof an atomic system to its magneticmoment. The inverse of the gyro-magnetic ratio is called the magneto-mechanical ratio.

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H

Haber, Fritz (1868–1934) Germanchemist who worked at the Karl-sruhe Technical Institute, where heperfected the *Haber process formaking ammonia in 1908. As a Jew,he left Germany in 1933 to go intoexile in Britain, working in Cam-bridge at the Cavendish Laboratory.For his Haber process, he wasawarded the 1918 Nobel Prize forchemistry.

Haber process An industrialprocess for producing ammonia byreaction of nitrogen with hydrogen:

N2 + 3H2 ˆ 2NH3

The reaction is reversible andexothermic, so that a high yield ofammonia is favoured by low temper-ature (see le chatelier’s principle).However, the rate of reaction wouldbe too slow for equilibrium to bereached at normal temperatures, soan optimum temperature of about450°C is used, with a catalyst of iron

containing potassium and alu-minium oxide promoters. The higherthe pressure the greater the yield, al-though there are technical difÜcul-ties in using very high pressures. Apressure of about 250 atmospheres iscommonly employed.

The process is of immense impor-tance for the Üxation of nitrogen forfertilizers. It was developed in 1908by Fritz *Haber and was developedfor industrial use by Carl Bosch(1874–1940), hence the alternativename Haber–Bosch process. The ni-trogen is obtained from liquid air.Formerly, the hydrogen was from*water gas and the water–gas shiftreaction (the Bosch process) but nowthe raw material (called synthesisgas) is obtained by steam *reformingnatural gas.

habit See crystal.

haem (heme) An iron-containingmolecule that binds with proteins as

CH

CHCH

CH

N N

N N

CH2

CH2

CH3

CH3CH3

CH3

COOHHOOC

Fe

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a *cofactor or *prosthetic group toform the haemoproteins. These are*haemoglobin, *myoglobin, and the*cytochromes. Essentially, haemcomprises a *porphyrin with its fournitrogen atoms holding the iron(II)atom as a chelate. This iron can re-versibly bind oxygen (as in haemo-globin and myoglobin) or (as in thecytochromes) conduct electrons byconversion between the iron(II) andiron(III) series.

haematin test (Teichmann test) Atest for blood using the presence ofcharacteristic haematin crystals. Itwas introduced in 1853 by LudwigTeichmann.

haematite A mineral form ofiron(III) oxide, Fe2O3. It is the mostimportant ore of iron and usually oc-curs in two main forms: as a massivered kidney-shaped ore (kidney ore)and as grey to black metallic crystalsknown as specular iron ore.Haematite is the major red colour-ing agent in rocks; the largest de-posits are of sedimentary origin. Inindustry haematite is also used as apolishing agent (jeweller’s rouge) andin paints.

haemochromogen test(Takayama test) A test used toconÜrm the presence of blood. It is a microcrystal test exploiting the characteristic appearance ofhaemochromogen crystals observedunder a microscope. The rest was in-troduced in Japan in 1912 by MasaoTakayama.

haemoerythrin A red iron-contain-ing respiratory pigment that occursin the blood of annelids and someother invertebrates. Its structure isessentially the same as that of*haemoglobin except the prostheticgroup has a different chemical com-position.

haemoglobin One of a group of

globular proteins occurring widely inanimals as oxygen carriers in blood.Vertebrate haemoglobin comprisestwo pairs of polypeptide chains,known as α-chains and β-chains(forming the globin protein), witheach chain folded to provide a bind-ing site for a *haem group. Each ofthe four haem groups binds oneoxygen molecule to form oxy-haemoglobin. Dissociation occurs inoxygen-depleted tissues: oxygen is re-leased and haemoglobin is reformed.The haem groups also bind other in-organic molecules, including carbonmonoxide (to form carboxyhaemo-globin). In vertebrates, haemoglobinis contained in the red blood cells(erythrocytes).

haemoglobinic acid A very weakacid formed inside red blood cellswhen hydrogen ions combine withhaemoglobin. The presence of thehydrogen ions, which are producedby the dissociation of carbonic acid,encourages oxyhaemoglobin to disso-ciate into haemoglobin and oxygen.The oxygen diffuses into the tissuecells and the haemoglobin acts as a*buffer for the excess hydrogen ions,which it takes up to form haemoglo-binic acid.

hafnium Symbol Hf. A silvery lus-trous metallic *transition element;a.n. 72; r.a.m. 178.49; r.d. 13.3; m.p.2227±20°C; b.p. 4602°C. The elementis found with zirconium and is ex-tracted by formation of the chlorideand reduction by the Kroll process. Itis used in tungsten alloys in Ülamentsand electrodes and as a neutron ab-sorber. The metal forms a passiveoxide layer in air. Most of its com-pounds are hafnium(IV) complexes;less stable hafnium(III) complexesalso exist. The element was Ürst re-ported by Urbain in 1911, and its ex-istence was Ünally established by

259 hafnium

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Dirk Coster (1889–1950) and Georgede Hevesey (1885–1966) in 1923.


Hahn, Otto (1879–1968) Germanchemist, who studied in London(with William *Ramsay) and Canada(with Ernest *Rutherford) before re-turning to Germany in 1907. In 1917,together with Lise *Meitner, he dis-covered protactinium. In the late1930s he collaborated with FritzStrassmann (1902–80) and in 1938bombarded uranium with slow neu-trons. Among the products was bar-ium, but it was Meitner (now inSweden) who the next year inter-preted the process as nuclear Üssion.In 1944 Hahn received the NobelPrize for chemistry.

hahnium See transactinide el-ements.

half cell An electrode in contactwith a solution of ions, forming partof a *cell. Various types of half cellexist, the simplest consisting of ametal electrode immersed in a solu-tion of metal ions. Gas half cells havea gold or platinum plate in a solutionwith gas bubbled over the metalplate. The commonest is the *hydro-gen half cell. Half cells can also beformed by a metal in contact with aninsoluble salt or oxide and a solution.The *calomel half cell is an exampleof this. Half cells are commonly re-ferred to as electrodes.

half chair See ring conforma-tions.

half-life See decay.

half sandwich See sandwich com-pound.

half-thickness The thickness of aspeciÜed material that reduces the in-tensity of a beam of radiation to halfits original value.

half-width Half the width of aspectrum line (or in some cases thefull width) measured at half its height.

halide A compound of a halogenwith another element or group. Thehalides of typical metals are ionic(e.g. sodium Ûuoride, Na+F–). Metalscan also form halides in which thebonding is largely covalent (e.g. alu-minium chloride, AlCl3). Organic com-pounds are also sometimes referredto as halides; e.g. the alkyl halides(see haloalkanes) and the *acylhalides. Halides are named Ûuorides,chlorides, bromides, or iodides.

halite (rock salt) Naturally occur-ring *sodium chloride (common salt,NaCl), crystallizing in the cubic sys-tem. It is chieÛy colourless or white(sometimes blue) when pure but thepresence of impurities may colour itgrey, pink, red, or brown. Haliteoften occurs in association with an-hydrite and gypsum.

Hall–Heroult cell An electrolyticcell used industrially for the extrac-tion of aluminium from bauxite. Thebauxite is Ürst puriÜed by dissolvingit in sodium hydroxide and Ülteringoff insoluble constituents. Alu-minium hydroxide is then precipi-tated (by adding CO2) and this isdecomposed by heating to obtainpure Al2O3. In the Hall–Heroult cell,the oxide is mixed with cryolite (tolower its melting point) and themolten mixture electrolysed usinggraphite anodes. The cathode is thelining of the cell, also of graphite.The electrolyte is kept in a moltenstate (about 850°C) by the current.Molten aluminium collects at thebottom of the cell and can be tappedoff. Oxygen forms at the anode, andgradually oxidizes it away. The cell isnamed after the US chemist CharlesMartin Hall (1863–1914), who discov-ered the process in 1886, and theFrench chemist Paul Heroult (1863–

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1914), who discovered it indepen-dently in the same year.

hallucinogen A drug or chemicalthat causes alterations in perception(usually visual), mood, and thought.Common hallucinogenic drugs in-clude *lysergic acid diethylamide(LSD) and mescaline. There is nocommon mechanism of action forthis class of compounds althoughmany hallucinogens are structurallysimilar to neurotransmitters in thecentral nervous system, such as sero-tonin and the catecholamines.

halo A broad ring appearing in theelectron diffraction, neutron diffrac-tion, or X-ray diffraction patterns ofmaterials that are not crystals. Haloesof this type occur in gases and liquidsas well as in noncrystalline solids.

haloalkanes (alkyl halides) Organiccompounds in which one or morehydrogen atoms of an alkane havebeen substituted by halogen atoms.Examples are chloromethane, CH3Cl,dibromoethane, CH2BrCH2Br, etc.Haloalkanes can be formed by directreaction between alkanes and halo-gens using ultraviolet radiation. Theyare usually made by reaction of an al-cohol with a halogen carrier.

halocarbons Compounds that con-tain carbon and halogen atoms and(sometimes) hydrogen. The simplestare compounds such as tetra-chloromethane (CCl4), tetrabro-momethane (CBr4), etc. The*haloforms are also simple halocar-bons. The *chloroÛuorocarbons(CFCs) contain carbon, chlorine, andÛuorine. Similar to these are hy-drochloroÛuorocarbons (HCFCs),which contain carbon, chlorine,Ûuorine, and hydrogen, and the hy-droÛuorocarbons (HFCs), which con-tain carbon, Ûuorine, and hydrogen.The *halons are a class of halocar-bons that contain bromine.

haloform reaction A reaction forproducing *haloforms from methylketones. An example is the produc-tion of chloroform from propanoneusing sodium chlorate(I) (or bleach-ing powder):

CH3COCH3 + 3NaOCl → CH3COCl3+ 3NaOH

The substituted ketone then reacts togive chloroform (trichloromethane):

CH3COCCl3 + NaOH → NaOCOCH3+ CHCl3

The reaction can also be used formaking carboxylic acids, sinceRCOCH3 gives the product NaOCOR.It is particularly useful for aromaticacids as the starting ketone can bemade by a Friedel–Crafts acylation.

The reaction of methyl ketoneswith sodium iodate(I) gives iodoform(triiodomethane), which is a yellowsolid with a characteristic smell. Thisreaction is used in the iodoform testto identify methyl ketones. It alsogives a positive result with a sec-ondary alcohol of the formulaRCH(OH)CH3 (which is Ürst oxidizedto a methylketone) or with ethanol(oxidized to ethanal, which also un-dergoes the reaction).

haloforms The four compoundswith formula CHX3, where X is ahalogen atom. They are chloroform(CHCl3), and, by analogy, Ûuoroform(CHF3), bromoform (CHBr3), and iod-oform (CHI3). The systematic namesare trichloromethane, triÛuoro-methane, etc.

halogenating agent See halo-genation.

halogenation A chemical reactionin which a halogen atom is intro-duced into a compound. Halogena-tions are described as chlorination,Ûuorination, bromination, etc., ac-cording to the halogen involved.Halogenation reactions may takeplace by direct reaction with the

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halogen. This occurs with alkanes,where the reaction involves free radi-cals and requires high temperature,ultraviolet radiation, or a chemicalinitiator; e.g.

C2H6 + Br2 → C2H5Br + HBrThe halogenation of aromatic com-pounds can be effected by elec-trophilic substitution using analuminium chloride catalyst:

C6H6 + Cl2 → C6H5Cl + HClHalogenation can also be carried outusing compounds, such as phospho-rus halides (e.g. PCl3) or sulphur di-halide oxides (e.g. SOCl2), which reactwith –OH groups. Such compoundsare called halogenating agents. Addi-tion reactions are also referred to ashalogenations; e.g.

C2H4 + Br2 → CH2BrCH2Br

halogens (group 17 elements) Agroup of elements in the *periodictable (formerly group VIIB): Ûuorine(F), chlorine (Cl), bromine (Br), iodine(I), and astatine (At). All have a char-acteristic electron conÜguration ofnoble gases but with outer ns2np5

electrons. The outer shell is thus oneelectron short of a noble-gas conÜgu-ration. Consequently, the halogensare typical nonmetals; they havehigh electronegativities – high elec-tron afÜnities and high ionization en-ergies. They form compounds bygaining an electron to complete thestable conÜguration; i.e. they aregood oxidizing agents. Alternatively,they share their outer electrons toform covalent compounds, with sin-gle bonds.

All are reactive elements with thereactivity decreasing down thegroup. The electron afÜnity decreasesdown the group and other propertiesalso show a change from Ûuorine toastatine. Thus, the melting and boil-ing points increase; at 20°C, Ûuorineand chlorine are gases, bromine a liq-

uid, and iodine and astatine aresolids. All exist as diatomic mol-ecules.

The name ‘halogen’ comes fromthe Greek ‘salt-producer’, and the el-ements react with metals to formionic halide salts. They also combinewith nonmetals, the activity decreas-ing down the group: Ûuorine reactswith all nonmetals except nitrogenand the noble gases helium, neon,and argon; iodine does not react withany noble gas, nor with carbon, ni-trogen, oxygen, or sulphur. The el-ements Ûuorine to iodine all reactwith hydrogen to give the acid, withthe activity being greatest forÛuorine, which reacts explosively.Chlorine and hydrogen react slowlyat room temperature in the dark(sunlight causes a free-radical chainreaction). Bromine and hydrogenreact if heated in the presence of acatalyst. Iodine and hydrogen reactonly slowly and the reaction is notcomplete. There is a decrease in oxi-dizing ability down the group fromÛuorine to iodine. As a consequence,each halogen will displace any halo-gen below it from a solution of itssalt, for example:

Cl2 + 2Br– → Br2 + 2Cl–

The halogens also form a wide vari-ety of organic compounds in whichthe halogen atom is linked to carbon.In general, the aryl compounds aremore stable than the alkyl com-pounds and there is decreasing resis-tance to chemical attack down thegroup from the Ûuoride to the iodide.

Fluorine has only a valency of 1, al-though the other halogens can havehigher oxidation states using theirvacant d-electron levels. There is alsoevidence for increasing metallic be-haviour down the group. Chlorineand bromine form compounds withoxygen in which the halogen atom isassigned a positive oxidation state.

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Only iodine, however, forms positiveions, as in I+NO3

–.

halon A compound obtained by re-placing the hydrogen atoms of a hy-drocarbon by bromine along withother halogen atoms (see halocar-bons), for instance halon 1211 is bro-mochlorodiÛuoromethane (CF2BrCl)and halon 1301 is bromotriÛuoro-methane (CF3Br). Halons are very sta-ble and unreactive and are widelyused in Üre extinguishers. There isconcern that they are being brokendown in the atmosphere to bromine,which reacts with ozone, leading todepletion of *ozone layer, and theiruse is being curtailed. Although more*chloroÛuorocarbons are present inthe atmosphere, halons are betweenthree and ten times more destructiveof ozone.

halothane (1-chloro-1-bromo-2,2,2,-triÛuoroethane) A colourless, non-Ûammable oily liquid, CHBrClCF3;b.p. 51°C. It smells like trichloro-methane and has a sickly burningtaste. Halothane is widely used as ageneral anaesthetic, often adminis-tered also with oxygen and dinitro-gen oxide.

Hamiltonian Symbol H. A functionused to express the energy of a sys-tem in terms of its momentum andpositional coordinates. In simplecases this is the sum of its kineticand potential energies. In Hamilton-ian equations, the usual equationsused in mechanics (based on forces)are replaced by equations expressedin terms of momenta. This methodof formulating mechanics (Hamilton-ian mechanics) was Ürst introducedby Sir William Rowan Hamilton(1805–65). The Hamiltonian operatoris used in quantum mechanics in the*Schrödinger equation.

Hammett equation An equationrelating the structure to the reactiv-

ity of side-chain derivatives of aro-matic compounds. It arises from acomparison between rate constantsfor various reactions with the rate ofhydrolysis of benzyl chloride on theone hand and a comparison betweenequilibrium constants (such as thedissociation constant of benzoic acid)on the other hand. The Hammettequation can be written in the formlog(k/k0) = ρlog(K/K0), where log(K/K0)refers to comparing dissociation con-stants to the dissociation constant,K0, of benzoic acid in water at 25°C,and log(k/k0) refers to comparingrates of reaction to the rate, k0, of hy-drolysis of benzyl chloride. The termlog(K/K0) = σ is called the substituentconstant, since the nature of the sub-stituent affects the strength of thebenzoic acid. If σ is positive, the sub-stituent is electron attracting, whileif σ is negative the substituent is elec-tron donating. ρ is a reaction con-stant, which is determined for agiven reaction by the slope of agraph of log(k/k0) against σ. The nu-merical value of ρ depends on tem-perature and the type of solvent.

The Hammett equation applies tometa- and para- substituents (pro-vided that resonance interactionfrom the substituents does not occur)but not to ortho-substituents.

Hantz–Widman system A systemfor naming heterocyclic compounds,independently introduced for nam-ing 5- and 6- membered nitrogenmonocyclic compounds by A.Hantzsch (1887) and O. Widman(1888). Subsequently it was extendedto other ring sizes and other hetero-atoms. It is based on preÜxes denot-ing the hetero atoms(s) (e.g. aza- fornitrogen, thia- for sulphur, oxa- foroxygen), and items denoting ring size and saturation. For example, 4-membered rings have ete for unsatu-rated rings and etane for saturated;

263 Hantz–Widman system

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5- membered rings have ole for un-saturated and olane for saturatedcompounds. Rules are given for nam-ing compounds with two or moredifferent heteroatoms and for num-bering the atoms. The system iswidely used both for monocycliccompounds and for heterocyclic com-ponents of polycyclic compounds.A• Information about IUPAC nomenclature

hapticity Symbol η. The number ofelectrons in a ligand that are directlycoordinated to a metal.

hard acid See hsab principle.

hard base See hsab principle.

hardening of oils The process ofconverting unsaturated esters of*fatty acids into (more solid) satu-rated esters by hydrogenation using anickel catalyst. It is used in the man-ufacture of margarine from vegetableoils.

hardness of water The presencein water of dissolved calcium or mag-nesium ions, which form a scumwith soap and prevent the formationof a lather. The main cause of hardwater is dissolved calcium hydrogen-carbonate (Ca(HCO3)2), which isformed in limestone or chalk regionsby the action of dissolved carbondioxide on calcium carbonate. Thistype is known as temporary hardnessbecause it is removed by boiling:

Ca(HCO3)2(aq) → CaCO3(s) + H2O(l)+ CO2(g)

The precipitated calcium carbonate isthe ‘fur’ (or ‘scale’) formed in kettles,boilers, pipes, etc. In some areas,hardness also results from dissolvedcalcium sulphate (CaSO4), which can-not be removed by boiling (perma-nent hardness).

Hard water is a considerable prob-lem in washing, reducing the efÜ-ciency of boilers, heating systems,

etc., and in certain industrialprocesses. Various methods of watersoftening are used. In public sup-plies, the temporary hardness can beremoved by adding lime (calcium hy-droxide), which precipitates calciumcarbonate

Ca(OH)2(aq) + Ca(HCO3)2(aq) →2CaCO3(s) + 2H2O(l)

This is known as the Clark process (oras ‘clarking’). It does not remove per-manent hardness. Both temporaryand permanent hardness can betreated by precipitating calcium car-bonate by added sodium carbonate –hence its use as a washing soda andin bath salts. Calcium (and other)ions can also be removed from waterby ion-exchange using zeolites (e.g.Permutit). This method is used insmall domestic water-softeners. An-other technique is not to remove theCa2+ ions but to complex them andprevent them reacting further. Fordomestic use polyphosphates (con-taining the ion P6O18

6–, e.g. Calgon)are added. Other sequestering agentsare also used for industrial water. Seealso sequestration.

Hargreaves process See potas-sium sulphate.

harmonic An oscillation having afrequency that is a simple multipleof a fundamental sinusoidal oscilla-tion. The fundamental frequency of asinusoidal oscillation is usually calledthe Ürst harmonic. The second har-monic has a frequency twice that ofthe fundamental, and so on.

harmonic oscillator A system thatoscillates with simple harmonic mo-tion. The harmonic oscillator is ex-actly soluble in both classicalmechanics and quantum mechanics.Many systems exist for which har-monic oscillators provide very goodapproximations. Atoms vibratingabout their mean positions in mol-

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ecules or crystal lattices at low tem-peratures can be regarded as good ap-proximations to harmonic oscillatorsin quantum mechanics. Even if a sys-tem is not exactly a harmonic oscilla-tor the solution of the harmonicoscillator is frequently a useful start-ing point for treating such systemsusing *perturbation theory. Compareanharmonic oscillator.

harpoon mechanism A mecha-nism suggested to explain the reac-tion between such metal atoms assodium or potassium and such halo-gen molecules as bromine, e.g. K +Br2 → KBr + Br. As the alkali atom ap-proaches the bromine molecule itsvalence electron moves to thebromine molecule (thus providing a‘harpoon’). There are then two ionswith a Coulomb attraction betweenthem. As a result the ions move to-gether and the reaction takes place.This mechanism, which has beenworked out quantitatively, explainswhy the reaction occurs far morereadily than might be expected tak-ing into account only mechanical col-lisions between the alkali-metalatoms and halogen molecules.

Hartree–Fock procedure A self-consistent Üeld (SCF) procedure usedto Ünd approximate *wave functionsand energy levels in many-electronatoms. This procedure was intro-duced by the English mathematicianand physicist Douglas Hartree in1928 and improved by the Sovietphysicist Vladimir Fock in 1930 (bytaking into account the Pauli exclu-sion principle). The initial wave func-tions can be taken to be hydrogenicatomic orbitals. The resulting equa-tions can be solved numerically usinga computer. The results of theHartree–Fock theory are sufÜcientlyaccurate to show that electron den-sity occurs in shells around atoms

and can be used quantitatively toshow chemical periodicity.

hashish See cannabis.

hassium Symbol Hs. A radioactive*transactinide element; a.n. 108. Itwas Ürst made in 1984 by Peter Arm-bruster and a team in Darmstadt,Germany. It can be produced by bom-barding lead-208 nuclei with iron-58nuclei. Only a few atoms have everbeen produced. The name comesfrom the Latinized form of Hesse, theGerman state where it was Ürst syn-thesized.


hazchem code (emergency actioncode; EAC) A code designed to be dis-played when hazardous chemicalsare transported or stored in bulk. It isused to help the emergency servicesto take action quickly in any acci-dent. The code consists of a numberfollowed by one or two letters. Thenumber indicates the type of sub-stance to be used in treating the acci-dent (e.g. stream of water, Üne spray,foam, dry agent). The Ürst letter indi-cates the type of protective clothingneeded along with information aboutthe possibility of violent reaction onwhether the substance should becontained or diluted. The second let-ter, where it exists, is letter E, indi-cating that people have to beevacuated from the neighbourhoodof the incident. In the UK, the code isusually displayed as part of a panel,which includes an international UNnumber for the substance, a tele-phone number for specialist advice,the company name, and a symbol in-dicating the danger (e.g., a skull andcrossbones for toxic substances).

HCFC (hydrochloroÛuorocarbon) Seehalocarbons.

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h.c.p. Hexagonal close packing. Seeclose packing.

headspace The space above a sam-ple held in a sealed container. Head-space analysis is used in forensicscience to investigate the volatileconstituents of a sample.

heat capacity (thermal capacity)The ratio of the heat supplied to anobject or specimen to its consequentrise in temperature. The speciÜc heatcapacity is the ratio of the heat sup-plied to unit mass of a substance toits consequent rise in temperature.The molar heat capacity is the ratioof the heat supplied to unit amountof a substance to its consequent risein temperature. In practice, heat ca-pacity (C) is measured in joules perkelvin, speciÜc heat capacity (c) inJ K–1 kg–1, and molar heat capacity(Cm) in J K–1 mol–1. For a gas, the val-ues of c and Cm are commonly giveneither at constant volume, when onlyits *internal energy is increased, orat constant pressure, which requiresa greater input of heat as the gas isallowed to expand and do workagainst the surroundings. The sym-bols for the speciÜc and molar heatcapacities at constant volume are cvand Cv, respectively; those for thespeciÜc and molar heat capacities atconstant pressure are cp and Cp.

heat engine A device for convert-ing heat into work. Engines usuallywork on cycles of operation, themost efÜcient of which would be the*Carnot cycle.

heat of atomization The energyrequired to dissociate one mole of agiven substance into atoms.

heat of combustion The energyliberated when one mole of a givensubstance is completely oxidized.

heat of crystallization The en-ergy liberated when one mole of a

given substance crystallizes from asaturated solution of the same sub-stance.

heat of dissociation The energyabsorbed when one mole of a givensubstance is dissociated into its con-stituent elements.

heat of formation The energy lib-erated or absorbed when one mole ofa compound is formed in their *stan-dard states from its constituent el-ements.

heat of neutralization The en-ergy liberated in neutralizing onemole of an acid or base.

heat of reaction The energy liber-ated or absorbed as a result of thecomplete chemical reaction of molaramounts of the reactants.

heat of solution The energy liber-ated or absorbed when one mole of agiven substance is completely dis-solved in a large volume of solvent(strictly, to inÜnite dilution).

heavy hydrogen See deuterium.

heavy metal A metal with a highrelative atomic mass. The term isusually applied to common transitionmetals, such as copper, lead, andzinc. These metals are a cause of en-vironmental *pollution (heavy-metalpollution) from a number of sources,including lead in petrol, industrialefÛuents, and leaching of metal ionsfrom the soil into lakes and rivers byacid rain.

heavy spar A mineral form of*barium sulphate, BaSO4.

heavy water (deuterium oxide)Water in which hydrogen atoms, 1H,are replaced by the heavier isotopedeuterium, 2H (symbol D). It is acolourless liquid, which forms hexag-onal crystals on freezing. Its physicalproperties differ from those of ‘nor-mal’ water; r.d. 1.105; m.p. 3.8°C; b.p.

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101.4°C. Deuterium oxide, D2O, oc-curs to a small extent (about 0.003%by weight) in natural water, fromwhich it can be separated by frac-tional distillation or by electrolysis. Itis useful in the nuclear industry be-cause of its ability to reduce the ener-gies of fast neutrons to thermalenergies and because its absorptioncross-section is lower than that of hy-drogen and consequently it does notappreciably reduce the neutron Ûux.In the laboratory it is used for *la-belling other molecules for studies ofreaction mechanisms. Water alsocontains the compound HDO.

hecto- Symbol h. A preÜx used inthe metric system to denote 100times. For example, 100 coulombs =1 hectocoulomb (hC).

Heisenberg, Werner Karl(1901–76) German physicist, who be-came a professor at the University ofLeipzig and, after World War II, atthe Kaiser Wilhelm Institute in Göt-tingen. In 1923 he was awarded theNobel Prize for his work on matrixmechanics. But he is best known forhis 1927 discovery of the *uncer-tainty principle.

Heisenberg uncertainty princi-ple See uncertainty principle.

helicate A type of inorganic mol-ecule containing a double helix ofbipyridyl-derived molecules formedaround a chain of up to Üve copper(I)ions. See also supramolecular chem-istry.

helium Symbol He. A colourlessodourless gaseous nonmetallic el-ement belonging to group 18 of theperiodic table (see noble gases); a.n.2; r.a.m. 4.0026; d. 0.178 g dm–3; m.p.–272.2°C (at 20 atm.); b.p. –268.93°C.The element has the lowest boilingpoint of all substances and can be so-lidiÜed only under pressure. Naturalhelium is mostly helium–4, with a

small amount of helium–3. There arealso two short-lived radioactive iso-topes: helium–5 and –6. It occurs inores of uranium and thorium and insome natural-gas deposits. It has a va-riety of uses, including the provisionof inert atmospheres for welding andsemiconductor manufacture, as a re-frigerant for superconductors, and asa diluent in breathing apparatus. It isalso used in Ülling balloons. Chemi-cally it is totally inert and has noknown compounds. It was discoveredin the solar spectrum in 1868 byJoseph Lockyer (1836–1920).


helium–neon laser A laser inwhich the medium is a mixture ofhelium and neon in the mole ratio1:5 respectively. An electric dischargeis used to excite a He atom to themetastable 1s12s1 conÜguration.Since the excitation energy coincideswith an excitation energy of neon,transfer of energy between heliumand neon atoms can readily occurduring collisions. These collisionslead to highly excited neon atomswith unoccupied lower energy states.Thus, population inversion occursgiving rise to laser action with awavelength of 633 nm. (Many otherspectral lines are also produced inthe process.)

Helmholtz, Hermann LudwigFerdinand von (1821–94) Germanphysiologist and physicist. In 1850 hemeasured the speed of a nerve im-pulse and in 1851 invented the oph-thalmoscope. Helmholtz discoveredthe conservation of energy (1847),giving many examples of its applica-tion, and also introduced the conceptof *free energy.

Helmholtz free energy See freeenergy.

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Helmholtz model See electricaldouble layer.

heme See haem.

hemiacetals See acetals.

hemicellulose A *polysaccharidefound in the cell walls of plants. Thebranched chains of this moleculebind to cellulose microÜbrils, form-ing a network of cross-linked Übres.

hemihedral form The form of acrystal in which only half the num-ber of faces required for the symme-try are present. Compare holohedralform.

hemihydrate A crystalline hydratecontaining two molecules of com-pound per molecule of water (e.g.2CaSO4.H2O).

hemiketals See ketals.

henry Symbol H. The *SI unit of in-ductance equal to the inductance of aclosed circuit in which an e.m.f. ofone volt is produced when the elec-tric current in the circuit varies uni-formly at a rate of one ampere persecond. It is named after the U.S.physicist Joseph Henry (1797–1878).

Henry’s law At a constant tempera-ture the mass of gas dissolved in aliquid at equilibrium is proportionalto the partial pressure of the gas. Thelaw, discovered in 1801 by the Britishchemist and physician WilliamHenry (1775–1836), is a special caseof the partition law. It applies only togases that do not react with the sol-vent.

heparin A glycosaminoglycan (mu-copolysaccharide) with anticoagulantproperties, occurring in vertebratetissues, especially the lungs andblood vessels.

heptadecanoic acid (margaricacid) A white crystalline carboxylicacid with a linear chain of carbon

atoms, C16H33COOH; m.p. 59–61°C;b.p. 227°C (100 mm Hg). It is presentin certain natural fats.

heptahydrate A crystalline hy-drate that has seven moles of waterper mole of compound.

heptane A liquid straight-chainalkane obtained from petroleum,C7H16; r.d. 0.684; m.p. –90.6°C; b.p.98.4°C. In standardizing *octanenumbers, heptane is given a valuezero.

heptaoxodiphosphoric(V) acidSee phosphoric(v) acid.

heptavalent (septivalent) Having avalency of seven.

herbal cannabinoids See cannabi-noids.

herbicide See pesticide.

Hermann–Mauguin system (in-ternational system) A notation used to describe the symmetry ofpoint groups. In contrast to the*SchoenÛies system, which is usedfor isolated molecules (e.g. in spec-troscopy), the Hermann–Mauguinsystem is used in *crystallography.Some of the categories are the sameas the SchoenÛies system. n is thesame group as Cn. nmm is the samegroup as Cnv. There are two ms be-cause of two distinct types of mirrorplane containing the n-fold axis. n22is the same group as Dn. The othercategories do not coincide with theSchoenÛies system. is a group withan n-fold rotation–inversion axis andincludes C3h as 6

_, S4 as 4

_, S6 as 3

_, and

S2 as 1_. n/m is the same group as Cnh

except that C3h is regarded as 6_. n2m

is the same group as Dnd, except thatD3h is regarded as 62m. n/m 2/m 2/m,abbreviated to n/mmm, is the samegroup as Dnh, except that D3h isregarded as 6

_2m. (Unlike the

SchoenÛies system, the Hermann–Mauguin system regards the three-

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fold axis as a special case.) As regardsthe cubic groups, Oh is denoted m3m(or 4/m 3

_2/m), O is denoted 432, Th is

denoted m3 (or 2/m 3_), Td is denoted

4_3m, and T is denoted 23. In the Her-

mann–Mauguin system all the cubicgroups have 3 as the second numberbecause of the three-fold axis that oc-curs in all cubic groups.

heroin (diacetylmorphine) A highlyaddictive drug produced by acetylat-ing *morphine. It is usually used asthe hydrochloride. In the UK it is aclass A drug but it can be prescribedas a painkiller under the name dia-morphine. The Marquis and Froedhetests are used to give an initial indica-tion of heroin.

hertz Symbol Hz. The *SI unit offrequency equal to one cycle persecond. It is named after Heinrich*Hertz.

Hess’s law If reactants can be con-verted into products by a series of re-actions, the sum of the heats of thesereactions (with due regard to theirsign) is equal to the heat of reactionfor direct conversion from reactantsto products. More generally, the over-all energy change in going from reac-tants to products does not depend onthe route taken. The law can be usedto obtain thermodynamic data thatcannot be measured directly. For ex-ample, the heat of formation ofethane can be found by consideringthe reactions:

2C(s) + 3H2(g) + 3½O2(g) → 2CO2(g)+ 3H2O(l)

The heat of this reaction is 2∆HC +3∆HH, where ∆HC and ∆HH are theheats of combustion of carbon andhydrogen respectively, which can bemeasured. By Hess’s law, this is equalto the sum of the energies for twostages:

2C(s) + 3H2(g) → C2H6(g)

(the heat of formation of ethane, ∆Hf)and

C2H6(g) + 3½O2 → 2CO2(g) + 3H2O(l)(the heat of combustion of ethane,∆HE). As ∆HE can be measured and as

∆Hf + ∆HE = 2∆Hc + 3∆HH

∆Hf can be found. Another exampleis the use of the *Born–Haber cycleto obtain lattice energies. The lawwas Ürst put forward in 1840 by theSwiss-born Russian chemist GermainHenri Hess (1802–50). It is sometimescalled the law of constant heat sum-mation and is a consequence of thelaw of conservation of energy.

hetero atom An odd atom in thering of a heterocyclic compound. Forinstance, nitrogen is the hetero atomin pyridine.

heterocyclic See cyclic.

heterogeneous Relating to two ormore phases, e.g. a heterogeneous*catalyst. Compare homogeneous.

heterolytic Üssion The breakingof a bond in a compound in whichthe two fragments are oppositelycharged ions. For example, HCl → H+

+ Cl–. Compare homolytic fission.

heteronuclear Denoting a mol-ecule in which the atoms are of dif-ferent elements.

heteropolar bond See chemicalbond.

heteropoly compound See clus-ter compound.

heteropolymer See polymer.

Heusler alloys Ferromagnetic al-loys containing no ferromagnetic el-ements. The original alloys containedcopper, manganese, and tin and wereÜrst made by Conrad Heusler (19th-century mining engineer).

hexachlorobenzene A colourlesscrystalline compound, C6Cl6; m.p.

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227°C. It is made by the chlorinationof benzene with an iron(III) chloridecatalyst or by treating hexachlorocy-clohexane with chlorine in hexa-chloroethane. It is used to preservewood and dress seeds, and in themanufacture of hexaÛuorobenzene.

hexacyanoferrate(II) (ferro-cyanide) A complex iron-containinganion, [Fe(CN6)]4–, used as a solutionof its potassium salt as a test for fer-ric iron (iron(III)), with which itforms a dark blue precipitate ofPrussian blue. The sodium salt isused as an anticaking agent in com-mon salt.

hexacyanoferrate(III) (ferri-cyanide) A complex iron-containinganion, [Fe(CN)6)]3–, used as a solutionof its potassium salt as a test for fer-rous iron (iron(II)), with which itforms a dark blue precipitate ofPrussian blue.

hexadecane (cetane) A colourlessliquid straight-chain alkane hydrocar-bon, C16H34, used in standardizing*cetane numbers of Diesel fuel.

hexadecanoate See palmitate.

hexadecanoic acid See palmiticacid.

hexagonal close packing Seeclose packing.

hexagonal crystal See crystal sys-tem.

hexahydrate A crystalline com-pound that has six moles of waterper mole of compound.

hexamethylenetetramine Seehexamine.

hexamine (hexamethylenete-tramine) A white crystalline com-pound, C6H12N4, made by thecondensation of methanal with am-monia. It has been used as a solidfuel for camping stoves and as an an-

tiseptic for treating urinary infec-tions in medicine. It is used in theproduction of *cyclonite.

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hexanedioate (adipate) A salt orester of hexanedioic acid.

hexanedioic acid (adipic acid) Acarboxylic acid, (CH2)4(COOH)2; r.d.1.36; m.p. 153°C; b.p. 265°C (100mmHg). It is used in the manufactureof *nylon 6,6. See also polymeriza-tion.

6-hexanelactam See caprolactam.

hexanitrohexaazaisowurtzitaneSee hniw.

hexanoate (caproate) A salt orester of hexanoic acid.

hexanoic acid (caproic acid) A liq-uid fatty acid, CH3(CH2)4COOH; r.d.0.93; m.p. –3.4°C; b.p. 205°C. Glyc-erides of the acid occur naturally incow and goat milk and in some veg-etable oils.

hexose A *monosaccharide thathas six carbon atoms in its mol-ecules.

hexyl group (hexyl radical) Theorganic group CH3CH2CH2CH2-CH2CH2–, derived from hexane.

HFC (hydroÛuorocarbon) See halo-carbons.

highest occupied molecular or-bital (HOMO) The orbital in a mol-ecule that has the highest energylevel occupied at the temperature ofabsolute zero. The highest occupiedmolecular orbital and the lowest un-occupied molecular orbital (LUMO)


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are the two *frontier orbitals of themolecule.

high frequency (HF) A radio fre-quency in the range 3–30 megahertz;i.e. having a wavelength in the range10–100 metres.

high-performance liquid chro-matography (HPLC) A sensitivetechnique for separating or analysingmixtures, in which the sample isforced through the chromatographycolumn under pressure.

high-resolution electron-lossspectroscopy (HRELS) A techniquefor obtaining information about mol-ecules absorbed on a surface by in-elastic scattering of electrons.

high-speed steel A steel that willremain hard at dull red heat and cantherefore be used in cutting tools forhigh-speed lathes. It usually con-tains 12–22% tungsten, up to 5%chromium, and 0.4–0.7% carbon. Itmay also contain small amounts ofvanadium, molybdenum, and othermetals.

Hilbert space A linear vector spacethat can have an inÜnite number ofdimensions. The concept is of inter-est in physics because the state of asystem in *quantum mechanics isrepresented by a vector in Hilbertspace. The dimension of the Hilbertspace has nothing to do with thephysical dimension of the system.The Hilbert space formulation ofquantum mechanics was put forwardby the Hungarian-born US mathe-matician John von Neumann(1903–57) in 1927. Other formula-tions of quantum mechanics, such as*matrix mechanics and *wave me-chanics, can be deduced from theHilbert space formulation. It isnamed after the German mathemati-cian David Hilbert (1862–1943), whoinvented the concept early in the20th century.

Hildebrand rule If the density ofthe vapour phase above a liquid isconstant, the molar entropy of vapor-ization is constant. This law does nothold if molecular association takesplace in the liquid or if the liquid issubject to quantum-mechanicaleffects, e.g. as in superÛuidity. Therule is named after the US chemistJoel Henry Hildebrand (1881–1983).

Hill reaction The release of oxygenfrom isolated illuminated chloro-plasts when suitable electron accep-tors (e.g. potassium ferricyanide) areadded to the surrounding water. Thereaction was discovered by RobertHill (1899–1991) in 1939; the electronacceptors substitute for NADP+, thenatural acceptor for the light-depend-ent reactions of *photosynthesis.

histidine See amino acid.

histochemistry The study of thedistribution of the chemical con-stituents of tissues by means of theirchemical reactions. It utilizes suchtechniques as staining, light and elec-tron microscopy, autoradiography,and chromatography.

HMX (octogen; cyclotetramethyl-enetetranitramine) A colourless crys-talline compound, C4H8N8O8; r.d. 1.9;m.p. 276–286°C. It has a structuresimilar to that of *cyclonite (RDX),but with an 8-membered ring ratherthan a six-membered ring. An ex-tremely powerful explosive, it is usedmainly for military purposes and as a

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rocket propellant. The name comesfrom ‘High Molecular Weight RDX’.

HNIW (hexanitrohexaazaisowurtzi-tane) A powerful explosive, C6N12O12.It has a three-dimensional bridgedstructure containing six N–NO2groups. Also known as CL20, it isextremely sensitive and so far hasnot been produced in bulk.

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Hoff, Jacobus Henrikus van’t(1852–1911) Dutch physical chemistwho Ürst recognized that a moleculecould exist in two mirror-imageforms. He proposed that these formsrotated the plane of polarization inopposite senses, and is generally re-garded as the founder of stereochem-istry. Van’t Hoff also did importantwork on other branches of physicalchemistry, especially chemical ther-modynamics. He was awarded the1901 Nobel Prize for chemistry.

Hofmann’s reaction (Hofmann re-arrangement) A reaction for makingprimary *amines from *aminesusing bromine or chlorine andsodium hydroxide:

RCONH2 → RNH2

The halogen replaces a hydrogenatom from the amido group to forma halo-amide. This then reacts withthe alkali to produce an isocyanate,which decomposes into the amineand carbon dioxide. The amine hasone carbon atom fewer than theamide from which it is produced.This technique is used to reduce thelength of carbon chains in moles-

cules (the Hofmann degradation).The reaction is named after the Ger-man chemist August Wilhelm vonHofmann (1818–92).

hole 1. A vacant electron positionin the lattice structure of a solid thatbehaves like a mobile positive*charge carrier. 2. A vacant electronposition in one of the inner orbitalsof an atom.

holmium Symbol Ho. A soft silverymetallic element belonging to the*lanthanoids; a.n. 67; r.a.m. 164.93;r.d. 8.795 (20°C); m.p. 1474°C; b.p.2695°C. It occurs in apatite, xeno-time, and some other rare-earth min-erals. There is one natural isotope,holmium–165; eighteen artiÜcial iso-topes have been produced. There areno uses for the element, which wasdiscovered by Per Cleve (1840–1905)and J. L. Soret in 1879.A• Information from the WebElements site

holohedral form The form of acrystal in which the full number offaces required for the symmetry arepresent. Compare holohedral form.

HOMO See highest occupied mo-lecular orbital.

homocyclic See cyclic.

homogeneous Relating to onlyone phase, e.g. a homogeneous mix-ture, a homogeneous *catalyst. Com-pare hetereogeneous.

homoleptic compound A chemi-cal complex with only one type ofligand, as in nickel carbonyl or leadtetraethyl.

homologous series A series of re-lated chemical compounds that havethe same functional group(s) but dif-fer in formula by a Üxed group ofatoms. For instance, the simple car-boxylic acids: methanoic (HCOOH),ethanoic (CH3COOH), propanoic


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(C2H5COOH), etc., form a homolo-gous series in which each memberdiffers from the next by CH2. Succes-sive members of such a series arecalled homologues.

homolytic Üssion The breaking ofa bond in a compound in which thefragments are uncharged free radi-cals. For example, Cl2 → Cl. + Cl..Compare heterolytic fission.

homonuclear Denoting a moleculein which the atoms are of the sameelement.

homopolar bond See chemicalbond.

homopolymer See polymer.

hormone A substance that is man-ufactured and secreted in very smallquantities into the bloodstream byan endocrine gland or a specializednerve cell and regulates the growthor functioning of a speciÜc tissue ororgan in a distant part of the body.For example, the hormone insulincontrols the rate and manner inwhich glucose is used by the body.

hornblende Any of a group ofcommon rock-forming minerals of the amphibole group with the generalized formula(Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2.Hornblendes consist mainly of cal-cium, iron, and magnesium silicate.

host–guest chemistry Seesupramolecular chemistry.

HPLC See high-performance liquidchromatography.

HRELS See high-resolution elec-tron-loss spectroscopy.

HSAB principle A method of classi-fying Lewis acids and bases (See acid)developed by Ralph Pearson in the1960s. The acronym stands for ‘hardand soft acids and bases’. It is basedin empirical measurements of stabil-

ity of compounds with certain lig-ands. Hard acids tend to complexwith halide ions in the order

F– > Cl– > Br– > I–

Soft acids complex in the oppositeorder. Compounds that complexwith hard acids are hard bases; onesthat more readily form complexeswith soft acids are called soft bases.In general, soft acids and bases aremore easily polarized than hard acidsand bases and consequently havemore covalent character in the bond.The idea is an extension of the *typeA and B metals concept to com-pounds other than metal complexes.

Hückel, Erich (1896–1980) Germanphysicist and theoretical chemistwho worked with Peter *Debye atZürich on the theory of electrolytes.Later he moved to Copenhagen towork with Niels *Bohr and here heproduced his work on bonding inaromatic molecules. See hückel ap-proximation; aromatic compound.

Hückel approximation A set ofapproximations used to simplify themolecular orbital analysis of conju-gated molecules, suggested by ErichHückel in 1931. In the Hückel theory,the σ orbitals are treated separatelyfrom the π orbitals, the shape of themolecule being determined by the σorbitals. The Hückel theory makesthe approximation that interactionsbetween non-neighbouring atoms are taken to be zero. It enables calcu-lations to be made for conjugatedmolecules. In particular, the *delocal-ization energy of such molecules canbe estimated. Hückel explained thestability of benzene associated witharomaticity in this way. The theorycan also be used to analyse delocal-ized bonding in solids.

Hückel rule See aromatic com-pound.

humectant A substance used to

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maintain moisture levels. Humec-tants are generally *hygroscopic. Forexample, glycerol is employed as ahumectant in confectionery, food-stuffs, and tobacco. Other polyhydricalcohols, such as mannitol and sor-bitol, are also used as humectant ad-ditives in the foodstuffs industry.

Humphreys series A series oflines in the *hydrogen spectrumwith the form

1/λ = R(1/62 – 1/n2), n = 7,8,9…,where λ is the wavelength associatedwith the lines and R is the Rydbergconstant. The Humphreys series wasdiscovered by C. J. Humphreys in1953 and lies in the far infrared.

Hund coupling cases See cou-pling.

Hund’s rules Empirical rules for in-terpreting atomic *spectra used todetermine the lowest energy level fora conÜguration of two equivalentelectrons (i.e. electrons with thesame n and l quantum numbers) in amany-electron *atom. (1) The lowestenergy state has the maximum *mul-tiplicity consistent with the *Pauliexclusion principle. (2) The lowestenergy state has the maximum totalelectron orbital angular momentumquantum number, consistent withrule (1). These rules were put forwardby the German physicist FriedrichHund (1896–1993) in 1925. Hund’srules are explained by quantumtheory involving the repulsion be-tween two electrons.

hyaluronic acid A *glycosamino-glycan (mucopolysaccharide) that ispart of the matrix of connective tis-sue. Hyaluronic acid binds cells to-gether and helps to lubricate joints.It may play a role in the migration ofcells at wounds; this activity ceaseswhen hyaluronidase breaks downhyaluronic acid.

hybrid orbital See orbital.

hydracid See binary acid.

hydrate A substance formed bycombination of a compound withwater. See water of crystallization.

hydrated alumina See aluminiumhydroxide.

hydrated aluminium hydroxideSee aluminium hydroxide.

hydration See solvation.

hydrazine A colourless liquid orwhite crystalline solid, N2H4; r.d. 1.01(liquid); m.p. 1.4°C; b.p. 113.5°C. It isvery soluble in water and soluble inethanol. Hydrazine is prepared bythe Raschig synthesis in which am-monia reacts with sodium(I) chlorate(sodium hypochlorite) to give NH2Cl,which then undergoes further reac-tion with ammonia to give N2H4. In-dustrial production must be carefullycontrolled to avoid a side reactionleading to NH4Cl. The compound is aweak base giving rise to two series ofsalts, those based on N2H5

+, whichare stable in water (sometimes writ-ten in the form N2H4.HCl rather thanN2H5

+Cl–), and a less stable and ex-tensively hydrolysed series based onN2H6

2+. Hydrazine is a powerful re-ducing agent and reacts violentlywith many oxidizing agents, henceits use as a rocket propellant. It re-acts with aldehydes and ketones togive *hydrazones.

hydrazoic acid See hydrogenazide.

hydrazones Organic compoundscontaining the group =C:NNH2,formed by condensation of substi-tuted hydrazines with with aldehy-des and ketones (see illustration).Phenylhydrazones contain the group=C:NNHC6H5.


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hydride A chemical compound ofhydrogen and another element or el-ements. Non-metallic hydrides (e.g.ammonia, methane, water) are cova-lently bonded. The alkali metals andalkaline earths (*s-block elements)form salt-like hydrides containingthe hydride ion H–, which producehydrogen on reacting with water. Hy-dride-forming *transition elementsform interstitial hydrides, with thehydrogen atoms ‘trapped’ within thegaps in the lattice of metal atoms.Complex hydrides, such as *lithiumtetrahydroaluminate(III), have hy-dride ions as *ligands; many are pow-erful reducing agents.

hydriodic acid See hydrogen io-dide.

hydrobromic acid See hydrogenbromide.

hydrocarbons Chemical com-pounds that contain only carbon andhydrogen. A vast number of differenthydrocarbon compounds exist, themain types being the *alkanes,*alkenes, *alkynes, and *arenes.

hydrochloric acid See hydrogenchloride.

hydrochloride See amine salts.

hydrochloroÛuorocarbon (HCFC)See halocarbons.

hydrocyanic acid See hydrogencyanide.

hydrodynamic radius The effec-tive radius of an ion in a solutionmeasured by assuming that it is abody moving through the solutionand resisted by the solution’s viscos-ity. If the solvent is water, the hydro-

dynamic radius includes all the watermolecules attracted to the ion. As aresult, it is possible for a small ion tohave a larger hydrodynamic radiusthan a large ion – if it is surroundedby more solvent molecules. Experi-ments involving *nuclear magneticresonance (NMR) and isotope tracersindicate that there is considerablemovement between solvent mol-ecules within the hydrodynamic ra-dius and the rest of the solution.

hydroÛuoric acid See hydrogenfluoride.

hydroÛuorocarbon (HFC) Seehalocarbons.

hydrogen Symbol H. A colourlessodourless gaseous chemical element;a.n. 1; r.a.m. 1.008; d. 0.0899 g dm–3;m.p. –259.14°C; b.p. –252.87°C. It isthe lightest element and the mostabundant in the universe. It is pre-sent in water and in all organic com-pounds. There are three isotopes:naturally occurring hydrogen con-sists of the two stable isotopes hydro-gen–1 (99.985%) and *deuterium. Theradioactive *tritium is made artiÜ-cially. The gas is diatomic and hastwo forms: orthohydrogen, in whichthe nuclear spins are parallel, andparahydrogen, in which they are an-tiparallel. At normal temperaturesthe gas is 25% parahydrogen. In theliquid it is 99.8% parahydrogen. Themain source of hydrogen is steam*reforming of natural gas. It can alsobe made by the Bosch process (seehaber process) and by electrolysis ofwater. The main use is in the Haberprocess for making ammonia. Hydro-gen is also used in various other

275 hydrogen

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R CO

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industrial processes, such as thereduction of oxide ores, the reÜningof petroleum, the production ofhydrocarbons from coal, and the hy-drogenation of vegetable oils. Consid-erable interest has also been shownin its potential use in a ‘hydrogenfuel economy’ in which primary en-ergy sources not based on fossil fuels(e.g. nuclear, solar, or geothermal en-ergy) are used to produce electricity,which is employed in electrolysingwater. The hydrogen formed isstored as liquid hydrogen or as metalhydrides. Chemically, hydrogen re-acts with most elements. It was dis-covered by Henry *Cavendish in1766.A• Information from the WebElements site

hydrogen acceptor See hydrogencarrier.

hydrogenation 1. A chemical re-action with hydrogen; in particular,an addition reaction in which hydro-gen adds to an unsaturated com-pound. Nickel is a good catalyst forsuch reactions. 2. The process ofconverting coal to oil by making thecarbon in the coal combine with hy-drogen to form hydrocarbons. See fis-cher–tropsch process; bergiusprocess.

hydrogen azide (hydrazoic acid;azoimide) A colourless liquid, HN3;r.d. 1.09; m.p. –80°C; b.p. 37°C. It ishighly toxic and a powerful reduc-tant, which explodes in the presenceof oxygen and other oxidizingagents. It may be prepared by the re-action of sodium amide and sodiumnitrate at 175°C followed by distilla-tion of a mixture of the resultingsodium azide and a dilute acid. Seealso azides.

hydrogen bond A type of electro-static interaction between moleculesoccurring in molecules that have

hydrogen atoms bound to electro-negative atoms (F, N, O). It can beregarded as a strong dipole–dipoleattraction caused by the electron-withdrawing properties of the elec-tronegative atom. Thus, in the watermolecule the oxygen atom attractsthe electrons in the O–H bonds. Thehydrogen atom has no inner shells ofelectrons to shield the nucleus, andthere is an electrostatic interactionbetween the hydrogen proton and alone pair of electrons on an oxygenatom in a neighbouring molecule.Each oxygen atom has two lone pairsand can make hydrogen bonds totwo different hydrogen atoms. Thestrengths of hydrogen bonds areabout one tenth of the strengths ofnormal covalent bonds. Hydrogenbonding does, however, have signiÜ-cant effects on physical properties.Thus it accounts for the unusualproperties of *water and for the rela-tively high boiling points of H2O, HF,and NH3 (compared with H2S, HCl,and PH3). It is also of great impor-tance in living organisms. Hydrogenbonding occurs between bases in thechains of DNA. It also occurs be-tween the C=O and N–H groups inproteins, and is responsible for main-taining the secondary structure.

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Hydrogen bonds are not purely elec-trostatic and can be shown to havesome covalent character.

hydrogen bromide A colourlessgas, HBr; m.p. –88.5°C; b.p. –67°C. Itcan be made by direct combinationof the elements using a platinum cat-alyst. It is a strong acid dissociatingextensively in solution (hydrobromicacid).

hydrogencarbonate (bicarbonate)A salt of *carbonic acid in which onehydrogen atom has been replaced; itthus contains the hydrogencarbonateion HCO3

–.

hydrogen carrier (hydrogen accep-tor) A molecule that accepts hydro-gen atoms or ions, becoming reducedin the process (see oxidation–reduction). The *electron transportchain, whose function is to generateenergy in the form of ATP during res-piration, involves a series of hydro-gen carriers, including *NAD and*FAD, which pass on the hydrogen(derived from the breakdown of glu-cose) to the next carrier in the chain.

hydrogen chloride A colourlessfuming gas, HCl; m.p. –114.8°C; b.p.–85°C. It can be prepared in the labo-ratory by heating sodium chloridewith concentrated sulphuric acid(hence the former name spirits ofsalt). Industrially it is made directlyfrom the elements at high tempera-ture and used in the manufacture ofPVC and other chloro compounds. Itis a strong acid and dissociates fullyin solution (hydrochloric acid).

hydrogen cyanide (hydrocyanicacid; prussic acid) A colourless liquidor gas, HCN, with a characteristicodour of almonds; r.d. 0.699 (liquid at22°C); m.p. –14°C; b.p. 26°C. It is anextremely poisonous substanceformed by the action of acids onmetal cyanides. Industrially, it is madeby catalytic oxidation of ammonia

and methane with air and is used inproducing acrylate plastics. Hydrogencyanide is a weak acid (Ka = 2.1 × 10–9

mol dm–3). With organic carbonylcompounds it forms *cyanohydrins.

hydrogen electrode See hydro-gen half cell.

hydrogen Ûuoride A colourlessliquid, HF; r.d. 0.99; m.p. –83°C; b.p.19.5°C. It can be made by the actionof sulphuric acid on calcium Ûuoride.The compound is an extremely corro-sive Ûuorinating agent, which attacksglass. It is unlike the other hydrogenhalides in being a liquid (a result of*hydrogen bond formation). It is alsoa weaker acid than the others be-cause the small size of the Ûuorineatom means that the H–F bond isshorter and stronger. Solutions of hy-drogen Ûuoride in water are knownas hydroÛuoric acid.

hydrogen half cell (hydrogen elec-trode) A type of *half cell in which ametal foil is immersed in a solutionof hydrogen ions and hydrogen gas isbubbled over the foil. The standardhydrogen electrode, used in measur-ing standard *electrode potentials,uses a platinum foil with a 1.0 M so-lution of hydrogen ions, the gas at 1atmosphere pressure, and a tempera-ture of 25°C. It is written Pt(s)|H2(g),H+(aq), the effective reaction being

H2 → 2H+ + 2e.

hydrogenic Describing an atom orion that has only one electron; for ex-ample, H, He+, Li2+, C5+. Hydrogenicatoms (or ions) do not involve elec-tron–electron interactions and areeasier to treat theoretically.

hydrogen iodide A colourless gas,HI; m.p. –51°C; b.p. –35.38°C. It canbe made by direct combination ofthe elements using a platinum cata-lyst. It is a strong acid dissociating ex-tensively in solution (hydroiodic acid

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or hydriodic acid). It is also a reduc-ing agent.

hydrogen ion See acids.

hydrogen molecule ion The sim-plest type of molecule (H2

+), consist-ing of two hydrogen nuclei and oneelectron. In the *Born–Oppenheimerapproximation, in which the nucleiare regarded as being Üxed, the*Schrödinger equation for the hy-drogen molecule ion can be solvedexactly. This enables ideas and ap-proximation techniques concernedwith chemical bonding to be testedquantitatively.

hydrogen peroxide A colourlessor pale blue viscous unstable liquid,H2O2; r.d. 1.44; m.p. –0.41°C; b.p.150.2°C. As with water, there is con-siderable hydrogen bonding in theliquid, which has a high dielectricconstant. It can be made in the labo-ratory by adding dilute acid to bar-ium peroxide at 0°C. Large quantitiesare made commercially by electroly-sis of KHSO4.H2SO4 solutions. An-other industrial process involvescatalytic oxidation (using nickel, pal-ladium, or platinum with an an-thraquinone) of hydrogen and waterin the presence of oxygen. Hydrogenperoxide readily decomposes in lightor in the presence of metal ions togive water and oxygen. It is usuallysupplied in solutions designated byvolume strength. For example, 20-volume hydrogen peroxide wouldyield 20 volumes of oxygen per vol-ume of solution. Although the *per-oxides are formally salts of H2O2, thecompound is essentially neutral.Thus, the acidity constant of the ion-ization

H2O2 + H2O ˆ H3O+ + HO2–

is 1.5 × 10–12 mol dm–3. It is a strongoxidizing agent, hence its use as amild antiseptic and as a bleachingagent for cloth, hair, etc. It has also

been used as an oxidant in rocketfuels.

hydrogen spectrum The atomicspectrum of hydrogen is character-ized by lines corresponding to radia-tion quanta of sharply deÜnedenergy. A graph of the frequencies atwhich these lines occur against theordinal number that characterizestheir position in the series of lines,produces a smooth curve indicatingthat they obey a formal law. In 1885J. J. Balmer (1825–98) discovered thelaw having the form:

1/λ = R(1/n12 + 1/n2

2)This law gives the so-called Balmerseries of lines in the visible spectrumin which n1 = 2 and n2 = 3,4,5…, λ isthe wavelength associated with thelines, and R is the *Rydberg constant.

In the Lyman series, discovered byTheodore Lyman (1874–1954), n1 = 1and the lines fall in the ultraviolet.The Lyman series is the strongest fea-ture of the solar spectrum as ob-served by rockets and satellites abovethe earth’s atmosphere. In thePaschen series, discovered by F.Paschen (1865–1947), n1 = 3 and thelines occur in the far infrared. TheBrackett series (n1 = 4), Pfund series(n1 = 5), and Humphreys series (n1 = 6)also occur in the far infrared.A• Balmer’s paper

hydrogensulphate (bisulphate) Asalt containing the ion HSO4

– or anester of the type RHSO4, where R isan organic group. It was formerlycalled hydrosulphate.

hydrogen sulphide (sulphurettedhydrogen) A gas, H2S, with an odourof rotten eggs; r.d. 1.54 (liquid); m.p.–85.5°C; b.p. –60.7°C. It is soluble inwater and ethanol and may be pre-pared by the action of mineral acidson metal sulphides, typically hy-drochloric acid on iron(II) sulphide

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(see kipp’s apparatus). Solutions inwater (known as hydrosulphuric acid)contain the anions HS– and minutetraces of S2– and are weakly acidic.Acid salts (those containing the HS–

ion) are known as hydrogensulphides(formerly hydrosulphides). In acid so-lution hydrogen sulphide is a mildreducing agent. Hydrogen sulphidehas an important role in traditionalqualitative chemical analysis, whereit precipitates metals with insolublesulphides. Hydrogen sulphide is ex-ceedingly poisonous (more toxic thanhydrogen cyanide). See also clausprocess.

hydrogensulphite (bisulphite) Asalt containing the ion –HSO3 or anester of the type RHSO3, where R isan organic group.

hydroiodic acid See hydrogen io-dide.

hydrolysis A chemical reaction of acompound with water. For instance,salts of weak acids or bases hydrolysein aqueous solution, as in

Na+–CH3COO– + H2O ˆ Na+ + OH– +CH3COOH

The reverse reaction of *esteriÜca-tion is another example. See alsosolvolysis.

hydromagnesite A mineral formof basic *magnesium carbonate,3MgCO3.Mg(OH)2.3H2O.

hydron The positive ion H+. Thename is used when it is not relevantto specify the isotope, as is usuallythe case in compounds in which thehydrogen is in natural abundance.When the isotope is relevant thenproton (1H+), deuteron (2H+), or triton(3H+) should be used.

hydronium ion See oxonium ion.

hydrophilic Having an afÜnity forwater. See lyophilic.

hydrophobic Lacking afÜnity forwater. See lyophobic.

hydroquinone See benzene-1,4-diol.

hydrosol A sol in which the contin-uous phase is water. See colloids.

hydrosulphate See hydrogensul-phate.

hydrosulphide See hydrogen sul-phide.

hydrosulphuric acid See hydro-gen sulphide.

hydroxide A metallic compoundcontaining the ion OH– (hydroxideion) or containing the group –OH (hy-droxyl group) bound to a metal atom.Hydroxides of typical metals arebasic; those of *metalloids are am-photeric.

hydroxoacid A type of acid inwhich the acidic hydrogen is on a hy-droxyl group attached to an atomthat is not attached to an oxo (=O)group. An example is

Si(OH4) + H2O → Si(OH)3(O)– + H3O+

Compare oxoacid.

hydroxonium ion See oxoniumion.

4-hydroxybutanoic acid(gammahydroxybutyric acid; GHB) Anaturally occurring carboxylic acid,HO(CH2)3COOH, found in smallamounts in most living things. It isused as a therapeutic drug to treat in-somnia, depression and alcoholism.GHB, as it is commonly known, isalso used as an illegal club drug andas a date-rape drug.

279 4-hydroxybutanoic acid

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OHCH2

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4-hydroxybutanoic acid lactone(γ-butyrolactone) A colourless liquid*lactone, C4H6O2; b.p. 206°C. It isused as a solvent and in the produc-tion of certain synthetic resins.

hydroxycerussite See lead(ii) car-bonate hydroxide.

hydroxyethanoic acid See gly-colic acid.

hydroxylamine A colourless solid,NH2OH, m.p. 33°C. It explodes onheating and may be employed as anoxidizing agent or reducing agent. Itis made by the reduction of nitratesor nitrites, and is used in makingnylon. With aldehydes and ketones itforms *oximes.A• Information about IUPAC nomenclature

hydroxylation The introduction ofa hydroxyl group (–OH) into an or-ganic compound. For example,alkenes can be hydroxylated usingpotassium permanganate or leadethanoate to give alcohols. In bio-chemistry, various enzymes canbring about hydroxylation.

hydroxyl group The group –OH ina chemical compound.

2-hydroxypropanoic acid See lac-tic acid.

hygroscopic Describing a sub-stance that can take up water fromthe atmosphere. See also deliques-cence.

hyperconjugation The interactionof sigma bonds with pi bonds in a

molecule, as in the interaction of amethyl group with the benzene ringin toluene. It is postulated to accountfor the stability of some carboniumions. Hyperconjugation is oftenthought of as a contribution of reso-nance structures in which a sigmabond is broken to give ions; e.g.C6H5CH2

–H+ in toluene. It has beencalled no-bond resonance.

hyperÜne structure See finestructure.

hypertonic solution A solutionthat has a higher osmotic pressurethan some other solution. Comparehypotonic solution.

hypochlorite See chlorates.

hypochlorous acid See chloric(i)acid.

hypophosphorus acid See phos-phinic acid.

hyposulphite See sulphinate.

hyposulphurous acid See sul-phinic acid.

hypothesis See laws, theories,and hypotheses.

hypotonic solution A solutionthat has a lower osmotic pressurethan some other solution. Comparehypertonic solution.

hypsochromic shift A shift of aspectral band to shorter wavelengthsas a result of substitution in a mol-ecule or as a result of a change in the physical conditions. Comparebathochromic shift.

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I

ice See water.

ice point The temperature atwhich there is equilibrium betweenice and water at standard atmos-pheric pressure (i.e. the freezing ormelting point under standard condi-tions). It was used as a Üxed point (0°)on the Celsius scale, but the kelvinand the International Practical Tem-perature Scale are based on the*triple point of water.

icosahedron A polyhedron having20 triangular faces with Üve edgesmeeting at each vertex. The symme-try of an icosahedron (known asicosahedral symmetry) has Üvefold ro-tation axes. It is impossible in *crys-tallography to have a periodic crystalwith the point group symmetry of anicosahedron (icosahedral packing).However, it is possible for short-range order to occur with icosahedralsymmetry in certain liquids andglasses because of the dense packingof icosahedra. Icosahedral symmetryalso occurs in certain *quasicrystals,such as alloys of aluminium andmanganese.

ideal crystal A single crystal with aperfectly regular lattice that containsno impurities, imperfections, orother defects.

ideal gas (perfect gas) A hypotheti-cal gas that obeys the *gas laws ex-actly. An ideal gas would consist ofmolecules that occupy negligiblespace and have negligible forces be-tween them. All collisions made be-tween molecules and the walls of thecontainer or between molecules andother molecules would be perfectlyelastic, because the molecules would

have no means of storing energy ex-cept as translational kinetic energy.

ideal solution See raoult’s law.

IE Ionization energy. See ionizationpotential.

ignition temperature The tem-perature to which a substance mustbe heated before it will burn in air.

Ilkovic equation A relation usedin polarography relating the diffu-sion current ia and the concentrationc. The Ilkovic equation has the formia = kc, where k is a constant.

imides Organic compounds con-taining the group –CO.NH.CO.– (theimido group).A• Information about IUPAC nomenclature

imido group See imides.

imines Compounds containing thegroup –NH– in which the nitrogenatom is part of a ring structure, orthe group =NH, in which the nitro-gen atom is linked to a carbon atomby a double bond. In either case, thegroup is referred to as an iminogroup.A• Information about IUPAC nomenclature

imino group See imines.

Imperial units The British systemof units based on the pound and theyard. The former f.p.s. system wasused in engineering and was looselybased on Imperial units; for allscientiÜc purposes *SI units are nowused. Imperial units are also being re-placed for general purposes by met-ric units.


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implosion An inward collapse of avessel, especially as a result of evacu-ation.

IMS See ion-mobility spectrometry.

incandescence The emission oflight by a substance as a result ofraising it to a high temperature.

InChI A type of *line notation intro-duced by IUPAC. The acronym standsfor International Chemical IdentiÜer.Originally, it was IChI (IUPAC chemi-cal identiÜer). An InChI can give alarge amount of chemical informa-tion displayed in layers and sublayersseparated by forward slashes. For ex-ample, the InChI for naphthalene is

InChI=1/C10H8/c-1-2-6-10-8-4-3-7-9(10)5-1/h1-8H

Here, InChI=1 indicates the versionof InChI used. The remainder of thestring is the main layer. It consists ofthree sublayers. These are (1) the mo-lecular formula (C10H8); (2) the atom-connection information (startingwith c), and (3) the hydrogens pre-sent (starting with h). Other layerscan be added for such information ascharge, stereochemistry, and isotopiccomposition.

A• An FAQ produced by the Chemistry

Department, Cambridge University• Information and downloads from the

IUPAC site

incommensurate lattice A latticewith long-range periodic order thathas two or more periodicities inwhich an irrational number gives theratio between the periodicities. Anexample of an incommensurate lat-tice occurs in certain magnetic sys-tems in which the ratio of themagnetic period to the atomic latticeis an irrational number. The phasetransition between a commensuratelattice and an incommensurate lat-

tice can be analysed using the*Frenkel–Kontorowa model.

indene A colourless Ûammable hy-drocarbon, C9H8; r.d. 0.996; m.p.–1.8°C; b.p. 182.6°C. Indene is anaromatic hydrocarbon with a Üve-membered ring fused to a benzenering. It is present in coal tar and isused as a solvent and raw materialfor making other organic com-pounds.

independent-particle model Amodel for electrons in a many-electron system in which the cor-relation between electrons is eitherignored or taken into account by re-garding an electron as moving in anaveraged-out potential that repre-sents the interactions between theelectron and all the other particles inthe system. Although the indepen-dent-particle model cannot describeall aspects of many-electron systems,it had some notable successes, suchas explaining the shell structure ofelectrons in atoms.

indeterminacy See uncertaintyprinciple.

indicator A substance used to showthe presence of a chemical substanceor ion by its colour. Acid–base indica-tors are compounds, such as phe-nolphthalein and methyl orange,that change colour reversibly, de-pending on whether the solution isacidic or basic. They are usually weakacids in which the un-ionized formHA has a different colour from thenegative ion A–. In solution the indi-cator dissociates slightly

HA ˆ H+ + A–

In acid solution the concentration ofH+ is high, and the indicator islargely undissociated HA; in alkalinesolutions the equilibrium is displacedto the right and A– is formed. Usefulacid–base indicators show a sharpcolour change over a range of about

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2 pH units. In titration, the point atwhich the reaction is complete is theequivalence point (i.e. the point atwhich equivalent quantities of acidand base are added). The end point isthe point at which the indicator justchanges colour. For accuracy, the twomust be the same. During a titrationthe pH changes sharply close to theequivalence point, and the indicatorused must change colour over thesame range.

Other types of indicator can beused for other reactions. Starch, forexample, is used in iodine titrationsbecause of the deep blue complex itforms. Oxidation–reduction indica-tors are substances that show a re-versible colour change betweenoxidized and reduced forms. See alsoadsorption indicator.

indigo A blue vat dye, C16H10N2O2.It occurs as the glucoside indican inthe leaves of plants of the genus In-digofera, from which it was formerlyextracted. It is now made syntheti-cally.

indium Symbol In. A soft silvery el-ement belonging to group 13 (for-merly IIIB) of the periodic table; a.n.49; r.a.m. 114.82; r.d. 7.31 (20°C);m.p. 156.6°C; b.p. 2080±2°C. It occursin zinc blende and some iron oresand is obtained from zinc Ûue dust intotal quantities of about 40 tonnesper annum. Naturally occurring in-dium consists of 4.23% indium–113(stable) and 95.77% indium–115 (half-life 6 × 1014 years). There are a fur-ther Üve short-lived radioisotopes.The uses of the metal are small –some special-purpose electroplatesand some special fusible alloys. Sev-eral semiconductor compounds areused, such as InAs, InP, and InSb.With only three electrons in its va-lency shell, indium is an electron ac-ceptor and is used to dope puregermanium and silicon; it forms sta-

ble indium(I), indium(II), and in-dium(III) compounds. The elementwas discovered in 1863 by FerdinandReich (1799–1882) and HieronymusRichter (1824–90).A• Information from the WebElements site

indole A yellow solid, C8H7N, m.p.52°C. Its molecules consist of a ben-zene ring fused to a nitrogen-contain-ing Üve-membered ring. It occurs insome plants and in coal tar, and isproduced in faeces by bacterial ac-tion. It is used in making perfumes.Indole has the nitrogen atom posi-tioned next to the fused benzenering. An isomer with the nitrogentwo atoms away from the fused ringis called isoindole.

283 induced-fit model

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Indole

induced emission (stimulatedemission) The emission of a photonby an excited atom or molecule in-duced by an incident photon of suit-able energy. The relation betweeninduced emission and *spontaneousemission is given by the *Einstein co-efÜcients. The process of inducedemission is essential for the opera-tion of lasers and masers.

induced-Üt model A proposedmechanism of interaction betweenan enzyme and a substrate. It postu-lates that exposure of an enzyme to asubstrate causes the *active site ofthe enzyme to change shape in orderto allow the enzyme and substrate to bind (see enzyme–substrate com-plex). This hypothesis is generallypreferred to the *lock-and-key mech-anism.


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inductive effect The effect of agroup or atom of a compound inpulling electrons towards itself or inpushing them away. Inductive effectscan be used to explain some aspectsof organic reactions. For instance,electron-withdrawing groups, such as–NO2, –CN, –CHO, –COOH, and thehalogens substituted on a benzenering, reduce the electron density onthe ring and decrease its susceptibil-ity to further (electrophilic) substitu-tion. Electron-releasing groups, suchas –OH, –NH2, –OCH3, and –CH3,have the opposite effect. See also elec-tronic effects.

industrial fermenter See bioreac-tor.

inelastic neutron scattering Atechnique for investigating the mo-tion of molecules by scattering neu-trons. The neutrons pick up or loseenergy as they move through a sam-ple of a liquid. The analysis of neu-tron scattering experiments enablesinformation to be obtained about theliquid.

inert gases See noble gases.

inert-pair effect An effect seen es-pecially in groups 13 and 14 of theperiodic table, in which the heavierelements in the group tend to formcompounds with a valency two lowerthan the expected group valency. It isused to account for the existence ofthallium(I) compounds in group 13and lead(II) in group 14. In formingcompounds, elements in thesegroups promote an electron from aÜlled s-level state to an empty p-level.The energy required for this is morethan compensated for by the extraenergy gain in forming two morebonds. For the heavier elements, thebond strengths or lattice energies in the compounds are lower thanthose of the lighter elements. Conse-quently the energy compensation is

less important and the lower valencestates become favoured.

infrared chemiluminescence Atechnique for studying chemical re-action mechanisms by measuringand analysing weak infrared emis-sions from product molecules formedin certain chemical reactions. If prod-ucts are formed with excess energy,this appears as excited vibrationalstates of the molecules, which decaywith emission of infrared radiation.Spectroscopic investigation of this ra-diation gives information about thestates in which the product mol-ecules were formed.

infrared radiation (IR) Electro-magnetic radiation with wavelengthslonger than that of red light butshorter than radiowaves, i.e. radia-tion in the wavelength range 0.7 mi-crometre to 1 millimetre. It wasdiscovered in 1800 by William Her-schel (1738–1822) in the sun’s spec-trum. The natural vibrationalfrequencies of atoms and moleculesand the rotational frequencies ofsome gaseous molecules fall in theinfrared region of the electromag-netic spectrum. The infrared absorp-tion spectrum of a molecule is highlycharacteristic of it and the spectrumcan therefore be used for molecularidentiÜcation. Glass is opaque to in-frared radiation of wavelengthgreater than 2 micrometres andother materials, such as germanium,quartz, and polyethylene, have to beused to make lenses and prisms. Pho-tographic Ülm can be made sensitiveto infrared up to about 1.2 µm.

infrared spectroscopy (IR spec-troscopy) A technique for chemicalanalysis and the determination ofstructure. It is based on the princi-ples that molecular vibrations occurin the infrared region of the electro-magnetic spectrum and functionalgroups have characteristic absorption

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frequencies. The frequencies of mostinterest range from 2.5 to 16 µm;however, in IR spectroscopy it iscommon to use the reciprocal of thewavelength, and thus this range be-comes 4000–625 cm–1. Examples oftypical vibrations are centred on2900 cm–1 for C–H stretching in alka-nes, 1600 cm–1 for N–H stretching inamino groups, and 2200 cm–1 forC≡C stretching in alkynes. In an IRspectrometer there is a source of IRlight, covering the whole frequencyrange of the instrument, which issplit into two beams of equal inten-sity. One beam is passed through thesample and the other is used as a ref-erence against which the Ürst is thencompared. The spectrum is usuallyobtained as a chart showing absorp-tion peaks, plotted against wave-length or frequency. The sample canbe a gas, liquid, or solid. See alsofourier-transform infrared.

Ingold, Sir Christopher Kelk(1893–1970) British organic chemistwho worked chieÛy in London. Hismain work was on reaction mecha-nisms and, particularly, on *elec-tronic effects in physical organicchemistry.

inhibition A reduction in the rateof a catalysed reaction by substancescalled inhibitors. Inhibitors may workby poisoning catalysts for the reac-tion or by removing free radicals in achain reaction. Enzyme inhibition af-fects biochemical reactions, in whichthe catalysts are *enzymes. Competi-tive inhibition occurs when the in-hibitor molecules resemble thesubstrate molecules and bind to the*active site of the enzyme, so pre-venting normal enzymatic activ-ity. Competitive inhibition can be reversed by increasing the con-centration of the substrate. In non-competitive inhibition the inhibitorbinds to a part of the enzyme or *en-

zyme–substrate complex other thanthe active site, known as an allostericsite. This deforms the active site sothat the enzyme cannot catalyse thereaction. Noncompetitive inhibitioncannot be reversed by increasing theconcentration of the substrate. Thetoxic effects of many substances areproduced in this way. Inhibition byreaction products (feedback inhibi-tion) is important in the control ofenzyme activity. See also allostericenzyme.

inner Describing a chemical com-pound formed by reaction of onepart of a molecule with another partof the same molecule. Thus, a lactamis an inner amide; a lactone is aninner ester.

inner-sphere mechanism Seeelectron-transfer reaction.

inner transition series See tran-sition elements.

inorganic chemistry The branchof chemistry concerned with com-pounds of elements other thancarbon. Certain simple carboncompounds, such as CO, CO2, CS2,and carbonates and cyanides, areusually treated in inorganic chem-istry.

insecticide See pesticide.

insertion reaction A type ofchemical reaction in which an atomor group is inserted between twoatoms in a molecule. A common ex-ample is the insertion of carbene:

R3C–X + CH2: → R3CCH2XThe opposite of an insertion reactionis an extrusion reaction.

insulin A protein hormone, se-creted by the β cells of the islets ofLangerhans in the pancreas, that pro-motes the uptake of glucose by bodycells, particularly in the liver andmuscles, and thereby controls its

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concentration in the blood. Insulinwas the Ürst protein whose amino-acid sequence was fully determined(in 1955). Underproduction of insulinresults in the accumulation of largeamounts of glucose in the blood andits subsequent excretion in the urine.This condition, known as diabetesmellitus, can be treated successfullyby insulin injections.

intensive variable A quantity in amacroscopic system that has a welldeÜned value at every point insidethe system and that remains (nearly)constant when the size of the systemis increased. Examples of intensivevariables are the pressure, tempera-ture, density, speciÜc heat capacity atconstant volume, and viscosity. Anintensive variable results when any*extensive variable is divided by anarbitrary extensive variable such asthe volume. A macroscopic systemcan be described by one extensivevariable and a set of intensive vari-ables.

intercalation cell A type of sec-ondary cell in which layered elec-trodes, usually made of metal oxidesor graphite, store positive ions be-tween the crystal layers of an elec-trode. In one type, lithium ions forman intercalation compound with agraphite electrode when the cell ischarged. During discharge, the ionsmove through an electrolyte to theother electrode, made of manganeseoxide, where they are more tightlybound. When the cell is beingcharged, the ions move back to theirpositions in the graphite. This back-wards and forwards motion of theions has led to the name rocking-chair cell for this type of system. Suchcells have the advantage that onlyminor physical changes occur to theelectrodes during the charging anddischarging processes and the elec-trolyte is not decomposed but simply

serves as a conductor of ions. Conse-quently, such cells can be rechargedmany more times than, say, a lead-acid accumulator, which eventuallysuffers from degeneration of the elec-trodes. Lithium cells, based on thisprinciple, have been used in portableelectronic equipment, such as cam-corders. They have also been consid-ered for use in electric vehicles.

intercalation compound A typeof compound in which atoms, ions,or molecules are trapped betweenlayers in a crystal lattice. There is noformal chemical bonding betweenthe host crystal and the trapped mol-ecules (see also clathrate). Such com-pounds are formed by *lamellarsolids and are often nonstoichiomet-ric; examples are graphitic oxide(graphite–oxygen) and the mineral*muscovite.

interhalogen A chemical com-pound formed between two *halo-gens. Interhalogens are highlyreactive and volatile, made by directcombination of the elements. Theyinclude compounds with two atoms(C1F, IBr, etc.), four atoms (C1F3, IF3,etc.), six atoms (BrF5, IF5, etc.) and IF7with eight atoms.

intermediate bond See chemicalbond.

intermediate coupling See j-jcoupling.

intermetallic compound A com-pound consisting of two or moremetallic elements present in deÜniteproportions in an alloy.

intermolecular forces Weakforces occurring between molecules.See van der waals’ force; hydrogenbond.

internal conversion A process inwhich an excited atomic nucleus de-cays to the *ground state and the en-ergy released is transferred by

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electromagnetic coupling to one ofthe bound electrons of that atomrather than being released as a pho-ton. The coupling is usually with anelectron in the K-, L-, or M-shell ofthe atom, and this conversion elec-tron is ejected from the atom with akinetic energy equal to the differencebetween the nuclear transition en-ergy and the binding energy of theelectron. The resulting ion is itself inan excited state and usually subse-quently emits an Auger electron oran X-ray photon.

internal energy Symbol U. Thetotal of the kinetic energies of theatoms and molecules of which a sys-tem consists and the potential ener-gies associated with their mutualinteractions. It does not include thekinetic and potential energies of thesystem as a whole nor their nuclearenergies or other intra-atomic ener-gies. The value of the absolute inter-nal energy of a system in anyparticular state cannot be measured;the signiÜcant quantity is the changein internal energy, ∆U. For a closedsystem (i.e. one that is not being re-plenished from outside its bound-aries) the change in internal energyis equal to the heat absorbed by thesystem (Q) from its surroundings, lessthe work done (W) by the system onits surroundings, i.e. ∆U = Q – W. Seealso energy; thermodynamics.

internal resistance The resistancewithin a source of electric current,such as a cell or generator. It can becalculated as the difference betweenthe e.m.f. (E) and the potential differ-ence (V) between the terminals di-vided by the current being supplied(I), i.e. r = (E – V)/I, where r is the in-ternal resistance.

interstellar molecules See astro-chemistry.

interstellar molecules See astro-chemistry.

interstitial See crystal defect.

interstitial compound A com-pound in which ions or atoms of anonmetal occupy interstitial posi-tions in a metal lattice. Such com-pounds often have metallicproperties. Examples are found inthe *carbides, *borides, and *sili-cides.

intersystem crossing A process inwhich a singlet excited electronicstate makes a transition to a tripletexcited state at the point where thepotential energy curves for the ex-cited singlet and triplet states cross.This transition is forbidden in the ab-sence of *spin–orbit coupling but oc-curs in the presence of spin–orbitcoupling. A triplet formed in thisway is frequently in an excited vibra-tional state. This excited triplet statecan reach its lowest vibrational stateby collisions with other molecules.The transition from this state to thesinglet state is forbidden in the ab-sence of spin–orbit coupling but al-lowed when there is spin–orbitcoupling. This gives rise to the slowemision of electromagnetic radiationknown as *phosphorescence.

intrinsic factor See vitamin b com-plex.

Invar A tradename for an alloy ofiron (63.8%), nickel (36%), and carbon(0.2%) that has a very low expansivityover a a restricted temperaturerange. It is used in watches and otherinstruments to reduce their sensitiv-ity to changes in temperature.

inverse Compton effect The gainin energy of low-energy photonswhen they are scattered by free elec-trons of much higher energy. As aconsequence, the electrons lose en-ergy. See also compton effect.

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inversion A chemical reaction in-volving a change from one opticallyactive conÜguration to the oppositeconÜguration. The Walden inversionis an example. See nucleophilic sub-stitution.

iodic acid Any of various oxoacidsof iodine, such as iodic(V) acid andiodic(VII) acid. When used without anoxidation state speciÜed, the termusually refers to iodic(V) acid (HIO3).

iodic(V) acid A colourless or verypale yellow solid, HIO3; r.d. 4.63; de-composes at 110°C. It is soluble inwater but insoluble in pure ethanoland other organic solvents. The com-pound is obtained by oxidizing io-dine with concentrated nitric acid,hydrogen peroxide, or ozone. It is astrong acid and a powerful oxidizingagent.

iodic(VII) acid (periodic acid) A hy-groscopic white solid, H5IO6, whichdecomposes at 140°C and is verysoluble in water, ethanol, andethoxyethane. Iodic(VII) acid may beprepared by electrolytic oxidation ofconcentrated solutions of iodic(V)acid at low temperatures. It is a weakacid but a strong oxidizing agent.

iodide See halide.

iodine Symbol I. A dark violet nonmetallic element belonging togroup 17 of the periodic table (seehalogens); a.n. 53; r.a.m. 126.9045;r.d. 4.94; m.p. 113.5°C; b.p. 184.35°C.The element is insoluble in water butsoluble in ethanol and other organicsolvents. When heated it gives a vio-let vapour that sublimes. Iodine isrequired as a trace element (see es-sential element) by living organ-isms; in animals it is concentrated inthe thyroid gland as a constituent ofthyroid hormones. The element ispresent in sea water and was for-merly extracted from seaweed. It isnow obtained from oil-well brines

(displacement by chlorine). There isone stable isotope, iodine–127, andfourteen radioactive isotopes. It isused in medicine as a mild antiseptic(dissolved in ethanol as tincture of io-dine), and in the manufacture of io-dine compounds. Chemically, it isless reactive than the other halogensand the most electropositive (metal-lic) halogen. In solution it can bedetermined by titration using thio-sulphate solution:

I2 + 252O32– → 2I– + SuO6

2–.The molecule forms an intense bluecomplex with starch, which is conse-quently used as an indicator. It wasdiscovered in 1812 by Courtois.


iodine(V) oxide (iodine pentoxide)A white solid, I2O5; r.d. 4.799; decom-poses at 300–350°C. It dissolves inwater to give iodic(V) acid and alsoacts as an oxidizing agent.

iodine value A measure of theamount of unsaturation in a fat orvegetable oil (i.e. the number of dou-ble bonds). It is obtained by Ündingthe percentage of iodine by weightabsorbed by the sample in a giventime under standard conditions.

iodoethane (ethyl iodide) A colour-less liquid *haloalkane, C2H5I; r.d.1.9; m.p. –108°C; b.p. 72°C. It is madeby reacting ethanol with a mixture ofiodine and red phosphorus.

iodoform See triiodomethane.

iodoform test See haloform reac-tion.

iodomethane (methyl iodide) Acolourless liquid haloalkane, CH3I;r.d. 2.28; m.p. –66.45°C; b.p. 42.4°C. Itcan be made by reacting methanolwith a mixture of iodine and redphosphorus.

ion An atom or group of atoms that

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has either lost one or more electrons,making it positively charged (acation), or gained one or more elec-trons, making it negatively charged(an anion). See also ionization.

ion association The electrostaticattraction between ions in solutionsof electrolytes that causes them to as-sociate in pairs. As a result, completedissociation does not take place andexperimentally found electrical con-ductivities are lower than those pre-dicted by the *Debye–Hückel theory.In 1926, Neils Bjerrum (1879–1958)proposed improving the Debye–Hückel theory by taking ion associa-tion into account. He found that thesigniÜcance of ion association in-creases as the relative permittivity of the solvent decreases.

ion exchange The exchange ofions of the same charge between asolution (usually aqueous) and a solidin contact with it. The process occurswidely in nature, especially in theabsorption and retention of water-soluble fertilizers by soil. For exam-ple, if a potassium salt is dissolved inwater and applied to soil, potassiumions are absorbed by the soil andsodium and calcium ions are releasedfrom it.

The soil, in this case, is acting as an ion exchanger. Synthetic ion-exchange resins consist of variouscopolymers having a cross-linkedthree-dimensional structure to whichionic groups have been attached. Ananionic resin has negative ions builtinto its structure and therefore ex-changes positive ions. A cationic resinhas positive ions built in and ex-changes negative ions. Ion-exchangeresins, which are used in sugar reÜn-ing to remove salts, are synthetic or-ganic polymers containing sidegroups that can be ionized. In anionexchange, the side groups are ion-ized basic groups, such as –NH3

+ to

which anions X– are attached. The ex-change reaction is one in which dif-ferent anions in the solution displacethe X– from the solid. Similarly,cation exchange occurs with resinsthat have ionized acidic side groupssuch as –COO– or –SO2O–, with posi-tive ions M+ attached.

Ion exchange also occurs with inor-ganic polymers such as *zeolites, inwhich positive ions are held at sitesin the silicate lattice. These are usedfor water-softening, in which Ca2+

ions in solution displace Na+ ions inthe zeolite. The zeolite can be regen-erated with sodium chloride solution.Ion-exchange membranes are used asseparators in electrolytic cells to re-move salts from sea water and inproducing deionized water. Ion-exchange resins are also used as thestationary phase in ion-exchangechromatography.

ion-exchange chromatographySee ion exchange.

ionic bond See chemical bond.

ionic crystal See crystal.

ionic product The product of theconcentrations of ions present in agiven solution taking the stoichiome-try into account. For a sodium chlo-ride solution the ionic product is[Na+][Cl–]; for a calcium chloride solu-tion it is [Ca2+][Cl–]2. In pure water,there is an equilibrium with a smallamount of self-ionization:

H2O ˆ H+ + OH–

The equilibrium constant of this dis-sociation is given by

KW = [H+][OH–]since the concentration [H2O] can be taken as constant. KW is referredto as the ionic product of water. Ithas the value 10–14 mol2 dm–6 at25°C. In pure water (i.e. no addedacid or added alkali) [H+] = [OH–] =10–7 mol dm–3. In this type of self-

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ionization of a solvent the ionic prod-uct is also called the autoprotolysisconstant. See also solubility product;ph scale.

ionic radius A value assigned tothe radius of an ion in a crystallinesolid, based on the assumption thatthe ions are spherical with a deÜnitesize. X-ray diffraction can be used tomeasure the internuclear distance incrystalline solids. For example, inNaF the Na – F distance is 0.231 nm,and this is assumed to be the sum ofthe Na+ and F– radii. By making cer-tain assumptions about the shieldingeffect that the inner electrons haveon the outer electrons, it is possibleto assign individual values to theionic radii – Na+ 0.096 nm; F– 0.135nm. In general, negative ions havelarger ionic radii than positive ions.The larger the negative charge, thelarger the ion; the larger the positivecharge, the smaller the ion. See alsohydrodynamic radius.

ionic strength Symbol I. A func-tion expressing the effect of thecharge of the ions in a solution,equal to the sum of the molality ofeach type of ion present multipliedby the square of its charge. I = Σmizi

2.

ionization The process of produc-ing *ions. Certain molecules (seeelectrolyte) ionize in solution; forexample, *acids ionize when dis-solved in water (see also solvation):

HCl → H+ + Cl–

Electron transfer also causes ioniza-tion in certain reactions; for exam-ple, sodium and chlorine react by thetransfer of a valence electron fromthe sodium atom to the chlorineatom to form the ions that constitutea sodium chloride crystal:

Na + Cl → Na+Cl–

Ions may also be formed when anatom or molecule loses one or moreelectrons as a result of energy gained

in a collision with another particle ora quantum of radiation (see pho-toionization). This may occur as aresult of the impact of *ionizing radi-ation or of thermal ionization andthe reaction takes the form

A → A+ + eAlternatively, ions can be formed byelectron capture, i.e.

A + e → A–

ionization energy (IE) See ioniza-tion potential.

ionization gauge A vacuum gaugeconsisting of a three-electrode systeminserted into the container in whichthe pressure is to be measured. Elec-trons from the cathode are attractedto the grid, which is positively bi-ased. Some pass through the grid butdo not reach the anode, as it is main-tained at a negative potential. Someof these electrons do, however, col-lide with gas molecules, ionizingthem and converting them to posi-tive ions. These ions are attracted tothe anode; the resulting anode cur-rent can be used as a measure of thenumber of gas molecules present.Pressure as low as 10–6 pascal can bemeasured in this way.

ionization potential (IP) Symbol I.The minimum energy required to re-move an electron from a speciÜedatom or molecule to such a distancethat there is no electrostatic interac-tion between ion and electron. Origi-nally deÜned as the minimumpotential through which an electronwould have to fall to ionize an atom,the ionization potential was meas-ured in volts. It is now, however,deÜned as the energy to effect an ion-ization and is conveniently measuredin electronvolts (although this is notan SI unit) or joules per mole. Thesynonymous term ionization energy(IE) is often used.

The energy to remove the least

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strongly bound electron is the Ürstionization potential. Second, third,and higher ionization potentials canalso be measured, although there issome ambiguity in terminology.Thus, in chemistry the second ioniza-tion potential is often taken to be theminimum energy required to removean electron from the singly chargedion; the second IP of lithium wouldbe the energy for the process

Li+ → Li2+ + eIn physics, the second ionization po-tential is the energy required to re-move an electron from the next tohighest energy level in the neutralatom or molecule; e.g.

Li → Li+ + e,where Li+ is an excited singlycharged ion produced by removingan electron from the K-shell.A• A table of values from NIST

ionizing radiation Radiation ofsufÜciently high energy to cause*ionization in the medium throughwhich it passes. It may consist of astream of high-energy particles (e.g.electrons, protons, alpha-particles) orshort-wavelength electromagnetic ra-diation (ultraviolet, X-rays, gamma-rays). This type of radiation can causeextensive damage to the molecularstructure of a substance either as aresult of the direct transfer of energyto its atoms or molecules or as a re-sult of the secondary electrons re-leased by ionization. In biologicaltissue the effect of ionizing radiationcan be very serious, usually as a con-sequence of the ejection of an elec-tron from a water molecule and theoxidizing or reducing effects of theresulting highly reactive species:

2H2O → e– + H2O + H2O*

H2O* → .OH + .H

H2O+ + H2O → .OH + H3O+

where the dot before a radical indi-cates an unpaired electron and an *denotes an excited species.

ion-microprobe analysis A tech-nique for analysing the surface com-position of solids. The sample isbombarded with a narrow beam (assmall as 2 µm diameter) of high-energy ions. Ions ejected from thesurface by sputtering are detected bymass spectrometry. The technique al-lows quantitative analysis of bothchemical and isotopic compositionfor concentrations as low as a fewparts per million.

ion-mobility spectrometry (IMS)A technique for detecting low con-centrations of speciÜc compounds,based on the rate at which their ionsmigrate through an electric Üeld. Theinstrument operates in the gas phaseat atmospheric pressure. The samplevapour enters an ionizing region,where ions can be produced by a va-riety of methods. In compact instru-ments the source is usually a smallamount of radioactive material. Theions are allowed in pulses into a drifttube, where they move to a detectorunder the inÛuence of a homoge-neous electric Üeld. The rate of move-ment depends on the way the ionsinteract with neutral molecules inthe tube and this depends on theion’s size and shape. The spectrum isa plot of detector signal against time,and is characteristic of the samplebeing ionized. Ion-mobility spectrom-eters are compact, sensitive, and fast-acting. They are widely used inscreening for drugs and explosives atairports, border crossings, etc. oftenthe technique is to wipe a swab overluggage and place it in the instru-ment. More sophisticated instru-ments combine IMS with gaschromatography or mass spectrome-try. The technique is sometimes

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referred to as gas-phase electro-phoresis.

ionophore A relatively small hy-drophobic molecule that facilitatesthe transport of ions across lipidmembranes. Most ionophores areproduced by microorganisms. Thereare two types of ionophore: channelformers, which combine to form achannel in the membrane throughwhich ions can Ûow; and mobile ioncarriers, which transport ions acrossa membrane by forming a complexwith the ion.

ion pair A pair of oppositelycharged ions produced as a result ofa single ionization; e.g.

HCl → H+ + Cl–.Sometimes a positive ion and an elec-tron are referred to as an ion pair, asin

A → A+ + e–.

ion pump A type of *vacuumpump that can reduce the pressurein a container to about 1 nanopascalby passing a beam of electronsthrough the residual gas. The gas isionized and the positive ions formedare attracted to a cathode within the container where they remaintrapped. The pump is only useful atvery low pressures, i.e. below about 1 micropascal. The pump has a lim-ited capacity because the absorbedions eventually saturate the surfaceof the cathode. A more effectivepump can be made by simultane-ously producing a Ülm of metal byion impact (sputtering), so that fresh surface is continuously pro-duced. The device is then known as a sputter-ion pump.

IP See ionization potential.

IR See infrared radiation.

iridium Symbol Ir. A silvery metal-lic *transition element (see also plat-

inum metals); a.n. 77; r.a.m. 192.20;r.d. 22.42; m.p. 2410°C; b.p. 4130°C.It occurs with platinum and ismainly used in alloys with platinumand osmium. The element forms arange of iridium(III) and iridium(IV)complexes. It was discovered in 1804by Smithson Tennant (1761–1815).


iron Symbol Fe. A silvery malleableand ductile metallic *transition el-ement; a.n. 26; r.a.m. 55.847; r.d.7.87; m.p. 1535°C; b.p. 2750°C. Themain sources are the ores *haematite(Fe2O3), *magnetite (Fe3O4), limonite(FeO(OH)nH2O), ilmenite (FeTiO3),siderite (FeCO3), and pyrite (FeS2).The metal is smelted in a *blast fur-nace to give impure *pig iron, whichis further processed to give *castiron, *wrought iron, and varioustypes of *steel. The pure element hasthree crystal forms: alpha-iron, stablebelow 906°C with a body-centred-cubic structure; gamma-iron, stablebetween 906°C and 1403°C with anonmagnetic face-centred-cubicstructure; and delta-iron, which isthe body-centred-cubic form above1403°C. Alpha-iron is ferromagneticup to its Curie point (768°C). The el-ement has nine isotopes (mass num-bers 52–60), and is the fourth mostabundant in the earth’s crust. It is re-quired as a trace element (see essen-tial element) by living organisms.Iron is quite reactive, being oxidizedby moist air, displacing hydrogenfrom dilute acids, and combiningwith nonmetallic elements. It formsionic salts and numerous complexeswith the metal in the +2 or +3 oxida-tion states. Iron(VI) also exists in theferrate ion FeO4

2–, and the elementalso forms complexes in which its ox-idation number is zero (e.g. Fe(CO)5).


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iron(II) chloride A green-yellowdeliquescent compound, FeCl2;hexagonal; r.d. 3.16; m.p. 670°C. Italso exists in hydrated forms:FeCl2.2H2O (green monoclinic; r.d.2.36) and FeCl2.4H2O (blue-greenmonoclinic deliquescent; r.d. 1.93).Anhydrous iron(II) chloride can bemade by passing a stream of dry hy-drogen chloride over the heatedmetal; the hydrated forms can bemade using dilute hydrochloric acidor by recrystallizing with water. It isconverted into iron(III) chloride bythe action of chlorine.

iron(III) chloride A black-brownsolid, FeCl3; hexagonal; r.d. 2.9; m.p.306°C; decomposes at 315°C. It alsoexists as the hexahydrate FeCl3.6H2O,a brown-yellow deliquescent crys-talline substance (m.p. 37°C; b.p.280–285°C). Iron(III) chloride is pre-pared by passing dry chlorine overiron wire or steel wool. The reactionproceeds with incandescence whenstarted and iron(III) chloride sublimesas almost black iridescent scales. Thecompound is rapidly hydrolysed inmoist air. In solution it is partlyhydrolysed; hydrolysis can be sup-pressed by the addition of hydro-chloric acid. The compound dissolvesin many organic solvents, formingsolutions of low electrical conductiv-ity: in ethanol, ethoxyethane, andpyridine the molecular weight cor-responds to FeCl3 but is higher inother solvents corresponding toFe2Cl6. The vapour is also dimerized.In many ways the compound resem-bles aluminium chloride, which itmay replace in Friedel–Crafts reac-tions.

iron(II) oxide A black solid, FeO;cubic; r.d. 5.7; m.p. 1420°C. It can beobtained by heating iron(II) oxalate;the carbon monoxide formed pro-duces a reducing atmosphere thuspreventing oxidation to iron(III)

oxide. The compound has the sodiumchloride structure, indicating itsionic nature, but the crystal lattice isdeÜcient in iron(II) ions and it is non-stoichiometric. Iron(II) oxide dis-solves readily in dilute acids.

iron(III) oxide A red-brown toblack insoluble solid, Fe2O3; trigonal;r.d. 5.24; m.p. 1565°C. There is also ahydrated form, Fe2O3.xH2O, which isa red-brown powder; r.d. 2.44–3.60.(See rusting.)

Iron(III) oxide occurs naturally as*haematite and can be prepared byheating iron(III) hydroxide or iron(II)sulphate. It is readily reduced onheating in a stream of carbonmonoxide or hydrogen.

iron pyrites See pyrite.

iron(II) sulphate An off-whitesolid, FeSO4.H2O; monoclinic; r.d.2.970. There is also a heptahydrate,FeSO4.7H2O; blue-green monoclinic;r.d. 1.898; m.p. 64°C. The heptahy-drate is the best known iron(II) saltand is sometimes called green vitriolor copperas. It is obtained by the ac-tion of dilute sulphuric acid on ironin a reducing atmosphere. The anhy-drous compound is very hygroscopic.It decomposes at red heat to giveiron(III) oxide, sulphur trioxide, andsulphur dioxide. A solution of iron(II)sulphate is gradually oxidized on ex-posure to air, a basic iron(III) sul-phate being deposited.

iron(III) sulphate A yellow hygro-scopic compound, Fe2(SO4)3; rhom-bic; r.d. 3.097; decomposes above480°C. It is obtained by heating anaqueous acidiÜed solution of iron(II)sulphate with hydrogen peroxide:

2FeSO4 + H2SO4 + H2O2 →Fe2(SO4)3 + 2H2O

On crystallizing, the hydrateFe2(SO4)3.9H2O is formed. The acidsulphate Fe2(SO4)3.H2SO4.8H2O is de-

293 iron(III) sulphate

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posited from solutions containing asufÜcient excess of sulphuric acid.

irreducible representation Arepresentation of a symmetry opera-tion of a group, which cannot be ex-pressed in terms of a representationof lower dimension. When the repre-sentation of the group is in matrixform (i.e. a set of matrices that multi-ply in the same way as the elementsof the group), the matrix represen-tation cannot be put into block-diagonal form by constructing a lin-ear combination of the basis func-tions. The importance of irreduciblerepresentations in *quantum me-chanics is that the energy levels ofthe system are labelled by the irre-ducible representations of the sym-metry group of the system, thusenabling *selection rules to be de-duced. In contrast to an irreduciblerepresentation, a reducible represen-tation can be expressed in terms of arepresentation of lower dimension,with a reducible matrix representa-tion that can be put into block diago-nal form by constructing a linearcombination of the basis functions.

irreversibility The property of asystem that precludes a change tothe system from being a *reversibleprocess. The paradox that althoughthe equations describing the bodiesin a system, such as Newton’s laws ofmotion, Maxwell’s equation, orSchrödinger’s equation are invariantunder time reversal, events involvingsystems made up from large num-bers of these bodies are not re-versible. The process of scramblingan egg is an example. The resolutionof this paradox requires the conceptof *entropy using *statistical me-chanics. Irreversibility occurs in thetransition from an ordered arrange-ment to a disordered arrangement,which is a natural trend, since

changes in a closed system occur inthe direction of increasing entropy.

irreversible process See irre-versibility; reversible process.

irreversible reaction See chemi-cal reaction.

IR spectroscopy See infraredspectroscopy.

isentropic process Any processthat takes place without a change of*entropy. The quantity of heat trans-ferred, δQ, in a reversible process isproportional to the change in en-tropy, δS, i.e. δQ = TδS, where T is thethermodynamic temperature. There-fore, a reversible *adiabatic processis isentropic, i.e. when δQ equalszero, δS also equals zero.

Ising model A model for magneticsystems in which atomic *spins haveto be aligned either parallel or an-tiparallel to a given direction. TheIsing model was introduced, andsolved in the case of one dimension,by E. Ising in 1925. Ising found thatin one dimension, in the absence ofan external magnetic Üeld, there isno spontaneous magnetization at anytemperature above absolute zero.The study of *phase transitions inthe Ising model in dimensionsgreater than one has been very im-portant to the general understandingof phase transitions. In two dimen-sions, the Ising model was Ürst solvedexactly by L. Onsager in 1944. Inthree dimensions, approximationtechniques, frequently involvingrenormalization have to be used.

ISIS/Draw A commonly used chem-ical drawing program for 2D and 3Dstructures, copyright of MDL Infor-mation Systems, Inc. The programhas certain additional features in-cluding calculation of molecularweight, calculation of percentages of

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elements present, IUPAC name gen-eration, and viewing in RasMol.

A• A limited version of ISIS/Draw at the

Elsevier MDL website (free registrationrequired)

iso- PreÜx denoting that a com-pound is an *isomer, e.g. isopentane(CH3CH(CH3)C2H5, 2-methylbutane) isan isomer of pentane.

isobar 1. A curve on a graph indi-cating readings taken at constantpressure. 2. One of two or more nu-clides that have the same number ofnucleons but different *atomic num-bers. Radium–88, actinium–89, andthorium–90 are isobars as each has a*nucleon number of 228.

isocyanate See cyanic acid.

isocyanic acid See cyanic acid.

isocyanide See isonitrile.

isocyanide test A test for primaryamines by reaction with an alcoholicsolution of potassium hydroxide andtrichloromethane.

RNH2 + 3KOH + CHCl3 → RNC +3KCl + 3H2O

The isocyanide RNC is recognized byits unpleasant smell. This reaction ofprimary amines is called the carby-lamine reaction.

isoelectric point The point atwhich a substance (such as a colloidor protein) has zero net electriccharge. Usually such substances arepositively or negatively charged, de-pending on whether hydrogen ionsor hydroxyl ions are predominantlyabsorbed. At the isoelectric point thenet charge on the substance is zero,as positive and negative ions are ab-sorbed equally. The substance has itsminimum conductivity at its isoelec-tric point and therefore coagulatesbest at this point. In the case of hy-drophilic substances, in which the

surrounding water prevents coagula-tion, the isoelectric point is at theminimum of stability. The isoelectricpoint is characterized by the value ofthe pH at that point. Above the iso-electric pH level the substance acts asa base and below this level it acts asan acid. For example, at the isoelec-tric point the pH of gelatin is 4.7. Pro-teins precipitate most readily at theirisoelectric points; this property canbe utilized to separate mixtures ofproteins or amino acids.

isoelectronic Denoting differentmolecules that have the same num-ber of electrons. For example N2 andCO are isoelectronic. The energylevel diagrams of isoelectronic mol-ecules are therefore similar.

isoenzyme See isozyme.

isoindole See indole.

isoleptic complex A metal com-plex in which all the ligands are thesame.

isoleucine See amino acid.

isomerism The existence of chemi-cal compounds (isomers) that havethe same molecular formulae but dif-ferent molecular structures or differ-ent arrangements of atoms in space.In structural (or constitutional iso-merism) isomerism the moleculeshave different molecular structures:i.e. they may be different types ofcompound or they may simply differin the position of the functionalgroup in the molecule. Structural iso-mers generally have different physi-cal and chemical properties. Instereoisomerism, the isomers havethe same formula and functionalgroups, but differ in the arrangementof groups in space. Optical isomerismis one form of this (see optical activ-ity). Another type is cis–trans iso-merism (formerly geometricalisomerism), in which the isomers

295 isomerism

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have different positions of groupswith respect to a double bond or ringor central atom (see illustration).Octahedral complexes can displaycis-trans isomerism if they have for-mulae of the type MX2Y4. Octahedral

complexes with formulae of the typeMX3Y3 can display a different type ofisomerism. If the three X ligands arein a plane that includes the metalatom and the three Y ligands are in adifferent plane at right angles, then

isomerism 296

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1-chloropropane

H C C C CI

H

H

H

H

H

H

H C C C H

H

H

CI

H

H

H

2-chloropropane

structural isomers in which the functional group has different positions

methoxymethane ethanol

structural isomers in which the functional groups are different

trans-but-2-ene cis-but-2-ene

cis–trans isomers in which the groups are distributed on a double bond

cis–trans isomers in a square-planar complex

H C O C H

H

H

H

H

H C C OH

H

H

H

H

H CH3

C C

CH3 H

CH3

C C

H H

CH3

keto form enol form

C C

O

C C

OH

CI I

CII

Pt

I CI

CII

Pt

keto–enol tautomerism

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the structure is a mer-isomer (merid-ional). If the three X ligands are allon one face of the octahedron andthe three Y ligands are on an oppo-site face, then it is a fac-isomer (fa-cial). See also ambidentate; e–zconvention.

isomers See isomerism.

isometric 1. (in crystallography)Denoting a system in which the axesare perpendicular to each other, as incubic crystals. 2. Denoting a line ona graph illustrating the way in whichtemperature and pressure are inter-related at constant volume.

isomorphism The existence of twoor more substances (isomorphs) thathave the same crystal structure, sothat they are able to form *solid solu-tions.

isonitrile (isocyanide; carbylamine)An organic compound containing thegroup –NC, in which the bonding isto the nitrogen atom.

297 isopolymorphism

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trans-isomer cis-isomer

X

Y

YM

Y

Y

X

mer-isomerfac-isomer

Y

X

Y

XM

Y

Y

X

X

Y

YM

Y

X

X

X

Y

YM

X

Y

Isomerism


iso-octane See octane; octanenumber.

isopleth A vertical line in a liq-uid–vapour phase diagram consistingof a line of constant composition ofthe whole system as the pressure ischanged. The word isopleth comesfrom the Greek for ‘equal abun-dance’. See also tie line.

isopoly compound See clustercompound.

isopolymorphism The property ofa substance with more than one crys-talline structure that is isomorphouswith the crystalline structures of an-other substance (see polymorphism).For example, antimony(III) oxide,Sb2O3, has both rhombic and octahe-dral structures and these are isomor-phous with the similar structures ofarsenic(III) oxide, As2O3; both oxidestherefore exhibit isopolymorphism.


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isoprene A colourless liquid diene,CH2:C(CH3)CH:CH2. The systematicname is 2-methylbuta-1,3-diene. Theisoprene structure is the fundamen-tal structural unit in *terpenes andnatural *rubber. The compound itselfis used in making synthetic rubbers.

isoquinoline See quinoline.

isotactic polymer See polymer.

isotherm 1. A line on a map orchart joining points or places ofequal temperature. 2. A curve on agraph representing readings taken atconstant temperature (e.g. the rela-tionship between the pressure andvolume of a gas at constant tempera-ture).

isothermal process Any processthat takes place at constant tempera-ture. In such a process heat is, if nec-essary, supplied or removed from thesystem at just the right rate to main-tain constant temperature. Compareadiabatic process.

isotonic Describing solutions thathave the same osmotic pressure.

isotope One of two or more atomsof the same element that have thesame number of protons in their nu-cleus but different numbers of neu-trons. Hydrogen (1 proton, noneutrons), deuterium (1 proton, 1neutron), and tritium (1 proton, 2neutrons) are isotopes of hydrogen.Most elements in nature consist of amixture of isotopes. See isotope sepa-ration.

isotope separation The separa-tion of the *isotopes of an elementfrom each other on the basis of slightdifferences in their physical proper-ties. For laboratory quantities themost suitable device is often themass spectrometer. On a larger scalethe methods used include gaseousdiffusion (widely used for separatingisotopes of uranium in the form of

the gas uranium hexaÛuoride), distil-lation (formerly used to produceheavy water), electrolysis (requiringcheap electrical power), thermal dif-fusion (formerly used to separate ura-nium isotopes, but now considereduneconomic) and centrifuging. Vari-ous laser methods (involving theexcitation of one isotope and itssubsequent separation by electro-magnetic means) have also beenemployed.

isotopic isomers See isotopomers.

isotopic number (neutron excess)The difference between the numberof neutrons in an isotope and thenumber of protons.

isotopologues Substances that dif-fer only in isotopic composition, e.g.CH3OH, CH2DOH, CHD2OH, CD3OH,CD3OD.

isotopomers (isotopic isomers)Molecules that have the same num-bers of each type of isotopic atombut in different arrangements in themolecule. For example, CH2DOH andCH3OD are isotopomers.

isotropic Denoting a mediumwhose physical properties are inde-pendent of direction. Compareanisotropic.

isozyme (isoenzyme) One of severalforms of an enzyme in an individualor population that catalyse the samereaction but differ from each otherin such properties as substrateafÜnity and maximum rates of en-zyme–substrate reaction (seemichaelis–menten curve).

itaconic acid A product of the fer-mentation of the Ülamentous fungusAspergillus niger. Itaconic acid is usedcommercially in the production ofadhesives and paints.

IUPAC International Union of Pureand Applied Chemistry. An interna-

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tional nongovernmental bodyformed in 1919 to foster worldwidecommunications in chemical science,both academic and industrial. IUPACis the international authority deÜn-ing recommended terminology,atomic weights, isotopic abundances,

and other data. The IUPAC rules forsystematic chemical nomenclatureare widely adopted. IUPAC also intro-duced the *InChI line notation forcompounds.A• The IUPAC home page

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J

Jablonski diagram A diagramthat represents the electronic energylevels (and their relative positions) ofa molecule. A Jablonski diagram en-ables radiative transitions betweenenergy levels in molecules, as well assuch phenomena as internal conver-sion, to be shown. The vertical axismeasures energy, with the electronicenergy levels in their lowest vibra-tional states being short horizontallines. The vibrational energy levels ofa particular electronic state aredrawn as shaded regions above thehorizontal line for that state. In aJablonski diagram the horizontal lo-cation of the electronic state is notrelated to the internuclear distancebetween the atoms in these states.

jacinth See zircon.

jade A hard semiprecious stoneconsisting either of jadeite ornephrite. Jadeite, the most valued ofthe two, is a sodium aluminium py-roxene, NaAlSi2O6. It is prized for itsintense translucent green colour butwhite, green and white, brown, andorange varieties also occur. The onlyimportant source of jadeite is in theMogaung region of upper Burma.Nephrite is one of the amphibolegroup of rock-forming minerals. Itoccurs in a variety of colours, includ-ing green, yellow, white, and black.Sources include the Siberia, Turk-istan, New Zealand, Alaska, China,and W USA.

jadeite See jade.

Jahn–Teller effect If a likely struc-ture of a nonlinear molecule or ionwould have degenerate orbitals (i.e.two molecular orbitals with the same

energy levels) the actual structure ofthe molecule or ion is distorted so asto split the energy levels (‘raise’ thedegeneracy). The effect is observed ininorganic complexes. For example,the ion [Cu(H2O)6]2+ is octahedral andthe six ligands might be expected tooccupy equidistant positions at thecorners of a regular octahedron. Infact, the octahedron is distorted,with four ligands in a square and twoopposite ligands further away. If the‘original’ structure has a centre ofsymmetry, the distorted structuremust also have a centre of symmetry.The effect was predicted theoreticallyby H. A. Jahn (1907–79) and EdwardTeller (1908–2003) in 1937.

jargoon See zircon.

jasper An impure variety of *chal-cedony. It is associated with iron oresand as a result contains iron oxideimpurities that give the mineral itscharacteristic red or reddish-browncolour. Jasper is used as a gemstone.

jet A variety of *coal that can be cutand polished and is used for jew-ellery, ornaments, etc.

jeweller’s rouge Red powderedhaematite, iron(III) oxide, Fe2O3. It isa mild abrasive used in metal clean-ers and polishes.

j-j coupling A type of *coupling oc-curring between electrons in atomsand nucleons in nuclei, in which the energies associated with thespin–orbit interactions are muchhigher than the energies associatedwith electrostatic repulsion. *Multi-plets of many-electron atoms havinga large atomic number are character-


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ized by j-j coupling. The multiplets ofmany atoms and nuclei are interme-diate between j-j coupling and *Rus-sell–Saunders coupling, a state calledintermediate coupling.

Joliot-Curie, Irène (1897–1956)French physicist, daughter of Marieand Pierre *Curie, who was educatedby her mother and her scientist asso-ciates. In 1921 she began work at theRadium Institute, becoming directorin 1946. In 1926 she married FrédéricJoliot (1900–58). They shared the1935 Nobel Prize for chemistry fortheir discovery of artiÜcial radioactiv-ity the previous year.

joliotium See transactinide el-ements.

JMol A commonly used molecularviewing program similar to RasMol.It can be used as an applet in a webpage.

A• Details and a download for JMol from

SourceForge

joule Symbol J. The *SI unit ofwork and energy equal to the workdone when the point of applicationof a force of one newton moves, inthe direction of the force, a distanceof one metre. 1 joule = 107 ergs =0.2388 calorie. It is named afterJames Prescott *Joule.

Joule, James Prescott (1818–89)British physicist. In 1840 he discov-ered the relationship between elec-tric current passing through a wire,its resistance, and the amount ofheat produced. In 1849 he gave an ac-count of the kinetic theory of gases,and a year later announced his best-known Ünding, the mechanicalequivalent of heat. Later, with

William Thomson (Lord *Kelvin), hediscovered the *Joule–Thomson ef-fect.

Joule’s law The *internal energyof a given mass of gas is independentof its volume and pressure, being afunction of temperature alone. Thislaw, which was formulated by JamesPrescott Joule, applies only to *idealgases (for which it provides a deÜni-tion of thermodynamic temperature)as in a real gas intermolecular forceswould cause changes in the internalenergy should a change of volumeoccur. See also joule–thomsoneffect.

Joule–Thomson effect (Joule–Kelvin effect) The change in temper-ature that occurs when a gas expandsthrough a porous plug into a regionof lower pressure. For most real gasesthe temperature falls under these cir-cumstances as the gas has to do inter-nal work in overcoming theintermolecular forces to enable theexpansion to take place. This is a de-viation from *Joule’s law. There isusually also a deviation from *Boyle’slaw, which can cause either a rise ora fall in temperature since any in-crease in the product of pressure andvolume is a measure of externalwork done. At a given pressure, thereis a particular temperature, calledthe inversion temperature of the gas,at which the rise in temperaturefrom the Boyle’s law deviation is bal-anced by the fall from the Joule’s lawdeviation. There is then no tempera-ture change. Above the inversiontemperature the gas is heated by ex-pansion, below it, it is cooled. The ef-fect was discovered by James PrescottJoule working in collaboration withWilliam Thomson (later Lord Kelvin).

301 Joule–Thomson effect

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K

kainite A naturally occurring dou-ble salt of magnesium sulphate andpotassium chloride, MgSO4.KCl.3H2O.

kalinite A mineral form of *alu-minium potassium sulphate(Al2(SO4)3.K2SO4.24H2O).

kaolin (china clay) A soft white claythat is composed chieÛy of the min-eral kaolinite (see clay minerals). It isformed during the weathering andhydrothermal alteration of otherclays or feldspar. Kaolin is mined inthe UK, France, the Czech Republic,and USA. Besides its vital importancein the ceramics industry it is alsoused extensively as a Üller in themanufacture of rubber, paper, paint,and textiles and as a constituent ofmedicines.

Kastle Meyer test (phenolph-thalein test) A presumptive test usedto indicate blood. Phenolphthaleinand hydrogen peroxide are used; re-action with haemoglobin in theblood gives a pink colour.

katharometer An instrument forcomparing the thermal conductivi-ties of two gases by comparing therate of loss of heat from two heatingcoils surrounded by the gases. The in-strument can be used to detect thepresence of a small amount of an im-purity in air and is also used as a de-tector in gas chromatography.

Kekulé, Friedrich August vonStradonitz (1829–96) Germanchemist, who became professor atGhent (1858) and later at Bonn(1867). He studied the structures oforganic molecules and was the Ürstto recognise that carbon atoms form

stable chains. Kekulé is best remem-bered for his proposed structure forbenzene in 1865, which he cor-rectly interpreted as having a sym-metrical ring of six carbon atoms.

Kekulé structure A proposedstructure of *benzene in which themolecule has a hexagonal ring of car-bon atoms linked by alternating dou-ble and single bonds. Kekuléstructures contribute to the reso-nance hybrid of benzene. The struc-ture was suggested in 1865 byFriedrich August *Kekulé.

kelvin Symbol K. The *SI unit ofthermodynamic *temperature equalto the fraction 1/273.16 of the ther-modynamic temperature of the*triple point of water. The magni-tude of the kelvin is equal to that ofthe degree Celsius (centigrade), but atemperature expressed in degreescelsius is numerically equal to thetemperature in kelvins less 273.15(i.e. °C = K – 273.15). The *absolutezero of temperature has a tempera-ture of 0 K (–273.15°C). The formername degree kelvin (symbol °K) be-came obsolete by international agree-ment in 1967. The unit is namedafter Lord Kelvin.

Kelvin, Lord (William Thomson;1824–1907) British physicist, born inBelfast, who became professor ofnatural philosophy at Glasgow Uni-versity in 1846. He carried out im-portant experimental work onelectromagnetism, inventing themirror galvanometer and contribut-ing to the development of telegra-phy. He also worked with James*Joule on the *Joule–Thomson (or


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Joule–Kelvin) effect. His main theo-retical work was in thermodynam-ics, in which he stressed theimportance of the conservation ofenergy. He also introduced the con-cept of absolute zero and the Kelvintemperature scale based on it; theunit of thermodynamic temperatureis named after him. In 1896 he wascreated Baron Kelvin of Largs.

keratin Any of a group of Übrous*proteins occurring in hair, feathers,hooves, and horns. Keratins havecoiled polypeptide chains that com-bine to form supercoils of severalpolypeptides linked by disulphidebonds between adjacent cysteineamino acids.

kerosine See petroleum.

Kerr effect The ability of certainsubstances to refract differently lightwaves whose vibrations are in two di-rections (see double refraction)when the substance is placed in anelectric Üeld. The effect, discoveredin 1875 by John Kerr (1824–1907), iscaused by the fact that certain mol-ecules have electric *dipoles, whichtend to be orientated by the appliedÜeld; the normal random motions ofthe molecules tends to destroy thisorientation and the balance is struckby the relative magnitudes of theÜeld strength, the temperature, andthe magnitudes of the dipole mo-ments.

The Kerr effect is observed in aKerr cell, which consists of a glasscell containing the liquid or gaseoussubstance; two capacitor plates areinserted into the cell and light ispassed through it at right angles tothe electric Üeld. There are twoprincipal indexes of refraction: no (the ordinary index) and ne (theextraordinary index). The differencein the velocity of propagation in thecell causes a phase difference, δ, be-tween the two waves formed from a

beam of monochromatic light, wave-length λ, such that

δ = (no – ne)x/λ,where x is the length of the lightpath in the cell. Kerr also showedempirically that the ratio

(no – ne)λ = BE2,where E is the Üeld strength and B isa constant, called the Kerr constant,which is characteristic of the sub-stance and approximately inverselyproportional to the thermodynamictemperature.

The Kerr shutter consists of a Kerrcell Ülled with a liquid, such as ni-trobenzene, placed between twocrossed polarizers; the electric Üeld isarranged to be perpendicular to theaxis of the light beam and at 45° tothe axis of the polarizers. In the ab-sence of a Üeld there is no opticalpath through the device. When theÜeld is switched on the nitrobenzenebecomes doubly refracting and apath opens between the crossed po-larizers.

ketals Organic compounds, similarto *acetals, formed by addition of analcohol to a ketone. If one moleculeof ketone (RR′CO) reacts with onemolecule of alcohol R″OH, then ahemiketal is formed. The rings of ke-tose sugars are hemiketals. Furtherreaction produces a full ketal(RR′C(OR″)2).A• Information about IUPAC nomenclature

ketamine A vetinary anaesthetic

303 ketamine

k

O

Cl

NH

CH3

Ketamine


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that is used illegally as a club drug. Itis a class A drug in the UK.

ketene (keten) 1. The compoundCH2=C=O (ethenone). It can be pre-pared by pyrolysis of propanone. Thecompound reacts readily with com-pounds containing hydroxyl oramino groups and is used as an acety-lating agent. 2. Any of a class ofcompounds of the type R1R2=C=O,where R1 and R2 are organic groups.Ketenes are reactive compounds andare often generated in a reactionmedium for organic synthesis.


keto–enol tautomerism A formof tautomerism in which a com-pound containing a –CH2–CO– group(the keto form of the molecule) is inequilibrium with one containing the–CH=C(OH)– group (the enol). It oc-curs by migration of a hydrogenatom between a carbon atom and theoxygen on an adjacent carbon. Seeisomerism.

keto form See keto–enol tau-tomerism.

ketohexose See monosaccharide.

ketol An organic compound thathas both an alcohol (-CH2OH) and aketo (=CO) group. Ketols are made bya condensation reaction between twoketones or by the oxidation of dihy-dric alcohols (glycols).

ketone body Any of three com-pounds, acetoacetic acid (3-oxobu-tanoic acid, CH3COCH2COOH),β-hydroxybutyric acid (3-hydroxybu-tanoic acid, CH3CH(OH)CH2COOH),and acetone or (propanone,CH3COCH3), produced by the liver asa result of the metabolism of bodyfat deposits. Ketone bodies are nor-mally used as energy sources by pe-ripheral tissues.

ketones Organic compounds thatcontain the carbonyl group (>C=O)linked to two hydrocarbon groups.The ketone group is a carbonyl groupwith two single bonds to other car-bon atoms. In systematic chemicalnomenclature, ketone names endwith the sufÜx -one. Examples arepropanone (acetone), CH3COCH3, andbutanone (methyl ethyl ketone),CH3COC2H5. Ketones can be made byoxidizing secondary alcohols to con-vert the C–OH group to C=O. Certainketones form addition compoundswith sodium hydrogensulphate(IV)(sodium hydrogensulphite). They alsoform addition compounds with hy-drogen cyanide to give *cyanohy-drins and with alcohols to give*ketals. They undergo condensationreactions to yield *oximes, *hydra-zones, phenylhydrazones, and *semi-carbazones. These are reactions thatthey share with aldehydes. Unlikealdehydes, they do not affectFehling’s solution or Tollens reagentand do not easily oxidize. Strong oxi-dizing agents produce a mixture ofcarboxylic acids; butanone, for exam-ple, gives ethanoic and propanoicacids.A• Information about IUPAC nomenclature

ketopentose See monosaccharide.

ketose See monosaccharide.

khat A plant, Catha edilis, found inEast Africa and the Arabian peninsu-lar. The leaves are chewed to give amildly stimulating and euphoric ef-fect. Its activity comes from two alka-loids: *cathine and, in fresh leaves,the more potent *cathinone. It is acontrolled substance in many coun-tries including the US. In the UK it isnot a controlled substance and isused by certain Somalian and Yemeniethnic groups.

kieselguhr (diatomaceous earth; di-

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atomite) A soft Üne-grained depositconsisting of the siliceous skeletal re-mains of diatoms, formed in lakesand ponds. Kieselguhr is used as anabsorbent, Ültering material, Üller,and insulator.

kieserite A mineral form of *mag-nesium sulphate monohydrate,MgSO4.H2O.

kilo- Symbol k. A preÜx used in themetric system to denote 1000 times.For example, 1000 volts = 1 kilovolt(kV).

kilogram Symbol kg. The *SI unitof mass deÜned as a mass equal tothat of the international platinum–iridium prototype kept by the Inter-national Bureau of Weights andMeasures at Sèvres, near Paris.

kimberlite A rare igneous rockthat often contains diamonds. It oc-curs as narrow pipe intrusions but isoften altered and fragmented. It con-sists of olivine and phlogopite mica,usually with calcite, serpentine, andother minerals. The chief occur-rences of kimberlite are in SouthAfrica, especially at Kimberley (afterwhich the rock is named), and in theYakutia area of Siberia.

kinematic viscosity Symbol ν.The ratio of the *viscosity of a liquidto its density. The SI unit is m2 s–1.

kinetic effect A chemical effectthat depends on reaction rate ratherthan on thermodynamics. For exam-ple, diamond is thermodynamicallyless stable than graphite; its apparentstability depends on the vanishinglyslow rate at which it is converted.*Overpotential in electrolytic cells isanother example of a kinetic effect.Kinetic isotope effects are changes inreaction rates produced by isotopesubstitution. For example, if the slowstep in a chemical reaction is thebreaking of a C–H bond, the rate for

the deuterated compound would beslightly lower because of the lowervibrational frequency of the C–Dbond. Such effects are used in investi-gating the mechanisms of chemicalreactions.

kinetic energy See energy.

kinetic isotope effect See kineticeffect.

kinetics The branch of physicalchemistry concerned with measuringand studying the rates of chemicalreactions. The main aim of chemicalkinetics is to determine the mecha-nism of reactions by studying therate under different conditions (tem-perature, pressure, etc.). See also acti-vated-complex theory; arrheniusequation.

kinetic theory A theory, largelythe work of Count *Rumford, JamesPrescott *Joule, and James Clerk*Maxwell, that explains the physicalproperties of matter in terms of themotions of its constituent particles.In a gas, for example, the pressure isdue to the incessant impacts of thegas molecules on the walls of thecontainer. If it is assumed that themolecules occupy negligible space,exert negligible forces on each otherexcept during collisions, are perfectlyelastic, and make only brief collisionswith each other, it can be shown thatthe pressure p exerted by one moleof gas containing n molecules each ofmass m in a container of volume V,will be given by:

p = nmc_

2/3V,where c

_c2 is the mean square speed

of the molecules. As according to the*gas laws for one mole of gas: pV =RT, where T is the thermodynamictemperature, and R is the molar *gasconstant, it follows that:

RT = nmc_

2/3Thus, the thermodynamic tempera-

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ture of a gas is proportional to themean square speed of its molecules.As the average kinetic *energy oftranslation of the molecules is mc

_2/2,

the temperature is given by:T = (mc

_2/2)(2n/3R)

The number of molecules in onemole of any gas is the *Avogadroconstant, NA; therefore in this equa-tion n = NA. The ratio R/NA is a con-stant called the *Boltzmann constant(k). The average kinetic energy oftranslation of the molecules of onemole of any gas is therefore 3kT/2.For monatomic gases this is propor-tional to the *internal energy (U) ofthe gas, i.e.

U = NA3kT/2and as k = R/NA

U = 3RT/2For diatomic and polyatomic gasesthe rotational and vibrational ener-gies also have to be taken into ac-count (see degrees of freedom).

In liquids, according to the kinetictheory, the atoms and molecules stillmove around at random, the temper-ature being proportional to their av-erage kinetic energy. However, theyare sufÜciently close to each otherfor the attractive forces between mol-ecules to be important. A moleculethat approaches the surface will ex-perience a resultant force tending tokeep it within the liquid. It is, there-fore, only some of the fastest movingmolecules that escape; as a result the average kinetic energy of thosethat fail to escape is reduced. In thisway evaporation from the surface of a liquid causes its temperature tofall.

In a crystalline solid the atoms,ions, and molecules are able only tovibrate about the Üxed positions of a*crystal lattice; the attractive forcesare so strong at this range that nofree movement is possible.

Kipp’s apparatus A laboratory ap-paratus for making a gas by the reac-tion of a solid with a liquid (e.g. thereaction of hydrochloric acid withiron sulphide to give hydrogen sul-phide). It consists of three intercon-nected glass globes arrangedvertically, with the solid in the mid-dle globe. The upper and lowerglobes are connected by a tube andcontain the liquid. The middle globehas a tube with a tap for drawing offgas. When the tap is closed, pressureof gas forces the liquid down in thebottom reservoir and up into the top,and reaction does not occur. Whenthe tap is opened, the release in pres-sure allows the liquid to rise into themiddle globe, where it reacts withthe solid. It is named after PetrusKipp (1808–64).

Kirchhoff, Gustav Robert(1824–87) German physicist, who in1850 became a professor at Breslauand four years later joined RobertBunsen at Heidelberg. In 1845,while still a student, he formulatedKirchhoff’s laws concerning elec-tric circuits. With Bunsen he workedon spectroscopy, a technique that ledthem to discover the elements cae-sium (1860) and rubidium (1861).

Kjeldahl’s method A method formeasuring the percentage of nitro-gen in an organic compound. Thecompound is boiled with concen-trated sulphuric acid and copper(II)sulphate catalyst to convert any ni-trogen to ammonium sulphate. Al-kali is added and the mixture heatedto distil off ammonia. This is passedinto a standard acid solution, and theamount of ammonia can then befound by estimating the amount ofunreacted acid by titration. Theamount of nitrogen in the originalspecimen can then be calculated. Themethod was developed by the Danishchemist Johan Kjeldahl (1849–1900).

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knocking The metallic sound pro-duced by a spark-ignition petrol en-gine under certain conditions. It iscaused by rapid combustion of theunburnt explosive mixture in thecombustion chambers ahead of theÛame front. As the Ûame travels fromthe sparking plug towards the pistonit compresses and heats the unburntgases ahead of it. If the Ûame frontmoves fast enough, normal combus-tion occurs and the explosive mix-ture is ignited progressively by theÛame. If it moves too slowly, ignitionof the last part of the unburnt gascan occur very rapidly before theÛame reaches it, producing a shockwave that travels back and forthacross the combustion chamber. Theresult is overheating, possible dam-age to the plugs, an undesirablenoise, and loss of power (probablydue to preignition caused by over-heated plugs). Knocking can beavoided by an engine design that in-creases turbulence in the combustionchamber and thereby increases Ûamespeed. It also can be avoided by re-ducing the compression ratio, butthis involves loss of efÜciency. Themost effective method is to use high-octane fuel (see octane number),which has a longer self-ignition delay than low-octane fuels. This canbe achieved by the addition of anantiknock agent, such as lead(IV)tetraethyl, to the fuel, which re-tards the combustion chain reac-tions. However, lead-free petrol isnow preferred to petrol containinglead tetraethyl owing to environ-mental dangers arising from lead inthe atmosphere. In the USA the addi-tion of lead compounds is now for-bidden. New formulae for petrol aredesigned to raise the octane numberwithout polluting the atmosphere.These new formulae include increas-ing the content of aromatics and oxy-genates (oxygen-containing

compounds, such as alcohols). How-ever, it is claimed that the presencein the atmosphere of incompletelyburnt aromatics constitutes a cancerrisk.

Knoevenagel reaction A reactionin which an aldehyde RCHO reactswith malonic acid (CH2(COOH)2) with subsequent loss of CO2 to givean unsaturated carboxylic acidRCH=CHCOOH. Thus, the chainlength is increased by 2. The reactionis base-catalysed; usually pyridine isused. The reaction is named afterEmil Knoevenagel (1865–1921).

knotane See molecular knot.

knot theory A branch of mathe-matics used to classify knots and en-tanglements. Knot theory hasapplications to the study of the prop-erties of polymers and the statisticalmechanics of certain models of phasetransitions.

Knudsen Ûow See molecularflow.

Kohlrausch equation An equa-tion that describes the molar conduc-tivities of strong electrolytes at lowconcentration, i.e. Λm = Λ0

m – Κc½,

where Λm is the molar conductivity,Λ0

m is the limiting molar conductivity(the molar conductivity in the limitof zero concentration when the ionsdo not interact with each other), Κ isa coefÜcient related to the stoichiom-etry of the electrolyte, and c is theconcentration of the electrolyte. It ispossible to express Λ0

m as a sum ofthe contribution of each of its ions.The Kohlrausch equation was Ürststated in the 19th century by the Ger-man chemist Friedrich Kohlrausch(1840–1910) as the result of a consid-erable amount of experimental work.With its characteristic c½ depend-ence, the equation is explained quan-titatively by the existence of an ionicatmosphere round the ion as

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analysed by the *Debye– Hückel–On-sager theory.

Kohlrausch’s law If a salt is dis-solved in water, the conductivity ofthe (dilute) solution is the sum of twovalues – one depending on the posi-tive ions and the other on the nega-tive ions. The law, which depends onthe independent migration of ions,was deduced experimentally byFriedrich Kohlrausch.

Kolbe’s method A method ofmaking alkanes by electrolysing a so-lution of a carboxylic acid salt. For asalt Na+RCOO–, the carboxylate ionslose electrons at the cathode to giveradicals:

RCOO– – e → RCOO.

These decompose to give alkyl radi-cals

RCOO. → R. + CO2

Two alkyl radicals couple to give analkane

R. + R. → RRThe method can only be used for hy-drocarbons with an even number ofcarbon atoms, although mixtures oftwo salts can be electrolysed to give amixture of three products. Themethod was discovered by the Ger-man chemist Herman Kolbe(1818–84), who electrolysed pen-tanoic acid (C4H9COOH) in 1849 andobtained a hydrocarbon, which he as-sumed was the substance ‘butyl’C4H9 (actually octane, C8H18).

Koopmans’ theorem The princi-ple that the ionization energy of amolecule is equal to the orbital en-ergy of the ejected electron. It is thebasis of the interpretation of spectrain *photoelectron spectroscopy.Koopmans’ theorem is an approxima-tion in that it ignores any reorganiza-tion of electrons in the ion formed. Itis named after the Dutch mathemati-

cian and economist Tjalling CharlesKoopmans (1910–85).

Kovar A tradename for an alloy ofiron, cobalt, and nickel with an ex-pansivity similar to that of glass. It istherefore used in making glass-to-metal seals, especially in circum-stances in which a temperaturevariation can be expected.

Kramers theorem The energy lev-els of a system, such as an atom thatcontains an odd number of spin-½particles (e.g. electrons), are at leastdouble *degenerate in the absence ofan external magnetic Üeld. This de-generacy, known as Kramers degen-eracy, is a consequence of timereversal invariance. Kramers theoremwas stated by the Dutch physicistHendrick Anton Kramers (1894–1952)in 1930. Kramers degeneracy is re-moved by placing the system in anexternal magnetic Üeld. The theoremholds even in the presence of crystalÜelds (see crystal-field theory) and*spin–orbit coupling.

Krebs, Sir Hans Adolf (1900–81)German-born British biochemist,who emigrated to Britain in 1933,working at ShefÜeld University be-fore moving to Oxford in 1954. Krebs is best known for the *Krebscycle, the basis of which he discov-ered in 1937. Details were lateradded by Fritz Lipmann (1899–1986),with whom Krebs shared the 1953Nobel Prize for physiology or medi-cine.

Krebs cycle (citric acid cycle; tricar-boxylic acid cycle; TCA cycle) Acyclical series of biochemical reac-tions that is fundamental to themetabolism of aerobic organisms, i.e.animals, plants, and many micro-organisms (see illustration). The en-zymes of the Krebs cycle are locatedin the mitochondria and are in closeassociation with the components of

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the *electron transport chain. Thetwo-carbon acetyl coenzyme A (acetylCoA) reacts with the four-carbon ox-aloacetate to form the six-carbon cit-rate. In a series of seven reactions,this is reconverted to oxaloacetateand produces two molecules of car-bon dioxide. Most importantly, thecycle generates one molecule ofguanosine triphosphate (GTP – equiv-alent to 1 ATP) and reduces threemolecules of the coenzyme *NAD toNADH and one molecule of the coen-zyme *FAD to FADH2. NADH andFADH2 are then oxidized by the elec-tron transport chain to generatethree and two molecules of ATP re-spectively. This gives a net yield of 12molecules of ATP per molecule ofacetyl CoA.

Acetyl CoA can be derived fromcarbohydrates (via *glycolysis), fats,or certain amino acids. (Other aminoacids may enter the cycle at differentstages.) Thus the Krebs cycle is thecentral ‘crossroads’ in the complex

system of metabolic pathways and isinvolved not only in degradation andenergy production but also in thesynthesis of biomolecules. It isnamed after its principal discoverer,Sir Hans Adolf *Krebs.

Kroll process A process for produc-ing certain metals by reducing thechloride with magnesium metal. Itcan be used for titanium, e.g.

TiCl4 + 2Mg → Ti + 2MgCl2.It is named after William Kroll, whodevised the process in 1940.

krypton Symbol Kr. A colourlessgaseous element belonging to group0 (the *noble gases) of the periodictable; a.n. 36; r.a.m. 83.80; d. 3.73g m–3; m.p. –156.6°C; b.p. –152.3°C.Krypton occurs in air (0.0001% by vol-ume) from which it can be extractedby fractional distillation of liquid air.Usually, the element is not isolatedbut is used with other inert gases inÛuorescent lamps, etc. The elementhas Üve natural isotopes (mass num-

309 krypton

k

-ketoglutarateα

4C

4C

4C

4C

4C6C

6C

GTPCO2

CO2

5CP

3C

2C

NADH + H+

NADH + H+

NADH + H+FADH2

glycolysis

pyruvate

acetyl CoA

oxaloacetate citrate

isocitrate

NAD+

NAD+

malate

fumarate

FADsuccinate

succinyl CoA

NAD+

GDP

Krebs cycle


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bers 78, 80, 82, 83, 84) and there areÜve radioactive isotopes (76, 77, 79,81, 85). Krypton–85 (half-life 10.76years) is produced in Üssion reactorsand it has been suggested that anequilibrium amount will eventuallyoccur in the atmosphere. The el-ement is practically inert and formsvery few compounds (certain

Ûuorides, such as KrF2, have been re-ported).A• Information from the WebElements site

Kupfer nickel A naturally occur-ring form of nickel arsenide, NiAs; animportant ore of nickel.

kurchatovium See transactinideelements.

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L

labelling The process of replacing astable atom in a compound with a ra-dioisotope of the same element toenable its path through a biologicalor mechanical system to be traced bythe radiation it emits. In some casesa different stable isotope is used andthe path is detected by means of amass spectrometer. A compound con-taining either a radioactive or stableisotope is called a labelled compoundand the atom used is a label. If a hy-drogen atom in each molecule of thecompound has been replaced by a tri-tium atom, the compound is called atritiated compound. A radioactive la-belled compound will behave chemi-cally and physically in the same wayas an otherwise identical stable com-pound, and its presence can easily bedetected using a Geiger counter. Thisprocess of radioactive tracing iswidely used in chemistry, biology,medicine, and engineering. For ex-ample, it can be used to follow thecourse of the reaction of a carboxylicacid with an alcohol to give an ester,e.g.

CH3COOH + C2H5OH →C2H5COOCH3 + H2O

To determine whether the noncar-bonyl oxygen in the ester comesfrom the acid or the alcohol, the re-action is performed with the labelledcompound CH3CO18OH, in which theoxygen in the hydroxyl group of theacid has been ‘labelled’ by using the18O isotope. It is then found that thewater product is H2

18O; i.e. the oxy-gen in the ester comes from the alco-hol, not the acid.

labile Describing a chemical com-

pound in which certain atoms orgroups can easily be replaced byother atoms or groups. The term isapplied to coordination complexes inwhich ligands can easily be replacedby other ligands in an equilibrium re-action.

lactams Organic compounds con-taining a ring of atoms in which thegroup –NH.CO.– forms part of thering. Lactams can be formed by reac-tion of an –NH2 group in one part ofa molecule with a –COOH group inthe other to give a cyclic amide (seeillustration). They can exist in an al-ternative tautomeric form, the lactimform, in which the hydrogen atomon the nitrogen has migrated to theoxygen of the carbonyl to give–N=C(OH)–. The pyrimidine baseuracil is an example of a lactam.A• Information about IUPAC nomenclature

Lactams

lactate A salt or ester of lactic acid(i.e. a 2-hydroxypropanoate).

lactic acid (2-hydroxypropanoicacid) A clear odourless hygroscopicsyrupy liquid, CH3CH(OH)COOH, witha sour taste; r.d. 1.206; m.p. 18°C; b.p.122°C. It is prepared by the hydroly-sis of ethanal cyanohydrin or the oxi-


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dation of propan-1,2-diol using dilutenitric acid. Lactic acid is manufac-tured by the fermentation of lactose(from milk) and used in the dyeingand tanning industries. It is an alphahydroxy *carboxylic acid. See also op-tical activity.

Lactic acid is produced from pyru-vic acid in active muscle tissue whenoxygen is limited and subsequentlyremoved for conversion to glucose bythe liver. During strenuous exerciseit may build up in the muscles, caus-ing cramplike pains. It is also pro-duced by fermentation in certainbacteria and is characteristic of sourmilk.

lactims See lactams.A• Information about IUPAC nomenclature

lactones Organic compounds con-taining a ring of atoms in which thegroup –CO.O– forms part of the ring.Lactones can be formed (or regardedas formed) by reaction of an –OHgroup in one part of a molecule witha –COOH group in the other to give a cyclic ester (see illustration).This type of reaction occurs with γ-hydroxy carboxylic acids such as thecompound CH2(OH)CH2CH2COOH (inwhich the hydroxyl group is on thethird carbon from the carboxylgroup). The resulting γ-lactone has a Üve-membered ring. Similarly, δ-lactones have six-membered rings. β-lactones, with a four-memberedring, are not produced directly from

β-hydroxy acids, but can be synthe-sized by other means.A• Information about IUPAC nomenclature

lactose (milk sugar) A sugar com-prising one glucose molecule linkedto a galactose molecule. Lactose ismanufactured by the mammarygland and occurs only in milk. Forexample, cows’ milk contains about4.7% lactose. It is less sweet than su-crose (cane sugar).

Ladenburg benzene An (erro-neous) structure for *benzene pro-posed by Albert Ladenburg (1842–1911), in which the six carbon atomswere arranged at the corners of a tri-angular prism and linked by singlebonds to each other and to the sixhydrogen atoms.

laevo form See optical activity.

laevorotatory Designating achemical compound that rotates theplane of plane-polarized light to theleft (anticlockwise for someone fac-ing the oncoming radiation). See opti-cal activity.

laevulose See fructose.

Lagrange multipliers (undeter-mined multipliers) Parameters, usu-ally denoted λ, introduced to assist inÜnding the maximum or minimumvalue of a function f of several vari-ables x1,x2,…xn, subject to some con-straint that connects the variables.An important application of themethod of Lagrange multipliers is itsuse in the derivation of the Boltz-mann distribution in statistical me-chanics, in which one of theLagrange multipliers is –1/kT, where kis the *Boltzmann constant and T isthe thermodynamic temperature. La-grange multipliers were introducedby the Italian-born French mathe-matician Joseph-Louis Lagrange(1736–1813).

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Lagrangian Symbol L. A functionused to deÜne a dynamical system interms of functions of coordinates, ve-locities, and times given by:

L = T – Vwhere T is the kinetic energy of thesystem and V is the potential energyof the system. The Lagrangian formu-lation of dynamics has the advantagethat it does not deal with many vec-tor quantities, such as forces and ac-celerations, but only with two scalarfunctions, T and V. This leads to greatsimpliÜcations. Lagrangian dynamicswas formulated by Joseph Louis La-grange (1736–1813).

LAH Lithium aluminium hydride;see lithium tetrahydro-aluminate(iii).

lake A pigment made by combiningan organic dyestuff with an inorganiccompound (usually an oxide, hydrox-ide, or salt). Absorption of the or-ganic compound on the inorganicsubstrate yields a coloured complex,as in the combination of a dyestuffwith a *mordant. Lakes are used inpaints and printing inks.

lambda point See superfluidity.

Lamb-dip spectroscopy A spec-troscopic technique enabling the cen-tres of absorption lines to bedetermined very precisely by makinguse of the Doppler shift associatedwith very rapidly moving molecules.An intense monochromatic beam ofradiofrequency electromagnetic radi-ation is passed through a sample of agas with the frequency being slightlyhigher than that of maximum ab-sorption. Only certain moleculesmoving at a certain speciÜc speedcan absorb radiation. The beam isthen reÛected back through the sam-ple so that radiation is absorbed bymolecules moving at exactly thissame speed, except that they aremoving away from the mirror. If the

radiation is exactly at the absorptionpeak, only molecules moving perpen-dicular to the line of the beam(which hence have no Doppler shift)absorb both in the initial path andthe reÛected path of the radiation.Since molecules being excited in theinitial path leave fewer molecules tobe excited in the return path thiscauses a less intense absorption to beobserved. As a result a dip appears inthe curve, thus enabling the absorp-tion peak to be found very accu-rately. Lamb-dip spectroscopy isnamed after Willis Eugene Lamb(1913–2008).

Lamb shift A small energy differ-ence between two levels (2S1/2 and2P1/2) in the *hydrogen spectrum. Theshift results from the quantum inter-action between the atomic electronand the electromagnetic radiation. Itwas Ürst explained by Willis EugeneLamb.

lamellar solids Solid substances inwhich the crystal structure has dis-tinct layers (i.e. has a layer lattice).The *micas are an example of thistype of compound. *Intercalationcompounds are lamellar compoundsformed by interposition of atoms,ions, etc., between the layers of anexisting element or compound. Forexample, graphite is a lamellar solid.With strong oxidizing agents (e.g. amixture of concentrated sulphuricand nitric acids) it forms a nonstoi-chiometric ‘graphitic oxide’, which isan intercalation compound havingoxygen atoms between the layers ofcarbon atoms. Substances of this typeare called graphitic compounds.

lamp black A Ünely divided (micro-crystalline) form of carbon made byburning organic compounds ininsufÜcient oxygen. It is used as ablack pigment and Üller.

Landau levels The energy levels

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found by *quantum mechanics offree electrons in a uniform magneticÜeld. These energy levels, namedafter the Soviet physicist Lev Davi-dovich Landau (1908–68), whoanalysed the problem in 1930, havediscrete values, which are integermultiples of heB/m, where h is thePlanck constant, e is the charge ofthe electron, m is the mass of theelectron, and B is the magnetic Ûuxdensity. Each Landau level is a highly*degenerate level, with each levelbeing Ülled by 2eB/h, where the factor2 is due to the spin of the electron.

Landé interval rule A rule used ininterpreting atomic spectra statingthat if the *spin–orbit coupling isweak in a given multiplet, the energydifferences between two successive Jlevels (where J is the total resultantangular momentum of the coupledelectrons) are proportional to thelarger of the two values of J. The rulewas stated by the German-born USphysicist Alfred Landé (1888–1975) in1923. It can be deduced from thequantum theory of angular momen-tum. In addition to assuming *Rus-sell–Saunders coupling, the Landéinterval rule assumes that the inter-actions between spin magnetic mo-ments can be ignored, an assumptionthat is not correct for very lightatoms, such as helium. Thus theLandé interval rule is best obeyed byatoms with medium atomic num-bers.

Langevin equation A type of ran-dom equation of motion (see sto-chastic process) used to study*Brownian movement. The Langevinequation can be written in the formv.= ξv + A(t), where v is the velocity of

a particle of mass m immersed in aÛuid and v

.is the acceleration of the

particle; ξv is a frictional force result-ing from the viscosity of the Ûuid,with ξ being a constant friction

coefÜcient, and A(t) is a random forcedescribing the average effect of theBrownian motion. The Langevinequation is named after the Frenchphysicist Paul Langevin (1872–1946).It is necessary to use statistical meth-ods and the theory of probability tosolve the Langevin equation.

Langmuir adsorption isothermAn equation used to describe theamount of gas adsorbed on a planesurface, as a function of the pressureof the gas in equilibrium with thesurface. The Langmuir adsorptionisotherm can be written:

θ = bp/(1 + bp),where θ is the fraction of the surfacecovered by the adsorbate, p is thepressure of the gas, and b is a con-stant called the adsorption coefÜ-cient, which is the equilibriumconstant for the process of adsorp-tion. The Langmuir adsorptionisotherm was derived by the USchemist Irving Langmuir (1881–1957), using the *kinetic theory ofgases and making the assumptionsthat:(1) the adsorption consists entirely ofa monolayer at the surface;(2) there is no interaction betweenmolecules on different sites and eachsite can hold only one adsorbed mol-ecule;(3) the heat of adsorption does notdepend on the number of sites and isequal for all sites.The Langmuir adsorption isotherm isof limited application since for realsurfaces the energy is not the samefor all sites and interactions betweenadsorbed molecules cannot be ig-nored.

Langmuir–Blodgett Ülm A Ülmof molecules on a surface that cancontain multiple layers of Ülm. ALangmuir–Blodgett Ülm with multi-ple layers can be made by dipping aplate into a liquid so that it is cov-

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ered by a monolayer and then repeat-ing the process. This process, calledthe Langmuir–Blodgett technique, en-ables a multilayer to be built up, onemonolayer at a time. Langmuir–Blodgett Ülms have many potentialpractical applications, including insu-lation for optical and semiconductordevices and selective membranes inbiotechnology.

Langmuir–Hinshelwood mecha-nism A possible mechanism for abimolecular process catalyzed at asolid surface. It is assumed that twomolecules are adsorbed on adjacentsites and that a reaction takes placevia an activated complex on the sur-face to yield the products of the reac-tion. This mechanism was suggestedby the US chemist Irving Langmuir(1881–1957) in 1921 and developedby the British chemist Sir Cyril Hin-shelwood (1897–1967) in 1926. Somebimolecular reactions at surfaces arein agreement with the predictions ofthis model. See also langmuir–ridealmechanism.

Langmuir isotherm See langmuiradsorption isotherm.

Langmuir–Rideal mechanism Apossible mechanism for a bimolecu-lar process catalyzed at a solid sur-face. It is envisaged that one of themolecules is adsorbed and then re-acts with a second molecule that isnot adsorbed. This mechanism wassuggested by the US chemist IrvingLangmuir (1881–1957) in 1921 anddeveloped by the British chemist SirEric Rideal in 1939. The Langmuir–Rideal mechanism is not commonbut certain reactions involving hy-drogen atoms probably occur by thismechanism.

lanolin An emulsion of puriÜedwool fat in water, containing choles-terol and certain terpene alcoholsand esters. It is used in cosmetics.

lansfordite A mineral form of*magnesium carbonate pentahy-drate, MgCO3.5H2O.

lanthanide contraction See lan-thanoids.

lanthanides See lanthanoids.

lanthanoid contraction See lan-thanoids.

lanthanoids (lanthanides; lan-thanons; rare-earth elements) A se-ries of elements in the *periodictable, generally considered to rangein proton number from cerium (58) to lutetium (71) inclusive. Thelanthanoids all have two outer s-electrons (a 6s2 conÜguration), followlanthanum, and are classiÜed to-gether because an increasing protonnumber corresponds to increase innumber of 4f electrons. In fact, the 4fand 5d levels are close in energy andthe Ülling is not smooth. The outerelectron conÜgurations are as fol-lows:57 lanthanum (La) 5d16s2

58 cerium (Ce) 4f5d16s2 (or 4f26s2)59 praseodymium (Pr) 4f36s2

60 neodymium (Nd) 4f46s2

61 promethium (Pm) 4f56s2

62 samarium (Sm) 4f66s2

63 europium (Eu) 4f76s2

64 gadolinium (Gd) 4f75d16s2

65 terbium (Tb) 4f96s2

66 dysprosium (Dy) 4f106s2

67 holmium (Ho) 4f116s2

68 erbium (Er) 4f126s2

69 thulium (Tm) 4f136s2

70 ytterbium (Yb) 4f146s2

71 lutetium (Lu) 4f145d16s2

Note that lanthanum itself doesnot have a 4f electron but it is gener-ally classiÜed with the lanthanoidsbecause of its chemical similarities,as are yttrium (Yt) and scandium (Sc).Scandium, yttrium, and lanthanumare d-block elements; the lanthanoidsand *actinoids make up the f-block.

The lanthanoids are sometimes

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simply called the rare earths, al-though strictly the ‘earths’ are theiroxides. Nor are they particularly rare:they occur widely, usually together.All are silvery very reactive metals.The f-electrons do not penetrate tothe outer part of the atom and thereis no f-orbital participation in bond-ing (unlike the d-orbitals of the main*transition elements) and the el-ements form few coordination com-pounds. The main compoundscontain M3+ ions. Cerium also has thehighly oxidizing Ce4+ state and eu-ropium and ytterbium have a M2+

state.The 4f orbitals in the atoms are not

very effective in shielding the outerelectrons from the nuclear charge. Ingoing across the series the increasingnuclear charge causes a contractionin the radius of the M3+ ion – from0.1061 nm in lanthanum to0.0848 nm in lutetium. This effect,the lanthanoid contraction (or lan-thanide contraction), accounts for thesimilarity between the transition el-ements zirconium and hafnium.

lanthanons See lanthanoids.

lanthanum Symbol La. A silverymetallic element belonging to group3 (formerly IIIA) of the periodic tableand often considered to be one of the*lanthanoids; a.n. 57; r.a.m. 138.91;r.d. 6.146 (20°C); m.p. 921°C; b.p.3457°C. Its principal ore is bastnasite,from which it is separated by an ion-exchange process. There are two nat-ural isotopes, lanthanum–139 (stable)and lanthanum–138 (half-life1010–1015 years). The metal, being py-rophoric, is used in alloys for lighterÛints and the oxide is used in someoptical glasses. The largest use of lan-thanum, however, is as a catalyst incracking crude oil. Its chemistry re-sembles that of the lanthanoids. Theelement was discovered by CarlMosander (1797–1858) in 1839.

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lapis lazuli A blue rock that iswidely used as a semiprecious stoneand for ornamental purposes. It iscomposed chieÛy of the deep bluemineral lazurite embedded in a ma-trix of white calcite and usually alsocontains small specks of pyrite. It oc-curs in only a few places in crys-talline limestones as a contactmetamorphic mineral. The chiefsource is Afghanistan; lapis lazulialso occurs near Lake Baikal inSiberia and in Chile. It was formerlyused to make the artists’ pigment ul-tramarine.

Laporte selection rule A selec-tion rule in atomic spectra statingthat spectral lines associated withelectric-dipole radiation must arisefrom transitions between states ofopposite parity. The Laporte selectionrule was discovered by O. Laporte in1924 and was explained by the appli-cation of group theory to the *quan-tum mechanics of atoms. In the caseof magnetic-dipole and quadrupoleradiation the selection rule for spec-tral lines is the opposite of the La-porte rule, i.e. transitions are onlyallowed between states of the sameparity in these cases.

Larmor precession A precessionof the motion of charged particles ina magnetic Üeld. It was Ürst deducedin 1897 by Sir Joseph Larmor(1857–1942). Applied to the orbitalmotion of an electron around the nu-cleus of an atom in a magnetic Üeldof Ûux density B, the frequency ofprecession is given by eB/4πmvµ,where e and m are the electroniccharge and mass respectively, µ isthe permeability, and v is the velocityof the electron. This is known as theLarmor frequency.

laser (light ampliÜcation by stimu-


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lated emission of radiation) A lightampliÜer (also called an opticalmaser) usually used to producemonochromatic coherent radiationin the infrared, visible, and ultravio-let regions of the *electromagneticspectrum. Lasers that operate in theX-ray region of the spectrum are alsobeing developed.

Nonlaser light sources emit radia-tion in all directions as a result of thespontaneous emission of photons bythermally excited solids (Ülamentlamps) or electronically excitedatoms, ions, or molecules (Ûuores-cent lamps, etc.). The emission ac-companies the spontaneous return ofthe excited species to the *groundstate and occurs randomly, i.e. the ra-diation is not coherent. In a laser, theatoms, ions, or molecules are Ürst‘pumped’ to an excited state andthen stimulated to emit photons bycollision of a photon of the same en-ergy. This is called stimulated emis-sion. In order to use it, it is Ürstnecessary to create a condition in theamplifying medium, called popula-tion inversion, in which the majorityof the relevant entities are excited.Random emission from one entitycan then trigger coherent emissionfrom the others that it passes. In thisway ampliÜcation is achieved.

The laser ampliÜer is converted toan oscillator by enclosing the ampli-fying medium within a resonator. Ra-diation then introduced along theaxis of the resonator is reÛected backand forth along its path by a mirrorat one end and by a partially trans-mitting mirror at the other end. Be-tween the mirrors the waves areampliÜed by stimulated emission.The radiation emerges through thesemitransparent mirror at one end as a powerful coherent monochro-matic parallel beam of light. Theemitted beam is uniquely parallel be-cause waves that do not bounce back

and forth between the mirrorsquickly escape through the sides ofthe oscillating medium withoutampliÜcation.

Some lasers are solid, others areliquid or gas devices. Population in-version can be achieved by opticalpumping with Ûashlights or withother lasers. It can also be achievedby such methods as chemical reac-tions and discharges in gases.

Lasers have found many uses sincetheir invention in 1960. In chemistry,their main use has been in the studyof photochemical reactions, in thespectroscopic investigation of mol-ecules, and in *femtochemistry. Seealso dye laser; four-level laser;pockels cell.

laser spectroscopy Spectroscopythat makes use of lasers. The beamsof coherent monochromatic radia-tion produced by lasers have severalsigniÜcant advantages comparedwith other spectroscopic techniques,particularly in those that employ the*Raman effect.

Lassaigne’s test A method of test-ing for the presence of a halogen, ni-trogen, or sulphur in an organiccompound. A sample is heated in atest tube with a pellet of sodium. Thehot tube is dropped into pure waterand the fragments ground up in amortar. The presence of a halogen(now in the form of a sodium halide)is detected by precipitation with sil-ver nitrate solution. Nitrogen isrevealed by the formation of a pre-cipitate of Prussian blue on heatingpart of the solution with iron(II) sul-phate solution containing hydrochlo-ric acid and a trace of iron(III) ions.Lead ethanoate or sodium nitroprus-side gives a precipitate with any sul-phur present.

latent heat Symbol L. The quantityof heat absorbed or released when asubstance changes its physical phase

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at constant temperature (e.g. fromsolid to liquid at the melting point orfrom liquid to gas at the boilingpoint). For example, the latent heatof vaporization is the energy a sub-stance absorbs from its surroundingsin order to overcome the attractiveforces between its molecules as itchanges from a liquid to a gas and inorder to do work against the externalatmosphere as it expands. In thermo-dynamic terms the latent heat is the*enthalpy of evaporation (∆H), i.e. L =∆H = ∆U + p∆V, where ∆U is thechange in the internal energy, p isthe pressure, and ∆V is the change involume.

The speciÜc latent heat (symbol l) isthe heat absorbed or released perunit mass of a substance in thecourse of its isothermal change ofphase. The molar latent heat is theheat absorbed or released per unitamount of substance during anisothermal change of state.

latex Natural rubber as it is ob-tained from a rubber tree or any sta-ble suspension in water of a similarsynthetic polymer. Synthetic latexesare used to make articles from rub-ber or plastics by such techniques asdipping (rubber gloves), spreading(waterproof cloth), and electrodeposi-tion (plastic-coated metal). They arealso employed in paints and adhe-sives.

Latimer diagram A simple dia-gram summarizing the standard po-tentials for an element in differentoxidation states. The differentspecies are written in a horizontalline in order of decreasing oxidationstate, with the most highly oxidizedspecies on the left. Arrows are writ-ten between adjacent species withthe standard potential in volts indi-cated above the arrow. Often the oxi-dation number is included in thediagram. The standard electrode po-

tential for nonadjacent species canbe calculated, but values for commoncouples are often indicated by addi-tional arrows, Latimer diagrams de-pend on pH and are commonly givenfor both acidic and alkaline condi-tions. See also frost diagram.

lattice The regular arrangement ofatoms, ions, or molecules in a crys-talline solid. See crystal lattice.

lattice energy A measure of thestability of a *crystal lattice, given bythe energy that would be releasedper mole if atoms, ions, or moleculesof the crystal were brought togetherfrom inÜnite distances apart to formthe lattice. See born–haber cycle.

lattice vibrations The periodic vi-brations of the atoms, ions, or mol-ecules in a *crystal lattice about theirmean positions. On heating, the am-plitude of the vibrations increasesuntil they are so energetic that thelattice breaks down. The temperatureat which this happens is the meltingpoint of the solid and the substancebecomes a liquid. On cooling, the am-plitude of the vibrations diminishes.At *absolute zero a residual vibrationpersists, associated with the *zero-point energy of the substance. Theincrease in the electrical resistance of a conductor is due to increasedscattering of the free conductionelectrons by the vibrating latticeparticles.

laughing gas See dinitrogenoxide.

lauric acid See dodecanoic acid.

Lavoisier, Antoine Laurent(1743–1794) French chemist, who col-lected taxes for the government inParis. In the 1770s he discovered oxy-gen and nitrogen in air and demol-ished the *phlogiston theory ofcombustion by demonstrating therole of oxygen in the process. In 1783

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he made water by burning hydrogenin oxygen (see cavendish, henry). Healso devised a rational nomenclaturefor chemical compounds. In 1794 hewas tried by the Jacobins as an oppo-nent of the Revolution (because ofhis tax-gathering), found guilty, andguillotined.

law of chemical equilibrium Seeequilibrium constant.

law of conservation of energySee conservation law.

law of conservation of mass Seeconservation law.

law of constant composition Seechemical combination.

law of deÜnite proportions Seechemical combination.

law of mass action See mass ac-tion.

law of multiple proportions Seechemical combination.

law of octaves (Newlands’ law)An attempt at classifying elementsmade by John Newlands (1837–98) in1863. He arranged 56 elements inorder of increasing atomic mass ingroups of eight, pointing out thateach element resembled the elementeight places from it in the list. Hedrew an analogy with the notes of amusical scale. Newlands’ octaveswere groups of similar elements dis-tinguished in this way: e.g. oxygenand sulphur; nitrogen and phospho-rus; and Ûuorine, chlorine, bromine,and iodine. In some cases it was nec-essary to put two elements in thesame position. The proposal was re-jected at the time. See periodic table.

A• John Newlands’ paper

law of reciprocal proportionsSee chemical combination.

lawrencium Symbol Lr. A radio-

active metallic transuranic elementbelonging to the *actinoids; a.n. 103;mass number of the Ürst discoveredisotope 257 (half-life 8 seconds). Anumber of very short-lived isotopeshave now been synthesized. The el-ement was identiÜed by AlbertGhiorso and associates in 1961. It was named after E. O. Lawrence(1901–58).


laws of chemical combinationSee chemical combination.

laws, theories, and hypothesesIn science, a law is a descriptive prin-ciple of nature that holds in all cir-cumstances covered by the wordingof the law. There are no loopholes inthe laws of nature and any excep-tional event that did not comply withthe law would require the existinglaw to be discarded or would have tobe described as a miracle. Epony-mous laws are named after their dis-coverers (e.g. *Boyle’s law); somelaws, however, are known by theirsubject matter (e.g. the law of conser-vation of mass), while other laws useboth the name of the discoverer andthe subject matter to describe them(e.g. Newton’s law of gravitation).

A description of nature that en-compasses more than one law buthas not achieved the uncontrovert-ible status of a law is sometimescalled a theory. Theories are oftenboth eponymous and descriptive ofthe subject matter (e.g. Einstein’stheory of relativity and Darwin’stheory of evolution).

A hypothesis is a theory or law thatretains the suggestion that it may notbe universally true. However, somehypotheses about which no doubtstill lingers have remained hypothe-ses (e.g. Avogadro’s hypothesis), forno clear reason. Clearly there is a de-

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gree of overlap between the threeconcepts.

layer lattice A crystal structure inwhich the atoms are chemicallybonded in plane layers, with rela-tively weak forces between atoms inadjacent layers. Graphite and micasare examples of substances havinglayer lattices (i.e. they are *lamellarsolids).

lazurite See lapis lazuli.

LCAO (linear combination of atomicorbitals) A molecular *orbitalformed by the linear combination ofatomic orbitals. The LCAO approxi-mation arises because with an elec-tron that is very close to a nucleus,the potential energy of the electronis dominated by the interaction be-tween the electron and the nucleus.Thus, very close to the nucleus of anatom, A, in a molecule, the *wavefunction of the molecule is very simi-lar to the wave function of the atomA. The LCAO approximation showsthe increase in electron density asso-ciated with chemical bonding. TheLCAO method takes account of thesymmetry of the molecule using sym-metry-adapted linear combinations(SALC).

LCP See liquid-crystal polymer.

L-dopa See dopa.

L–D process See basic-oxygenprocess.

leaching Extraction of soluble com-ponents of a solid mixture by perco-lating a solvent through it.

lead Symbol Pb. A heavy dull greysoft ductile metallic element belong-ing to *group 14 (formerly IVB) ofthe periodic table; a.n. 82; r.a.m.207.19; r.d. 11.35; m.p. 327.5°C; b.p.1740°C. The main ore is the sulphidegalena (PbS); other minor sources in-clude anglesite (PbSO4), cerussite

(PbCO3), and litharge (PbO). Themetal is extracted by roasting the oreto give the oxide, followed by reduc-tion with carbon. Silver is also recov-ered from the ores. Lead has a varietyof uses including building construc-tion, lead-plate accumulators, bullets,and shot, and is a constituent of suchalloys as solder, pewter, bearing met-als, type metals, and fusible alloys.Chemically, it forms compoundswith the +2 and +4 oxidation states,the lead(II) state being the more sta-ble.A• Information from the WebElements site

lead(II) acetate See lead(ii)ethanoate.

lead–acid accumulator An accu-mulator in which the electrodes aremade of lead and the electrolyte con-sists of dilute sulphuric acid. Theelectrodes are usually cast from alead alloy containing 7–12% of anti-mony (to give increased hardness andcorrosion resistance) and a smallamount of tin (for better castingproperties). The electrodes are coatedwith a paste of lead(II) oxide (PbO)and Ünely divided lead; after inser-tion into the electrolyte a ‘forming’current is passed through the cell to

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2e

Pb2+Pb2+Pb2+ Pb2+Pb2+Pb2+

2H2SO42H2SO42H2SO4 + 4H++ 4H++ 4H+ 2O2–2O2–2O2–

2PbSO42PbSO42PbSO42H2O2H2O2H2O

sulphuric acid

2SO42–2SO42–2SO42–

+–

lead (Pb) lead(IV) oxidePbO2

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convert the PbO on the negativeplate into a sponge of Ünely dividedlead. On the positive plate the PbO isconverted to lead(IV) oxide (PbO2).The equation for the overall reactionduring discharge is:

PbO2 + 2H2SO4 + Pb → 2PbSO4 +2H2O

The reaction is reversed duringcharging. Each cell gives an e.m.f. ofabout 2 volts and in motor vehicles a12-volt battery of six cells is usuallyused. The lead–acid battery produces80–120 kJ per kilogram. Comparenickel–iron accumulator.

lead(II) carbonate A white solid,PbCO3, insoluble in water; rhombic;r.d. 6.6. It occurs as the mineral*cerussite, which is isomorphouswith aragonite and may be preparedin the laboratory by the addition ofcold ammonium carbonate solutionto a cold solution of a lead(II) salt (ac-etate or nitrate). It decomposes at315°C to lead(II) oxide and carbondioxide.

lead(II) carbonate hydroxide(white lead; basic lead carbonate) Apowder, 2PbCO3.Pb(OH)2, insoluble inwater, slightly soluble in aqueouscarbonate solutions; r.d. 6.14; decom-poses at 400°C. Lead(II) carbonate hy-droxide occurs as the mineralhydroxycerussite (of variable compo-sition). It was previously manufac-tured from lead in processes usingspent tanning bark or horse manure,which released carbon dioxide. It iscurrently made by electrolysis ofmixed solutions (e.g. ammonium ni-trate, nitric acid, sulphuric acid, andacetic acid) using lead anodes. For thehighest grade product the lead mustbe exceptionally pure (known in thetrade as ‘corroding lead’) as smallamounts of metallic impurity impartgrey or pink discolorations. The ma-terial was used widely in paints, bothfor art work and for commerce, but

it has the disadvantage of reactingwith hydrogen sulphide in industrialatmospheres and producing blacklead sulphide. The poisonous natureof lead compounds has also con-tributed to the declining importanceof this material.

lead-chamber process An obso-lete method of making sulphuricacid by the catalytic oxidation of sul-phur dioxide with air using a potas-sium nitrate catalyst in water. Theprocess was carried out in lead con-tainers (which was expensive) andonly produced dilute acid. It was re-placed in 1876 by the *contactprocess.

lead dioxide See lead(iv) oxide.

lead(II) ethanoate (lead(II) ac-etate) A white crystalline solid,Pb(CH3COO)2, soluble in water andslightly soluble in ethanol. It exists asthe anhydrous compound (r.d. 3.25;m.p. 280°C), as a trihydrate,Pb(CH3COO)2.3H2O (monoclinic; r.d.2.55; loses water at 75°C), and as adecahydrate, Pb(CH3COO)2.10H2O(rhombic; r.d. 1.69). The commonform is the trihydrate. Its chief inter-est stems from the fact that it is solu-ble in water and it also forms avariety of complexes in solution. Itwas once known as sugar of lead be-cause of its sweet taste.

lead(IV) ethanoate (lead tetra-acetate) A colourless solid,Pb(CH3COO)4, which decomposes inwater and is soluble in pure ethanoicacid; monoclinic; r.d. 2.228; m.p.175°C. It may be prepared by dissolv-ing dilead(II) lead(IV) oxide in warmethanoic acid. In solution it behavesessentially as a covalent compound(no measurable conductivity) in con-trast to the lead(II) salt, which is aweak electrolyte.

lead(IV) hydride See plumbane.

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lead monoxide See lead(ii) oxide.

lead(II) oxide (lead monoxide) Asolid yellow compound, PbO, whichis insoluble in water; m.p. 886°C. Itexists in two crystalline forms:litharge (tetrahedral; r.d. 9.53) andmassicot (rhombic; r.d. 8.0). It can beprepared by heating the nitrate, andis manufactured by heating moltenlead in air. If the temperature used islower than the melting point of theoxide, the product is massicot; abovethis, litharge is formed. Variations inthe temperature and in the rate ofcooling give rise to crystal vacanciesand red, orange, and brown forms oflitharge can be produced. The oxideis amphoteric, dissolving in acids togive lead(II) salts and in alkalis togive *plumbates.

lead(IV) oxide (lead dioxide) Adark brown or black solid with a ru-tile lattice, PbO2, which is insolublein water and slightly soluble in con-centrated sulphuric and nitric acids;r.d. 9.375; decomposes at 290°C.Lead(IV) oxide may be prepared bythe oxidation of lead(II) oxide byheating with alkaline chlorates or ni-trates, or by anodic oxidation oflead(II) solutions. It is an oxidizingagent and readily reverts to thelead(II) oxidation state, as illustratedby its conversion to Pb3O4 and PbOon heating. It reacts with hydrochlo-ric acid to evolve chlorine. Lead(IV)oxide has been used in the manufac-ture of safety matches and waswidely used until the mid-1970s as anadsorbent for sulphur dioxide in pol-lution monitoring.

lead(II) sulphate A white crys-talline solid, PbSO4, which is virtuallyinsoluble in water and soluble in so-lutions of ammonium salts; r.d. 6.2;m.p. 1170°C. It occurs as the mineralanglesite; it may be prepared in thelaboratory by adding any solutioncontaining sulphate ions to solutions

of lead(II) ethanoate. The materialknown as basic lead(II) sulphate maybe made by shaking together lead(II)sulphate and lead(II) hydroxide inwater. This material has been used inwhite paint in preference to lead(II)carbonate hydroxide, as it is not sosusceptible to discoloration throughreaction with hydrogen sulphide.The toxicity of lead compounds hasled to a decline in the use of thesecompounds.

lead(II) sulphide A black crys-talline solid, PbS, which is insolublein water; r.d. 7.5; m.p. 1114°C. It oc-curs naturally as the metallic-lookingmineral *galena (the principal ore oflead). It may be prepared in the labo-ratory by the reaction of hydrogensulphide with soluble lead(II) salts.Lead(II) sulphide has been used as anelectrical rectiÜer.

lead tetra-acetate See lead(iv)ethanoate.

lead(IV) tetraethyl (tetraethyllead) A colourless liquid, Pb(C2H5)4,insoluble in water, soluble in ben-zene, ethanol, ether, and petroleum;r.d. 1.659; m.p. –137°C; b.p. 200°C. Itmay be prepared by the reaction ofhydrogen and ethene with lead but amore convenient laboratory and in-dustrial method is the reaction of asodium–lead alloy with chloroethane.A more recent industrial process isthe electrolysis of ethylmagnesiumchloride (the Grignard reagent) usinga lead anode and slowly running ad-ditional chloroethane onto the cath-ode. Lead tetraethyl is used in fuelfor internal-combustion engines(along with 1,2-dibromoethane) to in-crease the *octane number and re-duce knocking. The use of lead(IV)tetraethyl in petrol results in theemission of hazardous lead com-pounds into the atmosphere. Pres-sure from environmental groups hasencouraged a reduction in the use of

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lead(IV) tetraethyl and an increasinguse of lead-free petrol.

Leblanc process An obsoleteprocess for manufacturing sodiumcarbonate. The raw materials weresodium chloride, sulphuric acid,coke, and limestone (calcium carbon-ate), and the process involved twostages. First the sodium chloride washeated with sulphuric acid to givesodium sulphate:

2NaCl(s) + H2SO4(l) → Na2SO4(s) +2HCl(g)

The sodium sulphate was thenheated with coke and limestone:

Na2SO4 + 2C + CaCO3 → Na2CO3 +CaS + 2CO2

Calcium sulphide was a by-product,the sodium carbonate being ex-tracted by crystallization. Theprocess, invented in 1783 by theFrench chemist Nicolas Leblanc(1742–1806), was the Ürst for produc-ing sodium carbonate synthetically(earlier methods were from wood ashand other vegetable sources). By theend of the 19th century it had beenlargely replaced by the *Solvayprocess.

lechatelierite A mineral form of*silicon(IV) oxide, SiO2.

Le Chatelier’s principle If a sys-tem is in equilibrium, any changeimposed on the system tends to shiftthe equilibrium to nullify the effectof the applied change. The principle,which is a consequence of the law ofconservation of energy, was Ürststated in 1888 by Henri Le Chatelier(1850–1936). It is applied to chemicalequilibria. For example, in the gas re-action

2SO2 + O2 ˆ 2SO3

an increase in pressure on the reac-tion mixture displaces the equilib-rium to the right, since this reducesthe total number of molecules pre-

sent and thus decreases the pressure.The standard enthalpy change forthe forward reaction is negative (i.e.the reaction is exothermic). Thus, anincrease in temperature displaces theequilibrium to the left since thistends to reduce the temperature. The*equilibrium constant thus falls withincreasing temperature.

A• Le Chatelier’s original paper

Leclanché cell A primary *voltaiccell consisting of a carbon rod (theanode) and a zinc rod (the cathode)dipping into an electrolyte of a10–20% solution of ammonium chlo-ride. *Polarization is prevented byusing a mixture of manganese diox-ide mixed with crushed carbon, heldin contact with the anode by meansof a porous bag or pot; this reactswith the hydrogen produced. Thiswet form of the cell, devised in 1867by Georges Leclanché (1839–82), hasan e.m.f. of about 1.5 volts. The *drycell based on it is widely used intorches, radios, and calculators.

lectin Any of a group of proteins,derived from plants, that can bind tospeciÜc oligosaccharides on the sur-face of cells, causing the cells toclump together. Lectins can be usedto identify mutant cells in cell cul-tures and to determine blood groupsas they can cause the agglutinationof red blood cells. Lectins are foundin seeds of legumes and in other tis-sues, in which they are thought toact as a toxin.

LEED (low-energy electron diffrac-tion) A technique used to study thestructure of crystal surfaces andprocesses taking place on these sur-faces. The surface is bombarded witha monochromatic electron beam 10–4

to 10–3 m in diameter, with energiesbetween 6 and 600 V. The electronsare diffracted by the surface atoms

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and then collected on a Ûuorescentscreen. Both the surface structureand changes that occur afterchemisorption and surface reactionscan be investigated in this way. It isnecessary for the surface to be care-fully cleaned and kept at ultrahighvacuum pressure. Although manysurfaces are altered by the electronbeam and therefore cannot be stud-ied using this method, there areenough surfaces and surfaceprocesses that can be studied usingLEED to make it a very useful tech-nique. DifÜculties in interpretingLEED patterns arise as multiple-scattering theory, rather than single-scattering theory (as in X-ray or neu-tron scattering), is required. See alsoelectron diffraction.

Lennard-Jones potential A po-tential used to give an approximatedescription of the potential energyinteraction, V, of molecules as a func-tion of intermolecular distance r. Thegeneral form of the Lennard-Jonespotential is

V = Cn/rn – C6/r6,where Cn and C6 are coefÜcients thatdepend on the speciÜc molecules andn is greater than 6 so that at smallseparations the repulsion term domi-nates the interaction, the r–6 termbeing attractive. The value n = 12 isfrequently chosen. In this case theLennard-Jones potential is given by:

V = 4W[(r0/r)12 – (r0/r)6],where W is the depth of the potentialwell and r0 is the separation at whichV = 0. The minimum value of thewell occurs at the separation re =21/6r0. The representation of the re-pulsive part of the interaction by a1/r12 term is not realistic; a muchmore realistic term is the exponen-tial term, exp(–r/r0), as it is closer tothe exponential decay of the wave

functions and thus of their overlap,which describes the repulsion.

leucine See amino acid.

leuco form See dyes.

leucomalachite green test A*presumptive test for blood. Thereagent is the dye leucomalachitegreen dissolved in water along withsodium perborate (NaBO3). A blue-green colour indicates a positive re-sult.

lever rule A rule enabling the rela-tive amounts of two phases a and b,which are in equilibrium, to befound by a construction in a phasediagram. (For example, a can be gasand b can be liquid.) The distances laand lb along the horizontal *tie lineof the phase diagram are measured.The lever rule states that nala = nblb,where na is the amount of phase aand nb is the amount of phase b. Therule takes its name from the similarform of the rule, mala = mblb, relatingthe moments of two masses ma andmb about a pivot in a lever.

Lewis, Gilbert Newton (1875–1946) US physical chemist who spentmost of his career at Berkeley, Cali-fornia. His ideas on chemical bond-ing were extremely inÛuential, andhe introduced the idea of a stableoctet of electrons and of a covalentbond being a shared pair of elec-trons. He also introduced the conceptof Lewis acids and bases (see acid).

Lewis acid and base See acid.

Liebermann’s reaction A methodof testing for phenols. A small sam-ple of the test substance and a crystalof sodium nitrite are dissolved inwarm sulphuric acid. The solution isthen poured into excess aqueous al-kali, when the formation of a blue-green colour indicates the presenceof a phenol.

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Liebermann test A *presumptivetest sometimes used for cocaine andmorphine. The Liebermann reagentis a solution of potassium nitrite(KNO2) in sulphuric acid. With mor-phine a black colour is produced; co-caine gives a yellow colour.

Liebig, Justus von (1803–73) Ger-man organic chemist who worked atGessen in Frankfurt. Liebig was theÜrst to recognise that two differentchemical compounds can have thesame formula. He also developed amethod of analysing organic com-pounds by burning them and weigh-ing the carbon dioxide and waterproduced. With his students, heanalysed many compounds and wasextremely inÛuential in the develop-ment of organic chemistry.

Liebig condenser A laboratorycondenser having a straight glasstube surrounded by a coaxial glassjacket through which cooling wateris passed. The device is named afterthe German organic chemist Justusvon Liebig (1803–73).

ligand An ion or molecule that do-nates a pair of electrons to a metalatom or ion in forming a coordina-tion *complex. Molecules that func-tion as ligands are acting as Lewisbases (see acid). For example, in thecomplex hexaquocopper(II) ion[Cu(H2O)6]2+ six water molecules coor-dinate to a central Cu2+ ion. In thetetrachloroplatinate(II) ion [PtCl4]2–,four Cl– ions are coordinated to acentral Pt2+ ion. A feature of such lig-ands is that they have lone pairs ofelectrons, which they donate toempty metal orbitals. A certain classof ligands also have empty p- or d-orbitals in addition to their lone pairof electrons and can produce com-plexes in which the metal has lowoxidation state. A double bond isformed between the metal and theligand: a sigma bond by donation of

the lone pair from ligand to metal,and a pi bond by back donation ofelectrons on the metal to empty d-or-bitals on the ligand. Carbon monox-ide is the most important suchligand, forming metal carbonyls (e.g.Ni(CO)4).

The examples given above are ex-amples of monodentate ligands (liter-ally: ‘having one tooth’), in whichthere is only one point on each lig-and at which coordination can occur.Some ligands are polydentate; i.e.they have two or more possible coor-dination points. For instance, 1,2-di-aminoethane, H2NC2H4NH2, is abidentate ligand, having two coordi-nation points. Certain polydentateligands can form *chelates.

ligand-Üeld theory An extensionof *crystal-Üeld theory describing theproperties of compounds of transi-tion-metal ions or rare-earth ions inwhich covalent bonding between thesurrounding molecules (see ligand)and the transition-metal ions is takeninto account. This may involve usingvalence-bond theory or molecular-orbital theory. Ligand-Üeld theorywas developed extensively in the1930s. As with crystal-Üeld theory,ligand-Üeld theory indicates that en-ergy levels of the transition-metalions are split by the surrounding lig-ands, as determined by *grouptheory. The theory has been very suc-cessful in explaining the optical,spectroscopic, and magnetic proper-ties of the compounds of transition-metal and rare-earth ions.

ligase Any of a class of enzymesthat catalyse the formation of cova-lent bonds using the energy releasedby the cleavage of ATP. Ligases areimportant in the synthesis and repairof many biological molecules, includ-ing DNA, and are used in genetic en-gineering to insert foreign DNA intocloning vectors.

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light-dependent reaction Seephotosynthesis.

light-independent reaction Seephotosynthesis.

lignin A complex organic polymerthat is deposited within the celluloseof plant cell walls during secondarythickening. LigniÜcation makes thewalls woody and therefore rigid.

lignite See coal.

lime See calcium oxide.

limestone A sedimentary rock thatis composed largely of carbonateminerals, especially carbonates ofcalcium and magnesium. *Calciteand *aragonite are the chief miner-als; *dolomite is also present in thedolomitic limestones. There aremany varieties of limestones butmost are deposited in shallow water.Organic limestones (e.g. *chalk) areformed from the calcareous skele-tons of organisms; precipitated lime-stones include oolite, which iscomposed of ooliths – spherical bod-ies formed by the precipitation ofcarbonate around a nucleus; and clas-tic limestones are derived from frag-ments of pre-existing calcareousrocks.

limewater A saturated solution of*calcium hydroxide in water. Whencarbon dioxide gas is bubbledthrough limewater, a ‘milky’ precipi-tate of calcium carbonate is formed:

Ca(OH)2(aq) + CO2(g) → CaCO3(s) +H2O(l)

If the carbon dioxide continues to bebubbled through, the calcium car-bonate eventually redissolves to forma clear solution of calcium hydrogen-carbonate:

CaCO3(s) + CO2(g) + H2O(g) →Ca(HCO3)2(aq)

If cold limewater is used the originalcalcium carbonate precipitated has a

calcite structure; hot limewateryields an aragonite structure.

limit cycle See attractor.

limonite A generic term for agroup of hydrous iron oxides, mostly amorphous. *Goethite and*haematite are important con-stituents, together with colloidalsilica, clays, and manganese oxides.Limonite is formed by direct precipi-tation from marine or fresh water inshallow seas, lagoons, and bogs (thusit is often called bog iron ore) and byoxidation of iron-rich minerals. It isused as an ore of iron and as a pig-ment.

Lindemann–Hinshelwood mech-anism A mechanism for unimolecu-lar chemical reactions put forward bythe British physicist Frederick Linder-mann (1886–1957) in 1921 and exam-ined in more detail by the Britishchemist Sir Cyril Hinshelwood(1897–1967) in 1927. The mechanismpostulates that a molecule of A be-comes excited by colliding with an-other molecule of A, and that havingbeen excited there is a possibilitythat it undergoes unimoleculardecay. If the process of unimoleculardecay is sufÜciently slow, the reac-tion has a Ürst-order rate law, inagreement with experiment. The Lin-demann–Hinshelwood mechanismpredicts that if the concentration ofA is reduced, the reaction kinetics be-come second order. This change fromÜrst to second order agrees with ex-periment qualitatively, although itdoes not do so quantitatively. Themechanism fails quantitatively be-cause the molecule has to be excitedin a speciÜc way for a reaction totake place. The RRK and RRKM theo-ries improve on this deÜciency of theLindemann–Hinshelwood mecha-nism.

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linear combination of atomicorbitals See lcao.

linear molecule A molecule inwhich the atoms are in a straightline, as in carbon dioxide, O=C=O.Linear molecules have only two rota-tional degrees of freedom.

linear rotor See moment of iner-tia.

line notation A notation systemfor writing the structure of a chemi-cal compound as a string of letters,numbers, and symbols. Examples ofline notation are *Wiswesser line no-tation (WLN), *SMILES, *SYBYL linenotation (SLN), *ROSDAL, and*InChI.

line spectrum See spectrum.

linoleic acid A liquid polyunsatu-rated *fatty acid with two doublebonds, CH3(CH2)4CH:CHCH2-CH:CH(CH2)7COOH. Linoleic acid isabundant in plant fats and oils, e.g.linseed oil, groundnut oil, and soya-bean oil. It is an *essential fatty acid.

linolenic acid A liquid polyunsatu-rated *fatty acid with three doublebonds in its structure:CH3CH2CH:CHCH2CH:CHCH2CH:CH-(CH2)7COOH. It occurs in certainplant oils, e.g. linseed and soya-beanoil, and in algae. It is one of the *es-sential fatty acids.

linseed oil A pale yellow oilpressed from Ûax seed. It contains amixture of glycerides of fatty acids,including linoleic acid and linolenicacid. It is a *drying oil, used in oilpaints, varnishes, linoleum, etc.

Linz–Donawitz process See basic-oxygen process.

lipase An enzyme secreted by thepancreas and the glands of the smallintestine of vertebrates that catalysesthe breakdown of fats into fatty acidsand glycerol.

lipid Any of a diverse group of or-ganic compounds, occurring in livingorganisms, that are insoluble inwater but soluble in organic solvents,such as chloroform, benzene, etc.Lipids are broadly classiÜed into twocategories: complex lipids, which areesters of long-chain fatty acids andinclude the *glycerides (which con-stitute the *fats and *oils of animalsand plants), *glycolipids, *phospho-lipids, and *waxes; and simple lipids,which do not contain fatty acids andinclude the *steroids and *terpenes.

Lipids have a variety of functions inliving organisms. Fats and oils are aconvenient and concentrated meansof storing food energy in plants andanimals. Phospholipids and *sterols,such as cholesterol, are major com-ponents of cell membranes (see lipidbilayer). Waxes provide vital water-prooÜng for body surfaces. Terpenesinclude vitamins A, E, and K, andphytol (a component of chlorophyll)and occur in essential oils, such asmenthol and camphor. Steroids in-clude the adrenal hormones, sex hor-mones, and bile acids.

Lipids can combine with proteinsto form lipoproteins, e.g. in cellmembranes. In bacterial cell walls,lipids may associate with polysaccha-rides to form lipopolysaccharides.


of lipids

lipid bilayer The arrangement oflipid molecules in biological mem-branes, which takes the form of adouble sheet. Each lipid moleculecomprises a hydrophilic ‘head’ (hav-ing a high afÜnity for water) and ahydrophobic ‘tail’ (having a lowafÜnity for water). In the lipid bilayerthe molecules are aligned so thattheir hydrophilic heads face out-wards, forming the outer and innersurfaces of the membrane, while the

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hydrophobic tails face inwards, awayfrom the external aqueous environ-ment.

lipoic acid A vitamin of the *vita-min B complex. It is one of the*coenzymes involved in the decar-boxylation of pyruvate by the en-zyme pyruvate dehydrogenase. Goodsources of lipoic acid include liverand yeast.

lipolysis The breakdown of storagelipids in living organisms. Most long-term energy reserves are in the formof triglycerides in fats and oils. Whenthese are needed, e.g. during starva-tion, lipase enzymes convert thetriglycerides into glycerol and thecomponent fatty acids. These arethen transported to tissues and oxi-dized to provide energy.

lipoprotein See lipid.

lipowitz alloy A low-melting(70–74°C) alloy of bismuth (50%), lead(27%), tin (13%), and cadmium (10%).

liquation The separation of mix-tures of solids by heating to a tem-perature at which lower-meltingcomponents liquefy.

liquefaction of gases The conver-sion of a gaseous substance into a liq-uid. This is usually achieved by oneof four methods or by a combinationof two of them:(1) by vapour compression, providedthat the substance is below its *criti-cal temperature;(2) by refrigeration at constant pres-sure, typically by cooling it with acolder Ûuid in a countercurrent heatexchanger;(3) by making it perform work adia-batically against the atmosphere in areversible cycle;(4) by the *Joule–Thomson effect.

Large quantities of liqueÜed gasesare now used commercially, espe-

cially *liqueÜed petroleum gas andliqueÜed natural gas.

liqueÜed natural gas (LNG) Seeliquefied petroleum gas.

liqueÜed petroleum gas (LPG)Various petroleum gases, principallypropane and butane, stored as a liq-uid under pressure. It is used as anengine fuel and has the advantage ofcausing very little cylinder-head de-posits.

LiqueÜed natural gas (LNG) is a sim-ilar product and consists mainly ofmethane. However, it cannot beliqueÜed simply by pressure as it hasa low critical temperature of 190 Kand must therefore be cooled tobelow this temperature before it willliquefy. Once liqueÜed it has to bestored in well-insulated containers. Itprovides a convenient form in whichto ship natural gas in bulk from oilwells or gas-only wells to users. It isalso used as an engine fuel.

liquid A phase of matter betweenthat of a crystalline solid and a *gas.In a liquid, the large-scale three-dimensional atomic (or ionic or mo-lecular) regularity of the solid is ab-sent but, on the other hand, so is thetotal disorganization of the gas. Al-though liquids have been studied formany years there is still no compre-hensive theory of the liquid state. Itis clear, however, from diffractionstudies that there is a short-rangestructural regularity extending overseveral molecular diameters. Thesebundles of ordered atoms, molecules,or ions move about in relation toeach other, enabling liquids to havealmost Üxed volumes, which adoptthe shape of their containers.

liquid crystal A substance thatÛows like a liquid but has some orderin its arrangement of molecules. Ne-matic crystals have long molecules allaligned in the same direction, but

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otherwise randomly arranged. Cho-lesteric and smectic liquid crystalsalso have aligned molecules, whichare arranged in distinct layers. Incholesteric crystals, the axes of themolecules are parallel to the plane ofthe layers; in smectic crystals theyare perpendicular.

liquid-crystal polymer A polymerwith a liquid-crystal structure, thisbeing the most thermodynamicallystable. Liquid-crystal polymers con-tain long rigid chains and combinestrength with lightness. They are,however, difÜcult to produce com-mercially.

liquidus A line on a phase diagramabove which a substance is liquid.

l-isomer See optical activity.

L-isomer See absolute configura-tion.

litharge See lead(ii) oxide.

lithia See lithium oxide.

lithium Symbol Li. A soft silverymetal, the Ürst member of group 1(formerly IA) of the periodic table (see alkali metals); a.n. 3; r.a.m.6.939; r.d. 0.534; m.p. 180.54°C; b.p.1347°C. It is a rare element found inspodumene (LiAlSi2O6), petalite(LiAlSi4O10), the mica lepidolite, andcertain brines. It is usually extractedby treatment with sulphuric acid togive the sulphate, which is convertedto the chloride. This is mixed with asmall amount of potassium chloride,melted, and electrolysed. The stableisotopes are lithium–6 and lithium–7.Lithium–5 and lithium–8 are short-lived radioisotopes. The metal is usedto remove oxygen in metallurgy andas a constituent of some Al and Mgalloys. It is also used in batteries andis a potential tritium source for fu-sion research. Lithium salts are usedin psychomedicine. The element re-acts with oxygen and water; on heat-

ing it also reacts with nitrogen andhydrogen. Its chemistry differs some-what from that of the other group 1elements because of the small size ofthe Li+ ion.


lithium aluminium hydride Seelithium tetrahydroaluminate(iii).

lithium battery A type of voltaiccell containing lithium or lithiumcompounds. The most commonlyused has a metallic lithium anodeand a manganese dioxide (MnO2)cathode, the electrolyte being a solu-tion of lithium salts in an organic sol-vent. Batteries of this type have anoutput of about 3 volts. They aremore expensive than alkaline batter-ies, but last longer. Li–MnO2 batteriesare also produced in a Ûat disk formfor use in digital watches and othersmall portable devices. A number ofother more specialized lithium pri-mary batteries are available but arenot in general use.

The lithium-ion battery is arechargeable cell. The anode is car-bon and the cathode is a metal oxide(e.g. cobalt(IV) oxide, CoO2). The elec-trolyte is a lithium salt such as theborate (LiBO4) or chlorate (LiClO4) inan organic solvent. The action of thecell depends on movement of Li ionsbetween anode and cathode with oxi-dation of the cobalt ions duringcharging and reduction during dis-charge. Lithium-ion batteries arelight and have a low self-dischargerate, although the capacity does dete-riorate with age. They are extensivelyused in mobile phones, laptops, cam-corders, and similar devices, as wellas electric cars.

lithium carbonate A white solid,Li2CO3; r.d. 2.11; m.p. 723°C; decom-poses above 1310°C. It is producedcommercially by treating the ore

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with sulphuric acid at 250°C andleaching the product to give a solu-tion of lithium sulphate. The carbon-ate is then obtained by precipitationwith sodium carbonate solution.Lithium carbonate is used in the pre-vention and treatment of manic-depressive disorders. It is also usedindustrially in ceramic glazes.

lithium deuteride See lithium hy-dride.

lithium hydride A white solid,LiH; cubic; r.d. 0.82; m.p. 680°C; de-composes at about 850°C. It is pro-duced by direct combination of theelements at temperatures above500°C. The bonding in lithium hy-dride is believed to be largely ionic;i.e. Li+H– as supported by the factthat hydrogen is released from theanode on electrolysis of the moltensalt. The compound reacts violentlyand exothermically with water toyield hydrogen and lithium hydrox-ide. It is used as a reducing agent toprepare other hydrides and the 2Hisotopic compound, lithiumdeuteride, is particularly valuable fordeuterating a range of organic com-pounds. Lithium hydride has alsobeen used as a shielding material forthermal neutrons.

lithium hydrogencarbonate Acompound, LiHCO3, formed by thereaction of carbon dioxide with aque-ous lithium carbonate and knownonly in solution. It has found medici-nal uses similar to those of lithiumcarbonate and is sometimes includedin proprietary mineral waters.

lithium hydroxide A white crys-talline solid, LiOH, soluble in water,slightly soluble in ethanol and insolu-ble in ether. It is known as the mono-hydrate (monoclinic; r.d. 1.51) and inthe anhydrous form (tetragonal, r.d.1.46; m.p. 450°C; decomposes at924°C). The compound is made by re-

acting lime with lithium salts orlithium ores. Lithium hydroxide isbasic but has a closer resemblance togroup 2 hydroxides than to the othergroup 1 hydroxides (an example ofthe Ürst member of a periodic grouphaving atypical properties).

lithium oxide (lithia) A white crys-talline compound, Li2O; cubic; r.d.2.01; m.p. 1700°C. It can be obtainedfrom a number of lithium ores; themain uses are in lubricating greases,ceramics, glass and refractories, andas a Ûux in brazing and welding.

lithium sulphate A white orcolourless crystalline material,Li2SO4, soluble in water and insolublein ethanol. It forms a monohydrate(monoclinic; r.d. 1.88) and an anhy-drous form, which exists in α- (mono-clinic), β- (hexagonal) and γ- (cubic)forms; r.d. 2.23. The compound isprepared by the reaction of the hy-droxide or carbonate with sulphuricacid. It is not isomorphous withother group 1 sulphates and does notform alums.

lithium tetrahydroaluminate(III)(lithium aluminium hydride; LAH) Awhite or light grey powder, LiAlH4;r.d. 0.917; decomposes at 125°C. It isprepared by the reaction of excesslithium hydride with aluminiumchloride. The compound is soluble inethoxyethane, reacts violently withwater to release hydrogen, and iswidely used as a powerful reducingagent in organic chemistry. It shouldalways be treated as a serious Üre riskin storage.

litmus A water-soluble dye ex-tracted from certain lichens. It turnsred under acid conditions and blueunder alkaline conditions, the colourchange occurring over the pH range4.5–8.3 (at 25°C). It is not suitable fortitrations because of the wide rangeover which the colour changes, but is

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used as a rough *indicator of acidityor alkalinity, both in solution and aslitmus paper (absorbent paper soakedin litmus solution).

litre Symbol l. A unit of volume inthe metric system regarded as a spe-cial name for the cubic decimetre. Itwas formerly deÜned as the volumeof 1 kilogram of pure water at 4°C atstandard pressure, which is equiva-lent to 1.000 028 dm3.

lixiviation The separation of mix-tures by dissolving soluble con-stituents in water.

LNG See liquefied petroleum gas.

localization The conÜnement ofelectrons to a particular atom in amolecule or to a particular chemicalbond.

localized bond A *chemical bondin which the electrons forming thebond remain between (or close to)the linked atoms. Compare delocal-ization.

lock-and-key mechanism Amechanism proposed in 1890 byEmil Fischer (1852–1919) to explainbinding between the active site of anenzyme and a substrate molecule.The active site was thought to have aÜxed structure (the lock), which ex-actly matched the structure of aspeciÜc substrate (the key). Thus theenzyme and substrate interact toform an *enzyme–substrate complex.The substrate is converted to prod-ucts that no longer Üt the active siteand are therefore released, liberatingthe enzyme. Recent observationsmade by X-ray diffraction studieshave shown that the active site of anenzyme is more Ûexible than thelock-and-key theory would suggest.

lodestone See magnetite.

logarithmic scale 1. A scale ofmeasurement in which an increase

or decrease of one unit represents atenfold increase or decrease in thequantity measured. Decibels and pHmeasurements are common exam-ples of logarithmic scales of measure-ment. 2. A scale on the axis of agraph in which an increase of oneunit represents a tenfold increase inthe variable quantity. If a curve y = xn

is plotted on graph paper with loga-rithmic scales on both axes, the re-sult is a straight line of slope n, i.e.logy = nlogx, which enables n to bedetermined.

London formula A formula givingan expression for the induced-dipole–induced-dipole interactionbetween molecules (called the dis-persion interaction or London inter-action). The London formula for theinteraction energy V is given by V=–C/r6, where C = ⅔α′1α′2I1I2/(I1 + I2).Here α′1 and α′2 are the polarizabilityvolumes of molecule 1 and 2 respec-tively, I1 and I2 are the ionizationenergies of molecules 1 and 2 respec-tively, and r is the distance betweenthe molecules. The London formulais named after Fritz London (1900–54), who derived it. The interactiondescribed by the London formula isusually the dominant term in inter-molecular forces (unless hydrogenbonds are present).

lone pair A pair of electrons havingopposite spin in an orbital of anatom. For instance, in ammonia thenitrogen atom has Üve electrons,three of which are used in formingsingle bonds with hydrogen atoms.The other two occupy a Ülled atomicorbital and constitute a lone pair (seeillustration). The orbital containingthese electrons is equivalent to a sin-gle bond (sigma orbital) in spatial ori-entation, accounting for thepyramidal shape of the molecule. Inthe water molecule, there are twolone pairs on the oxygen atom. In

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long period 332

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H

H

H

N

Lone pair

considering the shapes of molecules,repulsions between bonds and lonepairs can be taken into account:

lone pair–lone pair > lonepair–bond > bond–bond.

long period See periodic table.

Lorentz–Lorenz equation A rela-tion between the *polarizability α ofa molecule and the refractive index n of a substance made up of mol-ecules with this polarizability. TheLorentz–Lorenz equation can be writ-ten in the form α = (3/4πN) [(n2–1/(n2 + 2)], where N is the number ofmolecules per unit volume. Theequation provides a link between amicroscopic quantity (the polarizabil-ity) and a macroscopic quantity (therefractive index). It was derived usingmacroscopic electrostatics in 1880 byHendrik Lorentz (1853–1928) and in-dependently by the Danish physicistLudwig Valentin Lorenz also in 1880.Compare clausius–mossotti equa-tion.

Loschmidt’s constant (Loschmidtnumber) The number of particlesper unit volume of an *ideal gas atSTP. It has the value 2.686 763(23) ×1025 m–3 and was Ürst worked out byJoseph Loschmidt (1821–95).

Lotka–Volterra mechanism Asimple chemical reaction mechanismproposed as a possible mechanism of*oscillating reactions. The process in-volves a conversion of a reactant Rinto a product P. The reactant Ûowsinto the reaction chamber at a con-

stant rate and the product is re-moved at a constant rate, i.e. the re-action is in a steady state (but not inchemical equilibrium). The mecha-nism involves three steps:

R + X → 2XX + Y → 2YY → PThe Ürst two steps involve *auto-

catalysis: the Ürst step is catalysed bythe reactant X and the second by thereactant Y. The kinetics of such a re-action can be calculated numerically,showing that the concentrations ofboth X and Y increase and decreaseperiodically with time. This resultsfrom the autocatalytic action. Ini-tially, the concentration of X is small,but, as it increases, there is a rapidincrease in the rate of the Ürst reac-tion because of the autocatalytic ac-tion of X. As the concentration of Xbuilds up, the rate of the second reac-tion also increases. Initially, the con-centration of Y is low but there is asudden surge in the rate of step 2, re-sulting from the autocatalytic actionof Y. This lowers the concentration ofX and slows down step 1, so the con-centration of X falls. Less X is nowavailable for the second step and theconcentration of Y also starts to fall.With this fall in the amount of Y, lessX is removed, and the Ürst reactionagain begins to increase. Theseprocesses are repeated, leading to re-peated rises and falls in the concen-trations of both X and Y. The cyclesare not in phase, peaks in the con-centration of Y occurring later thanpeaks in X.

In fact, known oscillating chemicalreactions have different mechanismsto the above, but the scheme illus-trates how oscillation may occur.This type of process is found in Üeldsother than chemistry; they were in-vestigated by the Italian mathemati-cian Vito Volterra (1860–1940) in


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models of biological systems (e.g.predator–prey relationships).

low-energy electron diffractionSee leed.

lowering of vapour pressure Areduction in the saturated vapourpressure of a pure liquid when asolute is introduced. If the solute is a solid of low vapour pressure, thedecrease in vapour pressure of the liquid is proportional to theconcentration of particles of solute;i.e. to the number of dissolved mol-ecules or ions per unit volume. To aÜrst approximation, it does not de-pend on the nature of the particles.See colligative properties; raoult’slaw.

lowest unoccupied molecularorbital (LUMO) The orbital in amolecule that has the lowest unoccu-pied energy level at the absolute zeroof temperature. The lowest unoccu-pied molecular orbital and the high-est occupied molecular orbital(HOMO) are the two *frontier or-bitals of the molecule.

Lowry–Brønsted theory See acid.

LSD See lysergic acid diethy-lamide.

L-series See absolute configura-tion.

lubrication The use of a substanceto prevent contact between solid sur-faces in relative motion in order toreduce friction, wear, overheating,and rusting. Liquid hydrocarbons(oils), either derived from petroleumor made synthetically, are the mostwidely used lubricants as they arerelatively inexpensive, are goodcoolants, provide the appropriaterange of viscosities, and are ther-mally stable. Additives include poly-meric substances that maintain thedesired viscosity as the temperatureincreases, antioxidants that prevent

the formation of a sludge, and alka-line-earth phenates that neutralizeacids and reduce wear.

At high temperatures, solid lubri-cants, such as graphite or molybde-num disulphide, are often used.SemiÛuid lubricants (greases) areused to provide a seal against mois-ture and dirt and to remain attachedto vertical surfaces. They are madeby adding gelling agents, such asmetallic soaps, to liquid lubricants.

luciferase See bioluminescence.

luciferin See bioluminescence.

lumen Symbol lm. The SI unit of lu-minous Ûux equal to the Ûux emittedby a uniform point source of 1 can-dela in a solid angle of 1 steradian.

luminescence The emission oflight by a substance for any reasonother than a rise in its temperature.In general, atoms of substances emit*photons of electromagnetic energywhen they return to the *groundstate after having been in an excitedstate (see excitation). The causes ofthe excitation are various. If the ex-citing cause is a photon, the processis called photoluminescence; if it isan electron it is called electrolumi-nescence. Chemiluminescence is lumi-nescence resulting from a chemicalreaction (such as the slow oxidationof phosphorus); *bioluminescence isthe luminescence produced by a liv-ing organism (such as a ÜreÛy). If theluminescence persists signiÜcantlyafter the exciting cause is removed itis called phosphorescence; if it doesnot it is called Ûuorescence. This dis-tinction is arbitrary since there mustalways be some delay; in some deÜni-tions a persistence of more than 10nanoseconds (10–8 s) is treated asphosphorescence.

luminol test A *presumptive testfor blood. The reagent is a mixture of3-aminophthalhydrazide, sodium car-

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bonate, and sodium perborate. Whensprayed with the reagent, traces ofblood (even old blood) emit a faintchemoluminescence.

LUMO See lowest unoccupied mo-lecular orbital.

lutetium Symbol Lu. A silverymetallic element belonging to the*lanthanoids; a.n. 71; r.a.m. 174.97;r.d. 9.8404 (20°C); m.p. 1663°C; b.p.3402°C. Lutetium is the least abun-dant of the elements and the littlequantities that are available havebeen obtained by processing othermetals. There are two natural iso-topes, lutetium–175 (stable) andlutetium–176 (half-life 2.2 × 1010

years). The element is used as a cata-lyst. It was Ürst identiÜed by GergesUrbain (1872–1938) in 1907.A• Information from the WebElements site

lux Symbol lx. The SI unit of illumi-nance equal to the illumination pro-duced by a luminous Ûux of 1 lumendistributed uniformly over an area of1 square metre.

lyate ion The ion formed by re-moving a hydron from a molecule ofa solvent. In water, for example, thehydroxide ion (OH–) is the lyate ion.

lye See potassium hydroxide.

Lyman series See hydrogen spec-trum.

lyonium ion The ion formed byadding a hydron (H+) to a solventmolecule. For example, in ethanol,C2H5OH2

+ is the lyonium ion.

lyophilic Having an afÜnity for asolvent (‘solvent-loving’; if the sol-vent is water the term hydrophilic isused). See colloids.

lyophobic Lacking any afÜnity fora solvent (‘solvent-hating’; if the sol-vent is water the term hydrophobic isused). See colloids.

lyotropic mesomorph Anarrangement taken by micellesformed from surfactant molecules inconcentrated solutions. A lyotropicmesomorph consists of long cylin-ders in a fairly close-packed hexago-nal arrangement. Lyotropicmesomorphs are sometimes calledliquid crystalline phases for micelles.

lysergic acid diethylamide (LSD)A chemical derivative of lysergic acidthat has potent hallucinogenic prop-erties (see hallucinogen). It occurs inthe cereal-fungus ergot and isclassiÜed as an ergot *alkaloid. LSDwas Ürst synthesized in 1943.

lysine See amino acid.

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M

macromolecular crystal A crys-talline solid in which the atoms areall linked together by covalentbonds. Carbon (in diamond), boronnitride, and silicon carbide are exam-ples of substances that have macro-molecular crystals. In effect, thecrystal is a large molecule (hence the alternative description giant-molecular), which accounts for thehardness and high melting point ofsuch materials.

macromolecule A very large mol-ecule. Natural and synthetic poly-mers have macromolecules, as dosuch substances as haemoglobin. Seealso colloids.

macroscopic Designating a sizescale very much larger than that ofatoms and molecules. Macroscopicobjects and systems are described byclassical physics although *quantummechanics can have macroscopicconsequences. Compare mesoscopic;microscopic.

Madelung constant A constantarising in calculations of the cohe-sion of ionic crystals. The electrosta-tic interaction per ion pair, U, isgiven by U(r) = – αe2/r, where α is theMadelung constant and e2/r is theCoulomb interaction between theions, with r being the lattice con-stant. The value of α depends on thetype of lattice. For the sodium chlo-ride lattice, α has a value of about1.75. A more realistic calculation ofcohesion is obtained if short-rangerepulsions with an inverse power laware included, i.e.

U(r) = αe2/r – C/rn,

where C and n are constants. Thevalue of α can be used in calculationsto determine C and n. It was Ürst cal-culated by Erwin Madelung in 1918.

Magic acid See superacid.

magic-angle spinning A tech-nique used in solid-state *nuclearmagnetic resonance (NMR) formaking the line widths smaller. Inmagic-angle spinning, both the dipole–dipole interaction and thechemical shift anisotropy have theangular dependence 1–3cos2θ, whereθ is the angle between the principalaxis of the molecule and the appliedmagnetic Üeld. The ‘magic angle’ isthe angle θ that satisÜes 1–3cos2θ = 0and is given by θ = 54.74°. In magic-angle spinning the material is spunvery rapidly at the magic angle to theapplied magnetic Üeld so that the di-pole–dipole interactions and chemi-cal shift anisotropies average to zero.It is necessary for the frequency ofspinning to be at least as large as thewidth of the spectrum. This tech-nique has been extensively used,with the spinning between 4 and5 kHz.

Magnadur Tradename for a ce-ramic material used to make perma-nent magnets. It consists of sinterediron oxide and barium oxide.

Magnalium Tradename for an alu-minium-based alloy of high reÛectiv-ity for light and ultraviolet radiationthat contains 1–2% of copper and be-tween 5% and 30% of magnesium.Strong and light, these alloys alsosometimes contain other elements,such as tin, lead, and nickel.


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magnesia See magnesium oxide.

magnesite A white, colourless, orgrey mineral form of *magnesiumcarbonate, MgCO3, crystallizing inthe trigonal system. It is formed as areplacement mineral of magnesium-rich rocks when carbon dioxide isavailable. Magnesite is mined both asan ore for magnesium and as asource of magnesium carbonate. Itoccurs in Austria, USA, Greece, Nor-way, India, Australia, and SouthAfrica.

magnesium Symbol Mg. A silverymetallic element belonging to group2 (formerly IIA) of the periodic table(see alkaline-earth metals); a.n. 12;r.a.m. 24.305; r.d. 1.74; m.p. 648.8°C;b.p. 1090°C. The element is found ina number of minerals, includingmagnesite (MgCO3), dolomite(MgCO3.CaCO3), and carnallite(MgCl2.KCl.6H2O). It is also present insea water, and it is an *essential el-ement for living organisms. Extrac-tion is by electrolysis of the fusedchloride. The element is used in anumber of light alloys (e.g. for air-craft). Chemically, it is very reactive.In air it forms a protective oxide coat-ing but when ignited it burns withan intense white Ûame. It also reactswith the halogens, sulphur, and ni-trogen. Magnesium was Ürst isolatedby Bussy in 1828.A• Information from the WebElements site

magnesium bicarbonate Seemagnesium hydrogencarbonate.

magnesium carbonate A whitecompound, MgCO3, existing in anhy-drous and hydrated forms. The an-hydrous material (trigonal; r.d. 2.96) is found in the mineral *mag-nesite. There is also a trihydrate,MgCO3.3H2O (rhombic; r.d. 1.85),which occurs naturally as nesque-honite, and a pentahydrate,

MgCO3.5H2O (monoclinic; r.d. 1.73),which occurs as lansfordite. Magne-sium carbonate also occurs in themixed salt *dolomite (CaCO3.MgCO3)and as basic magnesium carbon-ate in the two minerals artinite(MgCO3.Mg(OH)2.3H2O) and hydro-magnesite (3MgCO3.Mg(OH)2.3H2O).The anhydrous salt can be formed byheating magnesium oxide in astream of carbon dioxide:

MgO(s) + CO2(g) → MgCO3(s)Above 350°C, the reverse reaction

predominates and the carbonate de-composes. Magnesium carbonate isused in making magnesium oxideand is a drying agent (e.g. in tablesalt). It is also used as a medicalantacid and laxative (the basic car-bonate is used) and is a componentof certain inks and glasses.

magnesium chloride A whitesolid compound, MgCl2. The anhy-drous salt (hexagonal; r.d. 2.32; m.p.714°C; b.p. 1412°C) can be preparedby the direct combination of drychlorine with magnesium:

Mg(s) + Cl2(g) → MgCl2(s)The compound also occurs naturallyas a constituent of carnallite(KCl.MgCl2). It is a deliquescent com-pound that commonly forms thehexahydrate, MgCl2.6H2O (mono-clinic; r.d. 1.57). When heated, thishydrolyses to give magnesium oxideand hydrogen chloride gas. The fusedchloride is electrolysed to producemagnesium and it is also used forÜreprooÜng wood, in magnesia ce-ments and artiÜcial leather, and as alaxative.

magnesium hydrogencarbonate(magnesium bicarbonate) A com-pound, Mg(HCO3)2, that is stable onlyin solution. It is formed by the actionof carbon dioxide on a suspension ofmagnesium carbonate in water:

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MgCO3(s) + CO2(g) + H2O(l) →Mg(HCO3)2(aq)

On heating, this process is reversed.Magnesium hydrogencarbonate isone of the compounds responsiblefor temporary *hardness in water.

magnesium hydroxide A whitesolid compound, Mg(OH)2; trigonal;r.d. 2.36; decomposes at 350°C. Mag-nesium hydroxide occurs naturally asthe mineral brucite and can be pre-pared by reacting magnesium sul-phate or chloride with sodiumhydroxide solution. It is used in thereÜning of sugar and in the process-ing of uranium. Medicinally it isimportant as an antacid (milk ofmagnesia) and as a laxative.

magnesium oxide (magnesia) Awhite compound, MgO; cubic; r.d.3.58; m.p. 2800°C. It occurs naturallyas the mineral periclase and is pre-pared commercially by thermally de-composing the mineral *magnesite:

MgCO3(s) → MgO(s) + CO2(g)It has a wide range of uses, includingreÛective coatings on optical instru-ments and aircraft windscreens andin semiconductors. Its high meltingpoint makes it useful as a refractorylining in metal and glass furnaces.

magnesium peroxide A whitesolid, MgO2. It decomposes at 100°Cto release oxygen and also releasesoxygen on reaction with water:

2MgO2(s) + 2H2O → 2Mg(OH)2 + O2

The compound is prepared by react-ing sodium peroxide with magne-sium sulphate solution and is used asa bleach for cotton and silk.

magnesium sulphate A white sol-uble compound, MgSO4, existing asthe anhydrous compound (rhombic;r.d. 2.66; decomposes at 1124°C) andin hydrated crystalline forms. Themonohydrate MgSO4.H2O (mono-clinic; r.d. 2.45) occurs naturally as

the mineral kieserite. The common-est hydrate is the heptahydrate,MgSO4.7H2O (rhombic; r.d. 1.68),which is called Epsom salt(s), and oc-curs naturally as the mineral ep-somite. This is a white powder with abitter saline taste, which loses 6H2Oat 150°C and 7H2O at 200°C. It isused in sizing and ÜreprooÜng cottonand silk, in tanning leather, and inthe manufacture of fertilizers, explo-sives, and matches. In medicine, it isused as a laxative. It is also used inveterinary medicine for treatment oflocal inÛammations and infectedwounds.

magnetic moment The ratio be-tween the maximum torque (Tmax)exerted on a magnet, current-carrying coil, or moving charge situ-ated in a magnetic Üeld and thestrength of that Üeld. It is thus ameasure of the strength of a magnetor current-carrying coil. In the Som-merfeld approach this quantity (alsocalled electromagnetic moment ormagnetic area moment) is the ratioTmax/B. In the Kennelly approach thequantity (also called magnetic dipolemoment) is Tmax/H.

In the case of a magnet placed in amagnetic Üeld of Üeld strength H, themaximum torque Tmax occurs whenthe axis of the magnet is perpendicu-lar to the Üeld. In the case of a coil ofN turns and area A carrying a currentI, the magnetic moment can beshown to be m = T/B = NIA or m = T/H= µNIA. Magnetic moments are meas-ured in A m2.

An orbital electron has an orbitalmagnetic moment IA, where I is theequivalent current as the electronmoves round its orbit. It is given by I= qω/2π, where q is the electroniccharge and ω is its angular velocity.The orbital magnetic moment istherefore IA = qωA/2π, where A is theorbital area. If the electron is spin-

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ning there is also a spin magneticmoment (see spin); atomic nuclei alsohave magnetic moments.

magnetic quantum number Seeatom.

magnetism A group of phenomenaassociated with magnetic Üelds.Whenever an electric current Ûows amagnetic Üeld is produced; as the or-bital motion and the *spin of atomicelectrons are equivalent to tiny cur-rent loops, individual atoms createmagnetic Üelds around them, whentheir orbital electrons have a net*magnetic moment as a result oftheir angular momentum. The mag-netic moment of an atom is the vec-tor sum of the magnetic moments ofthe orbital motions and the spins ofall the electrons in the atom. Themacroscopic magnetic properties of asubstance arise from the magneticmoments of its component atomsand molecules. Different materialshave different characteristics in anapplied magnetic Üeld; there are fourmain types of magnetic behaviour:

(a) In diamagnetism the magnetiza-tion is in the opposite direction tothat of the applied Üeld, i.e. the sus-ceptibility is negative. Although allsubstances are diamagnetic, it is aweak form of magnetism and may bemasked by other, stronger, forms. Itresults from changes induced in theorbits of electrons in the atoms of asubstance by the applied Üeld, the di-rection of the change opposing theapplied Ûux. There is thus a weaknegative susceptibility (of the orderof –10–8 m3 mol–1) and a relative per-meability of slightly less than one.

(b) In paramagnetism the atoms ormolecules of the substance have netorbital or spin magnetic momentsthat are capable of being aligned inthe direction of the applied Üeld.They therefore have a positive (butsmall) susceptibility and a relative

permeability slightly in excess ofone. Paramagnetism occurs in allatoms and molecules with unpairedelectrons; e.g. free atoms, free radi-cals, and compounds of transitionmetals containing ions with unÜlledelectron shells. It also occurs in met-als as a result of the magnetic mo-ments associated with the spins ofthe conducting electrons.

(c) In ferromagnetic substances,within a certain temperature range,there are net atomic magnetic mo-ments, which line up in such a waythat magnetization persists after theremoval of the applied Üeld. Below acertain temperature, called the Curiepoint (or Curie temperature) an in-creasing magnetic Üeld applied to aferromagnetic substance will causeincreasing magnetization to a highvalue, called the saturation magneti-zation. This is because a ferromag-netic substance consists of small(1–0.1 mm across) magnetized re-gions called domains. The total mag-netic moment of a sample of thesubstance is the vector sum of themagnetic moments of the compo-nent domains. Within each domainthe individual atomic magnetic mo-ments are spontaneously aligned byexchange forces, related to whetheror not the atomic electron spins areparallel or antiparallel. However, inan unmagnetized piece of ferromag-netic material the magnetic mo-ments of the domains themselves arenot aligned; when an external Üeld isapplied those domains that arealigned with the Üeld increase in sizeat the expense of the others. In avery strong Üeld all the domains arelined up in the direction of the Üeldand provide the high observed mag-netization. Iron, nickel, cobalt, andtheir alloys are ferromagnetic. Abovethe Curie point, ferromagnetic ma-terials become paramagnetic.

(d) Some metals, alloys, and transi-

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tion-element salts exhibit anotherform of magnetism called antiferro-magnetism. This occurs below a cer-tain temperature, called the Néeltemperature, when an ordered arrayof atomic magnetic moments sponta-neously forms in which alternate mo-ments have opposite directions.There is therefore no net resultantmagnetic moment in the absence ofan applied Üeld. In manganeseÛuoride, for example, this antiparal-lel arrangement occurs below a Néeltemperature of 72 K. Below this tem-perature the spontaneous orderingopposes the normal tendency of themagnetic moments to align with theapplied Üeld. Above the Néel temper-ature the substance is paramagnetic.

A special form of antiferromagnet-ism is ferrimagnetism, a type of mag-netism exhibited by the *ferrites. Inthese materials the magnetic mo-ments of adjacent ions are antiparal-lel and of unequal strength, or thenumber of magnetic moments in onedirection is greater than those in theopposite direction. By suitable choiceof rare-earth ions in the ferrite lat-tices it is possible to design ferri-magnetic substances with speciÜcmagnetizations for use in electroniccomponents.

magnetite A black mineral form ofiron oxide crystallizing in the cubicsystem. It is a mixed iron(II)-iron(III)oxide, Fe3O4, and is one of the majorores of iron. It is strongly magneticand some varieties, known as lode-stone, are natural magnets; thesewere used as compasses in the an-cient world. Magnetite is widely dis-tributed and occurs as an accessorymineral in almost all igneous andmetamorphic rocks. The largest de-posits of the mineral occur in N Swe-den.

magnetochemistry The branch ofphysical chemistry concerned with

measuring and investigating themagnetic properties of compounds. Itis used particularly for studying tran-sition-metal complexes, many ofwhich are paramagnetic becausethey have unpaired electrons. Meas-urement of the magnetic susceptibil-ity allows the magnetic moment ofthe metal atom to be calculated, andthis gives information about thebonding in the complex.

magnetomechanical ratio Seegyromagnetic ratio.

magneton A unit for measuringmagnetic moments of nuclear,atomic, or molecular magnets. TheBohr magneton µB has the value ofthe classical magnetic moment of anelectron, given by

µB = eh/4πme = 9.274 × 10–24 A m2,where e and me are the charge andmass of the electron and h is thePlanck constant. The nuclear magne-ton, µN is obtained by replacing themass of the electron by the mass ofthe proton and is therefore given by

µN = µB.me/mp = 5.05 × 10–27 A m2.

Main-Smith–Stoner rule An em-pirical rule in the theory of atomicstructure stating that for a principalquantum number n the number ofelectronic quantum states that canhave the orbital quantum number l is2(2l + 1). This rule describes the sub-shells of atoms. It was put forwardon the basis of chemical evidence byJ. D. Main-Smith and independentlyon the basis of magnetic and spectro-scopic evidence by Edmund Stoner in1924. The Main-Smith–Stoner rule isa consequence of the *Pauli exclu-sion principle. The rule was one ofthe key developments that led to theenunciation of the Pauli exclusionprinciple in 1925.

malachite A secondary mineralform of copper carbonate–hydroxide,

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CuCO3.Cu(OH)2. It is bright green andcrystallizes in the monoclinic systembut usually occurs as aggregates ofÜbres or in massive form. It is gener-ally found with *azurite in associa-tion with the more important copperores and is itself mined as an ore ofcopper (e.g. in Zaïre). It is also used asan ornamental stone and as a gem-stone.

MALDI Matrix absorption laser des-orption ionization. A technique forproducing ions for mass spec-troscopy, used especially for largebiological species. The sample is ab-sorbed on an inert matrix fromwhich ions are desorbed by a laser.

maleic acid See butenedioic acid.

maleic anhydride A colourlesssolid, C4H2O3, m.p. 53°C, the anhy-dride of cis-butenedioic acid (maleicacid). It is a cyclic compound with aring containing four carbon atomsand one oxygen atom, made by thecatalytic oxidation of benzene or itsderivatives at high temperatures. It isused mainly in the manufacture ofalkyd and polyester resins andcopolymers.

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OH

OH

O

O

O

O

O

Maleic anhydride

malic acid (2-hydroxybutanedioicacid) A crystalline solid,HOOCCH(OH)CH2COOH. l-malic acidoccurs in living organisms as an in-termediate metabolite in the *Krebscycle and also (in certain plants) inphotosynthesis. It is found especiallyin the juice of unripe fruits, e.g.green apples.

malonic acid See propanedioicacid.

malt The product of the hydrolysisof starch by β-amylase that occursduring the germination of barley inbrewing. See also maltose.

maltose (malt sugar) A sugar con-sisting of two linked glucose mol-ecules that results from the action ofthe enzyme amylase on starch. Mal-tose occurs in barley seeds followinggermination and drying, which is thebasis of the malting process used inthe manufacture of beer and maltwhisky.

malt sugar See maltose.

mancude Describing an organiccompound that contains the maxi-mum possible number of noncumu-lative double bonds, as in the*annulenes. It is an acronym. maxi-mum non-cumulative double.

Mandelin test A *presumptivetest for amphetamines and alkaloids.The Mandelin reagent is a 1% solu-tion of ammonium vanadate(NH4VO3) in concentrated sulphuricacid. Different substances give differ-ent colours. Mescaline, for example,produces an orange colour, heroin abrown colour, and amphetamine ablue-green colour.

manganate(VI) A salt containingthe ion MnO4

2–. Manganate(VI) ionsare dark green; they are produced bymanganate(VII) ions in basic solution.

manganate(VII) (permanganate) Asalt containing the ion MnO4

–. Man-ganate(VII) ions are dark purple andstrong oxidizing agents.

manganese Symbol Mn. A greybrittle metallic *transition element,a.n. 25; r.a.m. 54.94; r.d. 7.2; m.p.1244°C; b.p. 1962°C. The mainsources are pyrolusite (MnO2) andrhodochrosite (MnCO3). The metal


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can be extracted by reduction of theoxide using magnesium (*Krollprocess) or aluminium (*Gold-schmidt process). Often the ore ismixed with iron ore and reduced inan electric furnace to produce ferro-manganese for use in alloy steels.The element is fairly electropositive;it combines with oxygen, nitrogen,and other nonmetals when heated(but not with hydrogen). Salts ofmanganese contain the element inthe +2 and +3 oxidation states. Man-ganese(II) salts are the more stable. Italso forms compounds in higher oxi-dation states, such as manganese(IV)oxide and manganate(VI) and man-ganate(VII) salts. The element wasdiscovered in 1774 by Karl *Scheele.A• Information from the WebElements site

manganese(IV) oxide (manganesedioxide) A black oxide made by heat-ing manganese(II) nitrate. The com-pound also occurs naturally aspyrolusite. It is a strong oxidizingagent, used as a depolarizing agent involtaic cells.

manganic compounds Com-pounds of manganese in its +3 oxida-tion state; e.g. manganic oxide ismanganese(III) oxide, Mn2O3.

manganin A copper alloy contain-ing 13–18% of manganese and 1–4%of nickel. It has a high electrical re-sistance, which is relatively insensi-tive to temperature changes. It istherefore suitable for use in resis-tance wire.

manganous compounds Com-pounds of manganese in its +2 oxida-tion state; e.g. manganous oxide ismanganese(II) oxide, MnO.

mannan See mannose.

Mannich reaction A reaction inwhich a primary or secondary aminereacts with methanal (formaldehyde)

and a carbonyl compound to producean amino-carbonyl compound. Ittakes place in two stages. First theamine reacts with methanol to forma Schiff base:

R2N + H2CO → R2N+=CH2.This then reacts with the carbonylcompound:

R2N+=CH2 + R1R2CHCOR3 →R2N–CH2–C(R1R2)COR3.

The reaction was Ürst reported byCarl Mannich in 1912.

mannitol A polyhydric alcohol,CH2OH(CHOH)4CH2OH, derived frommannose or fructose. It is the mainsoluble sugar in fungi and an impor-tant carbohydrate reserve in brownalgae. Mannitol is used as a sweet-ener in certain foodstuffs and as a di-uretic to relieve Ûuid retention.

mannose A *monosaccharide,C6H12O6, stereoisomeric with glu-cose, that occurs naturally only inpolymerized forms called mannans.These are found in plants, fungi, andbacteria, serving as food energystores.

manometer A device for measur-ing pressure differences, usually bythe difference in height of two liquidcolumns. The simplest type is the U-tube manometer, which consists of aglass tube bent into the shape of a U.If a pressure to be measured is fed toone side of the U-tube and the otheris open to the atmosphere, the differ-ence in level of the liquid in the twolimbs gives a measure of the un-known pressure.

many-body problem The prob-lem that it is very difÜcult to obtainexact solutions to systems involvinginteractions between more than twobodies – using either classical me-chanics or quantum mechanics. Tounderstand the physics of many-bodysystems it is necessary to make use of

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approximation techniques or modelsystems. For some problems, such asthe three-body problem in classicalmechanics, it is possible to obtainqualitative information about the sys-tem. If there are a great many bodiesinteracting, such as the molecules ina gas, the problem can be analysedusing the techniques of *statisticalmechanics.

marble A metamorphic rock com-posed of recrystallized *calcite or*dolomite. Pure marbles are whitebut such impurities as silica or clayminerals result in variations of col-our. Marble is extensively used forbuilding purposes and ornamentaluse; the pure white marble from Car-rara in Italy is especially prized bysculptors. The term is applied com-mercially to any limestone ordolomite that can be cut and pol-ished.

margaric acid See heptadecanoicacid.

marijuana See cannabis.

MarkofÜan process (Markovprocess) A random process (see sto-chastic process) in which the rateof change of a time-dependent quan-tity ∂a(t)/∂t depends on the instanta-neous value of the quantity a(t),where t is the time, but not on itsprevious history. If a random processcan be assumed to be a Markovprocess, an analysis of the process isgreatly simpliÜed enabling usefulequations in *nonequilibrium statis-tical mechanics and disordered solidsto be derived. Problems involvingMarkov processes are solved usingstatistical methods and the theory ofprobability. Markov processes arenamed after the Russian mathemati-cian Andrei Andreevich Markov(1856–1922).

Markovnikoff’s rule When anacid HA adds to an alkene, a mixture

of products can be formed if thealkene is not symmetrical. For in-stance, the reaction betweenC2H5CH:CH2 and HCl can giveC2H5CH2CH2Cl or C2H5CHClCH3. In general, a mixture of productsoccurs in which one predominatesover the other. In 1870, VladimirMarkovnikoff (1837–1904) proposedthe rule that the main product wouldbe the one in which the hydrogenatom adds to the carbon having thelarger number of hydrogen atoms(the latter product above). This oc-curs when the mechanism is *elec-trophilic addition, in which theÜrst step is addition of H+. The electron-releasing effect of the alkylgroup (C2H5) distorts the electron-distribution in the double bond, mak-ing the carbon atom furthest fromthe alkyl group negative. This is theatom attacked by H+ giving the car-bonium ion C2H5C+HCH3, which fur-ther reacts with the negative ion Cl–.

In some circumstances anti-Markovnikoff behaviour occurs, inwhich the opposite effect is found.This happens when the mechanisminvolves free radicals and is commonin addition of hydrogen bromidewhen peroxides are present.

Markush structure A generalizedformula or description for a relatedset of chemical compounds, used inpatent applications. It is named afterEugene Markush (1888–1968), anAmerican manufacturer of dyes andpharmaceuticals. In 1924 he wasawarded a patent for “The processfor manufacture of dyes which com-prises coupling with a halogen-substituted pyralazone, a diazotizedunsulphonated material selectedfrom the group consisting of aniline,homologues of aniline, and halogensubstitution products of aniline”.Note that the patent was forprocesses to produce a range of com-

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pounds, including ones that had notactually been synthesized. In 1925The US Patent OfÜce ruled that suchpatents were valid. Markush struc-tures can be described for com-pounds with substituents at severalpositions, and often many thousandsof possible compounds are deÜned inthis way. An important part of chem-ical database searching is the abilityto Ünd possible Markush structuresto rule out priority in the patent ap-plication. Chemical drawing pro-grams can represent such structures.For example, a bond to the centre ofa ring indicates substitution at anyposition on the ring.

Marquis test A widely used *pre-sumptive test that gives a variety ofcolour changes with a range of com-pounds. It is particularly useful fordetecting opiate alkaloids and foramphetamines and methampheta-mine. Marquis reagent is a mixtureof methanal (formaldehyde) solutionin water with sulphuric acid. Mesca-line gives an orange colouration.With morphine, a violet colour isproduced. Amphetamines give an or-ange-red colour and methampheta-mine gives an orange colour. The twocan be distinguished by the *Simontest. The mechanism involves attackof the aldehyde an a substituted aro-matic ring to form a carbocation. Fur-ther reaction forms a coloured dimerof the original molecule.

marsh gas Methane formed by rot-ting vegetation in marshes.

Marsh’s test A chemical test for ar-senic in which hydrochloric acid andzinc are added to the sample, arsinebeing produced by the nascent hy-drogen generated. Gas from the sam-ple is led through a heated glass tubeand, if arsine is present, it decom-poses to give a brown deposit ofarsenic metal. The arsenic is distin-guished from antimony (which gives

a similar result) by the fact that an-timony does not dissolve in sodiumchlorate(I) (hypochlorite). The testwas devised in 1836 by the Britishchemist James Marsh (1789–1846).

martensite A solid solution of car-bon in alpha-iron (see iron) formedwhen *steel is cooled too rapidly forpearlite to form from austenite. It isresponsible for the hardness ofquenched steel.

mascagnite A mineral form of*ammonium sulphate, (NH4)2SO4.

maser (microwave ampliÜcation bystimulated emission of radiation) Adevice for amplifying or generatingmicrowaves by means of stimulatedemission (see laser).

mass A measure of a body’s inertia,i.e. its resistance to acceleration. Ac-cording to Newton’s laws of motion,if two unequal masses, m1 and m2,are allowed to collide, in the absenceof any other forces both will experi-ence the same force of collision. Ifthe two bodies acquire accelerationsa1 and a2 as a result of the collision,then m1a1 = m2a2. This equation en-ables two masses to be compared. If one of the masses is regarded as a standard of mass, the mass of all other masses can be meas-ured in terms of this standard. The body used for this purpose is a 1-kg cylinder of platinum–iridiumalloy, called the international stan-dard of mass.

mass action The law of mass ac-tion states that the rate at which achemical reaction takes place at agiven temperature is proportional tothe product of the active masses ofthe reactants. The active mass of a re-actant is taken to be its molar con-centration. For example, for areaction

xA + yB → products

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the rate is given byR = k[A]x[B]y

where k is the *rate constant. Theprinciple was introduced by C. M.Guldberg and P. Waage in 1863. It isstrictly correct only for ideal gases. Inreal cases *activities can be used. Seealso equilibrium constant.

mass concentration See concen-tration.

massicot See lead(ii) oxide.

mass number See nucleon num-ber.

mass spectroscopy A techniqueused to determine relative atomicmasses and the relative abundance ofisotopes, and for chemical analysisand the study of ion reactions. In amass spectrometer a sample (usuallygaseous) is ionized and the positiveions produced are accelerated into ahigh-vacuum region containing elec-tric and magnetic Üelds. These ÜeldsdeÛect and focus the ions onto a de-tector. The Üelds can be varied in acontrolled way so that ions of differ-ent types can impinge on the detec-tor. A mass spectrum is thus obtainedconsisting of a series of peaks of vari-able intensity to which mass/charge(m/e) values can be assigned. The orig-inal ions are usually produced byelectron impact, although ion im-pact, photoionization, Üeld ioniza-tion, *electrospray ionization, and*MALDI are also used. For organicmolecules, the mass spectrum con-sists of a series of peaks, one corre-sponding to the parent ion and theothers to fragment ions produced bythe ionization process. Different mol-ecules can be identiÜed by their char-acteristic pattern of lines. Analysis ofmixtures can be done by gas chro-matography–mass spectroscopy (seegas chromatography). Other typesof mass spectrometer exist. In aquadrupole mass spectrometer the

ions pass along a region surroundedby four parallel rods. Variable volt-ages applied to the rods produce anoscillating electric Üeld. Varying thefrequency of osillation allows differ-ent ions to pass through to a detec-tor. In a time-of-Ûight massspectrometer the ions are acceleratedby an electric Üeld and then enter adrift tube through which they pass toa detector. Different types of ion aredistinguished by their time of Ûightin the drift tube.

masurium A former name for*technetium.

matrix (pl. matrices) 1. (in chem-istry) A continuous solid phase inwhich particles (atoms, ions, etc.) areembedded. Unstable species, such asfree radicals, can be trapped in anunreactive substrate, such as solidargon, and studied by spectroscopy.The species under investigation areseparated by the matrix, hence theterm matrix isolation for this tech-nique. 2. (in geology) The Üne-grained material of rock in which thecoarser-grained material is embed-ded. 3. (in mathematics) A set ofquantities in a rectangular array,used in certain mathematical opera-tions. The array is usually enclosed inlarge parentheses or in square brack-ets.

matrix mechanics A formulationof *quantum mechanics using matri-ces (see matrix) to represent statesand operators. Matrix mechanics wasthe Ürst formulation of quantum me-chanics to be stated (by WernerHeisenberg in 1925) and was devel-oped by Heisenberg and Max Born(1882–1970) and the German physi-cist Pascual Jordan (1902–80). It wasshown by Erwin Schrödinger in 1926to be equivalent to the *wave me-chanics formulation of quantum me-chanics.

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Matura diamond See zircon.

Maxwell, James Clerk (1831–79)British physicist, born in Edinburgh,who held academic posts at Ab-erdeen, London, and Cambridge. Inthe 1860s he was one of the foundersof the *kinetic theory of gases, buthis best-known work was a mathe-matical analysis of electromagneticradiation, published in 1864.

Maxwell–Boltzmann distribu-tion A law describing the distribu-tion of speeds among the moleculesof a gas. In a system consisting of Nmolecules that are independent ofeach other except that they exchangeenergy on collision, it is clearly im-possible to say what velocity any par-ticular molecule will have. However,statistical statements regarding cer-tain functions of the molecules wereworked out by James Clerk *Max-well and Ludwig *Boltzmann. Oneform of their law states that n = Nexp(–E/RT), where n is the num-ber of molecules with energy in ex-cess of E, T is the thermodynamictemperature, and R is the *gas con-stant.

Maxwell’s demon An imaginarycreature that is able to open and shuta partition dividing two volumes of agas in a container, when the two vol-umes are initially at the same tem-perature. The partition operated bythe demon is only opened to allowfast molecules through. Such aprocess would make the volume ofgas containing the fast moleculeshotter than it was at the start; thevolume of gas remaining would ac-cordingly become cooler. Thisprocess would be a violation of thesecond law of *thermodynamics andtherefore cannot occur. Maxwell’sdemon was invented by James ClerkMaxwell in a letter written in 1867 toshow that the second law of thermo-dynamics has its origins in *statisti-

cal mechanics, although the namewas suggested by the Scottish scien-tist Sir William Thomson(1824–1907; subsequently LordKelvin).

Maxwell’s thermodynamicequations Equations in thermody-namics for a given mass of a homo-geneous system, relating the entropy(S), pressure (p), volume (V), and ther-modynamic temperature (T). Thefour equations are:

(∂T/∂V)s = –(∂p/∂S)V;∂T/∂p)s = –(∂V/∂S)p;(∂V/∂T)p = –(∂S/∂p)T;(∂S/∂V)T = –(∂p/∂T)V.

Mayer f-function A quantity thatoccurs in the calculation of virialcoefÜcients; it is deÜned by f =exp(–V2/kT) – 1, where V2 is the two-body interaction potential energy, kis the *Boltzmann constant, and T isthe thermodynamic temperature. Itis related to the second virial coefÜ-cient B by:

B = (–NA/V)∫ fdr1dr2,where NA is the Avogadro numberand V is the volume of the system.This equation simpliÜed to:

B = –2πNA∫ ∞0 fr2dr

in the case of closed-shell atoms andoctahedral and tetrahedral mol-ecules. When particles are so farapart that the interaction V → 0,then f → 0 also, but when the parti-cles are so close together that the in-teraction V → ∞, then f → –1. Thisenables strong repulsive interactionsbetween particles to be analysed interms of f but not of V. The functionis named after the US physicistJoseph Mayer.

Mayer’s test A general *presump-tive test for cocaine, morphine,heroin, and other alkaloids. Mayer’sreagent is a solution of potassiummercury iodide in water. A positive

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result is indicated by a cream precipi-tate.

McLeod gauge A vacuum pressuregauge, devised by Herbert McLeod(1841–1923), in which a relativelylarge volume of a low-pressure gas iscompressed to a small volume in aglass apparatus. The volume is re-duced to an extent that causes thepressure to rise sufÜciently to sup-port a column of Ûuid high enoughto read. This simple device, which re-lies on *Boyle’s law, is suitable formeasuring pressures in the range 103

to 10–3 pascal.

McMillan–Mayer theory Atheory of solutions of nonelectrolytesdeveloped by the US scientists W. G.McMillan and J. E. Mayer in 1945.The theory shows that there is a one-to-one correspondence between theequations describing a nonideal gasand those describing dilute solutionsof nonelectrolytes. In particular, theyshowed that there is a correspon-dence between the pressure of thegas and the osmotic pressure of thesolution. This enables an expansionfor solutions to be written, which isanalogous to the virial expansion ofnonideal gases with analogues of thevirial coefÜcients. These coefÜcientscan be calculated with the analogueof potential being the potential ofmean force of N solute molecules inthe pure solvent. The McMillan–Mayer theory can also be extended todistribution functions.

MDA (methylenedioxyampheta-mine) A hallucinogenic drug,C10H13CO2, originally designed formedical use but now extensivelyused as a club drug. Its effects aresimilar to those of MDMA (See ec-stasy).

MDMA Methylenedioxymetham-phetamine. See ecstasy.

mean free path The average dis-

tance travelled between collisions bythe molecules in a gas, the electronsin a metallic crystal, the neutrons ina moderator, etc. According to the*kinetic theory the mean free pathbetween elastic collisions of gas mol-ecules of diameter d (assuming theyare rigid spheres) is 1/√2nπd2, wheren is the number of molecules perunit volume in the gas. As n is pro-portional to the pressure of the gas,the mean free path is inversely pro-portional to the pressure.

mean free time The average timethat elapses between the collisions ofthe molecules in a gas, the electronsin a crystal, the neutrons in a moder-ator, etc. See mean free path.

mechanical bonding Bondingthat involves a mechanical constraintpreventing two parts of a moleculeseparating, rather than a chemicallinkage based on transfer or sharingof electrons. It is found in *rotax-anes, *catenanes, and *molecularknots.

mechanism The way in which aparticular chemical reaction occurs,described in terms of the steps in-volved. For example, the hydrolysisof an alkyl chloride proceeds by theSN1 mechanism (see nucleophilicsubstitution).

Mecke’s test A *presumptive testfor amphetamines, methampheta-mines, and heroin. Mecke’s reagentconsists of 1 gram of selenious acidin 100 ml of concentrated sulphuricacid. Different substances give differ-ent results. Ecstasy, for example,gives a light blue colour, turning toturquoise, and then dark blue.Heroin gives a yellow colour chang-ing to green. LSD gives an olive-greencolour, changing to black. Mescalinegives a brownish-orange colour.

medium frequency (MF) A radiofrequency in the range 0.3–3 mega-

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hertz; i.e. having a wavelength in therange 100–1000 metres.

mega- Symbol M. A preÜx used inthe metric system to denote one mil-lion times. For example, 106 volts = 1megavolt (MV).

Meitner, Lise (1878–1968) Aus-trian-born Swedish physicist and ra-diochemist who worked in Berlinwith Otto *Hahn. Together they dis-covered protactinium. Meitner andHahn worked together on neutronbombardment of uranium. In the1930s, she escaped from Austria toSweden to avoid Nazi persecution. InStockholm, along with her nephewOtto Frisch (1904–79), she formulatedthe theory of nuclear Üssion.

meitnerium Symbol Mt. A radio-active *transactinide element; a.n.109. It was Ürst made in 1982 byPeter Armbruster and a team inDarmstadt, Germany, by bombardingbismuth-209 nuclei with iron-58 nu-clei. Only a few atoms have everbeen detected.


melamine A white crystalline com-pound, C3N6H6. Melamine is a cycliccompound having a six-memberedring of alternating C and N atoms,with three NH2 groups. It can becopolymerized with methanal to givethermosetting melamine resins,

which are used particularly for lami-nated coatings.

mellitic acid (benzenehexacar-boxylic acid) A colourless crystallinecompound, C6(COOH)6, m.p. 288°C.Its molecules consist of a benzenering in which all six hydrogen atomshave been substituted by carboxyl(–COOH) groups. It occurs naturallyin some lignite beds as honeystone(the aluminium salt), and is made byoxidizing charcoal with concentratednitric acid. It decomposes on heatingto form pyromellitic anhydride, usedin making epoxy resins. Condensa-tion products of mellitic acid are em-ployed in making a wide range ofdyes.

melting point (m.p.) The tempera-ture at which a solid changes into aliquid. A pure substance under stan-dard conditions of pressure (usually 1atmosphere) has a single repro-ducible melting point. If heat is grad-ually and uniformly supplied to asolid the consequent rise in tempera-ture stops at the melting point untilthe fusion process is complete.

Mendeleev, Dmitri Ivanovich(1834–1907) Russian chemist, whobecame professor of chemistry at StPetersburg in 1866. His most famouswork, published in 1869, was thecompilation of the *periodic table ofthe elements, based on the periodiclaw.

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Mendeleev’s law See periodiclaw.

mendelevium Symbol Md. A radio-active metallic transuranic elementbelonging to the *actinoids; a.n. 101;mass number of the Ürst discoverednuclide 256 (half-life 1.3 hours). Sev-eral short-lived isotopes have nowbeen synthesized. The element wasÜrst identiÜed by Albert Ghiorso,Glenn Seaborg (1912–99), and associ-ates in 1955.A• Information from the WebElements site

Mendius reaction A reaction inwhich an organic nitrile is reducedby nascent hydrogen (e.g. fromsodium in ethanol) to a primaryamine:

RCN + 2H2 → RCH2NH2

menthol A white crystalline ter-pene alcohol, C10H19OH; r.d. 0.89;m.p. 42°C; b.p. 103–104°C. It has aminty taste and is found in certainessential oils (e.g. peppermint) andused as a Ûavouring.

mercaptans See thiols.

mercapto group See thiols.

mercuric compounds Com-pounds of mercury in its +2 oxida-tion state; e.g. mercuric chloride ismercury(II) chloride, HgCl2.

mercurous compounds Com-pounds of mercury in its +1 oxida-tion state; e.g. mercury(I) chloride ismercurous chloride, HgCl.

mercury Symbol Hg. A heavy sil-very liquid metallic element belong-ing to the *zinc group; a.n. 80; r.a.m.200.59; r.d. 13.55; m.p. –38.87°C; b.p.356.58°C. The main ore is the sul-phide cinnabar (HgS), which can bedecomposed to the elements. Mer-cury is used in thermometers,barometers, and other scientiÜc ap-paratus, and in dental amalgams. The

element is less reactive than zinc andcadmium and will not displace hy-drogen from acids. It is also unusualin forming mercury(I) compoundscontaining the Hg2

2+ ion, as well asmercury(II) compounds containingHg2+ ions. It also forms a number ofcomplexes and organomercury com-pounds.A• Information from the WebElements site

mercury cell A primary *voltaiccell consisting of a zinc anode and acathode of mercury(II) oxide (HgO)mixed with graphite. The electrolyteis potassium hydroxide (KOH) satu-rated with zinc oxide, the overall re-action being:

Zn + HgO → ZnO + HgThe e.m.f. is 1.35 volts and the cellwill deliver about 0.3 ampere-hourper cm3.

mercury(I) chloride A white salt,Hg2Cl2; r.d. 7.15; sublimes at 400°C. Itis made by heating mercury(II) chlo-ride with mercury and is used incalomel cells (so called because thesalt was formerly called calomel) andas a fungicide.

mercury(II) chloride A white salt,HgCl2; r.d. 5.4; m.p. 276°C; b.p.302°C. It is made by reacting mer-cury with chlorine and used in mak-ing other mercury compounds.

mercury(II) fulminate A greycrystalline solid, Hg(CNO)2.½H2O,made by the action of nitric acid onmercury and treating the solutionformed with ethanol. It is used as adetonator for cartridges and can behandled safely only under cold water.

mercury(II) oxide A yellow or redoxide of mercury, HgO. The red formis made by heating mercury in oxy-gen at 350°C; the yellow form, whichdiffers from the red in particle size,is precipitated when sodium hydrox-

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ide solution is added to a solution ofmercury(II) nitrate. Both forms de-compose to the elements at hightemperature. The black precipitateformed when sodium hydroxide isadded to mercury(I) nitrate solutionis sometimes referred to asmercury(I) oxide (Hg2O) but is proba-bly a mixture of HgO and free mer-cury.

mercury(II) sulphide A red orblack compound, HgS, occurring nat-urally as the minerals cinnabar (red)and metacinnabar (black). It can beobtained as a black precipitate bybubbling hydrogen sulphide througha solution of mercury(II) nitrate. Thered form is obtained by sublimation.The compound is also called vermil-ion (used as a pigment).

mer isomer See isomerism.

mescaline A powerful hallucino-genic compound obtained from pey-ote – the Ûowering head of a type ofMexican cactus. Mescaline is a class Adrug in the UK. It can be detected bythe Mecke test.

349 metal

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O

O

O

CH3

CH3CH3

CH2

CH2

NH2

Mescaline

meso-isomer See optical activity.

mesomerism (mesomeric effect) Aformer name for *resonance in mol-ecules. See also electronic effects.

mesomorph See lyotropic meso-morph.

mesoscopic Designating a sizescale intermediate between those of

the *microscopic and the *macro-scopic states. Mesoscopic objects andsystems require quantum mechanicsto describe them.

meta- 1. PreÜx designating a ben-zene compound in which two sub-stituents are in the 1,3 positions onthe benzene ring. The abbreviationm- is used; for example, m-xylene is1,3-dimethylbenzene. Compareortho-; para-. 2. PreÜx designating alower oxo acid, e.g. metaphosphoricacid. Compare ortho-.

metabolic pathway See metabo-lism.

metabolism The sum of the chem-ical reactions that occur within livingorganisms. The various compoundsthat take part in or are formed bythese reactions are called metabo-lites. In animals many metabolitesare obtained by the digestion of food,whereas in plants only the basicstarting materials (carbon dioxide,water, and minerals) are externallyderived. The synthesis (*anabolism)and breakdown (*catabolism) of mostcompounds occurs by a number ofreaction steps, the reaction sequencebeing termed a metabolic pathway.Some pathways (e.g. *glycolysis) arelinear; others (e.g. the *Krebs cycle)are cyclic.

metabolite See metabolism.

metaboric acid See boric acid.

metal Any of a class of chemical el-ements that are typically lustroussolids that are good conductors ofheat and electricity. Not all metalshave all these properties (e.g. mer-cury is a liquid). In chemistry, metalsfall into two distinct types. Those ofthe s- and p-blocks (e.g. sodium andaluminium) are generally soft silveryreactive elements. They tend to formpositive ions and so are described aselectropositive. This is contrasted


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with typical nonmetallic behaviourof forming negative ions. The *transi-tion elements (e.g. iron and copper)are harder substances and generallyless reactive. They form coordinationcomplexes. All metals have oxidesthat are basic, although some, suchas aluminium, have *amphotericproperties.

metaldehyde A solid compound,C4O4H4(CH3)4, formed by polymeriza-tion of ethanal (acetaldehyde) in di-lute acid solutions below 0°C. Thecompound, a tetramer of ethanal, isused in slug pellets and as a fuel forportable stoves.

metal fatigue A cumulative effectcausing a metal to fail after repeatedapplications of stress, none of whichexceeds the ultimate tensile strength.The fatigue strength (or fatigue limit)is the stress that will cause failureafter a speciÜed number (usually 107)of cycles. The number of cycles re-quired to produce failure decreasesas the level of stress or strain in-creases. Other factors, such as corro-sion, also reduce the fatigue life.

metallic bond A chemical bond ofthe type holding together the atomsin a solid metal or alloy. In suchsolids, the atoms are considered to beionized, with the positive ions occu-pying lattice positions. The valenceelectrons are able to move freely (oralmost freely) through the lattice,

forming an ‘electron gas’. The bond-ing force is electrostatic attractionbetween the positive metal ions andthe electrons. The existence of freeelectrons accounts for the good elec-trical and thermal conductivities ofmetals. See also energy bands.

metallic crystal A crystalline solidin which the atoms are held togetherby *metallic bonds. Metallic crystalsare found in some *interstitial com-pounds as well as in metals and al-loys.

metallized dye See dyes.

metallocene See sandwich com-pound.

metallography The microscopicstudy of the structure of metals andtheir alloys. Both optical microscopesand electron microscopes are used inthis work.

metalloid (semimetal) Any of aclass of chemical elements interme-diate in properties between metalsand nonmetals. The classiÜcation isnot clear cut, but typical metalloidsare boron, silicon, germanium, ar-senic, and tellurium. They are electri-cal semiconductors and their oxidesare amphoteric.

metallurgy The branch of appliedscience concerned with the produc-tion of metals from their ores, thepuriÜcation of metals, the manufac-ture of alloys, and the use and perfor-mance of metals in engineeringpractice. Process metallurgy is con-cerned with the extraction and pro-duction of metals, while physicalmetallurgy concerns the mechanicalbehaviour of metals.

metamict state The amorphousstate of a substance that has lost itscrystalline structure as a result of theradioactivity of uranium or thorium.Metamict minerals are mineralswhose structure has been disrupted

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by this process. The metamictizationis caused by alpha-particles and therecoil nuclei from radioactive disinte-gration.

metaphosphoric acid See phos-phoric(v) acid.

metaplumbate See plumbate.

metastable state A condition of asystem in which it has a precariousstability that can easily be disturbed.It is unlike a state of stable equilib-rium in that a minor disturbance willcause a system in a metastable stateto fall to a lower energy level. A booklying on a table is in a state of stableequilibrium; a thin book standing onedge is in metastable equilibrium.Supercooled water is also in a meta-stable state. It is liquid below 0°C; agrain of dust or ice introduced into itwill cause it to freeze. An excitedstate of an atom or nucleus that hasan appreciable lifetime is alsometastable.

metastannate See stannate.

metathesis A type of reaction inwhich radicals are exchanged. In in-organic chemistry, it is also calleddouble decomposition. A simple ex-ample is

KCL + AgNO3 → KNO3 + AgCl.Metathesis of alkenes is an importanttype of reaction in synthetic organicchemistry. It involves exchange ofgroups. For example

RHC=CH2 + RHC=CH2 → RHC=CHR+ H2C=CH2.

Reactions of this type are catalysedby metal alkylides (containing anM=CR2 grouping) and the intermedi-ate is a four-membered ring contain-ing the metal ion. The catalysts mostoften used are the Schrock catalystsbased on molybdenum and theGrubbs catalysts based on ruthenium.The American chemists RichardSchrock and Robert Grubbs shared

the Nobel prize for chemistry in 2005for work in this field.

methacrylate A salt or ester ofmethacrylic acid (2-methylpropenoicacid).

methacrylate resins *Acrylicresins obtained by polymerizing 2-methylpropenoic acid or its esters.

methacrylic acid See 2-methyl-propenoic acid.

methadone A synthetic opioid,C21H27NO, used medically as an anal-gesic for chronic pains and also as asubstitute for heroin in the treat-ment of addiction. Methadone is it-self addictive and considerablequantities of ‘street’ methadone areused in the UK.

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CH3

O

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CH3

NCH3

CH3

Methadone

methamphetamine See ampheta-mines.

methanal (formaldehyde) A colour-less gas, HCHO; r.d. 0.815 (at –20°C);m.p. –92°C; b.p. –21°C. It is the sim-plest *aldehyde, made by the cat-alytic oxidation of methanol (500°C;silver catalyst) by air. It forms twopolymers: *methanal trimer andpolymethanal. See also formalin.

methanal trimer A cyclic trimerof methanal, C3O3H6, obtained by dis-tillation of an acidic solution ofmethanal. It has a six-membered ringof alternating –O– and –CH2– groups.


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methanation A method of manu-facturing methane from carbonmonoxide or dioxide by high-pressure catalytic hydrogenation. It is often used to improve thecaloriÜc value of town gas.

methane A colourless odourlessgas, CH4; m.p. –182.5°C; b.p. –164°C.Methane is the simplest hydrocar-bon, being the Ürst member of the*alkane series. It is the main con-stituent of natural gas (∼99%) and assuch is an important raw material forproducing other organic compounds.It can be converted into methanol bycatalytic oxidation.

methanide See carbide.

methanoate (formate) A salt orester of methanoic acid.

methanoic acid (formic acid) Acolourless pungent liquid, HCOOH;r.d. 1.2; m.p. 8°C; b.p. 101°C. It can bemade by the action of concentratedsulphuric acid on the sodium salt(sodium methanoate), and occurs nat-urally in ants and stinging nettles.Methanoic acid is the simplest of the*carboxylic acids.

methanol (methyl alcohol) Acolourless liquid, CH3OH; r.d. 0.79;m.p. –93.9°C; b.p. 64.96°C. It is madeby catalytic oxidation of methane(from natural gas) using air.Methanol is used as a solvent (seemethylated spirits) and as a rawmaterial for making methanal(mainly for urea–formaldehyderesins). It was formerly made by thedry distillation of wood (hence thename wood alcohol).

methionine See amino acid.

methoxy group The organicgroup CH3O–.

methyl acetate See methylethanoate.

methyl alcohol See methanol.

methylamine A colourless Ûamma-ble gas, CH3NH2; m.p. –93.5°C; b.p.–6.3°C. It can be made by a catalyticreaction between methanol and am-monia and is used in the manufac-ture of other organic chemicals.

methylated spirits A mixtureconsisting mainly of ethanol withadded methanol (∼9.5%), pyridine(∼0.5%), and blue dye. The additivesare included to make the ethanol un-drinkable so that it can be sold with-out excise duty for use as a solventand a fuel (for small spirit stoves).

methylation A chemical reactionin which a methyl group (CH3–) is in-troduced in a molecule. A particularexample is the replacement of a hy-drogen atom by a methyl group, as ina *Friedel–Crafts reaction.

methylbenzene (toluene) Acolourless liquid, CH3C6H5; r.d. 0.9;m.p. –95°C; b.p. 111°C. Methylben-zene is derived from benzene by re-placement of a hydrogen atom by amethyl group. It can be obtainedfrom coal tar or made from methyl-cyclohexane (extracted from crudeoil) by catalytic dehydrogenation. Itsmain uses are as a solvent and as araw material for producing TNT.

methyl bromide See bro-momethane.

2-methylbuta-1,3-diene See iso-prene.

methyl chloride Seechloromethane.

methyl cyanide See ethane-nitrile.

methylene The highly reactive*carbene, :CH2. The divalent CH2

group in a compound is the methyl-ene group.

methylene chloride Seedichloromethane.

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methylenedioxymethampheta-mine (MDMA) See ecstasy.

methyl ethanoate (methyl ac-etate) A colourless volatile fragrantliquid, CH3COOCH3; r.d. 0.92; m.p.–98°C; b.p. 54°C. A typical *ester, itcan be made from methanol andmethanoic acid and is used mainly asa solvent.

methyl ethyl ketone See bu-tanone.

methyl group (methyl radical) Theorganic group CH3–.

methylidyne See carbyne.

methyl methacrylate An ester ofmethacrylic acid (2-methylpropenoicacid), CH2:C(CH3)COOCH3, used inmaking *methacrylate resins.

methyl orange An organic dyeused as an acid–base *indicator. Itchanges from red below pH 3.1 toyellow above pH 4.4 (at 25°C) and isused for titrations involving weakbases.

methylphenols (cresols) Organiccompounds having a methyl groupand a hydroxyl group bound directlyto a benzene ring. There are threeisomeric methylphenols with the for-mula CH3C6H4OH, differing in therelative positions of the methyl andhydroxyl groups. A mixture of thethree can be obtained by distillingcoal tar and is used as a germicideand antiseptic.

2-methylpropenoic acid(methacrylic acid) A white crystallineunsaturated carboxylic acid,CH2:C(CH3)COOH, used in making*methacrylate resins.

methyl red An organic dye similarin structure and use to methyl or-ange. It changes from red below pH4.4 to yellow above pH 6.0 (at 25°C).

methyl violet A violet dye used asa chemical indicator and as a biologi-cal stain. It is also the colouring mat-ter in methylated spirits. It is amixture of compounds of rosaniline,made by oxidizing dimethylpheny-lamine with copper(II) chloride.

methylxanthines Derivatives ofxanthine in which one or more hy-drogen atoms have been substitutedby methyl groups. The common onesare the trimethylxanthine *caffeineand the dimethylxanthines *theo-phylline and *theobromine.

metol See aminophenol.

metre Symbol m. The SI unit oflength, being the length of the pathtravelled by light in vacuum during atime interval of 1/(2.99 792 458 × 108)second. This deÜnition, adopted bythe General Conference on Weightsand Measures in October, 1983, re-placed the 1967 deÜnition based onthe krypton lamp, i.e. 1 650 763.73wavelengths in a vacuum of the radi-ation corresponding to the transition

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between the levels 2p10 and 5d5 ofthe nuclide krypton–86. This deÜni-tion (in 1958) replaced the older deÜ-nition of a metre based on aplatinum–iridium bar of standardlength. When the *metric systemwas introduced in 1791 in France,the metre was intended to be oneten-millionth of the earth’s meridianquadrant passing through Paris. How-ever, the original geodetic surveysproved the impractibility of such astandard and the original platinummetre bar, the mètre des archives, wasconstructed in 1793.

metric system A decimal systemof units originally devised by a com-mittee of the French Academy,which included J. L. Lagrange andP. S. Laplace, in 1791. It was based onthe *metre, the gram deÜned interms of the mass of a cubic centime-tre of water, and the second. Thiscentimetre-gram-second system (seec.g.s. units) later gave way forscientiÜc work to the metre-kilo-gram-second system (see m.k.s. units)on which *SI units are based.

Meyer, Viktor (1848–97) Germanchemist who worked at Zürich andlater at Heidelberg on a wide rangeof chemical topics. He was the Ürst toprepare oximes and the sulphur com-pound theophene. He is known forhis method of measuring relativemolecular mass by determiningvapour density. Meyer also workedon stereochemistry and was the Ürstto identify steric hindrance in chemi-cal reactions.

mica Any of a group of silicate min-erals with a layered structure. Micasare composed of linked SiO4 tetrahe-dra with cations and hydroxyl group-ings between the layers. The generalformula is X2Y4–6Z8O20(OH,F)4, whereX = K,Na,Ca; Y = Al,Mg,Fe,Li; and

Z = Si,Al. The three main mica miner-als are: *muscovite, K2Al4(Si6Al2O20)(OH,F)4; *biotite, K2(Mg,Fe2+)6–4(Fe3+,Al,Ti)0–2-

(Si6–5Al2–3O20)(OH,F)4; lepidolite, K2(Li,Al)5–6(Si6–7Al2–1O20)-

(OH,F)4. Micas have perfect basal cleavage andthe thin cleavage Ûakes are Ûexibleand elastic. Flakes of mica are used aselectrical insulators and as the dielec-tric in capacitors.

micelle An aggregate of moleculesin a *colloid. For example, whensoap or other *detergents dissolve inwater they do so as micelles – smallclusters of molecules in which thenonpolar hydrocarbon groups are inthe centre and the hydrophilic polargroups are on the outside solvated bythe water molecules.

Michaelis–Menten curve A graphthat shows the relationship betweenthe concentration of a substrate andthe rate of the corresponding en-zyme-controlled reaction. The curveonly applies to enzyme reactions in-volving a single substrate. It was de-vised by Leonor Michaelis (1875–1949) and Maud Menten (1879–1960).The graph can be used to calculatethe Michaelis constant (Km), which isthe concentration of a substrate re-quired in order for an enzyme to actat half of its maximum velocity(Vmax). The Michaelis constant is ameasure of the afÜnity of an enzymefor a substrate. A low value corre-sponds to a high afÜnity, and viceversa. See also enzyme kinetics.A• Original paper on Michaelis–Menten

kinetics

micro- Symbol µ. A preÜx used inthe metric system to denote one mil-lionth. For example, 10–6 metre = 1micrometre (µm).

microbalance A sensitive *balance

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capable of weighing masses of theorder 10–6 to 10–9 kg.

microcrystal test See crystal test.

microscopic Designating a sizescale comparable to the subatomicparticles, atoms, and molecules.Microscopic objects and systems aredescribed by *quantum mechanics.Compare macroscopic; mesoscopic.

microscopic reversibility Theprinciple that in a reversible reactionthe mechanism in one direction isthe exact reverse of the mechanismin the other direction. See also de-tailed balance.

microwaves Electromagneticwaves with wavelengths in the range10–3 to 0.03 m.

microwave spectroscopy A sen-sitive technique for chemical analysisand the determination of molecularstructure (bond lengths, bond angles,and dipole moments), and also rela-tive atomic masses. It is based on theprinciple that microwave radiation(see microwaves) causes changes inthe rotational energy levels of mol-ecules and absorption consequentlyoccurs at characteristic frequencies.In a microwave spectrometer a mi-crowave source, usually a klystronvalve, produces a beam that is passedthrough a gaseous sample. The beamthen impinges on the detector, usu-ally a crystal detector, and the signal(wavelength against intensity) is dis-played, either as a printed plot or onan oscilloscope. As microwaves areabsorbed by air the instrument isevacuated.

migration 1. The movement of agroup, atom, or double bond fromone part of a molecule to another. 2. The movement of ions under theinÛuence of an electric Üeld.

milk of magnesia See magnesiumhydroxide.

milk sugar See lactose.

Miller indices A set of three num-bers that characterize a face of a crys-tal. The French mineralogist RenéJust Haüy (1743–1822) proposed thelaw of rational intercepts, whichstates that there is always a set ofaxes, known as crystal axes, that al-lows a crystal face to be character-ized in terms of intercepts of the facewith these axes. The reciprocals ofthese intercepts are small rationalnumbers. When the fractions arecleared there is a set of three inte-gers. These integers are known as theMiller indices of the crystal face afterthe British mineralogist William Hal-lowes Miller (1810–80), who pointedout that crystal faces could be charac-terized by these indices.

If a plane is parallel to one of thecrystal axes then its intercept is atinÜnity and hence its reciprocal is 0.If a face cuts a crystal axis on thenegative side of the origin then theintercept, and hence its reciprocal,i.e. the Miller index for that axis, arenegative. This is indicated by a barover the Miller index. For example,the Miller indices for the eight facesof an octahedron are (III), (I

_II), (II

_I),

(III_), (I

_I_I), (II

_I_), (I

_II_) and (I

_I_I_).

milli- Symbol m. A preÜx used inthe metric system to denote onethousandth. For example, 0.001 volt= 1 millivolt (mV).

Millon’s reagent A solution ofmercury(II) nitrate and nitrous acidused to test for proteins. The sampleis added to the reagent and heatedfor two minutes at 95°C; the forma-tion of a red precipitate indicates thepresence of protein in the sample.The reagent is named after Frenchchemist Auguste Millon (1812–67).

mineral A naturally occurring sub-stance that has a characteristic chem-ical composition and, in general, a

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crystalline structure. The term is alsooften applied generally to organicsubstances that are obtained by min-ing (e.g. coal, petroleum, and naturalgas) but strictly speaking these arenot minerals, being complex mix-tures without deÜnite chemical for-mulas. Rocks are composed ofmixtures of minerals. Minerals maybe identiÜed by the properties oftheir crystal system, hardness (meas-ured on the Mohs’ scale), relativedensity, lustre, colour, cleavage, andfracture. Many names of mineralsend in -ite.

mineral acid A common inorganicacid, such as hydrochloric acid, sul-phuric acid, or nitric acid.

mirabilite A mineral form of*sodium sulphate, Na2SO4.10H2O.

misch metal An alloy of cerium(50%), lanthanum (25%), neodymium(18%), praseodymium (5%), and otherrare earths. It is used alloyed withiron (up to 30%) in lighter Ûints, andin small quantities to improve themalleability of iron. It is also addedto copper alloys to make themharder, to aluminium alloys to makethem stronger, to magnesium alloysto reduce creep, and to nickel alloysto reduce oxidation.

Mitscherlich’s law (law of isomor-phism) Substances that have thesame crystal structure have similarchemical formulae. The law can beused to determine the formula of anunknown compound if it is isomor-phous with a compound of knownformula. It is named after EilhardMitscherlich (1794–1863).

mixture A system of two or moredistinct chemical substances. Homo-geneous mixtures are those in whichthe atoms or molecules are inter-spersed, as in a mixture of gases or ina solution. Heterogeneous mixtureshave distinguishable phases, e.g. a

mixture of iron Ülings and sulphur.In a mixture there is no redistribu-tion of valence electrons, and thecomponents retain their individualchemical properties. Unlike com-pounds, mixtures can be separatedby physical means (distillation, crys-tallization, etc.).

m.k.s. units A *metric system ofunits devised by A. Giorgi (and some-times known as Giorgi units) in 1901.It is based on the metre, kilogram,and second and grew from the ear-lier *c.g.s. units. The electrical unitchosen to augment these three basicunits was the ampere and the perme-ability of space (magnetic constant)was taken as 10–7 Hm–1. To simplifyelectromagnetic calculations themagnetic constant was later changedto 4π × 10–7 Hm–1 to give the rational-ized MKSA system. This system, withsome modiÜcations, formed the basisof *SI units, now used in all scientiÜcwork.

mmHg A unit of pressure equal tothat exerted under standard gravityby a height of one millimetre of mer-cury, or 133.322 pascals.

mobility (of an ion) Symbol u. Theterminal speed of an ion in an elec-tric Üeld divided by the Üeld strength.

mode The pattern of motion in a vi-brating body. If the body has severalcomponent particles, such as a mol-ecule consisting of several atoms, themodes of vibration are the differenttypes of molecular vibrations possi-ble.

moiety A characteristic part of amolecule. For example, in the esterC2H5COOCH3, the OCH3 can be re-garded as the alcohol (methanol)moiety.

moissanite A clear form of siliconcarbide used as an inexpensive sub-stitute for diamond. It is named after

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the French chemist Henri Moissan(1852–1907), who discovered it in1893.

molal concentration See concen-tration.

molality See concentration.

molar 1. Denoting that an exten-sive physical property is being ex-pressed per *amount of substance,usually per mole. For example, themolar heat capacity of a compound isthe heat capacity of that compoundper unit amount of substance; in SIunits it would be expressed in J K–1

mol–1. 2. Having a concentration ofone mole per dm3.

molar conductivity Symbol Λ.The conductivity of that volume ofan electrolyte that contains one moleof solution between electrodesplaced one metre apart.

molar heat capacity See heat ca-pacity.

molarity See concentration.

molar volume (molecular volume)The volume occupied by a substanceper unit amount of substance.

mole Symbol mol. The SI unit of*amount of substance. It is equal tothe amount of substance that con-tains as many elementary units asthere are atoms in 0.012 kg of car-bon–12. The elementary units maybe atoms, molecules, ions, radicals,electrons, etc., and must be speciÜed.1 mole of a compound has a massequal to its *relative molecular massexpressed in grams.

molecular beam A beam ofatoms, ions, or molecules at low pres-sure, in which all the particles aretravelling in the same direction andthere are few collisions betweenthem. They are formed by allowing agas or vapour to pass through anaperture into an enclosure, which

acts as a collimator by containingseveral additional apertures and vac-uum pumps to remove any particlesthat do not pass through the aper-tures. Molecular beams are used instudies of surfaces and chemical reac-tions and in spectroscopy.

molecular chaperone Any of agroup of proteins in living cells thatassist newly synthesized or dena-tured proteins to fold into their func-tional three-dimensional structures.The chaperones bind to the proteinand prevent improper interactionswithin the polypeptide chain, so thatit assumes the correct folded orienta-tion. This process requires energy inthe form of ATP.

molecular distillation Distillationin high vacuum (about 0.1 pascal)with the condensing surface so closeto the surface of the evaporating liq-uid that the molecules of the liquidtravel to the condensing surfacewithout collisions. This technique en-ables very much lower temperaturesto be used than are used with distilla-tion at atmospheric pressure andtherefore heat-sensitive substancescan be distilled. Oxidation of the dis-tillate is also eliminated as there isno oxygen present.

molecular Ûow (Knudsen Ûow)The Ûow of a gas through a pipe inwhich the mean free path of gas mol-ecules is large compared to the di-mensions of the pipe. This occurs atlow pressures; because most colli-sions are with the walls of the piperather than other gas molecules, theÛow characteristics depend on therelative molecular mass of the gasrather than its viscosity. The effectwas studied by M. H. C. Knudsen(1871–1949).

molecular formula See formula.

molecularity The number of mol-ecules involved in forming the acti-

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vated complex in a step of a chemicalreaction. Reactions are said to be uni-molecular, bimolecular, or trimolecu-lar according to whether 1, 2, or 3molecules are involved.

molecular knot (knotane) A typeof compound in which one or morechains of atoms in the molecule arelooped in the conÜguration of a knot.The molecule may have only oneclosed chain forming the knot. If theknot is a trefoil knot the compoundis chiral. Alternatively, molecularknots may have two or more sepa-rate loops tied together in a knot. Insuch cases there is no formal chemi-cal bonding between the rings andthey are held by *mechanical bond-ing. Molecular knots can be producedsynthetically and also occur naturallyin certain proteins.

molecular modelling The use ofcomputer software to produce simu-lations of molecular structures. Vari-ous chemical drawing programs existto allow a graphic representation ofchemical formulas in two dimen-sions. More sophisticated three-dimensional modelling programsalso exist. The information about themolecule is stored in a data Üle giv-ing the numbers and types of atomspresent and the coordinates of theseatoms. The software converts thisinto an on-screen three-dimensionalview of the molecular structure insome speciÜed format (e.g. ball-and-stick, wireframe, etc.). The structurecan be rotated on the screen and, de-pending on the software, calculationsmay be done on the molecule.

molecular orbital See orbital.

molecular-orbital theory Amethod of computational chemistryin which the electrons are not as-signed to individual bonds betweenatoms, but are treated as movingunder the inÛuence of the nuclei in

the whole molecule. The moleculehas a set of molecular orbitals (see or-bital). The usual technique is to ob-tain molecular orbitals by a linearcombination of atomic orbitals(LCAO). See also density-functiontheory; valence-bond theory.

molecular recognition The wayin which a molecule may have ahighly speciÜc response to anothermolecule or atom. It is a feature ofhost–guest chemistry. See supramol-ecular chemistry.

molecular sieve Porous crystallinesubstances, especially aluminosili-cates (see zeolite), that can be dehy-drated with little change in crystalstructure. As they form regularlyspaced cavities, they provide a highsurface area for the adsorption ofsmaller molecules.

The general formula of these sub-stances is MnO.Al2O3.xSiO2.yH2O,where M is a metal ion and n is twicethe reciprocal of its valency. Molecu-lar sieves are used as drying agentsand in the separation and puriÜca-tion of Ûuids. They can also be loadedwith chemical substances, which re-main separated from any reactionthat is taking place around them,until they are released by heating orby displacement with a morestrongly adsorbed substance. Theycan thus be used as cation exchangemediums and as catalysts and cata-lyst supports. They are also used asthe stationary phase in certain typesof *chromatography (molecular-sievechromatography).

molecular symmetry The set ofsymmetry operations (rotations,reÛections, etc.) that can be appliedto a molecule. This set forms the*point group of the molecule. For anisolated molecule the *SchoenÛiessystem rather than the *Hermann–Mauguin system is used to denote itssymmetry. Molecular symmetry is

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analysed systematically using *grouptheory. It is possible to make deÜnitestatements about certain propertiesof molecules, such as whether theycan have an electric dipole momentor exhibit optical activity on thebasis of symmetry.

molecular volume See molar vol-ume.

molecular weight See relativemolecular mass.

molecule One of the fundamentalunits forming a chemical compound;the smallest part of a chemical com-pound that can take part in a chemi-cal reaction. In most covalentcompounds, molecules consist ofgroups of atoms held together by co-valent or coordinate bonds. Covalentsubstances that form *macromolecu-lar crystals have no discrete mol-ecules (in a sense, the whole crystalis a molecule). Similarly, ionic com-pounds do not have single molecules,being collections of oppositelycharged ions.

mole fraction Symbol X. A meas-ure of the amount of a component ina mixture. The mole fraction of com-ponent A is given by XA = nA/N, wherenA is the amount of substance of A(for a given entity) and N is the totalamount of substance of the mixture(for the same entity).

molÜle format A format using aconnection table and atom coordi-nates to hold information aboutchemical structures. It was producedby Elsevier MDL and is widely used inchemical drawing programs. Fileshave the extension .mol.

Molisch’s test See alpha-naphtholtest.

molybdenum Symbol Mo. A sil-very hard metallic *transition el-ement; a.n. 42; r.a.m. 95.94; r.d.10.22; m.p. 2617°C; b.p. 4612°C. It is

found in molybdenite (MoS2), themetal being extracted by roasting togive the oxide, followed by reductionwith hydrogen. The element is usedin alloy steels. Molybdenum(IV) sul-phide (MoS2) is used as a lubricant.Chemically, it is unreactive, beingunaffected by most acids. It oxidizesat high temperatures and can be dis-solved in molten alkali to give arange of molybdates and polymolyb-dates. Molybdenum was discoveredin 1778 by Karl *Scheele.


moment of inertia Symbol I. Aquantity associated with a body thatis rotating about an axis. If a bodyconsists of i particles of mass m a per-pendicular distance r from an axis ofrotation, the moment of inertia I ofthe body about that axis of rotationis given by I = ∑ miri

2. The moment ofinertia of a body is an importantquantity because it is the analoguefor rotational motion of mass for lin-ear motion.

In a molecule all rotational motioncan be analysed using three perpen-dicular axes of rotation. Each of thesehas a moment of inertia associatedwith it. To a Ürst approximation, amolecule can be regarded as a rigidrotor, i.e. a body that is not distortedby its rotation. There are four typesof rigid rotor:In a spherical top all three momentsof inertia are equal (e.g. SF6).In a symmetric top two of the mo-ments of inertia are equal (e.g. NH3).In an asymmetric top all three mo-ments of inertia are different (e.g.H2O).In a linear rotor the moment of iner-tia about the axis of the molecule iszero (e.g. HCl, CO2).

The type of rotor depends on thesymmetry of the molecule. If a mol-ecule has cubic or icosahedral sym-

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metry it is a spherical top. If it has athreefold or higher axis of symmetryit is a symmetric top. If it does nothave a threefold or higher axis ofsymmetry it is an asymmetric top.All linear molecules (and hence alldiatomic molecules) are linear rotors.

In reality, molecules are not rigidrotors because of the centrifugalforces associated with the rotation.The effect of such forces can betaken into account by adding a cor-rection term to the rigid rotor model.

monatomic molecule A ‘mol-ecule’ consisting of only one atom(e.g. Ar or He), distinguished from di-atomic and polyatomic molecules.

Mond process A method of obtain-ing pure nickel by heating the im-pure metal in a stream of carbonmonoxide at 50–60°C. Volatile nickelcarbonyl (Ni(CO)4) is formed, and thiscan be decomposed at higher tem-peratures (180°C) to give pure nickel.The method was invented by the Ger-man–British chemist Ludwig Mond(1839–1909).

Monel metal An alloy of nickel(60–70%), copper (25–35%), and smallquantities of iron, manganese, sili-con, and carbon. It is used to makeacid-resisting equipment in thechemical industry.

monobasic acid An *acid that hasonly one acidic hydrogen atom in itsmolecules. Hydrochloric (HCl) and ni-tric (HNO3) acids are common exam-ples.

monoclinic See crystal system.

monoethanolamine Seeethanolamine.

monoglyceride See glyceride.

monohydrate A crystalline com-pound having one mole of water permole of compound.

monomer A molecule (or com-pound) that joins with others informing a dimer, trimer, or polymer.

monosaccharide (simple sugar) Acarbohydrate that cannot be splitinto smaller units by the action of di-lute acids. Monosaccharides areclassiÜed according to the number ofcarbon atoms they possess: trioseshave three carbon atoms; tetroses,four; pentoses, Üve; hexoses, six; etc.Each of these is further divided intoaldoses and ketoses, depending onwhether the molecule contains analdehyde group (–CHO) or a ketonegroup (–CO–). For example glucose,having six carbon atoms and an alde-hyde group, is an aldohexosewhereas fructose is a ketohexose.These aldehyde and ketone groupsconfer reducing properties on mono-saccharides: they can be oxidized toyield sugar acids. They also reactwith phosphoric acid to producephosphate esters (e.g. in *ATP),which are important in cell metabo-lism. Monosaccharides can exist as ei-ther straight-chain or ring-shapedmolecules. They also exhibit *opticalactivity, giving rise to both dextro-rotatory and laevorotatory forms.

monosodium glutamate (MSG) Awhite solid, C8H8NNaO4.H2O, usedextensively as a Ûavour enhancer, es-pecially in convenience foods. It is asalt of glutamic acid (an *aminoacid), from which it is prepared. Itcan cause an allergic reaction insome susceptible people who con-sume it.

monotropy See allotropy.

monovalent (univalent) Having avalency of one.

Monte Carlo method A numeri-cal method for solving mathematicaland physical problems that involvesthe random sampling of numbers.Applications of Monte Carlo methods

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include calculations in the theory ofliquids, phase transitions, and quan-tum mechanical systems. The namecomes from the casino at MonteCarlo and alludes to the use of ran-dom numbers in this technique.

montmorillonite A clay mineralof variable composition. It is a hy-drated aluminosilicate with a largecapacity for exchanging cations (seeion exchange) and is the principalconstituent of bentonite and fuller’searth. One type of montmorillonitereadily absorbs water, and anothertype swells in water to form a *gel.

mordant A substance used in cer-

tain dyeing processes. Mordants areoften inorganic oxides or salts, whichare absorbed on the fabric. Thedyestuff then forms a coloured com-plex with the mordant, the colourdepending on the mordant used aswell as the dyestuff. See also lake.

morphine An opiate that is themain active constituent of opium. Itis used medically in the relief of se-vere pain, and can be acetylated toproduce heroin. Morphine can be de-tected by the *Marquis test.

Morse potential The potential en-ergy of a diatomic molecule as afunction of the difference (r – re),

361 Morse potential

m

Glucose (an aldohexose)

aldehydegroupH

H

HO

H

H

OH

O

OH

OH

O1C

2C

3C

4C

5C

6CH

2OH

straight-chain form

Fructose (a ketohexose)

HO

H

H

O

H

OH

OH

2C

3C

4C

5C

6CH

2OH

6CH

2OH

5C

OH

H

3C

4C

HO

H

H

C2

H

OH

OH

C1

HO

α-glucose

belowplaneof ring

ring forms

β-glucose

6CH

2OH

5C

OH

H

3C

4C

HO

H

H

C2

H

OH

C1

H

O

aboveplaneof ring

OH

ketonegroup

1CH OH

H

straight-chain form ring form

H

4C

5C

HO

OH

HOCH2

C3

OH

H

OH

C2

CH2

OHO6 1

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where r is the variable interatomicdistance and re is the equilibrium in-teratomic distance. The Morse poten-tial U(r – re) is given by

De1 – exp[–β(–r – re)]2,where De is the dissociation energy atthe minimum of the curve (i.e. whenr = re) and β is a constant. The Morsepotential was used by the US physi-cist Philip M. Morse in 1929 in solv-ing the Schrödinger equation. TheMorse potential is a reasonably goodrepresentation of a potential-energyfunction except that as r approaches0, U does not approach inÜnity as itshould for a true potential energyfunction. ModiÜcations of the Morsepotential have been suggested to im-prove on this aspect.

mosaic gold See tin(iv) sulphide.

Moseley’s law The frequencies ofthe lines in the *X-ray spectra of theelements are related to the atomicnumbers of the elements. If thesquare roots of the frequencies ofcorresponding lines of a set of el-ements are plotted against theatomic numbers a straight line isobtained. The law was discovered byH. G. Moseley (1887–1915).

A• Original paper on the law

moss agate See agate.

Mössbauer effect An effect occur-ring when certain nuclides decaywith emission of gamma radiation.For an isolated nucleus, the gammaradiation would usually have aspread of energies because the en-ergy of the process is partitioned be-tween the gamma-ray photon andthe recoil energy of the nucleus. In1957 Rudolph Mössbauer (1929– )found that in certain solids, in whichthe emitting nucleus is held bystrong forces in the lattice, the recoilenergy is taken up by the whole lat-

tice. Since this may typically contain1010–1020 atoms, the recoil energy isnegligible and the energy of theemitted photon is sharply deÜned ina very narrow energy spread.

The effect is exploited in Möss-bauer spectroscopy in which agamma-ray source is mounted on amoving platform and a similar sam-ple is mounted nearby. A detectormeasures gamma rays scattered bythe sample. The source is movedslowly towards the sample at a vary-ing speed, so as to continuouslychange the frequency of the emittedgamma radiation by the Doppler ef-fect. A sharp decrease in the signalfrom the detector at a particularspeed (i.e. frequency) indicates reso-nance absorption in the sample nu-clei. The effect is used to investigatenuclear energy levels. In chemistry,Mössbauer spectroscopy can also giveinformation about the bonding andstructure of compounds becausechemical shifts in the resonance en-ergy are produced by the presence ofsurrounding atoms.

m.p. See melting point.

MSG See monosodium glutamate.

mucopolysaccharide See gly-cosaminoglycan.

mufÛe furnace An insulated fur-nace, usually electrically heated, usedfor producing controlled high tem-peratures. In the laboratory, mufÛefurnaces are used for drying solids,sintering, and studying high-temper-ature reactions. They typically oper-ate in the range 100–1200°C.

multicentre bond A bond formedbetween three, and sometimes more,atoms that contains only a single pairof electrons. The structure of *bo-ranes can be explained by consider-ing them to be *electron-deÜcientcompounds containing multicentrebonds.

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multidecker sandwich See sand-wich compound.

multiple bond A bond betweentwo atoms that contains more thanone pair of electrons. Such bonds pri-marily involve sigma bonding withsecondary contribution from pi bond-ing (or, sometimes, delta bonding).See orbital.

multiple proportions See chemi-cal combination.

multiplet 1. A spectral line formedby more than two (see doublet)closely spaced lines. 2. A group of el-ementary particles that are identicalin all respects except that of electriccharge.

multiplicity A quantity used inatomic *spectra to describe the en-ergy levels of many-electron atomscharacterized by *Russell–Saunderscoupling given by 2S + 1, where S isthe total electron *spin quantumnumber. The multiplicity of an en-ergy level is indicated by a left super-script to the value of L, where L is theresultant electron *orbital angularmomentum of the individual elec-tron orbital angular momenta l.

multipole interactions Interac-tions between arrays of point charges(multipoles) associated with potentialenergies that depend on distance.Multipole interactions are an impor-tant feature of intermolecular forces.An n-pole is a set of n charges havingan n-pole moment but not a lowermoment. A monopole is a singlecharge and the monopole moment isthe charge. A dipole consists of twoopposite charges and thus has nooverall charge (and hence no mono-pole moment). Higher multipoles arethe *quadrupole and the *octupole.The interaction potential energy be-tween multipoles falls off with dis-tance increasingly rapidly as theorder of the multipoles increases. If a

2m-pole interacts with a 2n-pole, theinteraction potential energy, V,varies with distance r as V =c/(rm + n – 1), where c is a constant. Therapid fall-off with distance, as theorder increases, is due to the set ofcharges appearing to tend towardsneutrality (as seen from outside). Themultipole interactions with thelargest interactions are: ion–ion,ion–dipole, dipole–dipole, and disper-sion interactions.

Mumetal The original trade namefor a ferromagnetic alloy, containing78% nickel, 17% iron, and 5% copper,that had a high permeability and alow coercive force. More modern ver-sions also contain chromium andmolybdenum. These alloys are usedin some transformer cores and forshielding various devices from exter-nal magnetic Üelds.

Muntz metal A form of *brass con-taining 60% copper, 39% zinc, andsmall amounts of lead and iron.Stronger than alpha-brass, it is usedfor hot forgings, brazing rods, andlarge nuts and bolts. It is named afterG. F. Muntz (1794–1857).

muscovite (white mica; potashmica) A mineral form of potassiumaluminosilicate, K2Al4(Si6Al2)O20-(OH,F)4; one of the most importantmembers of the *mica group of min-erals. It is chemically complex andhas a sheetlike crystal structure (seeintercalation compound). It is usu-ally silvery-grey in colour, sometimestinted with green, brown, or pink.Muscovite is a common constituentof certain granites and pegmatites. Itis also common in metamorphic andsedimentary rocks. It is widely usedin industry, for example in the man-ufacture of electrical equipment andas a Üller in rooÜng materials, wall-papers, and paint.

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mustard gas See sulphurmustard; nitrogen mustard.

mutarotation Change of opticalactivity with time as a result of spon-taneous chemical reaction.

myoglobin A globular protein oc-curring widely in muscle tissue as anoxygen carrier. It comprises a singlepolypeptide chain and a *haem

group, which reversibly binds a mol-ecule of oxygen. This is only relin-quished at relatively low externaloxygen concentrations, e.g. duringstrenuous exercise when muscle oxy-gen demand outpaces supply fromthe blood. Myoglobin thus acts as anemergency oxygen store.

myristic acid See tetradecanoicacid.

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N

NAD (nicotinamide adenine dinu-cleotide) A *coenzyme, derived fromthe B vitamin *nicotinic acid, thatparticipates in many biological dehy-drogenation reactions. NAD is charac-teristically loosely bound to theenzymes concerned. It normally car-ries a positive charge and can acceptone hydrogen atom and two elec-trons to become the reduced form,NADH. NADH is generated during theoxidation of food; it then gives up itstwo electrons (and single proton) tothe electron transport chain, therebyreverting to NAD+ and generatingthree molecules of ATP per moleculeof NADH.

NADP (nicotinamide adenine dinu-cleotide phosphate) differs from NADonly in possessing an additionalphosphate group. It functions in thesame way as NAD although anabolicreactions (see anabolism) generallyuse NADPH (reduced NADP) as a hy-drogen donor rather than NADH. En-

zymes tend to be speciÜc for eitherNAD or NADP as coenzyme.

nano- Symbol n. A preÜx used inthe metric system to denote 10–9. Forexample, 10–9 second = 1 nanosecond(ns).

nanotechnology The develop-ment and use of devices that have asize of only a few nanometres. Re-search has been carried out into verysmall components, which depend onelectronic effects and may involvemovement of a countable number ofelectrons in their action. Such de-vices would act much faster thanlarger components. Considerable in-terest has been shown in the produc-tion of structures on a molecularlevel by suitable sequences of chemi-cal reactions. It is also possible to ma-nipulate individual atoms on surfacesusing a variant of the *atomic forcemicroscope.

nanotube See buckminster-fullerene.

CONH2

site of reduction to NADH

phosphate groupattached here toform NADP

NH2NH2NH2

NNN

NNNNNN

NNN

+++NNN

OOO

H H

OHOHOH OHOHOH

CH2CH2CH2

HHH

OOO

O PPP O–

OOO

OOO CH2CH2CH2

H

OOO

HH

OHOHOH

H

OHOHOH

O PPP O–

NAD


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napalm A substance used in incen-diary bombs and Ûame throwers,made by forming a gel of petrol withaluminium soaps (aluminium salts oflong-chain carboxylic acids, such aspalmitic acid).

naphtha Any liquid hydrocarbon ormixture obtained by the fractionaldistillation of petroleum. It is gener-ally applied to higher *alkane frac-tions with nine or ten carbon atoms.Naphtha is used as a solvent and as astarting material for *cracking intomore volatile products, such aspetrol.

naphthalene A white volatilesolid, C10H8 (see formula); r.d. 1.025;m.p. 80.55°C; b.p. 218°C. Naphtha-lene is an aromatic hydrocarbon withan odour of mothballs and is ob-tained from crude oil. It is a raw ma-terial for making certain syntheticresins.

napalm 366

nα

β

Naphthalene

naphthols Two phenols derivedfrom naphthalene with the formulaC10H7OH, differing in the position ofthe –OH group. The most importantis naphthalen-2-ol (β-naphthol), with the –OH in the 2-position. It is a white solid (r.d. 1.28; m.p.123–124°C; b.p. 295°C) used in rub-ber as an antioxidant. Naphthalen-2-ol will couple with diazonium salts atthe 1-position to form red *azo com-pounds, a reaction used in testing forthe presence of primary amines (bymaking the diazonium salt andadding naphthalen-2-ol).

naphthyl group The group C10H7–

obtained by removing a hydrogenatom from naphthalene. There aretwo forms depending on whether thehydrogen is removed from the 1- or2-position.

nascent hydrogen A reactiveform of hydrogen generated in situ inthe reaction mixture (e.g. by the ac-tion of acid on zinc). Nascent hydro-gen can reduce elements andcompounds that do not readily reactwith ‘normal’ hydrogen. It was oncethought that the hydrogen was pre-sent as atoms, but this is not thecase. Probably hydrogen moleculesare formed in an excited state andreact before they revert to theground state.

natron A mineral form of hydratedsodium carbonate, Na2CO3.H2O.

Natta process See ziegler process.

natural abundance See abun-dance.

natural gas A naturally occurringmixture of gaseous hydrocarbonsthat is found in porous sedimentaryrocks in the earth’s crust, usually inassociation with *petroleum de-posits. It consists chieÛy of methane(about 85%), ethane (up to about10%), propane (about 3%), and butane.Carbon dioxide, nitrogen, oxygen,hydrogen sulphide, and sometimeshelium may also be present. Naturalgas, like petroleum, originates in thedecomposition of organic matter. It iswidely used as a fuel and also to pro-duce carbon black and some organicchemicals. Natural gas occurs onevery continent, the major reservesoccurring in the USA, Russia, Kazakh-stan, Turkmenistan, Ukraine, Alge-ria, Canada, and the Middle East. Seealso liquefied petroleum gas.

NBR See nitrile rubber.

NBS See N-bromosuccinimide.


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Néel temperature The tempera-ture above which an antiferro-magnetic substance becomesparamagnetic (see magnetism). Thesusceptibility increases with temper-ature, reaching a maximum at theNéel temperature, after which itabruptly declines. The phenomenonwas discovered around 1930 by L. E. F. Néel (1904–2000).

neighbouring-group participa-tion An effect in an organic chemi-cal reaction in which the reactivecentre interacts with a lone pair orwith electrons in other bonds in themolecule that are not conjugatedwith the centre. This may affect therate or the stereochemistry of theproducts. It is sometimes called an-chimeric assistance.

nematic crystal See liquidcrystal.

neodymium Symbol Nd. A soft sil-very metallic element belonging tothe *lanthanoids; a.n. 60; r.a.m.144.24; r.d. 7.004 (20°); m.p. 1021°C;b.p. 3068°C. It occurs in bastnasiteand monazite, from which it is recov-ered by an ion-exchange process.There are seven naturally occurringisotopes, all of which are stable, ex-cept neodymium–144, which isslightly radioactive (half-life1010–1015 years). Seven artiÜcial ra-dioisotopes have been produced. Themetal is used to colour glass violet-purple and to make it dichroic. It isalso used in misch metal (18%neodymium) and in neodymium–iron–boron alloys for magnets. It was discovered by Carl von Welsbach(1856–1929) in 1885.A• Information from the WebElements site

neon Symbol Ne. A colourlessgaseous element belonging to group18 (formerly group 0) of the periodictable (the *noble gases); a.n. 10;

r.a.m. 20.179; d. 0.9 g dm–3; m.p.–248.67°C; b.p. –246.05°C. Neon oc-curs in air (0.0018% by volume) and isobtained by fractional distillation ofliquid air. It is used in dischargetubes and neon lamps, in which ithas a characteristic red glow. It formshardly any compounds (neonÛuorides have been reported). The el-ement was discovered in 1898 by SirWilliam Ramsey and M. W. Travers.


neoprene A synthetic rubber madeby polymerizing the compound 2-chlorobuta-1,2-diene. Neoprene isoften used in place of natural rubberin applications requiring resistanceto chemical attack.

nephelometry The measurementof the turbidity (cloudiness) of a liq-uid. Nephalometers generally have alight source (often a laser) and a de-tector to measure the amount oflight scattered by suspended parti-cles. They are used in a number ofÜelds including water quality controland blood analysis (for protein con-tent).

nephrite See jade.

neptunium Symbol Np. A radio-active metallic transuranic elementbelonging to the *actinoids; a.n. 93;r.a.m. 237.0482. The most stable iso-tope, neptunium–237, has a half-lifeof 2.2 × 106 years and is produced insmall quantities as a by-product bynuclear reactors. Other isotopes havemass numbers 229–236 and 238–241.The only other relatively long-livedisotope is neptunium–236 (half-life 5× 103 years). The element was Ürstproduced by Edwin McMillan (1907–91) and Philip Abelson (1913–2004) in1940.


367 neptunium

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neptunium series See radioactiveseries.

Nernst, (Hermann) Walther(1864–1941) German physicalchemist noted for his work on elec-trochemistry and chemical thermo-dynamics. He is remembered as thediscoverer of the third law of ther-modynamics. Nernst was awardedthe 1920 Nobel prize for chemistry.

Nernst–Einstein equation Anequation relating the limiting molarconductivity Λm

0 (see kohlrausch’slaw) to the ionic diffusion coefÜ-cients, devised by Nernst and Albert*Einstein. The Nernst–Einstein equa-tion is

Λm0 = (F2/RT)(v+z+

2D+ + v–z–2D–),

where F is the Faraday constant, R isthe gas constant, T is the thermody-namic temperature, v+ and v– are thenumber of cations and anions performula unit of electrolyte, z+ and z–are the valences of the ions, and D+and D– are the diffusion coefÜcientsof the ions. An application of theNernst–Einstein equation is to calcu-late the ionic diffusion coefÜcientsfrom experimental determinations ofconductivity.

Nernst equation An equation thatrelates the *electrode potential E ofan electrode that is in contact withan ionic solution to the ionic concen-tration c. The equation is:

E = EŠ – RTzFlnc,where EŠ is the standard electrodepotential, R is the gas constant, T isthe absolute temperature, z is the va-lence of the ion, and F is the Faradayconstant. The equation was derivedby Walther *Nernst in 1889. It is fun-damental to the thermodynamics ofelectrochemical cells.

Nernst heat theorem A state-ment of the third law of *thermody-namics in a restricted form given by

Walter Nernst: if a chemical changetakes place between pure crystallinesolids at *absolute zero there is nochange of entropy.

nerve agents A class of highlytoxic compounds that act by affect-ing the action of acetylcholine, a neu-rotransmitter regulated by theenzyme acetylcholinesterase (AChE).The nerve agents, which areorganophosphates, bind to AChE andblock its action. Consequently thereis nothing to stop the build up ofacetylcholine, and exposure to smallamounts of nerve gas can kill in min-utes. There are two series of agents.The G-series was produced by Ger-man scientists in the 1930s and 40s(the G stands for German). The mainones are *tabun (GA), *sarin (GB),*soman (GD), and *cyclosarin (GF). Afurther series of nerve agents is theV-series. The most common of theseis *VX, which was discovered at Por-ton Down in the UK in 1952. Nerveagents are classiÜed as weapons ofmass destruction by the UN.

nesquehonite A mineral form of*magnesium carbonate trihydrate,MgCO3.3H2O.

Nessler’s reagent A solution ofmercury(II) iodide (HgI2) in potassiumiodide and potassium hydroxidenamed after Julius Nessler(1827–1905). It is used in testing forammonia, with which it forms abrown coloration or precipitate.

neutral Describing a compound orsolution that is neither acidic norbasic. A neutral solution is one thatcontains equal numbers of both pro-tonated and deprotonated forms ofthe solvent.

neutralization The process inwhich an acid reacts with a base toform a salt and water.

neutron A neutral hadron that is

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stable in the atomic nucleus but de-cays into a proton, an electron, andan antineutrino with a mean life of12 minutes outside the nucleus. Itsrest mass is slightly greater than thatof the proton, being 1.674 9286(10) ×10–27 kg. Neutrons occur in all atomicnuclei except normal hydrogen. Theneutron was Ürst reported in 1932 bythe British physicist James Chadwick(1891–1974).

neutron activation analysis Seeactivation analysis.

neutron diffraction The scatter-ing of neutrons by atoms in solids,liquids, or gases. This process hasgiven rise to a technique, analogousto *X-ray diffraction techniques,using a Ûux of thermal neutronsfrom a nuclear reactor to study solid-state phenomena. Thermal neutronshave average kinetic energies ofabout 0.025 eV (4 × 10–21 J) givingthem an equivalent wavelength ofabout 0.1 nanometre, which is suit-able for the study of interatomic in-terference. There are two types ofinteraction in the scattering of neu-trons by atoms: one is the interactionbetween the neutrons and theatomic nucleus, the other is the in-teraction between the *magnetic mo-ments of the neutrons and the spinand orbital magnetic moments of theatoms. The latter interaction has pro-vided valuable information on anti-ferromagnetic and ferrimagneticmaterials (see magnetism). Interac-tion with the atomic nucleus givesdiffraction patterns that complementthose from X-rays. X-rays, whichinteract with the extranuclear elec-trons, are not suitable for investigat-ing light elements (e.g. hydrogen),whereas neutrons do give diffractionpatterns from such atoms becausethey interact with nuclei.

neutron number Symbol N. Thenumber of neutrons in an atomic nu-

cleus of a particular nuclide. It isequal to the difference between the*nucleon number and the *atomicnumber.

Newlands’ law See law of oc-taves.

Newman projection A type of*projection in which a molecule isviewed along a bond. See conforma-tion.

newton Symbol N. The *SI unit offorce, being the force required togive a mass of one kilogram an accel-eration of 1 m s–2. It is named afterSir Isaac Newton (1642–1727).

Newtonian Ûuid A Ûuid in whichthe velocity gradient is directly pro-portional to the shear stress. If twoÛat plates of area A are separated bya layer of Ûuid of thickness d andmove relative to each other at a ve-locity v, then the rate of shear is v/dand the shear stress is F/A, where F isthe force applied to each. For a New-tonian Ûuid F/A = µv/d, where µ is theconstant of proportionality and iscalled the Newtonian viscosity. Manyliquids are Newtonian Ûuids over awide range of temperatures and pres-sures. However, some are not; theseare called non-Newtonian Ûuids. Insuch Ûuids there is a departure fromthe simple Newtonian relationships.For example, in some liquids the vis-cosity increases as the velocity gradi-ent increases, i.e. the faster the liquidmoves the more viscous it becomes.Such liquids are said to be dilatantand the phenomenon they exhibit iscalled dilatancy. It occurs in somepastes and suspensions. More com-mon, however, is the opposite effectin which the viscosity depends notonly on the velocity gradient but alsoon the time for which it has beenapplied. These liquids are said toexhibit thixotropy. The faster athixotropic liquid moves the less vis-

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cous it becomes. This property isused in nondrip paints (which aremore viscous on the brush than onthe wall) and in lubricating oils(which become thinner when theparts they are lubricating start tomove). Another example is the non-Newtonian Ûow of macromoleculesin solution or in polymer melts. Inthis case the shearing force F is notparallel to the shear planes and thelinear relationship does not apply. In general, the many types of non-Newtonian Ûuid are somewhat com-plicated and no theory has beendeveloped to accommodate themfully.

NHOMO Next-to-highest occupiedmolecular orbital. See subjacent or-bitals.

niacin See nicotinic acid.

Nichrome Tradename for a groupof nickel–chromium alloys used forwire in heating elements as they pos-sess good resistance to oxidation andhave a high resistivity. Typical isNichrome V containing 80% nickeland 19.5% chromium, the balanceconsisting of manganese, silicon, andcarbon.

nickel Symbol Ni. A malleable duc-tile silvery metallic *transition el-ement; a.n. 28; r.a.m. 58.70; r.d. 8.9;m.p. 1450°C; b.p. 2732°C. It is foundin the minerals pentlandite (NiS),pyrrhoite ((Fe,Ni)S), and garnierite((Ni,Mg)6(OH)6Si4O11.H2O). Nickel isalso present in certain iron me-teorites (up to 20%). The metal isextracted by roasting the ore to givethe oxide, followed by reductionwith carbon monoxide and puriÜca-tion by the *Mond process. Alterna-tively electolysis is used. Nickel metalis used in special steels, in Invar, and,being ferromagnetic, in magnetic al-loys, such as *Mumetal. It is also aneffective catalyst, particularly for hy-

drogenation reactions (see also raneynickel). The main compounds areformed with nickel in the +2 oxida-tion state; the +3 state also exists (e.g.the black oxide, Ni2O3). Nickel wasdiscovered by Axel Cronstedt(1722–65) in 1751.


nickel arsenide structure A typeof ionic crystal structure in whichthe anions have a distorted hexago-nal close packed arrangement withthe cations occupying the octahedralholes. Each type of ion has a coordi-nation number of 6. Examples ofcompounds with this structure areNiAs, NiS, FeS, and CoS.


nickel–cadmium cell Seenickel–iron accumulator.

nickel carbonyl A colourlessvolatile liquid, Ni(CO)4; m.p. –25°C;b.p. 43°C. It is formed by direct com-bination of nickel metal with carbonmonoxide at 50–60°C. The reaction isreversed at higher temperatures, andthe reactions are the basis of the*Mond process for purifying nickel.The nickel in the compound has anoxidation state of zero, and the com-pound is a typical example of a com-plex with pi-bonding *ligands, inwhich Ülled d-orbitals on the nickeloverlap with empty p-orbitals on thecarbon.

nickelic compounds Compoundsof nickel in its +3 oxidation state; e.g.nickelic oxide is nickel(III) oxide(Ni2O3).

nickel–iron accumulator (Edisoncell; NIFE cell) A *secondary cell de-vised by Thomas Edison (1847–1931)having a positive plate of nickeloxide and a negative plate of ironboth immersed in an electrolyte of

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potassium hydroxide. The reactionon discharge is

2NiOOH.H2O + Fe → 2Ni(OH)2 +Fe(OH)2,

the reverse occurring during charg-ing. Each cell gives an e.m.f. of about1.2 volts and produces about 100 kJper kilogram during each discharge.The nickel–cadmium cell is a similardevice with a negative cadmium elec-trode. It is often used as a *dry cell.Compare lead–acid accumulator.

nickelous compounds Com-pounds of nickel in its +2 oxidationstate; e.g. nickelous oxide is nickel(II)oxide (NiO).

nickel(II) oxide A green powder,NiO; r.d. 6.6. It can be made by heat-ing nickel(II) nitrate or carbonatewith air excluded.

nickel(III) oxide (nickel peroxide;nickel sesquioxide) A black or greypowder, Ni2O3; r.d. 4.8. It is made byheating nickel(II) oxide in air andused in *nickel–iron accumulators.

nickel silver See german silver.

Nicol prism A device for producingplane-polarized light (see polarizer).It consists of two pieces of calcite cutwith a 68° angle and stuck togetherwith Canada balsam. The extraordi-nary ray (see double refraction)passes through the prism while theordinary ray suffers total internalreÛection at the interface betweenthe two crystals, as the refractiveindex of the calcite is 1.66 for the or-dinary ray and that of the Canadabalsam is 1.53. ModiÜcations of theprism using different shapes and ce-ments are used for special purposes.It was devised in 1828 by WilliamNicol (1768–1851).

nicotinamide See nicotinic acid.

nicotinamide adenine dinu-cleotide See nad.

nicotinamide adenine dinu-cleotide phosphate (NADP) Seenad.

nicotine A colourless poisonous*alkaloid present in tobacco. It isused as an insecticide.

nicotinic acid (niacin) A vitamin ofthe *vitamin B complex. It can bemanufactured by plants and animalsfrom the amino acid tryptophan. Theamide derivative, nicotinamide, is acomponent of the coenzymes *NADand NADP. These take part in manymetabolic reactions as hydrogen ac-ceptors. DeÜciency of nicotinic acidcauses the disease pellagra in hu-mans. Good sources are liver andgroundnut and sunÛower meals.

nido-structure See borane.

nielsbohrium See transactinideelements.

NIFE cell See nickel–iron accumu-lator.

ninhydrin A brown crystalline sub-stance, C9H4O3.H2O (see formula). Itreacts with amino acids to give a bluecolour. Ninhydrin is commonly usedin chromatography to analyse theamino-acid content of proteins. It is

371 ninhydrin

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N

O

OH

Nicotinic acid

O

O

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OH

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also used in forensic science to de-velop latent Üngerprints.

niobium Symbol Nb. A soft ductilegrey-blue metallic transition el-ement; a.n. 41; r.a.m. 92.91; r.d. 8.57;m.p. 2468°C; b.p. 4742°C. It occurs inseveral minerals, including niobite(Fe(NbO3)2), and is extracted by sev-eral methods including reduction ofthe complex Ûuoride K2NbF7 usingsodium. It is used in special steelsand in welded joints (to increasestrength). Niobium–zirconium alloysare used in superconductors. Chemi-cally, the element combines with thehalogens and oxidizes in air at 200°C.It forms a number of compounds andcomplexes with the metal in oxida-tion states 2, 3, or 5. The elementwas discovered by Charles Hatchett(c. 1765–1847) in 1801 and Ürst iso-lated by Christian Blomstrand(1826–97) in 1864. Formerly, it wascalled columbium.A• Information from the WebElements site

nitrate A salt or ester of nitric acid.

nitrating mixture A mixture ofconcentrated sulphuric and nitricacids, used to introduce a nitro group(–NO2) into an organic compound. Itsaction depends on the presence ofthe nitronium ion, NO2

+. It is mainlyused to introduce groups into themolecules of *aromatic compounds(the nitro group can subsequently beconverted into or replaced by others)and to make commercial *nitro com-pounds, such as the explosives cellu-lose trinitrate (nitrocellulose),glyceryl trinitrate (nitroglycerine),trinitrotoluene (TNT), and picric acid(trinitrophenol).

nitration A type of chemical reac-tion in which a nitro group (–NO2) isadded to or substituted in a mol-ecule. Nitration can be carried out bya mixture of concentrated nitric and

sulphuric acids (see nitrating mix-ture).

nitre (saltpetre) Commercial *potas-sium nitrate; the name was formerlyapplied to natural crustlike efÛores-cences, occurring in some arid re-gions.

nitre cake See sodium hydrogen-sulphate.

nitrene A species of the type HN: orRN:, analogous to a *carbene. The ni-trogen atom has four nonbondingelectrons, two of which are a lonepair. The other two may have paral-lel spins (a triplet state) or antiparal-lel spins (a singlet state).

nitric acid A colourless corrosivepoisonous liquid, HNO3; r.d. 1.50;m.p. –42°C; b.p. 83°C. Nitric acid maybe prepared in the laboratory by thedistillation of a mixture of an alkali-metal nitrate and concentratedsulphuric acid. The industrial pro-duction is by the oxidation of ammo-nia to nitrogen monoxide, theoxidation of this to nitrogen dioxide,and the reaction of nitrogen dioxidewith water to form nitric acid and ni-trogen monoxide (which is recycled).The Ürst reaction (NH3 to NO) iscatalysed by platinum or platinum/rhodium in the form of Üne wiregauze. The oxidation of NO and theabsorption of NO2 to form the prod-uct are noncatalytic and proceedwith high yields but both reactionsare second-order and slow. Increasesin pressure reduce the selectivity ofthe reaction and therefore ratherlarge gas absorption towers are re-quired. In practice the absorbing acidis refrigerated to around 2°C and acommercial ‘concentrated nitric acid’at about 67% is produced.

Nitric acid is a strong acid (highlydissociated in aqueous solution) anddilute solutions behave much likeother mineral acids. Concentrated ni-

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tric acid is a strong oxidizing agent.Most metals dissolve to form nitratesbut with the evolution of nitrogenoxides. Concentrated nitric acid alsoreacts with several nonmetals to givethe oxo acid or oxide. Nitric acid isgenerally stored in dark brown bot-tles because of the photolytic decom-position to dinitrogen tetroxide. Seealso nitration.

nitric oxide See nitrogen monox-ide.

nitrides Compounds of nitrogenwith a more electropositive element.Boron nitride is a covalent compoundhaving macromolecular crystals. Cer-tain electropositive elements, such aslithium, magnesium, and calcium,react directly with nitrogen to formionic nitrides containing the N3– ion.Transition elements form a range ofinterstitial nitrides (e.g. Mn4N, W2N),which can be produced by heatingthe metal in ammonia.

nitriding The process of hardeningthe surface of steel by producing alayer of iron nitride. One techniqueis to heat the metal in ammonia gas.Another is to dip the hot metal in abath of molten sodium cyanide.

nitriÜcation A chemical process inwhich nitrogen (mostly in the formof ammonia) in plant and animalwastes and dead remains is oxidizedat Ürst to nitrites and then to ni-trates. These reactions are effectedmainly by the bacteria Nitrosomonasand Nitrobacter respectively. Unlikeammonia, nitrates are readily takenup by plant roots; nitriÜcation istherefore a crucial part of the *nitro-gen cycle. Nitrogen-containing com-pounds are often applied to soilsdeÜcient in this element, as fertilizer.Compare denitrification.

nitrile rubber A copolymer ofbuta-1,3-diene and propenonitrile. Ni-trile rubbers are commercially im-

portant synthetic rubbers because oftheir resistance to oil and many sol-vents. See also buna rubber.

nitriles (cyanides) Organic com-pounds containing the group –CNbound to an organic group. Nitrilesare made by reaction between potas-sium cyanide and haloalkanes in al-coholic solution, e.g.

KCN + CH3Cl → CH3CN + KClAn alternative method is dehydrationof amides

CH3CONH2 – H2O → CH3CNThey can be hydrolysed to amidesand carboxylic acids and can be re-duced to amines.


nitrite A salt or ester of nitrousacid. The salts contain the dioxoni-trate (III) ion, NO2

–, which has a bondangle of 115°.nitroalkane (nitroparafÜn) A typeof organic compound of general for-mula CnH2n+1NO2 the nitroalkanesare colourless, pleasant-smelling liq-uids made by treating a haloalkanewith silver nitrate. They can be re-duced to amines by the action of tinand hydrochloric acid. Lower ni-troalkanes such as nitromethane(CH3NO2, b.p. 100°C) are used ashigh-performance fuels (for internalcombustion engines and rockets), aschemical intermediates, and as polarsolvents.

nitrobenzene A yellow oily liquid,C6H5NO2; r.d. 1.2; m.p. 6°C; b.p.211°C. It is made by the *nitration ofbenzene using a mixture of nitricand sulphuric acids.

nitrocellulose See cellulose ni-trate.

nitro compounds Organic com-pounds containing the group –NO2(the nitro group) bound to a carbon

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atom. Nitro compounds are made by*nitration reactions. They can be re-duced to aromatic amines (e.g. ni-trobenzene can be reduced tophenylamine). See also explosive.

nitrogen Symbol N. A colourlessgaseous element belonging to *group15 (formerly VB) of the periodictable; a.n. 7; r.a.m. 14.0067; d. 1.2506g dm–3; m.p. –209.86°C; b.p. –195.8°C.It occurs in air (about 78% by volume)and is an essential constituent of pro-teins and nucleic acids in living or-ganisms (see nitrogen cycle).Nitrogen is obtained for industrialpurposes by fractional distillation ofliquid air. Pure nitrogen can be ob-tained in the laboratory by heating a

metal azide. There are two naturalisotopes: nitrogen–14 and nitro-gen–15 (about 3%). The element isused in the *Haber process for mak-ing ammonia and is also used to pro-vide an inert atmosphere in weldingand metallurgy. The gas is diatomicand relatively inert – it reacts withhydrogen at high temperatures andwith oxygen in electric discharges. Italso forms *nitrides with certainmetals. Nitrogen was discovered in1772 by Daniel Rutherford (1749–1819).


nitrogen cycle One of the majorcycles of chemical elements in the

nitrogen 374

n

nitrogen fixationby lightning

protein inplants

protein inanimals

nitrites

nitrification

feeding

death death

nitrogen inbacteria

nitrogen inthe

atmosphere

nitrates inthe soil

denitrification

nitrification

nitrogen fixationnitrogen fixationnitrogen fixationby bacteriaby bacteriaby bacteria

oxides ofnitrogen in

theatmosphere

nitrificationnitrificationnitrification

ammonia indead

organicmatter

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environment. Nitrates in the soil aretaken up by plant roots and maythen pass along food chains into ani-mals. Decomposing bacteria convertnitrogen-containing compounds (es-pecially ammonia) in plant and ani-mal wastes and dead remains backinto nitrates, which are released intothe soil and can again be taken up byplants (see nitrification). Though ni-trogen is essential to all forms of life,the huge amount present in the at-mosphere is not directly available tomost organisms (compare carboncycle). It can, however, be assimi-lated by some specialized bacteria(see nitrogen fixation) and is thusmade available to other organismsindirectly. Lightning Ûashes alsomake some nitrogen available toplants by causing the combination ofatmospheric nitrogen and oxygen toform oxides of nitrogen, which enterthe soil and form nitrates. Some ni-trogen is returned from the soil tothe atmosphere by denitrifying bac-teria (see denitrification).

nitrogen dioxide See dinitrogentetroxide.

nitrogen Üxation A chemicalprocess in which atmospheric nitro-gen is assimilated into organic com-pounds in living organisms andhence into the *nitrogen cycle. Theability to Üx nitrogen is limited tocertain bacteria (e.g. Azotobacter, Ana-baena). Rhizobium bacteria are able toÜx nitrogen in association with cellsin the roots of leguminous plants,such as peas and beans, in whichthey form characteristic root nod-ules; cultivation of legumes is there-fore one way of increasing soilnitrogen. Various chemical processesare used to Üx atmospheric nitrogenin the manufacture of *fertilizers.These include the *Birkeland–Eydeprocess, the cyanamide process (see

calcium dicarbide), and the *Haberprocess.

nitrogen monoxide (nitric oxide)A colourless gas, NO; m.p. –163.6°C;b.p. –151.8°C. It is soluble in water,ethanol, and ether. In the liquid statenitrogen monoxide is blue in colour(r.d. 1.26). It is formed in many reac-tions involving the reduction of ni-tric acid, but more convenientreactions for the preparation of rea-sonably pure NO are reactions ofsodium nitrite, sulphuric acid, and ei-ther sodium iodide or iron(II) sul-phate. Nitrogen monoxide reactsreadily with oxygen to give nitrogendioxide and with the halogens to givethe nitrosyl halides XNO (X = F,Cl,Br).It is oxidized to nitric acid by strongoxidizing agents and reduced to dini-trogen oxide by reducing agents. Themolecule has one unpaired electron,which accounts for its paramagnet-ism and for the blue colour in the liq-uid state. This electron is relativelyeasily removed to give the nitrosylion NO+, which is the ion present insuch compounds as NOClO4, NOBF4,NOFeCl4, (NO)2PtCl6 and a ligand incomplexes, such as Co(CO)3NO.

In mammals and other vertebrates,nitrogen monoxide is now known toplay several important roles. For ex-ample, it acts as a gaseous mediatorin producing such responses as dila-tion of blood vessels, relaxation ofsmooth muscle, and inhibition ofplatelet aggregation. In certain cellsof the immune system it is convertedto the peroxynitrite ion (–O–O–N=O),which has activity against tumourcells and pathogens.

nitrogen mustards A group of ni-trogen compounds similar to *sul-phur mustard. They were used aschemotherapy agents in cancer treat-ment. Like sulphur mustard, they arepowerful blistering agents. Largequantities were made during World

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War II, although none were used incombat.

nitrogenous base A basic com-pound containing nitrogen. The termis used especially of organic ringcompounds, such as adenine, gua-nine, cytosine, and thymine, whichare constituents of nucleic acids. Seeamine salts.

nitroglycerine An explosive madeby reacting 1,2,3-trihydroxypropane(glycerol) with a mixture of concen-trated sulphuric and nitric acids.Despite its name and method ofpreparation, it is not a nitro com-pound, but an ester of nitric acid,CH2(NO3)CH(NO3)CH2(NO3). It is usedin dynamites.

nitro group See nitro compounds.

nitronium ion See nitryl ion.

nitroparafÜn See nitroalkane.

nitrosamines A group of carcino-genic compounds with the generalformula RR′NNO, where R and R′ areside groups with a variety of possiblestructures. Nitrosamines, which are acomponent of cigarette smoke, causecancer in a number of organs, partic-ularly in the liver, kidneys, andlungs. An example of a nitrosamineis dimethylnitrosamine, which hastwo methyl side groups (CH3–).

nitrosonium ion The positive ionNO+, present in certain salts such asthe chlorate (NO+ClO4) and theboroÛuoride (NO+BF4).

nitrosyl ion The ion NO+. See ni-trogen monoxide.

nitrous acid A weak acid, HNO2,known only in solution and in thegas phase. It is prepared by the ac-tion of acids upon nitrites, preferablyusing a combination that removesthe salt as an insoluble precipitate(e.g. Ba(NO2)2 and H2SO4). The solu-tions are unstable and decompose on

heating to give nitric acid and nitro-gen monoxide. Nitrous acid can func-tion both as an oxidizing agent(forms NO) with I– and Fe2+, or as areducing agent (forms NO3

–) with, forexample, Cu2+; the latter is mostcommon. It is widely used (preparedin situ) for the preparation of diazo-nium compounds in organic chem-istry. The full systematic name isdioxonitric(III) acid.

nitrous oxide See dinitrogenoxide.

nitryl ion (nitronium ion) The ionNO2

+, found in mixtures of nitric acidand sulphuric acid and solutions ofnitrogen oxides in nitric acid. Nitrylsalts, such as NO2

+ClO4–, can be iso-

lated but are extremely reactive. Nit-ryl ions generated in situ are used for*nitration in organic chemistry.

NMR See nuclear magnetic reso-nance.

nobelium Symbol No. A radioactivemetallic transuranic element belong-ing to the *actinoids; a.n. 102; massnumber of most stable element 254(half-life 55 seconds). Seven isotopesare known. The element was ÜrstidentiÜed with certainty by AlbertGhiorso and Glenn Seaborg (1912–99)in 1966.


noble gases (inert gases; raregases; group 18 elements) A groupof monatomic gaseous elementsforming group 18 (formerly group 0)of the *periodic table: helium (He),neon (Ne), argon (Ar), krypton (Kr),xenon (Xe), and radon (Rn). The elec-tron conÜguration of helium is 1s2.The conÜgurations of the others ter-minate in ns2np6 and all inner shellsare fully occupied. The elements thusrepresent the termination of a periodand have closed-shell conÜguration

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and associated high ionization ener-gies (He 2370 to Rn 1040 kJ mol–1)and lack of chemical reactivity. Beingmonatomic the noble gases arespherically symmetrical and havevery weak interatomic interactionsand consequent low enthalpies of va-porization. The behaviour of thelighter members approaches that ofan ideal gas at normal temperatures;with the heavier members increasingpolarizability and dispersion forceslead to easier liquefaction underpressure. Four types of ‘compound’have been described for the noblegases but of these only one can becorrectly described as compounds inthe normal sense. One type consistsof such species as HHe+, He2

+, Ar2+,

HeLi+, which form under highly ener-getic conditions, such as those in arcsand sparks. They are short-lived andonly detected spectroscopically. Asecond group of materials describedas inert-gas–metal compounds do nothave deÜned compositions and aresimply noble gases adsorbed onto thesurface of dispersed metal. The thirdtype, previously described as ‘hy-drates’ are in fact clathrate com-pounds with the noble gas moleculetrapped in a water lattice. True com-pounds of the noble gases were Ürstdescribed in 1962 and severalÛuorides, oxyÛuorides, Ûuoroplati-nates, and Ûuoroantimonates of*xenon are known. A few kryptonÛuorides and a radon Ûuoride arealso known although the short half-life of radon and its intense alpha ac-tivity restrict the availability ofinformation. Apart from argon, thenoble gases are present in the atmos-phere at only trace levels. Heliummay be found along with natural gas(up to 7%), arising from the radio-active decay of heavier elements (viaalpha particles).

noble metal A metal characterized

by it lack of chemical reactivity, par-ticularly to acids and atmosphericcorrosion. Examples include gold,palladium, platinum, and rhodium.

no-bond resonance See hypercon-jugation.

NOE See nuclear overhauser ef-fect.

nonahydrate A crystalline com-pound that has nine moles of waterper mole of compound.

nonanoic acid (perargonic acid) Aclear oily liquid carboxylic acid, CH3(CH2)7 COOH; r.d. 0.9; m.p. 12.5°C;b.p. 254°C. It is found as esters in oilof pelargonium and certain esters areused as Ûavourings.

nonbenzenoid aromatics Aro-matic compounds that have ringsother than benzene rings. Examplesare the cyclopentadienyl anion,C5H5

–, and the tropylium cation,C7H7

+. See also annulenes.

noncompetitive inhibition Seeinhibition.

nonequilibrium statistical me-chanics The statistical mechanics ofsystems not in thermal equilibrium.One of the main purposes of non-equilibrium statistical mechanics isto calculate *transport coefÜcientsand inverse transport coefÜcients,such as conductivity and viscosity,from Ürst principles and to provide abasis for transport theory. The non-equilibrium systems easiest to under-stand are those near thermalequilibrium. For systems far fromequilibrium, such possibilities as*chaos and self-organization canarise due to nonlinearity.

nonequilibrium thermodynam-ics The thermodynamics of systemsnot in thermal equilibrium. The non-equilibrium systems easiest to under-stand are those near thermal

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equilibrium; these systems are de-scribed by the *Onsager relations.For systems far from equilibrium,more complicated patterns, such as*chaos and *self-organization, canarise due to nonlinearity. Which be-haviour is observed depends on thevalue of certain parameters in thesystem. The transition from one typeof behaviour to another as the para-meters are altered occurs at *bifurca-tions.

nonferrous metal Any metalother than iron or any alloy that doesnot contain iron. In commercialterms this usually means aluminium,copper, lead, nickel, tin, zinc, ortheir alloys.

nonmetal An element that is not a*metal. Nonmetals can either be in-sulators or semiconductors. At lowtemperatures nonmetals are poorconductors of both electricity andheat as few free electrons movethrough the material. If the conduc-tion band is near to the valence band(see energy bands) it is possible fornonmetals to conduct electricity athigh temperatures but, in contrast tometals, the conductivity increaseswith increasing temperature. Non-metals are electronegative elements,such as carbon, nitrogen, oxygen,phosphorus, sulphur, and the halo-gens. They form compounds thatcontain negative ions or covalentbonds. Their oxides are either neu-tral or acidic.

nonpolar compound A com-pound that has covalent moleculeswith no permanent dipole moment.Examples of nonpolar compoundsare methane and benzene.

nonpolar solvent See solvent.

nonreducing sugar A sugar thatcannot donate electrons to othermolecules and therefore cannot actas a reducing agent. Sucrose is the

most common nonreducing sugar.The linkage between the glucose andfructose units in sucrose, which in-volves aldehyde and ketone groups,is responsible for the inability of su-crose to act as a *reducing sugar.

nonrelativistic quantum theorySee quantum theory.

nonrenewable energy sourcesSee renewable energy sources.

nonstoichiometric compound(Berthollide compound) A chemicalcompound in which the elements donot combine in simple ratios. For ex-ample, rutile (titanium(IV) oxide) isoften deÜcient in oxygen, typicallyhaving a formula TiO1.8.

noradrenaline (norepinephrine) Ahormone produced by the adrenalglands and also secreted from nerveendings in the sympathetic nervoussystem as a chemical transmitter ofnerve impulses. Many of its generalactions are similar to those of*adrenaline, but it is more concernedwith maintaining normal body activ-ity than with preparing the body foremergencies.

Nordhausen sulphuric acid Seedisulphuric(vi) acid.

norepinephrine See noradrena-line.

normal Having a concentration ofone gram equivalent per dm3.

normal mode of vibration Abasic vibration of a polyatomic mol-ecule. All vibrational motion of apolyatomic molecule can be treatedas a superposition of the normalmodes of vibration of the molecule.If N is the number of atoms in a mol-ecule, the number of modes of vibra-tion is 3N – 5 for a linear moleculeand 3N – 6 for a nonlinear molecule.Each of these vibrational modes has acharacteristic frequency, although it

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is possible for some of them to be de-generate. For example, in a linear tri-atomic molecule there are fournormal modes of vibration since3N – 5 = 4 for N = 3. These vibrationalmodes are: (a) symmetric stretching(breathing) vibrations; (b) antisym-metric stretching vibrations; and (c)two bending vibrations, which aredegenerate.

N.T.P. See s.t.p.

n-type semiconductor See semi-conductor.

nuclear magnetic resonance(NMR) The absorption of electromag-netic radiation at a suitable precisefrequency by a nucleus with anonzero magnetic moment in an ex-ternal magnetic Üeld. The phenome-non occurs if the nucleus hasnonzero *spin, in which case it be-haves as a small magnet. In an exter-nal magnetic Üeld, the nucleus’smagnetic moment vector precessesabout the Üeld direction but only cer-tain orientations are allowed byquantum rules. Thus, for hydrogen(spin of ½) there are two possiblestates in the presence of a Üeld, eachwith a slightly different energy. Nu-clear magnetic resonance is the ab-sorption of radiation at a photonenergy equal to the difference be-tween these levels, causing a transi-tion from a lower to a higher energystate. For practical purposes, the dif-ference in energy levels is small andthe radiation is in the radiofrequencyregion of the electromagnetic spec-trum. It depends on the Üeldstrength.

NMR can be used for the accuratedetermination of nuclear moments.It can also be used in a sensitive formof magnetometer to measure mag-netic Üelds. In medicine, magneticresonance imaging (MRI) has been de-veloped, in which images of tissue

are produced by magnetic-resonancetechniques.

The main application of NMR is asa technique for chemical analysisand structure determination, knownas NMR spectroscopy. It depends onthe fact that the electrons in a mol-ecule shield the nucleus to some ex-tent from the Üeld, causing differentatoms to absorb at slightly differentfrequencies (or at slightly differentÜelds for a Üxed frequency). Sucheffects are known as chemical shifts.There are two methods of NMR spec-troscopy. In continuous wave (CW)NMR, the sample is subjected to astrong Üeld, which can be varied in acontrolled way over a small region. Itis irradiated with radiation at a Üxedfrequency, and a detector monitorsthe Üeld at the sample. As the Üeldchanges, absorption corresponding totransitions occurs at certain values,and this causes oscillations in theÜeld, which induce a signal in the de-tector. Fourier transform (FT) NMRuses a Üxed magnetic Üeld and the sample is subjected to a high-intensity pulse of radiation coveringa range of frequencies. The signalproduced is analysed mathematicallyto give the NMR spectrum. The mostcommon nucleus studied is 1H. Forinstance, an NMR spectrum ofethanol (CH3CH2OH) has three peaksin the ratio 3:2:1, corresponding tothe three different hydrogen-atomenvironments. The peaks also have aÜne structure caused by interactionbetween spins in the molecule. Othernuclei can also be used for NMR spec-troscopy (e.g. 13C, 14N, 19F) althoughthese generally have lower magneticmoment and natural abundance thanhydrogen. See also electron paramag-netic resonance; endor.

nuclear magneton See magneton.

nuclear Overhauser effect (NOE)An effect in *nuclear magnetic reso-

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nance (NMR) used to increase the in-tensities of resonance lines. The largepopulation differences between thestates given by the Boltzmann distri-bution gives rise to strong intensitiesof resonance lines. In the nuclearOverhauser effect, spin relaxation isused to transfer population differ-ence from one type of nucleus to an-other type of nucleus so that theintensities of the resonance lines ofthe second type of nucleus are in-creased. An example of such an en-hancement, which is widely used, isbetween protons and 13C, in whichcase it is possible to attain enhance-ment of about three times. In theNOE proton, irradiation is used toproduce the enhancement of the 13Cline. Another use of the NOE is to de-termine the distance between pro-tons.

nucleic acid A complex organiccompound in living cells that con-sists of a chain of *nucleotides. Thereare two types: *DNA (deoxyribonu-cleic acid) and *RNA (ribonucleicacid).

A• Information on IUPAC nomenclature

nucleon A *proton or a *neutron.

nucleon number (mass number)Symbol A. The number of *nucleonsin an atomic nucleus of a particularnuclide.

nucleophile An ion or moleculethat can donate electrons. Nucle-ophiles are often oxidizing agentsand Lewis bases. They are either neg-ative ions (e.g. Cl–) or molecules thathave electron pairs (e.g. NH3). In or-ganic reactions they tend to attackpositively charged parts of a mol-ecule. Compare electrophile.

nucleophilic addition A type ofaddition reaction in which the Ürststep is attachment of a *nucleophile

to a positive (electron-deÜcient) partof the molecule. *Aldehydes and *ke-tones undergo reactions of this typebecause of polarization of the car-bonyl group (carbon positive).

nucleophilic substitution A typeof substitution reaction in which a*nucleophile displaces another groupor atom from a compound. For exam-ple, in

CR3Cl + OH– → CR3OH + Cl–

the nucleophile is the OH– ion. Thereare two possible mechanisms of nu-cleophilic substitution. In SN1 reac-tions, a positive carbonium ion isÜrst formed:

CR3Cl → CR3+ + Cl–

This then reacts with the nucleophileCR3

+ + OH– → CR3OHThe CR3

+ ion is planar and the OH–

ion can attack from either side. Con-sequently, if the original molecule isoptically active (the three R groupsare different) then a racemic mixtureof products results.

The alternative mechanism, theSN2 reaction, is a concerted reactionin which the nucleophile approachesfrom the side of the R groups as theother group (Cl in the example)leaves. In this case the conÜgurationof the molecule is inverted. If theoriginal molecule is optically active,the product has the opposite activity,an effect known as Walden inversion.The notations SN1 and SN2 refer tothe kinetics of the reactions. In theSN1 mechanism, the slow step is theÜrst one, which is unimolecular (andÜrst order in CR3Cl). In the SN2 reac-tion, the process is bimolecular (andsecond order overall).

nucleoside An organic compoundconsisting of a nitrogen-containing*purine or *pyrimidine base linkedto a sugar (ribose or deoxyribose). Anexample is *adenosine. Compare nu-cleotide.

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nucleosynthesis The synthesis ofchemical elements by nuclearprocesses. There are several ways inwhich nucleosynthesis can takeplace. Primordial nucleosynthesistook place very soon after the bigbang, when the universe was ex-tremely hot. This process was respon-sible for the cosmic abundancesobserved for light elements, such ashelium. Explosive nucleosynthesiscan also occur during the explosionof a supernova. However, stellar nu-cleosynthesis, which takes place inthe centre of stars at very high tem-peratures, is now the principal formof nucleosynthesis. The exact processoccurring in stellar nucleosynthesisdepends on the temperature, density,and chemical composition of thestar. The synthesis of helium fromprotons and of carbon from heliumcan both occur in stellar nucleosyn-thesis.

nucleotide An organic compoundconsisting of a nitrogen-containing*purine or *pyrimidine base linkedto a sugar (ribose or deoxyribose) anda phosphate group. *DNA and *RNAare made up of long chains of nu-cleotides (i.e. polynucleotides). Com-pare nucleoside.

nucleus The central core of anatom that contains most of its mass.It is positively charged and consistsof one or more nucleons (protons orneutrons). The positive charge of thenucleus is determined by the numberof protons it contains (see atomicnumber) and in the neutral atom thisis balanced by an equal number ofelectrons, which move around thenucleus. The simplest nucleus is thehydrogen nucleus, consisting of oneproton only. All other nuclei alsocontain one or more neutrons. The

neutrons contribute to the atomicmass (see nucleon number) but notto the nuclear charge. The most mas-sive nucleus that occurs in nature isuranium–238, containing 92 protonsand 146 neutrons. The symbol usedfor this *nuclide is 238

92U, the upperÜgure being the nucleon number andthe lower Ügure the atomic number.In all nuclei the nucleon number (A)is equal to the sum of the atomicnumber (Z) and the neutron number(N), i.e. A = Z + N.

nuclide A type of atom as charac-terized by its *atomic number and its*neutron number. An *isotope refersto a series of different atoms thathave the same atomic number butdifferent neutron numbers (e.g. ura-nium–238 and uranium–235 are iso-topes of uranium), whereas a nucliderefers only to a particular nuclearspecies (e.g. the nuclides ura-nium–235 and plutonium–239 are Üs-sile). The term is also used for thetype of nucleus.

nylon Any of various syntheticpolyamide Übres having a protein-like structure formed by the conden-sation between an amino group ofone molecule and a carboxylic acidgroup of another. There are threemain nylon Übres, nylon 6, nylon 6,6,and nylon 6,10. Nylon 6, for exampleEnkalon and Celon, is formed by theself-condensation of 6-aminohexa-noic acid. Nylon 6,6, for example Brinylon, is made by polycondensationof hexanedioic acid (adipic acid) and1,6-diaminohexane (hexamethylene-diamine) having an average formulaweight between 12 000 and 15 000.Nylon 6,10 is prepared by polymeriz-ing decanedioic acid and 1,6-diamino-hexane.

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O

occlusion 1. The trapping of smallpockets of liquid in a crystal duringcrystallization. 2. The absorption ofa gas by a solid such that atoms ormolecules of the gas occupy intersti-tial positions in the solid lattice. Pal-ladium, for example, can occludehydrogen.

ochre A yellow or red mineral formof iron(III) oxide, Fe2O3, used as a pig-ment.

octadecanoate See stearate.

octadecanoic acid See stearicacid.

octadecenoic acid A straight-chain unsaturated fatty acid with theformula C17H33COOH. Cis-octadec-9-enoic acid (see oleic acid) has the for-mula CH3(CH2)7CH:CH(CH2)7COOH.The glycerides of this acid are foundin many natural fats and oils.

octahedral See complex.

octahydrate A crystalline hydratethat has eight moles of water permole of compound.

octane A straight-chain liquid*alkane, C8H18; r.d. 0.7; m.p.–56.79°C; b.p. 125.66°C. It is presentin petroleum. The compound is iso-meric with 2,2,4-trimethylpentane,((CH3)3CCH2CH(CH3)2, iso-octane). Seeoctane number.

octane number A number thatprovides a measure of the ability of afuel to resist knocking when it isburnt in a spark-ignition engine. It isthe percentage by volume of iso-octane (C8H18; 2,2,4-trimethylpen-tane) in a blend with normal heptane(C7H16) that matches the knocking

behaviour of the fuel being tested ina single cylinder four-stroke engineof standard design. Compare cetanenumber.

octanitrocubane See cubane.

octanoic acid (caprylic acid) Acolourless liquid straight-chainsaturated *carboxylic acid,CH3(CH2)6COOH; b.p. 239.3°C.

octavalent Having a valency ofeight.

octave See law of octaves.

octet A stable group of eight elec-trons in the outer shell of an atom(as in an atom of a noble gas).

octogen See hmx.

octupole A set of eight pointcharges that has zero net charge anddoes not have either a dipole mo-ment or a quadrupole moment. Anexample of an octupole is a methanemolecule (CH4). Octupole interactionsare much smaller than quadrupoleinteractions and very much smallerthan dipole interactions.

ohm Symbol Ω. The derived *SIunit of electrical resistance, beingthe resistance between two points ona conductor when a constant poten-tial difference of one volt, applied be-tween these points, produces acurrent of one ampere in the conduc-tor. The former international ohm(sometimes called the ‘mercuryohm’) was deÜned in terms of the re-sistance of a column of mercury. Theunit is named after Georg Ohm(1787–1854).

oil Any of various viscous liquids


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that are generally immiscible withwater. Natural plant and animal oilsare either volatile mixtures of ter-penes and simple esters (e.g. *essen-tial oils) or are *glycerides of fattyacids. Mineral oils are mixtures of hy-drocarbons (e.g. *petroleum).

oil of vitriol See sulphuric acid.

oil of wintergreen Methyl salicy-late (methyl 2-hydroxybenzoate,C8H8O3), a colourless aromatic liquidester, b.p. 223°C. It occurs in the es-sential oils of some plants, and ismanufactured from salicylic acid. It iseasily absorbed through the skin andused in medicine for treating muscu-lar and sciatic pain. Because of its at-tractive smell it is also used inperfumes and food Ûavourings.

oil sand (tar sand; bituminous sand)A sandstone or porous carbonaterock that is impregnated with hydro-carbons. The largest deposit of oilsand occurs in Alberta, Canada (theAthabasca tar sands); there are alsodeposits in the Orinoco Basin ofVenezuela, Russia, USA, Madagascar,Albania, Trinidad, and Romania.

oil shale A Üne-grained carbona-ceous sedimentary rock from whichoil can be extracted. The rock con-tains organic matter – kerogen –which decomposes to yield oil whenheated. Deposits of oil shale occur onevery continent, the largest knownreserves occurring in Colorado, Utah,and Wyoming in the USA. Commer-cial production of oil from oil shale isgenerally considered to be uneco-nomic unless the price of petroleumrises above the recovery costs for oilfrom oil shale. However, threats ofdeclining conventional oil resourceshave resulted in considerable interestand developments in recovery tech-niques.

oleaginous Producing or contain-ing oil or lipids. Oleaginous microor-

ganisms, which normally contain20–25% oil, are of interest in biotech-nology as alternative sources of con-ventional oils or as possible sourcesfor novel oils. The majority of theoils produced by oleaginous eukary-otic microorganisms are similar toplant oils. One possibility under con-sideration is the production of oilsand fats from waste material, to beused in animal feed.

oleate A salt or ester of *oleic acid.

oleÜnes See alkenes.

oleic acid An unsaturated *fattyacid with one double bond,CH3(CH2)7CH:CH(CH2)7COOH; r.d. 0.9;m.p. 13°C. Oleic acid is one of themost abundant constituent fattyacids of animal and plant fats, oc-curring in butterfat, lard, tallow,groundnut oil, soya-bean oil, etc. Its systematic chemical name is cis-octadec-9-enoic acid.

oleum See disulphuric(vi) acid.

oligonucleotide A short polymerof *nucleotides.

oligopeptide See peptide.

oligosaccharide A carbohydrate (atype of *sugar) whose molecules con-tain a chain of up to 20 united mono-saccharides. Oligosaccharides areformed as intermediates during thedigestion of *polysaccharides, suchas cellulose and starch.

olivine An important group ofrock-forming silicate minerals crys-tallizing in the orthorhombic system.Olivine conforms to the general for-mula (Mg,Fe)2SiO4 and comprises acomplete series from pure magne-sium silicate (forsterite, Mg2SiO4) topure iron silicate (fayalite, Fe2SiO4). Itis green, brown-green, or yellow-green in colour. A gem variety ofolivine is peridot.

one-pot synthesis A method of

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synthesizing organic compounds inwhich the materials used are mixedtogether in a single vessel and al-lowed to react, rather than conduct-ing the reaction in a sequence ofseparate stages.

onium ion An ion formed byadding a proton to a neutral mol-ecule, e.g. the hydroxonium ion(H3O+) or the ammonium ion (NH4

+).

Onsager relations In a systemthat is not at equilibrium, variouschanges are occurring. For example,there may be a Ûow of energy fromone part of the system to anotherand, at the same time, a Ûow of mass(diffusion). Flows of this type are cou-pled, i.e. each depends on the other.Equations exist of the type

J1 = L11X1 + L12X2

J2 = L21X1 + L22X2

Here, J1 is the Ûow of energy and J2the Ûow of matter. X1 is the ‘force’producing energy Ûow and X2 thatproducing matter Ûow. L11 is thecoefÜcient for thermal conductanceand L22 the coefÜcient for diffusion.The coefÜcients L12 and L21 representcoupling of the Ûows with eachother. Equations of this type can begeneralized to any number of Ûowsand are known as the phenomeno-logical relations. In Onsager’s theorythe coupling coefÜcients are equal,i.e. L12 = L21, etc. These are known asreciprocal relations. It follows that:

(∂J1/∂X2)X1 = (∂J2/∂X1)X2.The theory was developed by theSwedish physicist Lars Onsager in1931.

opal A hydrous amorphous form ofsilica. Many varieties of opal occur,some being prized as gemstones.Common opal is usually milk whitebut the presence of impurities maycolour it yellow, green, or red. Pre-cious opals, which are used as gem-

stones, display the property ofopalescence – a characteristic inter-nal play of colours resulting from theinterference of light rays within thestone. Black opal has a black back-ground against which the colours aredisplayed. The chief sources of pre-cious opals are Australia and Mexico.Geyserite is a variety deposited bygeysers or hot springs. Another vari-ety, diatomite, is made up of theskeletons of diatoms.

open chain See chain.

open-hearth process A tradi-tional but now obsolete method formanufacturing steel by heating to-gether scrap, pig iron, etc., in a re-fractory-lined shallow open furnaceheated by burning producer gas inair. It has been replaced by the*basic-oxygen process.

operator A mathematical entitythat performs a speciÜc operation ona function to transform the functioninto another function. For example,the square root sign √ and the differ-entiation symbol d/dx are operators.

opiate One of a group of drugs de-rived from *opium, which depressbrain function (a narcotic action).Opiates include *morphine and itssynthetic derivatives, such as *heroinand codeine. They are used in medi-cine chieÛy to relieve pain, but theuse of morphine and heroin isstrictly controlled since they cancause drug dependence and toler-ance.

opioid Any one of a group of sub-stances that produce pharmacologi-cal and physiological effects similarto those of morphine. Opioids arenot necessarily structurally similar tomorphine, although a subgroup ofopioids, the *opiates, are morphine-derived compounds.

opium A substance obtained from

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the unripe seed head of the opiumpoppy (Papaver somniferum). It con-tains a number of alkaloids, includ-ing *morphine and *codeine. Seeopiate.

optical activity The ability of cer-tain substances to rotate the plane ofplane-polarized light as it passesthrough a crystal, liquid, or solution.It occurs when the molecules of thesubstance are asymmetric, so thatthey can exist in two different struc-tural forms each being a mirrorimage of the other (see chirality el-ement). The two forms are opticalisomers or enantiomers. The exis-tence of such forms is also known asenantiomorphism (the mirror imagesbeing enantiomorphs). One form willrotate the light in one direction andthe other will rotate it by an equalamount in the other. The two possi-ble forms are described as *dextroro-tatory or *laevorotatory according tothe direction of rotation. PreÜxes areused to designate the isomer: (+)-(dextrorotatory), (–)- (laevorotatory),and (±)- (racemic mixture) are nowpreferred to, and increasingly usedfor, the former d-, l-, and dl-, respec-tively. An equimolar mixture of thetwo forms is not optically active. It is called a racemic mixture (or ra-cemate). In addition, certain mol-ecules can have a meso form in

385 optical activity

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)+

Optical activity

Opioid

which one part of the molecule is amirror image of the other. Such mol-ecules are not optically active.

Molecules that show optical activ-ity have no plane of symmetry. Thecommonest case of this is in organiccompounds in which a carbon atomis linked to four different groups. Anatom of this type is said to be a chiralcentre. Asymmetric molecules show-ing optical activity can also occur ininorganic compounds. For example,an octahedral complex in which thecentral ion coordinates to six differ-ent ligands would be optically active.Many naturally occurring compoundsshow optical isomerism and usuallyonly one isomer occurs naturally. Forinstance, glucose is found in the dex-trorotatory form. The other isomer,(–)- or l-glucose, can be synthesized inthe laboratory, but cannot be synthe-sized by living organisms. See also ab-solute configuration.


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optical brightener See brighten-ers.

optical glass Glass used in themanufacture of lenses, prisms, andother optical parts. It must be homo-geneous and free from bubbles andstrain. Optical crown glass may con-tain potassium or barium in place ofthe sodium of ordinary crown glassand has a refractive index in therange 1.51 to 1.54. Flint glass con-tains lead oxide and has a refractiveindex between 1.58 and 1.72. Higherrefractive indexes are obtained byadding lanthanoid oxides to glasses;these are now known as lanthanumcrowns and Ûints.

optical isomers See optical activ-ity.

optical maser See laser.

optical pumping See laser.

optical rotary dispersion (ORD)The effect in which the amount ofrotation of plane-polarized light byan optically active compound de-pends on the wavelength. A graph ofrotation against wavelength has acharacteristic shape showing peaksor troughs.

optical rotation Rotation of plane-polarized light. See optical activity.

optoacoustic spectroscopy Aspectroscopic technique in whichelectromagnetic radiation is absorbedby materials that generate soundwaves. This technique has been usedparticularly in gases. The principleunderlying optoacoustic spectroscopyis that the absorbed electromagneticradiation is converted into motion,which is associated with the produc-tion of sound waves. See also photo-acoustic spectroscopy.

orbit The path of an electron as ittravels round the nucleus of an atom.See orbital.

orbital A region in which an elec-tron may be found in an atom ormolecule. In the original *Bohrtheory of the atom the electronswere assumed to move around thenucleus in circular orbits, but furtheradvances in quantum mechanics ledto the view that it is not possible togive a deÜnite path for an electron.According to *wave mechanics, theelectron has a certain probability ofbeing in a given element of space.Thus for a hydrogen atom the elec-tron can be anywhere from close tothe nucleus to out in space but themaximum probability in sphericalshells of equal thickness occurs in aspherical shell around the nucleuswith a radius equal to the Bohr ra-dius of the atom. The probabilities ofÜnding an electron in different re-gions can be obtained by solving theSchrödinger wave equation to givethe wave function ψ, and the proba-bility of location per unit volume isthen proportional to |ψ|2. Thus theidea of electrons in Üxed orbits hasbeen replaced by that of a probabilitydistribution around the nucleus – anatomic orbital (see illustration). Alter-natively, the orbital can be thoughtof as an electric charge distribution(averaged over time). In representingorbitals it is convenient to take a sur-face enclosing the space in which theelectron is likely to be found with ahigh probability.

The possible atomic orbitals corre-spond to subshells of the atom. Thusthere is one s-orbital for each shell(orbital quantum number l = 0). This is spherical. There are three p-orbitals (corresponding to the threevalues of l) and Üve d-orbitals. Theshapes of orbitals depend on thevalue of l. For instance, p-orbitalseach have two lobes; most d-orbitalshave four lobes.

In molecules, the valence electronsmove under the inÛuence of two nu-

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clei (in a bond involving two atoms)and there are corresponding molecu-lar orbitals for electrons (see illustra-tion). It is convenient in consideringthese to regard them as formed byoverlap of atomic orbitals. In a hydro-gen molecule the s-orbitals on thetwo atoms overlap and form a mo-

lecular orbital between the two nu-clei. This is an example of a sigma or-bital. In a double bond, as in ethene,one bond is produced by overlapalong the line of axes to form asigma orbital. The other is producedby sideways overlap of the lobes ofthe p-orbitals (see illustration). The

387 orbital

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symmetricals-orbital

x

y

z z

x

y

py pzpx

y

x

z

three equivalent p-orbitals, each having 2 lobes

Atomic orbitals

p-orbitals

H

H H

H

CC

pi orbital

H

H

C C

H

H

sigma orbitalhybrid sp3-orbitals

Molecular orbitals: formation of the double bond in ethene

Orbital


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resulting molecular orbital has twoparts, one on each side of the sigmaorbital – this is a pi orbital. It is alsopossible for a delta orbital to form bylateral overlap of two d-orbitals. Infact, the combination of two atomicorbitals produces two molecular or-bitals with different energies. Theone of lower energy is the bondingorbital, holding the atoms together;the other is the antibonding orbital,which would tend to push the atomsapart. In the case of valence elec-trons, only the lower (bonding) or-bital is Ülled.

In considering the formation ofmolecular orbitals it is often usefulto think in terms of hybrid atomic or-bitals. For instance, carbon has in itsouter shell one s-orbital and three p-orbitals. In forming methane (orother tetrahedral molecules) thesecan be regarded as combining to givefour equivalent sp3 hybrid orbitals,each with a lobe directed to a cornerof a tetrahedron. It is these that over-lap with the s-orbitals on the hydro-gen atoms. In ethene, two p-orbitalscombine with the s-orbital to givethree sp2 hybrids with lobes in aplane pointing to the corners of anequilateral triangle. These form thesigma orbitals in the C–H and C–Cbonds. The remaining p-orbitals (oneon each carbon) form the pi orbital.In ethyne, sp2 hybridization occurs togive two hybrid orbitals on eachatom with lobes pointing along theaxis. The two remaining p-orbitals oneach carbon form two pi orbitals. Hy-brid atomic orbitals can also involved-orbitals. For instance, square-planarcomplexes use sp2d hybrids; octa-hedral complexes use sp3d2.

orbital quantum number Seeatom.

ORD See optical rotary dispersion.

order In the expression for the rateof a chemical reaction, the sum of

the powers of the concentrations isthe overall order of the reaction. Forinstance, in a reaction

A + B → Cthe rate equation may have the form

R = k[A][B]2

This reaction would be described asÜrst order in A and second order in B.The overall order is three. The orderof a reaction depends on the mecha-nism and it is possible for the rate tobe independent of concentration(zero order) or for the order to be afraction. See also molecularity;pseudo order.

ore A naturally occurring mineralfrom which a metal and certainother elements (e.g. phosphorus) canbe extracted, usually on a commer-cial basis. Metals may be present inores in the native form, but morecommonly they occur combined asoxides, sulphides, sulphates, silicates,etc.

ore dressing See beneficiation.

oregonator A type of chemical re-action mechanism that causes an*oscillating reaction. It is the type ofmechanism responsible for the *B–Zreaction, and involves Üve steps ofthe form:

A + Y → XX + Y → CA + X → 2X + Z2X → DZ → YAutocatalysis occurs as in the

*Lotka–Volterra mechanism and the*brusselator. The mechanism wasnamed after Oregon in America,where the research group that dis-covered it is based.

organic chemistry The branch ofchemistry concerned with com-pounds of carbon.

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organo- PreÜx used before thename of an element to indicate com-pounds of the elements containingorganic groups (with the elementbound to carbon atoms). For exam-ple, lead(IV) tetraethyl is an organo-lead compound.

organometallic compound Acompound in which a metal atom orion is bound to an organic group.Organometallic compounds mayhave single metal–carbon bonds, as in the aluminium alkyls (e.g.Al(CH3)3). In some cases, the bondingis to the pi electrons of a doublebond, as in complexes formed be-tween platinum and ethene, or tothe pi electrons of a ring, as in *fer-rocene.

A• Information about IUPAC nomenclature• Further information about

nomenclature

organophosphorus compoundA compound that has at least onecarbon–phosphorus bond. Organo-phosphorus compounds are used aspesticides (as in malathion andparathion) and as catalysts and sol-vents. Phosphate esters, sometimesincluded in this category, are em-ployed for Üre-prooÜng textiles.

Orgel diagram A diagram showinghow the energy levels of a transition-metal atom split when it is placed ina ligand Üeld. The vertical axis showsthe energy and the horizontal axisshows the strength of the ligandÜeld, with zero ligand Üeld strengthat the centre of the horizontal axis.That the splitting for dn is the sameas dn + 5 and the opposite of d10 – n isreadily seen on an Orgel diagram,both for octahedral and tetrahedralÜelds. The spectroscopic, optical, andmagnetic properties of complexes oftransition metals are made clear insuch diagrams. Orgel diagrams are

named after Leslie Orgel, who devel-oped them in 1955.

origin of elements The nuclearprocesses that give rise to chemicalelements. There is not one singleprocess that can account for all theelements. The abundance of thechemical elements is determined notjust by the stability of the nuclei ofthe atoms but also how readily thenuclear processes leading to the exis-tence of these atoms occur.

Most of the helium in the universewas produced by fusion in the earlyuniverse when the temperature andthe pressure were very high. Most ofthe elements between helium andiron were made in nuclear fusion re-actions inside stars. Since iron is atthe bottom of the energy valley ofstability, energy needs to be put intoa nucleus heavier than iron for a fu-sion reaction to occur. Inside starssome heavy elements are built up bythe s-process, where s stands forslow, in which high-energy neutronsare absorbed by a nucleus, with theresulting nucleus undergoing betadecay to produce a nucleus with ahigher atomic number. Some heavyelements are produced by the r-process, where r stands for rapid,which occurs in supernova explo-sions when a great deal of gravita-tional energy is released when alarge star that has exhausted its nu-clear fuel collapses to a neutron star,with the release of a very large num-ber of neutrons.

ornithine (Orn) An *amino acid,H2N(CH2)3CH(NH2)COOH, that is nota constituent of proteins but is im-portant in living organisms as an in-termediate in the reactions of the*urea cycle and in arginine synthesis.

ornithine cycle See urea cycle.

orpiment A natural yellow mineralform of arsenic(III) sulphide, As2S3.

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The name is also used for the syn-thetic compound, which is used as apigment.

ortho- 1. PreÜx indicating that abenzene compound has two substi-tuted groups in the 1,2 positions (i.e.on adjacent carbon atoms). The ab-breviation o- is used; for example o-dichlorobenzene is 1,2-dichloro-benzene. Compare meta-; para-. 2. PreÜx formerly used to indicatethe most hydrated form of an acid.For example, phosphoric(V) acid,H3PO4, was called orthophosphoricacid to distinguish it from the lowermetaphosphoric acid, HPO3 (which isactually (HPO3)n). 3. PreÜx denotingthe form of diatomic molecules inwhich nuclei have parallel spins, e.g.orthohydrogen (see hydrogen). Com-pare para-.

orthoboric acid See boric acid.

orthoclase See feldspars.

orthohelium A form of heliumonce thought to exist as of one oftwo species of the element. Becausethe spectrum of atomic helium con-sists of transitions between singletstates (including transitions from theground state) and transitions be-tween triplet states but not transi-tions between singlet and tripletstates, early spectroscopists thoughtthat two species of helium existed.The other form was called para-helium.

orthohydrogen See hydrogen.

orthophosphoric acid See phos-phoric(v) acid.

orthoplumbate See plumbate.

orthorhombic See crystal system.

orthosilicate See silicate.

orthostannate See stannate.

oscillating reaction (clock reac-tion) A type of chemical reaction in

which the concentrations of theproducts and reactants change peri-odically, either with time or with po-sition in the reacting medium. Thus,the concentration of a componentmay increase with time to a maxi-mum, decrease to a minimum, thenincrease again, and so on, continuingthe oscillation over a period of time.Systems are also known in which spi-rals and other patterns spreadthrough the reacting medium,demonstrating a periodic spatial vari-ation. Oscillating chemical reactionshave certain features in common.They all occur under conditions farfrom chemical equilibrium and all in-volve *autocatalysis, i.e. a product ofa reaction step acts as a catalyst forthat step. This autocatalysis drivesthe oscillation by a process of posi-tive feedback. Moreover, oscillatingchemical reactions are associatedwith the phenomenon known asbistability. In this, a reaction may bein a steady-state condition, with reac-tants Ûowing into a reaction zonewhile products are Ûowing out of it.Under these conditions, the concen-trations in the reaction zone may notchange with time, although the reac-tion is not in a state of chemical equi-librium. Bistable systems have twopossible stable steady states. Interac-tion with an additional substance inthe reaction medium causes the sys-tem to oscillate between the states asthe concentrations change. Oscillat-ing chemical reactions are thought tooccur in a number of biochemicalprocesses. For example, they occur inglycolysis, in which ATP is producedby enzyme-catalysed reactions. Theyare also known to regulate therhythm of the heartbeat. Most havehighly complex reaction mecha-nisms. See also lotka–volterra mech-anism; brusselator; oregonator;chaotic reaction.

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osmiridium A hard white naturallyoccurring alloy consisting principallyof osmium (17–48%) and iridium(49%). It also contains small quanti-ties of platinum, rhodium, and ruthe-nium. It is used for making smallitems subject to wear, e.g. electricalcontacts or the tips of pen nibs.

osmium Symbol Os. A hard blue-white metallic *transition element;a.n. 76; r.a.m. 190.2; r.d. 22.57; m.p.3045°C; b.p. 5027°C. It is found asso-ciated with platinum and is used incertain alloys with platinum and irid-ium (see osmiridium). Osmium formsa number of complexes in a range ofoxidation states. It was discovered bySmithson Tennant (1761–1815) in1804.A• Information from the WebElements site

osmium(IV) oxide (osmium tetrox-ide) A yellow solid, OsO4, made byheating osmium in air. It is used asan oxidizing agent in organic chem-istry, as a catalyst, and as a Üxative inelectron microscopy.

osmometer See osmosis.

osmosis The passage of a solventthrough a semipermeable membraneseparating two solutions of differentconcentrations. A semipermeablemembrane is one through which themolecules of a solvent can pass butthe molecules of most solutes can-not. There is a thermodynamic ten-dency for solutions separated by sucha membrane to become equal in con-centration, the water (or other sol-vent) Ûowing from the weaker to thestronger solution. Osmosis will stopwhen the two solutions reach equalconcentration, and can also bestopped by applying a pressure to theliquid on the stronger-solution sideof the membrane. The pressure re-quired to stop the Ûow from a puresolvent into a solution is a character-

istic of the solution, and is called theosmotic pressure (symbol Π). Osmoticpressure depends only on the con-centration of particles in the solu-tion, not on their nature (i.e. it is a*colligative property). For a solutionof n moles in volume V at thermody-namic temperature T, the osmoticpressure is given by ΠV = nRT, whereR is the gas constant. Osmotic-pressure measurements are used inÜnding the relative molecular massesof compounds, particularly macro-molecules. A device used to measureosmotic pressure is called an os-mometer.

osmotic pressure See osmosis.

Ostwald, Friedrich Wilhelm(1853–1932) German physicalchemist. In 1887 he went to Leipzigwhere he worked on a wide range oftopics including hydrolysis, viscosity,ionization, and catalysis. Ostwald washighly inÛuential in the developmentof physical chemistry as a subject. Hewas awarded the 1909 Nobel Prizefor chemistry.

Ostwald ripening A process usedin crystal growth in which a mixtureof large and small crystals are both incontact with a solvent. The largecrystals grow and the small crystalsdisappear. This occurs because thereis a higher energy associated withthe smaller crystals. When they dis-solve the heat associated with thishigher energy is released, enablingrecrystallization to occur on the largecrystals. Ostwald ripening is used insuch applications as photography, re-quiring crystals with speciÜc proper-ties.

Ostwald’s dilution law An ex-pression for the degree of dissocia-tion of a weak electrolyte. Forexample, if a weak acid dissociates inwater

HA ˆ H+ + A–

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the dissociation constant Ka is givenby

Ka = α2n/(1 – α)Vwhere α is the degree of dissociation,n the initial amount of substance (be-fore dissociation), and V the volume.If α is small compared with 1, thenα2 = KV/n; i.e. the degree of dissocia-tion is proportional to the squareroot of the dilution. The law was Ürstput forward by Wilhelm Ostwald toaccount for electrical conductivitiesof electrolyte solutions.

outer-sphere mechanism Seeelectron-transfer reaction.

overpotential (overvoltage) A po-tential that must be applied in anelectrolytic cell in addition to thetheoretical potential required to lib-erate a given substance at an elec-trode. The value depends on theelectrode material and on the cur-rent density. It is a kinetic effect oc-curring because of the signiÜcantactivation energy for electron trans-fer at the electrodes, and is particu-larly important for the liberation ofsuch gases as hydrogen and oxygen.For example, in the electrolysis of asolution of zinc ions, hydrogen (EŠ =0.00 V) would be expected to be liber-ated at the cathode in preference tozinc (EŠ = –0.76 V). In fact, the highoverpotential of hydrogen on zinc(about 1 V under suitable conditions)means that zinc can be deposited in-stead. The relation between the cur-rent and the overpotential is given bythe *Butler–Volmer equation.

overtones See harmonic.

oxalate A salt or ester of *oxalicacid.

oxalic acid (ethanedioic acid) Acrystalline solid, (COOH)2, that isslightly soluble in water. Oxalic acidis strongly acidic and very poisonous.

It occurs in certain plants, e.g. sorreland the leaf blades of rhubarb.

oxaloacetic acid A compound,HO2CCH2COCO2H, that plays an inte-gral role in the *Krebs cycle. Theanion, oxaloacetate, reacts with theacetyl group from acetyl coenzyme Ato form citrate.

oxazole A heterocyclic compoundhaving a nitrogen atom and an oxy-gen atom in a Üve-membered ring,C3H3NO.

outer-sphere mechanism 392

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oxfuel A liquid fuel containingadded alcohols or ethers to act as anadditional source of oxygen duringcombustion of the fuel. It has beenclaimed that such additives help tolower the concentration of carbonmonoxide in engine emissions.

oxidant See oxidizing agent.

oxidation See oxidation–reduction.

oxidation number (oxidationstate) See oxidation–reduction.

oxidation–reduction (redox) Orig-inally, oxidation was simply regardedas a chemical reaction with oxygen.The reverse process – loss of oxygen– was called reduction. Reaction withhydrogen also came to be regarded asreduction. Later, a more general ideaof oxidation and reduction was devel-oped in which oxidation was loss ofelectrons and reduction was gain ofelectrons. This wider deÜnition cov-ered the original one. For example,in the reaction

4Na(s) + O2(g) → 2Na2O(s)the sodium atoms lose electrons to


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give Na+ ions and are oxidized. At thesame time, the oxygen atoms gainelectrons and are reduced. ThesedeÜnitions of oxidation and reduc-tion also apply to reactions that donot involve oxygen. For instance in

2Na(s) + Cl2(g) → 2NaCl(s)the sodium is oxidized and the chlo-rine reduced. Oxidation and reduc-tion also occurs at the electrodes in*cells.

This deÜnition of oxidation and re-duction applies only to reactions inwhich electron transfer occurs – i.e.to reactions involving ions. It can beextended to reactions between cova-lent compounds by using the conceptof oxidation number (or state). This isa measure of the electron controlthat an atom has in a compoundcompared to the atom in the pure el-ement. An oxidation number consistsof two parts:(1) Its sign, which indicates whetherthe control has increased (negative)or decreased (positive).(2) Its value, which gives the numberof electrons over which control haschanged.

The change of electron control maybe complete (in ionic compounds) orpartial (in covalent compounds). Forexample, in SO2 the sulphur has anoxidation number +4, having gainedpartial control over 4 electrons com-pared to sulphur atoms in pure sul-phur. The oxygen has an oxidationnumber –2, each oxygen having lostpartial control over 2 electrons com-pared to oxygen atoms in gaseousoxygen. Oxidation is a reaction in-volving an increase in oxidationnumber and reduction involves a de-crease. Thus in

2H2 + O2 → 2H2Othe hydrogen in water is +1 and theoxygen –2. The hydrogen is oxidizedand the oxygen is reduced.

The oxidation number is used in

naming inorganic compounds. Thusin H2SO4, sulphuric(VI) acid, the sul-phur has an oxidation number of +6.Compounds that tend to undergo re-duction readily are *oxidizing agents;those that undergo oxidation are *re-ducing agents.

oxidation state See oxidation–reduction.

oxidative deamination A reac-tion involved in the catabolism ofamino acids that assists their excre-tion from the body. An example of an oxidative deamination is the conversion of glutamate to α-ketoglutarate, a reaction catalysed by the enzyme glutamate dehydroge-nase.

oxidative decarboxylation Thereaction in the *Krebs cycle in whichoxygen, derived from two water mol-ecules, is used to oxidize two carbonatoms to two molecules of carbondioxide. The two carbon atoms resultfrom the decarboxylation reactionsthat occur during the Krebs cycle asthe six-carbon compound citrate isconverted to the four-carbon com-pound oxaloacetate.

oxides Binary compounds formedbetween elements and oxygen. Ox-ides of nonmetals are covalent com-pounds having simple molecules (e.g.CO, CO2, SO2) or giant molecular lat-tices (e.g. SiO2). They are typicallyacidic or neutral. Oxides of metalsare ionic, containing the O2– ion.They are generally basic or *ampho-teric. Various other types of ionicoxide exist (see ozonides; peroxides;superoxides).

oxidizing acid An acid that can actas a strong oxidizing agent as well asan acid. Nitric acid is a common ex-ample. It is able to attack metals,such as copper, that are below hydro-gen in the electromotive series, byoxidizing the metal:

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2HNO3 + Cu → CuO + H2O + 2NO2

This is followed by reaction betweenthe acid and the oxide:

2HNO3 + CuO → Cu(NO3)2 + H2O

oxidizing agent (oxidant) A sub-stance that brings about oxidation inother substances. It achieves this bybeing itself reduced. Oxidizing agentscontain atoms with high oxidationnumbers; that is the atoms have suf-fered electron loss. In oxidizing othersubstances these atoms gain elec-trons.

oximes Compounds containing thegroup C:NOH, formed by reaction ofan aldehyde or ketone with hydroxy-lamine (H2NOH) (see illustration).Ethanal (CH3CHO), for example,forms the oxime CH3CH:NOH.A• Information about IUPAC nomenclature

oxo- PreÜx indicating the presenceof oxygen in a chemical compound.

oxoacid An acid in which theacidic hydrogen is part of a hydroxylgroup bound to an atom that isbound to an oxo group (=O). Sul-phuric acid is an example. Comparehydroxoacid.

3-oxobutanoic acid (acetoaceticacid) A colourless syrupy liquid,CH3COCH2COOH. It is an unstablecompound, decomposing intopropanone and carbon dioxide. Theacid can be prepared from its ester,*ethyl 3-oxobutanoate.

oxonium ion An ion of the typeR3O+, in which R indicates hydrogenor an organic group especially theion H3O+, which is formed when*acids dissociate in water. This is also

called the hydroxonium ion or thehydronium ion.

oxo process An industrial processfor making aldehydes by reaction be-tween alkanes, carbon monoxide,and hydrogen (cobalt catalyst usinghigh pressure and temperature).

oxyacetylene burner A weldingor cutting torch that burns a mixtureof oxygen and acetylene (ethyne) in aspecially designed jet. The Ûame tem-perature of about 3300°C enables allferrous metals to be welded. For cut-ting, the point at which the steel isto be cut is preheated with the oxy-acetylene Ûame and a powerful jet ofoxygen is then directed onto thesteel. The oxygen reacts with the hotsteel to form iron oxide and the heatof this reaction melts more iron,which is blown away by the force ofthe jet.

oxycodone An opioid, C18H21N2,similar in structure to *codeine butwith a –OH group in codeine re-placed by a carbonyl group. It is ananalgesic often used for the treat-ment of chronic pain. It is also usedillegally and is a controlled substancein most countries.

oxygen Symbol O. A colourlessodourless gaseous element belongingto *group 16 (formerly VIB) of the pe-riodic table; a.n. 8; r.a.m. 15.9994; d.1.429 g dm–3; m.p. –218.4°C; b.p.–183°C. It is the most abundant el-ement in the earth’s crust (49.2% byweight) and is present in the atmos-phere (28% by volume). Atmosphericoxygen is of vital importance for allorganisms that carry out aerobic res-piration. For industrial purposes it isobtained by fractional distillation of

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ketone hydroxylamine

– H2O

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liquid air. It is used in metallurgicalprocesses, in high-temperatureÛames (e.g. for welding), and inbreathing apparatus. The commonform is diatomic (dioxygen, O2);there is also a reactive allotrope*ozone (O3). Chemically, oxygen re-acts with most other elements form-ing *oxides. The element wasdiscovered by Joseph Priestley in1774.A• Information from the WebElements site

oxygenates Oxygen-containingorganic compounds, such as ethanoland acetone, present in motor fuels.

oxyhaemoglobin See haemoglo-bin.

ozonation The formation of*ozone (O3) in the earth’s atmos-phere. In the upper atmosphere(stratosphere) about 20–50 km abovethe surface of the earth, oxygen mol-ecules (O2) dissociate into their con-stituent atoms under the inÛuence ofultraviolet light of short wavelength(below about 240 nm). These atomscombine with oxygen molecules toform ozone (see ozone layer). Ozoneis also formed in the lower atmos-phere from nitrogen oxides andother pollutants by photochemicalreactions (see photochemical smog).

ozone (trioxygen) A colourless gas,O3, soluble in cold water and in alka-lis; m.p. –192.7°C; b.p. –111.9°C. Liq-uid ozone is dark blue in colour andis diamagnetic (dioxygen, O2, is para-magnetic). The gas is made by pass-ing oxygen through a silent electricdischarge and is usually used in mix-tures with oxygen. It is produced inthe stratosphere by the action ofhigh-energy ultraviolet radiation onoxygen and its presence there acts asa screen for ultraviolet radiation (seeozone layer). Ozone is also one of

the greenhouse gases (see green-house effect). It is a powerful oxi-dizing agent and is used to formozonides by reaction with alkenesand subsequently by hydrolysis tocarbonyl compounds.

ozone hole See ozone layer.

ozone layer (ozonosphere) A layerof the *earth’s atmosphere in whichmost of the atmosphere’s ozone isconcentrated. It occurs 15–50 kmabove the earth’s surface and is virtu-ally synonymous with the stratos-phere. In this layer most of the sun’sultraviolet radiation is absorbed bythe ozone molecules, causing a risein the temperature of the stratos-phere and preventing vertical mixingso that the stratosphere forms a sta-ble layer. By absorbing most of thesolar ultraviolet radiation the ozonelayer protects living organisms onearth. The fact that the ozone layer isthinnest at the equator is believed toaccount for the high equatorial inci-dence of skin cancer as a result of ex-posure to unabsorbed solarultraviolet radiation. In the 1980s itwas found that depletion of theozone layer was occurring over boththe poles, creating ozone holes. Thisis thought to have been caused by aseries of complex photochemical re-actions involving nitrogen oxidesproduced from aircraft and, more se-riously, *chloroÛuorocarbons (CFCs)and *halons. CFCs rise to the stratos-phere, where they react with ultravi-olet light to release chlorine atoms;these atoms, which are highly reac-tive, catalyse the destruction ofozone. Use of CFCs is now much re-duced in an effort to reverse thishuman-induced damage to the ozonelayer.

ozonides 1. A group of compoundsformed by reaction of ozone with al-kali metal hydroxides and formally

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containing the ion O3–. 2. Unstable

compounds formed by the additionof ozone to the C=C double bond inalkenes. See ozonolysis.

ozonolysis A reaction of alkeneswith ozone to form an ozonide. Itwas once used to investigate the

structure of alkenes by hydrolysingthe ozonide to give aldehydes or ke-tones. For instance

R2C:CHR′ → R2CO + R′CHOThese could be identiÜed, and thestructure of the original alkene deter-mined.

ozonolysis 396

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P

PAGE (polyacrylamide gel electro-phoresis) A type of *electrophore-sis used to determine the size andcomposition of proteins. Proteins areplaced on a matrix of polyacrylamidegel and an electric Üeld is applied.The protein molecules migrate to-wards the positive pole, the smallermolecules moving at a faster ratethrough the pores of the gel. The pro-teins are then detected by applying astain.

PAH Polyaromatic hydrocarbon. Seeparticulate matter.

palladium Symbol Pd. A soft whiteductile *transition element (see alsoplatinum metals); a.n. 46; r.a.m.106.4; r.d. 12.02; m.p. 1552°C; b.p.3140±1°C. It occurs in some copperand nickel ores and is used in jew-ellery and as a catalyst for hydro-genation reactions. Chemically, itdoes not react with oxygen at normaltemperatures. It dissolves slowly inhydrochloric acid. Palladium is ca-pable of occluding 900 times its ownvolume of hydrogen. It forms fewsimple salts, most compounds beingcomplexes of palladium(II) with somepalladium(IV). It was discovered byWilliam Woolaston (1766–1828) in1803.


palmitate (hexadecanoate) A saltor ester of palmitic acid.

palmitic acid (hexadecanoic acid)A 16-carbon saturated fatty acid,CH3(CH2)14COOH; r.d. 0.85; m.p.63°C; b.p. 390°C. Glycerides of

palmitic acid occur widely in plantand animal oils and fats.

pantothenic acid A vitamin of the*vitamin B complex. It is a con-stituent of coenzyme A, which per-forms a crucial role in the oxidationof fats, carbohydrates, and certainamino acids. DeÜciency rarely occursbecause the vitamin occurs in manyfoods, especially cereal grains, peas,egg yolk, liver, and yeast.

OHCH2

NH

CH2

CH2

OH

H OH

CH3CH3 O O

Pantothenic acid

papain A protein-digesting enzymeoccurring in the fruit of the West In-dian papaya tree (Carica papaya). It isused as a digestant and in the manu-facture of meat tenderizers.

paper chromatography A tech-nique for analysing mixtures by*chromatography, in which the sta-tionary phase is absorbent paper. Aspot of the mixture to be investigatedis placed near one edge of the paperand the sheet is suspended verticallyin a solvent, which rises through thepaper by capillary action carrying thecomponents with it. The componentsmove at different rates, partly be-cause they absorb to different extentson the cellulose and partly because ofpartition between the solvent andthe moisture in the paper. The paperis removed and dried, and the differ-ent components form a line of spotsalong the paper. Colourless sub-stances are detected by using ultra-


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violet radiation or by spraying with asubstance that reacts to give acoloured spot (e.g. ninhydrin gives ablue coloration with amino acids).The components can be identiÜed bythe distance they move in a giventime.

para- 1. PreÜx designating a ben-zene compound in which two sub-stituents are in the 1,4 positions, i.e.directly opposite each other, on thebenzene ring. The abbreviation p- isused; for example, p-xylene is 1,4-dimethylbenzene. Compare ortho-;meta-. 2. PreÜx denoting the form ofdiatomic molecules in which the nu-clei have opposite spins, e.g. parahy-drogen (see hydrogen). Compareortho-.

parafÜn See petroleum.

parafÜns See alkanes.

parafÜn wax See petroleum.

paraformaldehyde See methanal.

parahelium See orthohelium.

parahydrogen See hydrogen.

paraldehyde See ethanal.

parallel spins Neighbouring spin-ning electrons in which the *spins,and hence the magnetic moments, ofthe electrons are aligned in the samedirection.

paramagnetism See magnetism.

Paraquat Tradename for an or-ganic herbicide (see dipyridyl) usedto control broadleaved weeds andgrasses. It is poisonous to humans,having toxic effects on the liver,lungs, and kidneys if ingested.Paraquat is not easily broken downand can persist in the environmentadsorbed to soil particles. See also pes-ticide.

parent See daughter.

partially permeable membrane

A membrane that is permeable to thesmall molecules of water and certainsolutes but does not allow the pas-sage of large solute molecules. Thisterm is preferred to semipermeablemembrane when describing biologi-cal membranes. See osmosis.

partial pressure See dalton’s law.

particle in a box A system inquantum mechanics used to illus-trate important features of quantummechanics, such as quantization ofenergy levels and the existence of*zero-point energy. A particle with amass m is allowed to move betweentwo walls having the coordinates x =0 and x = L. The potential energy ofthe particle is taken to be zero be-tween the walls and inÜnite outsidethe walls. The time-independent*Schrödinger equation is exactly sol-uble for this problem, with the ener-gies En being given by n2h2/8mL2, n =1,2,…, and the wave functions ψnbeing given by ψn = (2/L)½ sin(nπx/L),where n are the quantum numbersthat label the energy levels and h isthe *Planck constant. The particle ina box can be used as a rough modelfor delocalized electrons in a mol-ecule or solid. Thus it can be used toexplain the colours of moleculeswith conjugated double bonds. It canalso explain the colour imparted tocrystals by F-centres. The problem ofa particle in a box can also be solvedin two and three dimensions, en-abling relations between symmetryand degenerate energy levels to beseen.

particulate matter (PM) Matterpresent as small particles. Particulatematter, also known as particulates, isproduced as an airborne pollutant inmany processes. It may be inorganic,e.g. silicate or carbon, or organic, e.g.polyaromatic hydrocarbons (PAHs).Particulates are classiÜed by size; e.g.PM10, with particles less than 10 µm,

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and PM2.5, with particles less than2.5 µm.

parting agent See release agent.

partition If a substance is in con-tact with two different phases then,in general, it will have a differentafÜnity for each phase. Part of thesubstance will be absorbed or dis-solved by one and part by the other,the relative amounts depending onthe relative afÜnities. The substanceis said to be partitioned between thetwo phases. For example, if two im-miscible liquids are taken and a thirdcompound is shaken up with them,then an equilibrium is reached inwhich the concentration in one sol-vent differs from that in the other.The ratio of the concentrations is the partition coefÜcient of the sys-tem. The partition law states thatthis ratio is a constant for given liq-uids.

partition coefÜcient See parti-tion.

partition function The quantity ZdeÜned by

Z = ∑exp(–Ei/kT),where the sum is taken over allstates i of the system. Ei is the energyof the ith state, k is the Boltzmannconstant, and T is the thermody-namic temperature. Z is a quantity offundamental importance in equilib-rium statistical mechanics. For a sys-tem in which there are nontrivialinteractions, it is very difÜcult to cal-culate the partition function exactly.For such systems it is necessary touse approximation techniques. Thepartition function links results at theatomic level to thermodynamics,since Z is related to the Helmholtzfree energy F by F = kTlnZ.

pascal The *SI unit of pressureequal to one newton per squaremetre. It is named after the French

mathematician Blaise Pascal(1623–62).

Paschen–Back effect An effect onatomic line spectra that occurs whenthe atoms are placed in a strong mag-netic Üeld. Spectral lines that givethe anomalous *Zeeman effect whenthe atoms are placed in a weakermagnetic Üeld have a splitting pat-tern in a very strong magnetic Üeld.The Paschen–Back effect is namedafter the German physicists LouisCarl Heinrich Friedrich Paschen(1865–1947) and Ernest E. A. Back(1881–1959), who discovered it in1912. In the quantum theory ofatoms the Paschen–Back effect is ex-plained by the fact that the energiesof precession of the electron’s orbitalangular momentum l and the spinangular momentum s about the di-rection of the magnetic Üeld H aregreater than the energies of couplingbetween l and s. In the Paschen–Backeffect the orbital magnetic momentand the spin magnetic moment pre-cess independently about the direc-tion of H.

Paschen series See hydrogen spec-trum.

passive Describing a solid that hasreacted with another substance toform a protective layer, so that fur-ther reaction stops. The solid is saidto have been ‘rendered passive’. Forexample, aluminium reacts sponta-neously with oxygen in air to form athin layer of *aluminium oxide,which prevents further oxidation.Similarly, pure iron forms a protec-tive oxide layer with concentrated ni-tric acid and is not dissolved further.

Pasteur, Louis (1822–95) Frenchchemist and microbiologist, whoheld appointments in Strasbourg(1849–54) and Lille (1854–57), beforereturning to Paris to the Ecole Nor-male and the Sorbonne. From 1888

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to his death he was director of thePasteur Institute. In 1848 he discov-ered *optical activity, in 1860 relat-ing it to molecular structure. In 1856he began work on fermentation, andby 1862 was able to disprove the exis-tence of spontaneous generation. Heintroduced pasteurization (originallyfor wine) in 1863. He went on tostudy disease and developed vaccinesagainst cholera (1880), anthrax(1882), and rabies (1885).

Patterson function A function,denoted P(x,y,z), used in the analysisof the results of *X-ray crystallogra-phy. P(x,y,z) is deÜned by

P(x,y,z) = Σhkl

|Fhkl|cos2π(hx + ky + lz),

where h, k, and l are the Miller in-dices of the crystal. This function isplotted as a contour map with max-ima of the contours at vector dis-tances from the origin, i.e. vectorsfrom the origin to the point (x,y,z),where the maximum occurs, corre-sponding to vector distances betweenpairs of maxima in electron density.Thus, the Patterson function enablesinteratomic vectors, which give thelengths and directions betweenatomic centres, to be determined.This technique, introduced by A. L.Patterson in 1934, is called Pattersonsynthesis and is most useful if theunit cell contains heavy atoms.

Patterson synthesis See patter-son function.

Pauli, Wolfgang Ernst (1900–58)Austrian-born Swiss physicist. Afterstudying with Niels *Bohr and Max*Born, he taught at Heidelberg and,Ünally Zurich. His formulation in1924 of the *Pauli exclusion princi-ple explained the electronic make-upof atoms. For this work he wasawarded the 1945 Nobel Prize forphysics. In 1931 he predicted the ex-istence of the neutrino.

Pauli exclusion principle The

principle that no two electrons in anatom can have all four quantumnumbers the same. It was Ürst formu-lated in 1925 by Wolfgang *Pauli andmore generally applies to the quan-tum states of all elementary particleswith half-integral spin.

Pauling, Linus Carl (1901–94) USchemist. After spending two years inEurope, he became a professor at theCalifornian Institute of Technology.His original work was on chemicalbonding; in the mid-1930s he turnedto the structure of proteins, forwhich he was awarded the 1954Nobel Prize for chemistry. He wasalso an active campaigner against nu-clear weapons and in 1962 wasawarded the Nobel Peace Prize.

Pauling’s rules A set of rules thatenables the structures of many com-plex ionic crystals to be understood.These rules involve the sizes andelectrical charges of the ions andwere proposed by Linus *Pauling in1929. They work well for mainlyionic crystals, such as silicates, butless well for crystals, such as sul-phides, in which the bonding ismainly covalent. Pauling put forwardhis rules on the basis of empirical ob-servations and calculations of crystalenergies. A number of extensions andmodiÜcations have been proposed.

p-block elements The block of el-ements in the periodic table consist-ing of the main groups 13 (B to Tl),14 (C to Pb), 15 (N to Bi), 16 (O to Po),17 (F to At) and 18 (He to Rn). Theouter electronic conÜgurations ofthese elements all have the formnp2npx where x = 1 to 6. Members atthe top and on the right of the p-block are nonmetals (C, N, P, O, F, S,Cl, Br, I, At). Those on the left and atthe bottom are metals (Al, Ga, In, Tl,Sn, Pb, Sb, Bi, Po). Between the two,from the top left to bottom right, lie

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an ill-deÜned group of metalloid el-ements (B, Si, Ge, As, Te).

PCB See polychlorinated biphenyl.

PCP See phencyclidine.

peacock ore See bornite.

pearl ash See potassiumcarbonate.

pearlite See steel.

pelargonic acid See nonanoicacid.

penicillin An antibiotic derivedfrom the mould Penicillium notatum;speciÜcally it is known as penicillin Gand belongs to a class of similar sub-stances called penicillins. They pro-duce their effects by disruptingsynthesis of the bacterial cell wall,and are used to treat a variety of in-fections caused by bacteria.

401 pentavalent

p

S

N

CH3

CH3

O

OOH

NH

R

O

Penicillin

Penning ionization A photochem-ical process that produces a posi-tively charged ion. For atoms A andB, the process of Penning ionizationis written:

A* + B → A + B+ + e–,where A* denotes that the atom Ahas absorbed a photon, thus acquir-ing enough energy for the process totake place, and e– is an electron. Anexample of Penning ionization is theionization of mercury by argon:

Ar* + Hg → Ar + Hg+ + e–,which occurs because the energy ofthe metastable state of argon ishigher than the ionization energy of

mercury. The process of Penning ion-ization was discovered by F. M. Pen-ning in 1927.

Penrose tiling A method of tiling atwo-dimensional plane using tileswith Üvefold symmetry. Since thistype of symmetry is not allowed incrystallography, two types of tile areneeded, which are called ‘fat’ and‘skinny’. Adjacent tiles have to obeycertain matching rules. Penrosetiling, named after the British mathe-matician and physicist Sir Roger Pen-rose (1931– ), who put forward thisidea in 1974, is the two-dimensionalanalogue of a *quasicrystal.

pentaerythritol A white crys-talline compound, C(CH2OH)4; m.p.260°C; b.p. 276°C (30 mmHg). It isused in making the explosive pen-taerythritol trinitrate and in produc-ing resins and other organicproducts.

pentaerythritol tetranitrate(PETN) A powerful high explosivemade from pentaerythritol,C(CH2ONO2)4.

pentahydrate A crystalline hy-drate that has Üve moles of water permole of compound.

pentane A straight-chain alkanehydrocarbon, C5H12; r.d. 0.63; m.p.–129.7°C; b.p. 36.1°C. It is obtainedby distillation of petroleum.

pentanedioic acid (glutaric acid)A simple dicarboxylic acid,HOOC(CH2)3COOH; m.p. 96°C; b.p.200°C. It is used in the production ofcertain polymers.

pentanoic acid (valeric acid) Acolourless liquid *carboxylic acid,CH3(CH2)3COOH; r.d. 0.9; m.p. –34°C;b.p. 186.05°C. It is used in the per-fume industry.

pentavalent (quinquevalent) Hav-ing a valency of Üve.


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pentlandite A mineral consistingof a mixed iron–nickel sulphide,(Fe,Ni)9S8, crystallizing in the cubicsystem; the chief ore of nickel. It isyellowish-bronze in colour with ametallic lustre. The chief occurrenceof the mineral is at Sudbury in On-tario, Canada.

pentose A sugar that has Üve car-bon atoms per molecule. See mono-saccharide.

pentose phosphate pathway(pentose shunt) A series of biochemi-cal reactions that results in the con-version of glucose 6-phosphate toribose 5-phosphate and generatesNADPH, which provides reducingpower for other metabolic reactions,such as synthesis of fatty acids. Ri-bose 5-phosphate and its derivativesare components of such molecules asATP, coenzyme A, NAD, FAD, DNA,and RNA. In plants the pentose phos-phate pathway also plays a role inthe synthesis of sugars from carbondioxide. In animals the pathway oc-curs at various sites, including theliver and adipose tissue.

pentyl group (pentyl radical) Theorganic group CH3CH2CH2CH2CH2–,derived from pentane.

pepsin An enzyme that catalysesthe breakdown of proteins topolypeptides in the vertebrate stom-ach. It is secreted as an inactive pre-cursor, pepsinogen.

peptide Any of a group of organiccompounds comprising two or moreamino acids linked by peptide bonds.

These bonds are formed by the reac-tion between adjacent carboxyl(–COOH) and amino (–NH2) groupswith the elimination of water (see il-lustration). Dipeptides contain twoamino acids, tripeptides three, and soon. *Polypeptides contain more thanten and usually 100–300. Naturallyoccurring oligopeptides (of less thanten amino acids) include the tripep-tide glutathione and the pituitaryhormones vasopressin and oxytocin,which are octapeptides. Peptides alsoresult from protein breakdown, e.g.during digestion.


of peptides

peptide mapping (peptide Ünger-printing) The technique of formingtwo-dimensional patterns of peptides(on paper or gel) by partial hydrolysisof a protein followed by electro-phoresis and chromatography. Thepeptide pattern (or Üngerprint) pro-duced is characteristic for a particu-lar protein and the technique can beused to separate a mixture of pep-tides.

peptidoglycan A macromoleculethat is a component of the cell wallof bacteria; it is not found in eukary-otes. Consisting of chains of aminosugars (N-acetylglucosamine and N-acetylmuramic acid) linked to atripeptide (of alanine, glutamic acid,and lysine or diaminopimelic acid), itconfers strength and shape to thecell wall.

per- PreÜx indicating that a chemi-

pentlandite 402

p

amino acid 1 amino acid 2 dipeptide

peptide bond

H R O

H N C C OH

H

+

H R′ O

H N C C

H

OH

H R O

H N C C

H

H R′ O

N C C

H

OH + H2O

water

Peptide bond


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cal compound contains an excess ofan element, e.g. a peroxide.

perchlorate See chlorates.

perchloric acid See chloric(vii)acid.

perdisulphuric acid See peroxo-sulphuric(vi) acid.

perfect gas See ideal gas; gas.

perfect solution See raoult’s law.

pericyclic reaction A type of con-certed chemical reaction that pro-ceeds through a cyclic conjugatedtransition state. Pericyclic reactionsinclude *cheletropic reactions, some*cycloadditions, *sigmatropic reac-tions, and *electrocyclic reactions.They can be understood using *fron-tier orbital theory or the *Woodward–Hoffmann rules.

peridot See olivine.

period 1. The time taken for onecomplete cycle of an oscillating sys-tem or wave. 2. See periodic table.

period doubling A mechanism fordescribing the transition to *chaos incertain dynamical systems. If theforce on a body produces a regularorbit with a speciÜc *period a suddenincrease in the force can suddenlydouble the period of the orbit andthe motion becomes more complex.The original simple motion is calleda one-cycle, while the more compli-cated motion after the period dou-bling is called a two-cycle. Theprocess of period doubling can con-tinue until a motion called an n-cycleis produced. As n increases to inÜnitythe motion becomes non-periodic.The period-doubling route to chaosoccurs in many systems involvingnonlinearity, including lasers andcertain chaotic chemical reactions.The period-doubling route to chaoswas postulated and investigated bythe US physicist Mitchell Feigen-

baum in the early 1980s. Routes tochaos other than period doublingalso exist.

periodic acid See iodic(vii) acid.

periodic law The principle thatthe physical and chemical propertiesof elements are a periodic function oftheir proton number. The conceptwas Ürst proposed in 1869 by Dimitri*Mendeleev, using relative atomicmass rather than proton number, asa culmination of efforts to rationalizechemical properties by JohannDöbereiner (1817), John Newlands(1863), and Lothar Meyer (1864). Oneof the major successes of the periodiclaw was its ability to predict chemi-cal and physical properties of undis-covered elements and unknowncompounds that were later conÜrmedexperimentally. See periodic table.

periodic table A table of elementsarranged in order of increasing pro-ton number to show the similaritiesof chemical elements with relatedelectronic conÜgurations. (The origi-nal form was proposed by DimitriMendeleev in 1869 using relativeatomic masses.) In the modern shortform, the *lanthanoids and *acti-noids are not shown. The elementsfall into vertical columns, known asgroups. Going down a group, theatoms of the elements all have thesame outer shell structure, but an in-creasing number of inner shells. Tra-ditionally, the alkali metals wereshown on the left of the table andthe groups were numbered IA toVIIA, IB to VIIB, and 0 (for the noblegases). All the elements in the middleof the table are classiÜed as *transi-tion elements and the nontransitionelements are regarded as main-groupelements. Because of confusion inthe past regarding the numbering ofgroups and the designations of sub-groups, modern practice is to num-ber the groups across the table from

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1 to 18 (see Appendix). Horizontalrows in the table are periods. TheÜrst three are called short periods;the next four (which include transi-tion elements) are long periods.Within a period, the atoms of all theelements have the same number ofshells, but with a steadily increasingnumber of electrons in the outershell. The periodic table can also bedivided into four blocks dependingon the type of shell being Ülled: the*s-block, the *p-block, the *d-block,and the *f-block.

There are certain general featuresof chemical behaviour shown in theperiodic table. In moving down agroup, there is an increase in metal-lic character because of the increasedsize of the atom. In going across a pe-riod, there is a change from metallic(electropositive) behaviour to non-metallic (electronegative) because ofthe increasing number of electronsin the outer shell. Consequently,metallic elements tend to be thoseon the left and towards the bottomof the table; nonmetallic elementsare towards the top and the right.

There is also a signiÜcant differ-ence between the elements of thesecond short period (lithium toÛuorine) and the other elements intheir respective groups. This is be-cause the atoms in the second periodare smaller and their valence elec-trons are shielded by a small 1s2

inner shell. Atoms in the other peri-ods have inner s- and p-electronsshielding the outer electrons fromthe nucleus. Moreover, those in thesecond period only have s- and p-orbitals available for bonding. Heav-ier atoms can also promote electronsto vacant d-orbitals in their outershell and use these for bonding. Seealso diagonal relationship; inert-pair effect.A• The WebElements table produced by

Mark Winter at the University ofShefÜeld

• Over 50 different forms of the periodictable in the Chemogenesis web book byMark R. Leach

periplanar See torsion angle.

Perkin, Sir William Henry(1838–1907) British chemist, whowhile still a student accidentally pro-duced mauvine, the Ürst aniline dyeand the Ürst dyestuff to be synthe-sized. Perkin built a factory to pro-duce it, and made a fortune.

Permalloys A group of alloys ofhigh magnetic permeability consist-ing of iron and nickel (usually40–80%) often with small amounts ofother elements (e.g. 3–5% molybde-num, copper, chromium, or tung-sten). They are used in thin foils inelectronic transformers, for magneticshielding, and in computer memo-ries.

permanent gas A gas, such as oxy-gen or nitrogen, that was formerlythought to be impossible to liquefy.A permanent gas is now regarded asone that cannot be liqueÜed by pres-sure alone at normal temperatures(i.e. a gas that has a critical tempera-ture below room temperature).

permanent hardness See hard-ness of water.

permanganate See manga-nate(vii).

permonosulphuric(VI) acid Seeperoxosulphuric(vi) acid.

Permutit Tradename for a *zeoliteused for water softening.

perovskite structure A type ofionic crystal structure shown by com-pounds of the type ABO3. There is acubic arrangement with A atoms sur-rounded by 12 O atoms and B atomssurrounded by 6 O atoms. Examples

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of the perovskite structure includeCaTiO3, BaTiO3, and SrTiO3. A• An interactive version of the structure

peroxides A group of inorganiccompounds that contain the O2

2– ion.They are notionally derived from hy-drogen peroxide, H2O2, but theseions do not exist in aqueous solutiondue to extremely rapid hydrolysis toOH–.A• Information about IUPAC nomenclature

peroxodisulphuric acid See per-oxosulphuric(vi) acid.

peroxomonosulphuric(VI) acidSee peroxosulphuric(vi) acid.

peroxosulphuric(VI) acid Theterm commonly refers to peroxo-monosulphuric(VI) acid, H2SO5, which is also called permonosul-phuric(VI) acid and Caro’s acid. It is acrystalline compound made by theaction of hydrogen peroxide on con-centrated sulphuric acid. It decom-poses in water and the crystalsdecompose, with melting, above45°C. The compound peroxodisul-phuric acid, H2S2O8, also exists (for-merly called perdisulphuric acid). It ismade by the high-current electrolysisof sulphate solutions. It decomposesat 65°C (with melting) and is hydrol-ysed in water to give the mono acidand sulphuric acid. Both peroxo acidsare very powerful oxidizing agents.See also sulphuric acid (for structuralformulas).

persistent Describing a pesticideor other pollutant that is not readilybroken down and can persist for longperiods, causing damage in the envi-ronment. For example, the herbi-cides *Paraquat and *DDT can persistin the soil for many years after theirapplication.

persistent organic pollutant

(POP) A toxic substance that is notbiodegradable and persists in the en-vironment. POPs can concentrate asthey move up the food chain and aregenerally deleterious to health. In2001, an international conference inSweden resulted in an agreement(the Stockholm Convention) to reduceor eliminate the 12 POPs of greatestconcern, and to add other chemicalsto the list in the future. The 12 chem-icals were the pesticides aldrin,chlordane, *DDT, dieldrin, endrin,heptachlor, mirex, and toxaphene,together with the industrial chemicalhexachlorobenzene and the groups*polychlorinated biphenyls, *diox-ins, and *furans.

A• OfÜcial Stockholm Convention site

Perspex Tradename for a form of*polymethylmethacrylate.

perturbation theory A methodused in calculations in both classicalphysics (e.g. planetary orbits) andquantum mechanics (e.g. atomicstructure), in which the system is di-vided into a part that is exactly calcu-lable and a small term, whichprevents the whole system frombeing exactly calculable. The tech-nique of perturbation theory enablesthe effects of the small term to becalculated by an inÜnite series (whichin general is an asymptotic series).Each term in the series is a ‘correc-tion term’ to the solutions of the ex-actly calculable system. In classicalphysics, perturbation theory can beused for calculating planetary orbits.In quantum mechanics, it can beused to calculate the energy levels inmolecules.

PES See photoelectron spec-troscopy.

PESM See photoelectron spectro-microscopy.

405 PESM

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pesticide Any chemical compoundused to kill pests that destroy agricul-tural production or are in some wayharmful to humans. Pesticides in-clude herbicides (such as *2,4-D and*Paraquat), which kill unwantedplants or weeds; insecticides (such aspyrethrum), which kill insect pests;fungicides, which kill fungi; and ro-denticides (such as *warfarin), whichkill rodents. The problems associatedwith pesticides are that they are veryoften nonspeciÜc and may there-fore be toxic to organisms that arenot pests; they may also be non-biodegradable, so that they persist inthe environment and may accumu-late in living organisms (see bioaccu-mulation). Organophosphorusinsecticides, such as malathion andparathion, are biodegradable but canalso damage the respiratory and ner-vous systems in humans as well askilling useful insects, such as bees.They act by inhibiting the action ofthe enzyme cholinesterase. Organo-chlorine insecticides, such as dield-rin, aldrin, and *DDT, are verypersistent and not easily biodegrad-able.

peta- Symbol P. A preÜx used in themetric system to denote one thou-sand million million times. For exam-ple, 1015 metres = 1 petametre (Pm).

petrochemicals Organic chemicalsobtained from petroleum or naturalgas.

petrolatum See petroleum jelly.

petroleum A naturally occurringoil that consists chieÛy of hydrocar-bons with some other elements, suchas sulphur, oxygen, and nitrogen. Inits unreÜned form petroleum isknown as crude oil (sometimes rockoil). Petroleum is believed to havebeen formed from the remains of liv-ing organisms that were deposited,together with rock particles and bio-

chemical and chemical precipitates,in shallow depressions, chieÛy in ma-rine conditions. Under burial andcompaction the organic matter wentthrough a series of processes beforebeing transformed into petroleum,which migrated from the source rockto become trapped in large under-ground reservoirs beneath a layer ofimpermeable rock. The petroleumoften Ûoats above a layer of waterand is held under pressure beneath alayer of *natural gas.

Petroleum reservoirs are discov-ered through geological exploration:commercially important oil reservesare detected by exploratory narrow-bore drilling. The major known re-serves of petroleum are in SaudiArabia, Russia, China, Kuwait, Iran,Iraq, Mexico, USA, United Arab Emi-rates, Libya, and Venezuela. The oil isactually obtained by the sinking ofan oil well. Before it can be used it isseparated by fractional distillation inoil reÜneries. The main fractions ob-tained are:(1) ReÜnery gas A mixture ofmethane, ethane, butane, andpropane used as a fuel and for mak-ing other organic chemicals.(2) Gasoline A mixture of hydrocar-bons containing 5 to 8 carbon atoms,boiling in the range 40–180°C. It isused for motor fuels and for makingother chemicals.(3) Kerosine (or parafÜn oil) A mix-ture of hydrocarbons having 11 or 12carbon atoms, boiling in the range160–250°C. Kerosine is a fuel for jetaircraft and for oil-Üred domesticheating. It is also cracked to producesmaller hydrocarbons for use inmotor fuels.(4) Diesel oil (or gas oil) A mixture of hydrocarbons having 13 to 25carbon atoms, boiling in the range220–350°C. It is a fuel for diesel en-gines.

The residue is a mixture of higher

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hydrocarbons. The liquid compo-nents are obtained by vacuum distil-lation and used in lubricating oils.The solid components (parafÜn wax)are obtained by solvent extraction.The Ünal residue is a black tar con-taining free carbon (asphalt or bitu-men).

petroleum ether A colourlessvolatile Ûammable mixture of hydro-carbons (not an ether), mainly pen-tane and hexane. It boils in the range30–70°C and is used as a solvent.

petroleum jelly (petrolatum) Asemi-solid mixture of hydrocarbonsextracted from petroleum. It is usedin skincare products and cosmetics,and as a lubricant. Petroleum iswidely available under the trade-name Vaseline.

pewter An alloy of lead and tin. Itusually contains 63% tin; pewtertankards and food containers shouldhave less than 35% of lead so that thelead remains in solid solution withthe tin in the presence of weak acidsin the food and drink. Copper issometimes added to increase ductil-ity and antimony is added if a hardalloy is required.

peyote See mescaline.

Pfund series See hydrogen spec-trum.

PGA See phosphoglyceric acid.

pH See ph scale.

phane See cyclophane.

phase A homogeneous part of aheterogeneous system that is sepa-rated from other parts by a distin-guishable boundary. A mixture of iceand water is a two-phase system. Asolution of salt in water is a single-phase system.

phase diagram A graph showingthe relationship between solid, liq-

uid, and gaseous *phases over arange of conditions (e.g. temperatureand pressure). See steel (illustra-tion).

phase rule For any system at equi-librium, the relationship P + F = C + 2holds, where P is the number of dis-tinct phases, C the number of compo-nents, and F the number of degreesof freedom of the system. The rela-tionship derived by Josiah Willard*Gibbs in 1876, is often called theGibbs phase rule.

phase space For a system with ndegrees of freedom, the 2n-dimen-sional space with coordinates (q1, q2,…, qn, p1, p2, …, pn), where the qs de-scribe the degrees of freedom of thesystem and the ps are the corre-sponding momenta. Each point rep-resents a state of the system. In a gasof N point particles, each particle hasthree positional coordinates andthree corresponding momentum co-ordinates, so that the phase spacehas 6N-dimensions. If the particleshave internal degrees of freedom,such as the vibrations and rotationsof molecules, then these must be in-cluded in the phase space, which isconsequently of higher dimensionthan that for point particles. As thesystem changes with time the repre-sentative points trace out a curve inphase space known as a trajectory.See also attractor; configurationspace; statistical mechanics.

phase transition A change in afeature that characterizes a system.Examples of phase transitions arechanges from solid to liquid, liquid togas, and the reverse changes. Otherexamples of phase transitions in-clude the transition from a paramag-net to a ferromagnet (see magnetism)and the transition from a normallyconducting metal to a superconduc-tor. Phase transitions can occur by al-

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tering such variables as temperatureand pressure.

Phase transitions can be classiÜedby their order. If there is non-zero*latent heat at the transition it issaid to be a Ürst-order transition. Ifthe latent heat is zero it is said to bea second-order transition.

Some models describing phasetransitions, particularly in low-dimensional systems, are amenableto exact mathematical solutions.Techniques for investigating transi-tions may include the feature of uni-versality, in which very differentphysical systems behave in the sameway near a phase transition.

phencyclidine (PCP) A hallucino-genic drug, originally used as a veti-nary anaesthetic. It is usually used asa powder (known as ‘angel dust’).

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Phencyclidine

phenol (carbolic acid) A white crys-talline solid, C6H5OH; r.d. 1.1; m.p.43°C; b.p. 182°C. It is made by the*cumene process or by the *Raschigprocess. Phenol reacts readily to formsubstituted derivatives. It is an im-portant industrial chemical used inmaking Nylon, phenolic and epoxyresins, dyestuffs, explosives, andpharmaceuticals. See also phenols.

phenol–formaldehyde resin Aclass of resins produced by polymer-izing phenols with formaldehyde(methanol) using acid or basic cata-lysts. They are generally cross-linked

thermosetting materials. *Bakelite isthe original example.

phenolic resins Synthetic resinsmade by copolymerizing phenolswith aldehydes. *Phenol–formalde-hyde resins are the commonest type.

phenolphthalein A dye used as anacid-base *indicator. It is colourlessbelow pH 8 and red above pH 9.6. Itis used in titrations involving weakacids and strong bases. It is also usedas a laxative.

phenolphthalein test See kastlemeyer test.

phenols Organic compounds thatcontain a hydroxyl group (–OH)bound directly to a carbon atom in abenzene ring. Unlike normal alco-hols, phenols are acidic because ofthe inÛuence of the aromatic ring.Thus, phenol itself (C6H5OH) ionizesin water:

C6H5OH → C6H5O– + H+

Phenols are made by fusing a sul-phonic acid salt with sodium hydrox-ide to form the sodium salt of thephenol. The free phenol is liberatedby adding sulphuric acid.

phenomenological relations Seeonsager relations.

phenoxy resins Thermoplasticmaterials made by condensation ofphenols, used mainly for packaging.

phenylalanine See amino acid.

phenylamine (aniline; aminoben-zene) A colourless oily liquid aro-matic *amine, C6H5NH2, with an‘earthy’ smell; r.d. 1.0217; m.p.–6.3°C; b.p. 184.1°C. The compoundturns brown on exposure to sunlight.It is basic, forming the phenylammo-nium (or anilinium) ion, C6H5NH3

+,with strong acids. It is manufacturedby the reduction of nitrobenzene orby the addition of ammonia tochlorobenzene using a copper(II) salt


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catalyst at 200°C and 55 atm. Thecompound is used extensively in therubber industry and in the manufac-ture of drugs and dyes.

phenylammonium ion The ionC6H5NH3

+, derived from *pheny-lamine.

N-phenylethanamide See ac-etanilide.

phenylenediamine See di-aminobenzene.

phenylethene (styrene) A liquidhydrocarbon, C6H5CH:CH2; r.d. 0.9;m.p. –31°C; b.p. 145°C. It can bemade by dehydrogenating ethylben-zene and is used in making poly-styrene.

phenyl group The organic groupC6H5–, present in benzene.

phenylhydrazine A toxic colour-less dense liquid, C6H5NHNH2, b.p.240°C, which turns brown on expo-sure to air. It is a powerful reducingagent, made from *diazonium saltsof benzene. It is used to identify alde-hydes and ketones, with which itforms condensation products called*hydrazones. With glucose and simi-lar sugars it forms osazones. For such tests, the nitro derivative 2,4-dinitrophenylhydrazine (DNP) isoften preferred as this generallyforms crystalline derivatives that canbe identiÜed by their melting points.Phenylhydrazine is also used to makedyes and derivatives of *indole.

phenylhydrazones See hydra-zones.

phenylmethanol (benzyl al-cohol) A liquid aromatic alcohol,C6H5CH2OH; r.d. 1.04; m.p. –15.3°C;b.p. 205.4°C. It is used mainly as asolvent.

phenylmethylamine See benzyl-amine.

phenyl methyl ketone (aceto-phenone) A colourless, crystallineketone with an odour of bitter al-monds, C6H5COCH3, m.p. 20°C. It ismade by treating benzene withethanoyl chloride in the presence ofaluminium chloride. It is used as asolvent and in making perfumes.

3-phenylpropenoic acid See cin-namic acid.

Phillips process A process formaking high-density polyethene bypolymerizing ethene at high pressure(30 atmospheres) and 150°C. Thecatalyst is chromium(III) oxide sup-ported on silica and alumina.

phlogiston theory A formertheory of combustion in which allÛammable objects were supposed to contain a substance called phlogis-ton, which was released when theobject burned. The existence of thishypothetical substance was proposedin 1669 by Johann Becher, who calledit ‘combustible earth’ (terra pinguis:literally ‘fat earth’). For example, ac-cording to Becher, the conversion ofwood to ashes by burning was ex-plained on the assumption that theoriginal wood consisted of ash andterra pinguis, which was released onburning. In the early 18th centuryGeorg Stahl renamed the substancephlogiston (from the Greek for‘burned’) and extended the theory toinclude the calcination (and corro-sion) of metals. Thus, metals werethought to be composed of calx (apowdery residue) and phlogiston;when a metal was heated, phlogistonwas set free and the calx remained.The process could be reversed byheating the metal over charcoal (asubstance believed to be rich in phlo-giston, because combustion almosttotally consumed it). The calx wouldabsorb the phlogiston released by theburning charcoal and become metal-lic again.

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The theory was Ünally demolishedby Antoine Lavoisier, who showed bycareful experiments with reactionsin closed containers that there wasno absolute gain in mass – the gainin mass of the substance wasmatched by a corresponding loss inmass of the air used in combustion.After experiments with Priestley’s de-phlogisticated air, Lavoisier realizedthat this gas, which he named oxy-gen, was taken up to form a calx(now called an oxide). The role ofoxygen in the new theory was almostexactly the opposite of phlogiston’srole in the old. In combustion andcorrosion phlogiston was released; inthe modern theory, oxygen is takenup to form an oxide.

phonochemistry The use of high-frequency sound (ultrasound, with afrequency greater than about 20 kHz)to induce or accelerate certain typesof chemical reaction.

phosgene See carbonyl chloride.

phosphagen A compound foundin animal tissues that provides a re-serve of chemical energy in the formof high-energy phosphate bonds. Themost common phosphagens arecreatine phosphate, occurring in ver-tebrate muscle and nerves, and argi-nine phosphate, found in mostinvertebrates. During tissue activity(e.g. muscle contraction) phospha-gens give up their phosphate groups,thereby generating *ATP from ADP.The phosphagens are then reformedwhen ATP is available.

phosphates Salts based formallyon phosphorus(V) oxoacids and inparticular salts of *phosphoric(V)acid, H3PO4. A large number of poly-meric phosphates also exist, contain-ing P–O–P bridges. These are formedby heating the free acid and its saltsunder a variety of conditions; as wellas linear polyphosphates, cyclic

polyphosphates and cross-linkedpolyphosphates or ultraphosphatesare known.

phosphatides See phospholipids.

phosphide A binary compound ofphosphorus with a more electroposi-tive element. Phosphides show awide range of properties. Alkali andalkaline earth metals form ionicphosphides, such as Na3P and Ca3P2,which are readily hydrolysed bywater. The other transition-metalphosphides are inert metallic-lookingsolids with high melting points andelectrical conductivities.

phosphine A colourless highlytoxic gas, PH3; m.p. –133°C; b.p.–87.7°C; slightly soluble in water.Phosphine may be prepared by react-ing water or dilute acids with cal-cium phosphide or by reactionbetween yellow phosphorus and con-centrated alkali. Solutions of phos-phine are neutral but phosphinedoes react with some acids to givephosphonium salts containing PH4

+

ions, analogous to the ammoniumions. Phosphine prepared in the labo-ratory is usually contaminated withdiphosphine and is spontaneouslyÛammable but the pure compound isnot so. Phosphine can function as aligand in binding to transition-metalions. Dilute gas mixtures of very purephosphine and the rare gases areused for doping semiconductors.

phosphinic acid (hypophosphorusacid) A white crystalline solid,H3PO2; r.d. 1.493; m.p. 26.5°C; decom-poses above 130°C. It is soluble inwater, ethanol, and ethoxyethane.Salts of phosphinic acid may be pre-pared by boiling white phosphoruswith the hydroxides of group I orgroup II metals. The free acid is madeby the oxidation of phosphine withiodine. It is a weak monobasic acid inwhich it is the –O–H group that is

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ionized to give the ion H2PO2–. The

acid and its salts are readily oxidizedto the orthophosphate and conse-quently are good reducing agents.

phosphite See phosphonic acid.

phosphodiester bond The cova-lent bond that links a phosphategroup and a sugar group, by meansof an oxygen bridge, in the sugar–phosphate backbone of a nucleic acidmolecule. See dna.

phosphoglyceric acid (PGA; 3-phosphoglycerate) See glycerate 3-phosphate.

phospholipids (phosphatides) Agroup of lipids having both a phos-phate group and one or more fattyacids. Glycerophospholipids (or phos-phoglycerides) are based on *glyc-erol; the three hydroxyl groups areesteriÜed with two fatty acids and aphosphate group, which may itselfbe bound to one of a variety of sim-ple organic groups (see illustration).Sphingolipids are based on the alco-hol sphingosine and contain only onefatty acid linked to an amino group.With their hydrophilic polar phos-phate groups and long hydrophobichydrocarbon ‘tails’, phospholipidsreadily form membrane-like struc-tures in water. They are a major com-ponent of cell membranes.

phosphonate See phosphonicacid.

phosphonic acid (phosphorousacid; orthophosphorous acid) Acolourless to pale-yellow deliques-cent crystalline solid, H3PO3; r.d.1.65; m.p. 73.6°C; decomposes at200°C; very soluble in water and sol-uble in alcohol. Phosphonic acid maybe crystallized from the solution ob-tained by adding ice-cold water tophosphorus(III) oxide or phosphorustrichloride. The structure of this ma-terial is unusual in that it containsone direct P–H bond and is more cor-rectly written (HO)2HPO. The acid isdibasic, giving rise to the ions H2PO3

–

and HPO32– (phosphonates; formerly

phosphites), and has moderate reduc-ing properties. On heating it givesphosphine and phosphoric(V) acid.

phosphonium ion The ion PH4+,

or the corresponding organic deriva-tives of the type R3PH+, RPH3

+. Thephosphonium ion PH4

+ is formallyanalogous to the ammonium ionNH4

+ but PH3 has a much lower pro-ton afÜnity than NH3 and reaction ofPH3 with acids is necessary for theproduction of phosphonium salts.

phosphonium salts See phospho-nium ion.

phosphor A substance that is ca-

411 phosphor

p

O

H3C

H3C CH

CH

O

fatty acid

fatty acid

glycerol phosphate group choline

H2C

O

O– CH3

CH3

CH3

(CH2)14(CH2)14(CH2)14 CCC OOO CH2CH2CH2

(CH2)7(CH2)7(CH2)7 (CH2)7(CH2)7(CH2)7 CCC OOO CCC HHH

OOO OOO CH2CH2CH2 CH2CH2CH2 NNNPPP

Structure of phosphatidylcholine (lecithin), a glycerophospholipid


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pable of *luminescence (includingphosphorescence). Phosphors that re-lease their energy after a short delayof between 10–10 and 10–4 second aresometimes called scintillators.

phosphor bronze An alloy of cop-per containing 4% to 10% of tin and0.05% to 1% of phosphorus as a deoxi-dizing agent. It is used particularlyfor marine purposes and where it isexposed to heavy wear, as in gearwheels. See also bronze.

phosphorescence See lumines-cence.

phosphoric(V) acid (orthophos-phoric acid) A white rhombic solid,H3PO4; r.d. 1.834; m.p. 42.35°C; loseswater at 213°C; very soluble in waterand soluble in ethanol. Phosphoric(V)acid is very deliquescent and is gen-erally supplied as a concentratedaqueous solution. It is the most com-mercially important derivative ofphosphorus, accounting for over 90%of the phosphate rock mined. It ismanufactured by two methods; thewet process, in which the productcontains some of the impurities origi-nally present in the rock and applica-tions are largely in the fertilizerindustry, and the thermal process,which produces a much purer prod-uct suitable for the foodstuffs and de-tergent industries. In the wet processthe phosphate rock, Ca3(PO4)2, istreated with sulphuric acid and thecalcium sulphate removed either asgypsum or the hemihydrate. In thethermal process, molten phosphorusis sprayed and burned in a mixtureof air and steam. Phosphoric(V) acidis a weak tribasic acid, which is bestvisualized as (HO)3PO. Its full system-atic name is tetraoxo-phosphoric(V)acid. It gives rise to three series ofsalts containing phosphate(V) ionsbased on the anions [(HO)2PO2]–,[(HO)PO3]2–, and PO4

3–. These salts areacidic, neutral, and alkaline in char-

acter respectively and phosphate ionsoften feature in buffer systems.There is also a wide range of higheracids and acid anions in which thereis some P–O–P chain formation. Thesimplest of these is pyrophosphoricacid (technically heptaoxodiphos-phoric(V) acid), H4P2O7, produced byheating phosphoric(V) acid (solid) andphosphorus(III) chloride oxide.Metaphosphoric acid is a glassy poly-meric solid (HPO2)x.

phosphorous acid See phosphonicacid.

phosphorus Symbol P. A non-metallic element belonging to*group 15 (formerly VB) of the peri-odic table; a.n. 15; r.a.m. 30.9738; r.d.1.82 (white), 2.34 (red); m.p. 44.1°C(α-white); b.p. 280°C (α-white). It oc-curs in various phosphate rocks,from which it is extracted by heatingwith carbon (coke) and silicon(IV)oxide in an electric furnace (1500°C).Calcium silicate and carbon monox-ide are also produced. Phosphorushas a number of allotropic forms.The α-white form consists of P4 tetra-hedra (there is also a β-white formstable below –77°C). If α-white phos-phorus is dissolved in lead andheated at 500°C a violet form is ob-tained. Red phosphorus, which is acombination of violet and whitephosphorus, is obtained by heatingα-white phosphorus at 250°C with airexcluded. There is also a black al-lotrope, which has a graphite-likestructure, made by heating whitephosphorus at 300°C with a mercurycatalyst. The element is highly reac-tive. It forms metal *phosphides andcovalently bonded phosphorus(III)and phosphorus(V) compounds. Phos-phorus is an *essential element forliving organisms. It is an importantconstituent of tissues (especiallybones and teeth) and of cells, beingrequired for the formation of nucleic

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acids and energy-carrying molecules(e.g. ATP) and also involved in variousmetabolic reactions. The elementwas discovered by Hennig Brand (c. 1630–92) in 1669.


phosphorus(III) bromide (phos-phorus tribromide) A colourless fum-ing liquid, PBr3; r.d. 2.85; m.p. –40°C;b.p. 173°C. It is prepared by passingbromine vapour over phosphorus butavoiding an excess, which would leadto the phosphorus(V) bromide. Likethe other phosphorus(III) halides,PBr3 is pyramidal in the gas phase. Inthe liquid phase the P–Br bonds arelabile; for example, PBr3 will reactwith PCl3 to give a mixture of prod-ucts in which the halogen atomshave been redistributed. Phospho-rus(III) bromide is rapidly hydrolysedby water to give phosphonic acid andhydrogen bromide. It reacts readilywith many organic hydroxyl groupsand is used as a reagent for introduc-ing bromine atoms into organic mol-ecules.

phosphorus(V) bromide (phos-phorus pentabromide) A yellowreadily sublimable solid, PBr5, whichdecomposes below 100°C and is solu-ble in benzene and carbon tetrachlo-ride (tetrachloromethane). It may beprepared by the reaction of phospho-rus(III) bromide with bromine or thedirect reaction of phosphorus withexcess bromine. It is very readily hy-drolysed to give hydrogen bromideand phosphoric(V) acid. An interest-ing feature of this material is that inthe solid state it has the structure[PBr4]+Br–. It is used in organic chem-istry as a brominating agent.

phosphorus(III) chloride (phos-phorus trichloride) A colourless fum-ing liquid, PCl3; r.d. 1.57; m.p.–112°C; b.p. 75.5°C. It is soluble in

ether and in carbon tetrachloride butreacts with water and with ethanol.It may be prepared by passing chlo-rine over excess phosphorus (excesschlorine contaminates the productwith phosphorus(V) chloride). Themolecule is pyramidal in the gasphase and possesses weak electron-pair donor properties. It is hy-drolysed violently by water tophosphonic acid and hydrogen chlo-ride. Phosphorus(III) chloride is animportant starting point for the syn-thesis of a variety of inorganic andorganic derivatives of phosphorus.

phosphorus(V) chloride (phos-phorus pentachloride) A yellow-white rhombic solid, PCl5, whichfumes in air; r.d. 4.65; m.p. 166.8°C(under pressure); sublimes at160–162°C. It is decomposed bywater to give hydrogen chloride andphosphoric(V) acid. It is soluble in or-ganic solvents. The compound maybe prepared by the reaction of chlo-rine with phosphorus(III) chloride.Phosphorus(V) chloride is structurallyinteresting in that in the gas phase ithas the expected trigonal bipyrami-dal form but in the solid phase it con-sists of the ions [PCl4]+[PCl6]–. Thesame ions are detected when phos-phorus(V) chloride is dissolved inpolar solvents. It is used in organicchemisty as a chlorinating agent.

phosphorus(III) chloride oxide(phosphorus oxychloride; phosphorylchloride) A colourless fuming liquid,POCl3; r.d. 1.67; m.p. 2°C; b.p.105.3°C. It may be prepared by thereaction of phosphorus(III) chloridewith oxygen or by the reaction ofphosphorus(V) oxide with phospho-rus(V) chloride. Its reactions are verysimilar to those of phosphorus(III)chloride. Hydrolysis with water givesphosphoric(V) acid. Phosphorus(III)chloride oxide has a distorted tetra-hedral shape and can act as a donor

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towards metal ions, thus giving riseto a series of complexes.

phosphorus cycle The cycling ofphosphorus between the biotic andabiotic components of the environ-ment. Inorganic phosphates (PO4

3–,HPO4

2–, or H2PO4–) are absorbed by

plants from the soil and bodies ofwater and eventually pass into ani-mals through food chains. Within liv-ing organisms phosphates are builtup into nucleic acids and other or-ganic molecules. When plants andanimals die, phosphates are releasedand returned to the abiotic environ-ment through the action of bacteria.On a geological time scale, phos-phates in aquatic environments even-tually become incorporated into andform part of rocks; through a gradualprocess of erosion, these phosphatesare returned to the soil, seas, rivers,and lakes. Phosphorus-containingrocks are mined for the manufactureof fertilizers, which provide an addi-tional supply of inorganic phosphateto the abiotic environment.

phosphorus(III) oxide (phospho-rus trioxide) A white or colourlesswaxy solid, P4O6; r.d. 2.13; m.p.23.8°C; b.p. 173.8°C. It is soluble inether, chloroform, and benzene butreacts with cold water to give phos-phonic acid, H3PO3, and with hotwater to give phosphine and phos-phoric(V) acid. The compound isformed when phosphorus is burnedin an oxygen-deÜcient atmosphere(about 50% yield). As it is difÜcult toseparate from white phosphorus bydistillation, the mixture is irradiatedwith ultraviolet radiation to convertexcess white phosphorus into the redform, after which the oxide can beseparated by dissolution in organicsolvents. Although called a trioxidefor historical reasons, phosphorus(III)oxide consists of P4O6 molecules oftetrahedral symmetry in which each

phosphorus atom is linked to thethree others by an oxygen bridge.The chemistry is very complex.Above 210°C it decomposes into redphosphorus and polymeric oxides. Itreacts with chlorine and bromine togive oxo-halides and with alkalis togive phosphonates (see phosphonicacid).

phosphorus(V) oxide (phosphoruspentoxide; phosphoric anhydride) Awhite powdery and extremely deli-quescent solid, P4O10; r.d. 2.39; m.p.580°C (under pressure); sublimes at300°C. It reacts violently with waterto give phosphoric(V) acid. It is pre-pared by burning elemental phos-phorus in a plentiful supply ofoxygen, then puriÜed by sublimation.The hexagonal crystalline form con-sists of P4O10 molecular units; thesehave the phosphorus atoms arrangedtetrahedrally, each P atom linked tothree others by oxygen bridges andhaving in addition one terminal oxy-gen atom. The compound is used as adrying agent and as a dehydratingagent; for example, amides are con-verted into nitrites and sulphuricacid is converted to sulphur trioxide.

phosphorus oxychloride Seephosphorus(iii) chloride oxide.

phosphorus pentabromide Seephosphorus(v) bromide.

phosphorus pentachloride Seephosphorus(v) chloride.

phosphorus tribromide See phos-phorus(iii) bromide.

phosphorus trichloride See phos-phorus(iii) chloride.

phosphorus trioxide See phos-phorus(iii) oxide.

phosphoryl chloride See phospho-rus(iii) chloride oxide.

photoacoustic spectroscopy Aspectroscopic technique in which

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spectra of opaque materials, such aspowders, are obtained by exposingthe material to light that is modu-lated at acoustic frequencies. Thisproduces a photoacoustic signal, themagnitude of which can be related tothe ultraviolet or infrared absorptioncoefÜcient of the material. See alsooptoacoustic spectroscopy.

photochemical reaction A chem-ical reaction caused by light or ultra-violet radiation. The incidentphotons are absorbed by reactantmolecules to give excited moleculesor free radicals, which undergo fur-ther reaction.

photochemical smog A noxioussmog produced by the reaction of ni-trogen oxides with hydrocarbons inthe presence of ultraviolet light fromthe sun. The reaction is very complexand one of the products is ozone.

photochemistry The branch ofchemistry concerned with *photo-chemical reactions.

photochromism A change of col-our occurring in certain substanceswhen exposed to light. Photochromicmaterials are used in sunglasses thatdarken in bright sunlight.

photoconductive effect See pho-toelectric effect.

photoelectric effect The libera-tion of electrons from a substance ex-posed to electromagnetic radiation.The number of such electrons (pho-toelectrons) emitted depends on theintensity of the radiation. The kineticenergy of the electrons emitted de-pends on the frequency of the radia-tion. The effect is a quantum processin which the radiation is regarded asa stream of *photons, each having anenergy hf, where h is the Planck con-stant and f is the frequency of the ra-diation. A photon can only eject anelectron if the photon energy ex-

ceeds the *work function, φ, of thesolid, i.e. if hf0 = φ an electron will beejected; f0 is the minimum frequency(or threshold frequency) at whichejection will occur. For many solidsthe photoelectric effect occurs at ul-traviolet frequencies or above, butfor some materials (having low workfunctions) it occurs with light. Themaximum kinetic energy, Em, of thephotoelectron is given by *Einstein’sequation: Em = hf – φ.

Apart from the liberation of elec-trons from atoms, other phenomenaare also referred to as photoelectriceffects. These are the photoconduc-tive effect and the photovoltaic ef-fect. In the photoconductive effect,an increase in the electrical conduc-tivity of a semiconductor is caused byradiation as a result of the excitationof additional free charge carriers bythe incident photons. Photoconduc-tive cells, using such photosensitivematerials as cadmium sulphide, arewidely used as radiation detectorsand light switches (e.g. to switch onstreet lighting).

In the photovoltaic effect, an e.m.f.is produced between two layers ofdifferent materials as a result of irra-diation. The effect is made use of inphotovoltaic cells, most of whichconsist of p–n semiconductor junc-tions. When photons are absorbednear a p–n junction new free chargecarriers are produced (as in photo-conductivity); however, in the photo-voltaic effect the electric Üeld in thejunction region causes the newcharge carriers to move, creating aÛow of current in an external circuitwithout the need for a battery.

photoelectron An electron emit-ted from a substance by irradiationas a result of the *photoelectric ef-fect or *photoionization.

photoelectron spectromicros-copy (PESM) A technique for inves-

415 photoelectron spectromicroscopy

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tigating the composition of surfaces,based on ionization of atoms at thesurfaces. Ionizing radiation (such asX-rays or ultraviolet radiation) is usedto eject electrons from atoms in thesurface. The ejected electrons arethen focused, thus enabling an imageof the surface to be constructed.

photoelectron spectroscopy(PES) A technique for determiningthe *ionization potentials of mol-ecules. In ultraviolet photoelectronspectroscopy (UPS) the sample is agas or vapour irradiated with a nar-row beam of ultraviolet radiation(usually from a helium source at 58.4nm, 21.21 eV photon energy). Thephotoelectrons produced in accor-dance with *Einstein’s equation arepassed through a slit into a vacuumregion, where they are deÛected bymagnetic or electrostatic Üelds togive an energy spectrum. The photo-electron spectrum obtained haspeaks corresponding to the ioniza-tion potentials of the molecule (andhence the orbital energies). The tech-nique also gives information on thevibrational energy levels of the ionsformed. X-ray photoelectron spec-troscopy (XPS), also known as ESCA(electron spectroscopy for chemicalanalysis), is a similar analytical tech-nique in which a beam of X-rays isused. In this case, the electronsejected are from the inner shells ofthe atoms. Peaks in the electron spec-trum for a particular element showcharacteristic chemical shifts, whichdepend on the presence of otheratoms in the molecule. See also koop-mans’ theorem.

photoemission The process inwhich electrons are emitted by a sub-stance as a result of irradiation. Seephotoelectric effect; photoioniza-tion.

photography The process of form-ing a permanent record of an image

on specially treated Ülm or paper. Innormal black-and-white photographya camera is used to expose a Ülm orplate to a focused image of the scenefor a speciÜed time. The Ülm or plateis coated with an emulsion contain-ing silver salts and the exposure tolight causes the silver salts to breakdown into silver atoms; where thelight is bright dark areas of silver areformed on the Ülm after develop-ment (by a mild reducing agent) andÜxing. The negative so formed isprinted, either by a contact processor by projection. In either case lightpassing through the negative Ülmfalls on a sheet of paper also coatedwith emulsion. Where the negative isdark, less light passes through andthe resulting positive is light in thisarea, corresponding with a light areain the original scene. As photo-graphic emulsions are sensitive to ul-traviolet and X-rays, they are widelyused in studies involving these formsof electromagnetic radiation. See alsocolour photography.

photoionization The *ionizationof an atom or molecule as a result ofirradiation by electromagnetic radia-tion. For a photoionization to occurthe incident photon of the radiationmust have an energy in excess of the*ionization potential of the speciesbeing irradiated. The ejected photo-electron will have an energy, E, givenby E = hf – I, where h is the Planckconstant, f is the frequency of the in-cident radiation, and I is the ioniza-tion potential of the irradiatedspecies.

photoluminescence See lumines-cence.

photolysis A chemical reactionproduced by exposure to light or ul-traviolet radiation. Photolytic reac-tions often involve free radicals, theÜrst step being homolytic Üssion of achemical bond. (See flash photoly-

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sis.) The photolysis of water, usingenergy from sunlight absorbed bychlorophyll, produces gaseous oxy-gen, electrons, and hydrogen ionsand is a key reaction in *photosyn-thesis.

photon A particle with zero restmass consisting of a *quantum ofelectromagnetic radiation. The pho-ton may also be regarded as a unit ofenergy equal to hf, where h is the*Planck constant and f is the fre-quency of the radiation in hertz. Pho-tons travel at the speed of light. Theyare required to explain the photo-electric effect and other phenomenathat require light to have particlecharacter.

photosensitive substance 1. Anysubstance that when exposed to elec-tromagnetic radiation produces aphotoconductive, photoelectric, orphotovoltaic effect. 2. Any sub-stance, such as the emulsion of aphotographic Ülm, in which electro-

magnetic radiation produces a chem-ical change.

photosynthesis The chemicalprocess by which green plants syn-thesize organic compounds fromcarbon dioxide and water in the pres-ence of sunlight. It occurs in thechloroplasts (most of which are inthe leaves) and there are two princi-pal types of reactions. In the light-dependent reactions, which requirethe presence of light, energy fromsunlight is absorbed by *photosyn-thetic pigments (chieÛy the greenpigment *chlorophyll) and used tobring about the photolysis of water:

H2O → 2H+ + 2e– + ½O2.The electrons released by this reac-tion pass along a series of electroncarriers (see electron transportchain); as they do so they lose theirenergy, which is used to convert ADPto ATP in the process of photophos-phorylation. The electrons and pro-tons produced by the photolysis ofwater are used to reduce NADP:

417 photosynthesis

p

ATP

ADP

electrons

Light-dependent reactions

electrontransportsystem

e– e–

Cyclicphotophos-phorylation

chlorophyll

light

ATP

ADP

oxygen

Photolysisof water

water

Noncyclicphotophos-phorylation

2H

2H+

H2O

2OH–H

2O

2OH

O2

NADPH+H+

ATP

ADP

3CATP

ADP

glucose

12

NADP+

2H

ADP ATP

5C

ribulosebisphosphate

carbondioxide

CO2

glyceraldehyde3-phosphate

Calvincycle

glycerate3-phosphate

Light-independent reactions

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2H+ + 2e– + NADP+ → NADPH + H+.The ATP and NADPH produced dur-ing the light-dependent reactionsprovide energy and reducing power,respectively, for the ensuing light-independent reactions (formerlycalled the ‘dark reaction’), whichnevertheless cannot be sustainedwithout the ATP generated by thelight-dependent reactions. Duringthese reactions carbon dioxide is re-duced to carbohydrate in a metabolicpathway known as the *Calvin cycle.Photosynthesis can be summarizedby the equation:

CO2 + 2H2O → [CH2O] + H2O + O2.Since virtually all other forms of

life are directly or indirectly depend-ent on plants for food, photosynthe-sis is the basis for all life on earth.Furthermore virtually all the atmos-pheric oxygen has originated fromoxygen released during photosynthe-sis.

photosynthetic pigments Theplant pigments responsible for thecapture of light energy during thelight-dependent reactions of *photo-synthesis. The green pigment*chlorophyll is the principal light re-ceptor, absorbing blue and red light.

pH scale A logarithmic scale for ex-pressing the acidity or alkalinity of asolution. To a Ürst approximation,the pH of a solution can be deÜned as–log10c, where c is the concentrationof hydrogen ions in moles per cubicdecimetre. A neutral solution at 25°C

has a hydrogen-ion concentration of10–7 mol dm–3, so the pH is 7. A pHbelow 7 indicates an acid solution;one above 7 indicates an alkaline so-lution. More accurately, the pH de-pends not on the concentration ofhydrogen ions but on their *activity,which cannot be measured experi-mentally. For practical purposes, thepH scale is deÜned by using a hydro-gen electrode in the solution of inter-est as one half of a cell, with areference electrode (e.g. a calomelelectrode) as the other half cell. ThepH is then given by (E – ER)F/2.303RT,where E is the e.m.f. of the cell andER the standard electrode potential ofthe reference electrode, and F theFaraday constant. In practice, a glasselectrode is more convenient than ahydrogen electrode.

pH stands for ‘potential of hydro-gen’. The scale was introduced bySøren Sørensen (1868–1939) in 1909.

phthalic acid A colourless crystalline dicarboxylic acid,C6H4(COOH)2; r.d. 1.6; m.p. 207°C.The two –COOH groups are substi-tuted on adjacent carbon atoms ofthe ring, the technical name beingbenzene-1,2-dicarboxylic acid. Theacid is made from phthalic anhydride(benzene-1,2-dicarboxylic anhydride,C8H4O3), which is made by the cata-lytic oxidation of naphthalene. Theanhydride is used in making plasti-cizers and polyester resins.

phthalic anhydride See phthalicacid.

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phthalic ester An ester of phthalicacid. Phthalic esters are made by re-acting phthalic anhydride with an al-cohol in the presence of sulphuricacid. They are high boiling-point liq-uids used as plasticizers and in themanufacture of polymers and alkydresins. The dimethyl ester is used asan insect repellent.

phthalocyanine A synthetic com-pound having molecules with fourisoindole rings linked by –N= bridges.The structure is similar to that of the*porphyrins. It can form complexeswith central metal ions. Copperphthalocyanines are used as dyes.

physical chemistry The branch ofchemistry concerned with the effectof chemical structure on physicalproperties. It includes chemical ther-modynamics and electrochemistry.

physisorption See adsorption.

phytostigmine See eserine.

pi-adduct An adduct that is formed

by donation of an electron pair be-tween a pi orbital and a sigma or piorbital.

piano stool See sandwich com-pound.

pi bond See orbital.

pico- Symbol p. A preÜx used in themetric system to denote 10–12. For ex-ample, 10–12 farad = 1 picofarad (pF).

picrate A salt or ester of picric acid.

picric acid (2,4,6-trinitrophenol) Ayellow highly explosive nitro com-pound, C6H2(NO2)3; r.d. 1.8; m.p.122°C.

pi electron An electron in a pi or-bital. See orbital.

pig iron The impure form of ironproduced by a blast furnace, which iscast into pigs (blocks) for convertingat a later date into cast iron, steel,etc. The composition depends on theores used, the smelting procedure,and the use to which the pigs willlater be put.

pinacol rearrangement A reac-tion in which the diol pinacol, (CH3)2COH(CH3)2COH, converts to the ke-tone pinacolone, CH3COC(CH3)3,under acid conditions, with loss of amolecule of water. A methyl groupmoves from one carbon atom to anadjacent one in order to stabilize an intermediate carbocation. Thereaction, also called the pinacol-pinacolone rearrangement, gives itsname to a class of similar rearrange-ments.

pi orbital See orbital.

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piperidine A saturated heterocycliccompound having a nitrogen atom ina six-membered ring, C5H11N; r.d.0.86; m.p. –7°C; b.p. 106°C. The struc-ture is present in many alkaloids

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CH2

CH2

Piperidine

pipette A graduated tube used fortransferring measured volumes ofliquid or, sometimes, gases.

Pirani gauge An instrument usedto measure low pressures (1–10–4

torr; 100–0.01 Pa). It consists of anelectrically heated Ülament, which isexposed to the gas whose pressure isto be measured. The extent to whichheat is conducted away from the Üla-ment depends on the gas pressure,which thus controls its equilibriumtemperature. Since the resistance ofthe Ülament is dependent on its tem-perature, the pressure is related tothe resistance of the Ülament. TheÜlament is arranged to be part of aWheatstone bridge circuit and thepressure is read from a microamme-ter calibrated in pressure units. Asthe effect depends on the thermalconductivity of the gas, the calibra-tion has to be made each time thepressure of a different gas is meas-ured.

pirsonnite A mineral consisting of a hydrated mixed carbonate ofsodium and calcium, Na2CO3.CaCO3.2H2O.

pitch A black or dark-brown residueresulting from the distillation of coaltar, wood tar, or petroleum (bitu-men). The term is also sometimesused for the naturally occurring pe-

troleum residue (asphalt). Pitch isused as a binding agent (e.g. in roadtars), for waterprooÜng (e.g. inrooÜng felts), and as a fuel.

pitchblende See uraninite.

pK value A measure of thestrength of an acid on a logarithmicscale. The pK value is given bylog10(1/Ka), where Ka is the acid disso-ciation constant. pK values are oftenused to compare the strengths of dif-ferent acids.

Planck, Max Karl Ernst Ludwig(1858–1947) German physicist, whobecame a professor at Berlin Univer-sity in 1892. Here he formulated the*quantum theory, which had itsbasis in a paper of 1900. One of themost important scientiÜc discoveriesof the century, this work earned himthe 1918 Nobel Prize for physics.

Planck constant Symbol h. Thefundamental constant equal to theratio of the energy of a quantum ofenergy to its frequency. It has thevalue 6.626 0755(40) × 10–34 J s. It isnamed after Max Planck. In quan-tum-mechanical calculations the ra-tionalized Planck constant (or Diracconstant) = h/2π = 1.054 589 ×10–34 Js is frequently used.

plane-polarized light See polar-ization of light.

plaster of Paris The hemihydrateof *calcium sulphate, 2CaSO4.H2O,prepared by heating the mineral gyp-sum. When ground to a Üne powderand mixed with water, plaster ofParis sets hard, forming interlockingcrystals of gypsum. The setting re-sults in an increase in volume and sothe plaster Üts tightly into a mould. Itis used in pottery making, as a castfor setting broken bones, and as aconstituent of the plaster used in thebuilding industry.

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synthetic resin to make it Ûexible. Seeplastics.

plastics Materials that can beshaped by applying heat or pressure.Most plastics are made from poly-meric synthetic *resins, although afew are based on natural substances(e.g. cellulose derivatives or shellac).They fall into two main classes. Ther-moplastic materials can be repeatedlysoftened by heating and hardenedagain on cooling. Thermosetting ma-terials are initially soft, but change ir-reversibly to a hard rigid form onheating. Plastics contain the syn-thetic resin mixed with such addi-tives as pigments, plasticizers (toimprove Ûexibility), antioxidants andother stabilizers, and Üllers. SeeChronology overleaf.

plastocyanin A blue copper-containing protein that is found in chloroplasts and acts as an elec-tron carrier molecule in the light-dependent reactions of *photo-synthesis. Plastocyanin consists ofamino acid groups in associationwith a copper molecule which givesthis compound a blue colour.

plastoquinone A quinone, foundin chloroplasts, that functions as one of the carrier molecules of the electron transport chain in thelight-dependent reactions of *photo-synthesis.

platinum Symbol Pt. A silverywhite metallic *transition element(see also platinum metals); a.n. 78;r.a.m. 195.09; r.d. 21.45; m.p. 1772°C;b.p. 3827±100°C. It occurs in somenickel and copper ores and is alsofound native in some deposits. Themain source is the anode sludge ob-tained in copper–nickel reÜning. Theelement is used in jewellery, labora-tory apparatus (e.g. thermocouples,electrodes, etc.), electrical contacts,and in certain alloys (e.g. with irid-

ium or rhodium). It is also a hy-drogenation catalyst. The elementdoes not oxidize nor dissolve inhydrochloric acid. Most of its com-pounds are platinum(II) orplatinum(IV) complexes.


platinum black Black Ünely di-vided platinum metal produced byvacuum evaporation and used as anabsorbent and a catalyst. See alsoadams catalyst.

platinum metals The three mem-bers of the second and third transi-tion series immediately proceedingsilver and gold: ruthenium (Ru),rhodium (Rh), and palladium (Pd);and osmium (Os), iridium (Ir), andplatinum (Pt). These elements, to-gether with iron, cobalt, and nickel,were formerly classed as group VIIIof the periodic table. The platinum-group metals are relatively hard andresistant to corrosion and are used injewellery and in some industrial ap-plications (e.g. electrical contacts).They have certain chemical similari-ties that justify classifying them to-gether. All are resistant to chemicalattack. In solution they form a vastrange of complex ions. They alsoform coordination compounds withcarbon monoxide and other pi-bonding ligands. A number of com-plexes can be made in which a hy-drogen atom is linked directly to themetal. The metals and their organiccompounds have considerable cata-lytic activity. See also transition el-ements.

Platonic hydrocarbons Hydrocar-bons that have the same moleculargeometry as the Üve Platonic solids,i.e. the tetrahedron, the cube, the oc-tahedron, the dodecahedron and theicosahedron. These hydrocarbonshave carbon atoms at the vertices of

421 Platonic hydrocarbons

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422

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PLASTICS

1851 Scottish chemist Charles Macintosh (1766–1843) makes ebonite (fromrubber).

1855 British chemist Alexander Parkes (1813–90) patents Parkesine, a plasticmade from nitrocellulose, methanol, and wood pulp; it is later called ‘celluloid’.

1860 British chemist Charles Williams (1829–1910) prepares isoprene (syntheticrubber).

1868 US printer John Hyatt (1837–1920) develops commercial process for makingcelluloid.

1884 French chemist Hilaire de Chardonnet (1839–1924) develops process formaking rayon.

1892 British chemists Edward Bevan (1856–1921) and Charles Cross (1855–1935)develop the viscose process for making rayon.

1899 British chemist Frederick Kipping (1863–1949) discovers silicone plastics.

1901 German chemists Krische and Spitteler make formaldehyde–casein plastic(Galalith).

1905 Belgian-born US chemist Leo Baekland (1863–1944) invents Bakelite.

1912 Swiss chemist Jacques Brandenberger produces Cellophane (viscose cellulosefilm).

1913 US Formica Insulation company markets plastic laminate made fromformaldehyde resins.

1918 Hans John prepares urea–formaldehyde resin.

1926 German chemist Hermann Staudinger (1881–1965) discovers the polymericnature of plastics.

1930 US chemist Waldo Semon develops PVC (polyvinyl chloride).

1930 Canadian chemist William Chalmers discovers polymethylmethacrylate(Perspex and Plexiglass).

1930 German chemists at IG Farbenindustrie produce polystyrene.

1931 Wallace Carothers invents nylon.

1938 US chemist Roy Plunkett produces polytetrafluoroethene (PTFE).

1939 British company ICI develops commercial process for making polyethene.

1941 British chemists John Whinfield (1901–66) and J. Dickson develop Terylene(Dacron).

1941 German company IG Farbenindustrie produces polyurethane.

1943 US Dow Corning company produces silicone plastics.

1947 British chemists produce acrylic fibres.

1953 German chemist Karl Ziegler (1896–1973) discovers catalyst for makinghigh-density polyethene.

1954 Italian chemist Giulio Natta (1903–79) develops industrial process formaking high-density polyethene (using Ziegler catalyst).

1989 Italian company Ferruzzi produces biodegradable plastic (based on starch).


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the polyhedra and single bonds alongthe edges. In fact, there are only twoknown Platonic hydrocarbons:cubane (C8H8) and dodecahedrane(C20H20). The hydrocarbon based on atetrahedron (C4H4) does not exist be-cause of angle strain, but substitutedderivatives C4X4 are known. Anglestrain is also the reason for thenonexistence of octahedrane (C6H6)and icosahedrane (C12H12).

pleochroic Denoting a crystal thatappears to be of different colours, de-pending on the direction from whichit is viewed. It is caused by polariza-tion of light as it passes through ananisotropic medium.

plumbago See carbon.

plumbane (lead(IV) hydride) An ex-tremely unstable gas, PbH4, said to beformed by the action of acids onmagnesium–lead alloys. It was Ürstreported in 1924, although doubtshave since been expressed about the existence of the compound. Itdemonstrates the declining stabilityof the hydrides in group 14. More sta-ble organic derivatives are known;e.g. trimethyl plumbane, (CH3)3PbH.

plumbate A compound formed byreaction of lead oxides (or hydrox-ides) with alkali. The oxides of leadare amphoteric (weakly acidic) andreact to give plumbate ions. With thelead(IV) oxide, reaction with moltenalkali gives the plumbate(IV) ion

PbO2 + 2OH– → PbO32– + H2O

In fact, various ions are present inwhich the lead is bound to hydroxidegroups, the principal one being thehexahydroxoplumbate(IV) ionPb(OH)62–. This is the negative ionpresent in crystalline ‘trihydrates’ ofthe type K2PbO3.3H2O. Lead(II) oxidegives the trihydroxoplumbate(II) ionin alkaline solutions

PbO(s) + OH–(aq) + H2O(l) →Pb(OH)32–(aq)

Plumbate(IV) compounds were for-merly referred to as orthoplumbates(PbO4

4–) or metaplumbates (PbO32–).

Plumbate(II) compounds were calledplumbites.

plumbic compounds Compoundsof lead in its higher (+4) oxidationstate; e.g. plumbic oxide is lead(IV)oxide, PbO2.

plumbite See plumbate.

plumbous compounds Com-pounds of lead in its lower (+2) oxida-tion state; e.g. plumbous oxide islead(II) oxide, PbO.

plutonium Symbol Pu. A dense sil-very radioactive metallic transuranicelement belonging to the *actinoids;a.n. 94; mass number of most stableisotope 244 (half-life 7.6 × 107 years);r.d. 19.84; m.p. 641°C; b.p. 3232°C.Thirteen isotopes are known, by farthe most important being pluto-nium–239 (half-life 2.44 × 104 years),which undergoes nuclear Üssion withslow neutrons and is therefore a vitalpower source for nuclear weaponsand some nuclear reactors. About 20tonnes of plutonium are producedannually by the world’s nuclear reac-tors. The element was Ürst producedby Seaborg, McMillan, Kennedy, andWahl in 1940.A• Information from the WebElements site

PM See particulate matter.

pnicogens See pnictogens.

pnictides See pnictogens.

pnictogens (pnicogens) The el-ements of group 15 of the periodictable, i.e. nitrogen (N), arsenic (As),antimony (Sb), and bismuth (Bi). Theelements As, Sb, and Bi form a rangeof solid-state ternary compoundswith interesting electrical and mag-

423 pnictogens

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netic properties. Examples are LaSiP3,K6Bi2Sn23, and Yb14MnSb11. Com-pounds of this type are known aspnictides.

Pockels cell An electro-optical de-vice used to produce population in-version in lasers and a pulse ofradiation. In a Pockels cell, crystals ofammonium dihydrogenphosphateare used to convert plane-polarizedlight to circularly polarized lightwhen a potential difference is ap-plied. A Pockels cell can be madepart of a laser cavity. When this hap-pens, a change of polarization occurswhen light is reÛected from a mirrorand the light that is polarized in oneplane is converted into light polar-ized in the perpendicular plane. Theeffect is that the reÛected light doesnot stimulate emission. When thePockels cell is turned off the polariza-tion does not occur and the energy inthe cavity can be released in theform of a pulse of stimulated radia-tion. See also laser.

point group A group of symmetryelements that leave a point un-changed, used in classifying mol-ecules. Compare space group.

poise A *c.g.s. unit of viscosityequal to the tangential force in dynesper square centimetre required tomaintain a difference in velocity ofone centimetre per second betweentwo parallel planes of a Ûuid sepa-rated by one centimetre. 1 poise isequal to 10–1 N s m–2.

poison 1. Any substance that is in-jurious to the health of a living or-ganism. 2. A substance that preventsthe activity of a catalyst. 3. A sub-stance that absorbs neutrons in a nu-clear reactor and therefore slowsdown the reaction. It may be addedintentionally for this purpose or maybe formed as a Üssion product andneed to be periodically removed.

polar compound A compoundthat is either ionic (e.g. sodium chlo-ride) or that has molecules with alarge permanent dipole moment (e.g.water).

polarimeter (polariscope) An in-strument used to determine theangle through which the plane of po-larization of plane-polarized light isrotated on passing through an opti-cally active substance. Essentially, apolarimeter consists of a light source,a polarizer (e.g. a sheet of Polaroid)for producing plane-polarized light, atransparent cell containing the sam-ple, and an analyser. The analyser isa polarizing material that can be ro-tated. Light from the source is plane-polarized by the polarizer and passesthrough the sample, then throughthe analyser into the eye or onto alight-detector. The angle of polariza-tion is determined by rotating theanalyser until the maximum trans-mission of light occurs. The angle ofrotation is read off a scale. Simpleportable polarimeters are used for es-timating the concentrations of sugarsolutions in confectionary manufac-ture.

polariscope (polarimeter) A deviceused to study optically active sub-stances (see optical activity). Thesimplest type of instrument consistsof a light source, collimator, polar-izer, and analyser. The specimen is placed between polarizer andanalyser, so that any rotation of theplane of polarization of the light canbe assessed by turning the analyser.

polarizability Symbol α. A meas-ure of the response of a molecule toan external electric Üeld. When amolecule is placed in an externalelectric Üeld, the displacement ofelectric charge induces a dipole inthe molecule. If the electric Üeldstrength is denoted E and the electri-cal dipole moment induced by this

Pockels cell 424

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electric Üeld p, the polarizability α isdeÜned by p = αE. To calculate thepolarizability from Ürst principles itis necessary to use the quantum me-chanics of molecules. However, ifregarded as a parameter, the polariz-ability α provides a link betweenmicroscopic and macroscopic theo-ries as in the *Clausius–Mossottiequation and the *Lorentz–Lorenzequation.

polarization 1. See polarizationof light. 2. The formation of prod-ucts of the chemical reaction in a*voltaic cell in the vicinity of theelectrodes resulting in increased re-sistance to current Ûow and, fre-quently, to a reduction in the e.m.f.of the cell. See also depolarization.3. The partial separation of electriccharges in an insulator subjected toan electric Üeld. 4. The separation ofcharge in a polar *chemical bond.

polarization of light The processof conÜning the vibrations of theelectric vector of light waves to onedirection. In unpolarized light theelectric Üeld vibrates in all directionsperpendicular to the direction ofpropagation. After reÛection or trans-mission through certain substances(see polaroid) the electric Üeld isconÜned to one direction and the ra-diation is said to be plane-polarizedlight. The plane of plane-polarizedlight can be rotated when it passesthrough certain substances (see opti-cal activity).

In circularly polarized light, the tipof the electric vector describes a cir-cular helix about the direction ofpropagation with a frequency equalto the frequency of the light. Themagnitude of the vector remains con-stant. In elliptically polarized light,the vector also rotates about the di-rection of propagation but the ampli-tude changes; a projection of thevector on a plane at right angles to

the direction of propagation de-scribes an ellipse. Circularly and el-liptically polarized light are producedusing a retardation plate.

polarizer A device used to plane-polarize light (see polarization oflight). *Nicol prisms or *Polaroidcan be used as polarizers. If a polar-izer is placed in front of a source ofunpolarized light, the transmittedlight is plane-polarized in a speciÜcdirection. As the human eye is un-able to detect that light is polarized,it is necessary to use an *analyser todetect the direction of polarization.Crossing a polarizer and analysercauses extinction of the light, i.e. ifthe plane of polarization of the polar-izer and the plane of the analyser areperpendicular, no light is transmittedwhen the polarizer and analyser arecombined. Both a polarizer and ananalyser are components of a *po-larimeter.

polar molecule A molecule thathas a dipole moment; i.e. one inwhich there is some separation ofcharge in the *chemical bonds, sothat one part of the molecule has apositive charge and the other a nega-tive charge.

polarography An analytical tech-nique having an electrochemicalbasis. A dropping-mercury electrodeis used as the cathode along with alarge nonpolarizable anode, and a di-lute solution of the sample. The drop-ping-mercury electrode consists of anarrow tube through which mercuryis slowly passed into the solution soas to form small drops at the end ofthe tube, which fall away. In this waythe cathode can have a low surfacearea and be kept clean. A variable po-tential is applied to the cell and aplot of current against potential (apolarogram) made. As each chemicalspecies is reduced at the cathode (inorder of their electrode potentials) a

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step-wise increase in current is ob-tained. The height of each step is pro-portional to the concentration of thecomponent. The technique is usefulfor detecting trace amounts of metalsand for the investigation of solvatedcomplexes.

Polaroid A doubly refracting ma-terial that plane-polarizes unpolar-ized light passed through it. Itconsists of a plastic sheet strained ina manner that makes it birefringentby aligning its molecules. Sunglassesincorporating a Polaroid material ab-sorb light that is vibrating horizon-tally – produced by reÛection fromhorizontal surfaces – and thus reduceglare.

polar solvent See solvent.

pollutant Any substance, producedand released into the environment asa result of human activities, that hasdamaging effects on living organ-isms. Pollutants may be toxic sub-stances (e.g. *pesticides) or naturalconstituents of the atmosphere (e.g.carbon dioxide) that are present inexcessive amounts. See pollution.

pollution An undesirable change inthe physical, chemical, or biologicalcharacteristics of the natural environ-ment, brought about by man’s activi-ties. It may be harmful to human ornonhuman life. Pollution may affectthe soil, rivers, seas, or the atmos-phere. There are two main classes of *pollutants: those that are bio-degradable (e.g. sewage), i.e. can be rendered harmless by naturalprocesses and need therefore causeno permanent harm if adequatelydispersed or treated; and those thatare nonbiodegradable (e.g. *heavymetals (such as lead) in industrialefÛuents and *DDT and other chlori-nated hydrocarbons used as pesti-cides), which eventually accumulatein the environment and may be con-

centrated in food chains. Other formsof pollution in the environment in-clude noise (e.g. from jet aircraft,trafÜc, and industrial processes) andthermal pollution (e.g. the release ofexcessive waste heat into lakes orrivers causing harm to wildlife). Re-cent pollution problems include thedisposal of radioactive waste; *acidrain; *photochemical smog; increas-ing levels of human waste; high lev-els of carbon dioxide and othergreenhouse gases in the atmosphere(see greenhouse effect); damage tothe *ozone layer by nitrogen oxides,*chloroÛuorocarbons (CFCs), and*halons; and pollution of inland wa-ters by agricultural *fertilizers andsewage efÛuent, causing eutrophica-tion. Attempts to contain or preventpollution include strict regulationsconcerning factory emissions, theuse of smokeless fuels, the banningof certain pesticides, the increasinguse of lead-free petrol, restrictions onthe use of chloroÛuorocarbons, andthe introduction, in some countries,of catalytic converters to cut pollu-tants in car exhausts.

polonium Symbol Po. A rare radio-active metallic element of group 16(formerly VIB) of the periodic table;a.n. 84; r.a.m. 210; r.d. 9.32; m.p.254°C; b.p. 962°C. The element oc-curs in uranium ores to an extent ofabout 100 micrograms per 1000 kilo-grams. It has over 30 isotopes, morethan any other element. The longest-lived isotope is polonium–209 (half-life 103 years). Polonium hasattracted attention as a possible heatsource for spacecraft as the energyreleased as it decays is 1.4 × 105

J kg–1 s–1. It was discovered by MarieCurie in 1898 in a sample of pitch-blende.A• Information from the WebElements site

poly- PreÜx indicating a polymer,

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e.g. polyethene. Sometimes bracketsare used in polymer names to in-dicate the repeated unit, e.g.poly(ethene).

polyacrylamide A polymerformed from 2-propenamide(CH2:CHCONH2). Polyacrylamide gelsare made using a cross-linking agentto form three-dimensional matrices.They are used in gel electrophoresis(see page).

polyacrylamide gel electro-phoresis See page.

polyamide A type of condensationpolymer produced by the interactionof an amino group of one moleculeand a carboxylic acid group of an-other molecule to give a protein-likestructure. The polyamide chains arelinked together by hydrogen bond-ing.

polyatomic molecule A moleculeformed from several atoms (e.g. pyri-dine, C5H5N, or dinitrogen tetroxide,N2O4), as distinguished from di-atomic and monatomic molecules.

polybasic acid An acid with morethan one replaceable hydrogen atom.Examples include the dibasic sul-phuric acid (H2SO4) and tribasic phos-phoric(V) (orthophosphoric) acid(H3PO4). Replacement of all the hy-drogens by metal atoms forms nor-mal salts. If not all the hydrogens arereplaced, *acid salts are formed.

polychlorinated biphenyl (PCB)Any of a number of derivatives ofbiphenyl (C6H5C6H5) in which someof the hydrogen atoms on the ben-zene rings have been replaced bychlorine atoms. Polychlorinatedbiphenyls are used in the manufac-ture of certain polymers as electricalinsulators. They are highly toxic andare suspected to be carcinogenic;their increasing use has caused con-

cern because they have been shownto accumulate in the food chain.

polychloroethene (PVC; polyvinylchloride) A tough white solid ma-terial, which softens with the appli-cation of a plasticizer, manufacturedfrom chloroethene by heating in aninert solvent using benzoyl peroxideas an initiator, or by the free-radicalmechanism initiated by heatingchloroethene in water with potas-sium persulphate or hydrogen perox-ide. The polymer is used in a varietyof ways, being easy to colour and re-sistant to Üre, chemicals, andweather.

polycyclic Denoting a compoundthat has two or more rings in its mol-ecules. Polycyclic compounds maycontain single rings (as in phenylben-zene, C6H5.C6H5) or fused rings (as innaphthalene, C10H8).

polydioxoboric(III) acid See boricacid.

polyene An unsaturated hydrocar-bon that contains two or more dou-ble carbon–carbon bonds in itsmolecule.

polyester A condensation polymerformed by the interaction of polyhy-dric alcohols and polybasic acids.Linear polyesters are saturated ther-moplastics and linked by dipole–dipole attraction as the carbonylgroups are polarized. They are exten-sively used as Übres (e.g. Terylene).Unsaturated polyesters readilycopolymerize to give thermosettingproducts. They are used in the manu-facture of glass-Übre products. See alsoalkyd resin.

polyethene (polyethylene; poly-thene) A Ûexible waxy translucentpolyalkene thermoplastic made in avariety of ways producing a polymerof varying characteristics. In the ICIprocess, ethene containing a trace of

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oxygen is subjected to a pressure inexcess of 1500 atmospheres and atemperature of 200°C. Low-densitypolyethene (r.d. 0.92) has a formulaweight between 50 000 and 300 000,softening at a temperature around110°C, while the high-density poly-thene (r.d. 0.945–0.96) has a formulaweight up to 3 000 000, softeningaround 130°C. The low-density poly-mer is less crystalline, being more at-actic. Polyethene is used as aninsulator; it is acid resistant and iseasily moulded and blown. Seephillips process; ziegler process.

polyethylene See polyethene.

polyhydric alcohol (polyol) An*alcohol that has several hydroxylgroups per molecule.

polymer A substance having largemolecules consisting of repeatedunits (the monomers). See Featurepp. 430–431.


polymerization A chemical reac-tion in which molecules join to-gether to form a polymer. If thereaction is an addition reaction, theprocess is addition polymerization;condensation reactions cause con-densation polymerization, in which asmall molecule is eliminated duringthe reaction. Polymers consisting of asingle monomer are homopolymers;those formed from two differentmonomers are copolymers.

polymethanal A solid polymer ofmethanal, formed by evaporation ofan aqueous solution of methanal.

polymethylmethacrylate A clearthermoplastic acrylic material madeby polymerizing methyl meth-acrylate. The technical name ispoly(methyl 2-methylpropenoate). Itis used in such materials as Perspex.

polymorphism The existence ofchemical substances in two (dimor-phism) or more physical forms. Seeallotropy.

polyol See polyhydric alcohol.

polypeptide A *peptide compris-ing ten or more amino acids.Polypeptides that constitute proteinsusually contain 100–300 amino acids.Shorter ones include certain antibiot-ics, e.g. gramicidin, and some hor-mones, e.g. ACTH, which has 39amino acids. The properties of apolypeptide are determined by thetype and sequence of its constituentamino acids.

polypropene (polypropylene) Anisotactic polymer existing in bothlow and high formula-weight forms.The lower-formula-weight polymer ismade by passing propene at moder-ate pressure over a heated phos-phoric acid catalyst spread on aninert material at 200°C. The reactionyields the trimer and tetramer. Thehigher-formula-weight polymer isproduced by passing propene into aninert solvent, heptane, which con-tains a trialkyl aluminium and a tita-nium compound. The product is amixture of isotactic and atacticpolypropene, the former being themajor constituent. Polypropene isused as a thermoplastic mouldingmaterial.

polypropylene See polypropene.

polysaccharide Any of a group ofcarbohydrates comprising longchains of monosaccharide (simple-sugar) molecules. Homopolysaccha-rides consist of only one type ofmonosaccharide; heteropolysaccha-rides contain two or more differenttypes. Polysaccharides may have mo-lecular weights of up to several mil-lion and are often highly branched.Some important examples are starch,glycogen, and cellulose.

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polystyrene A clear glasslike ma-terial manufactured by free-radicalpolymerization of phenylethene(styrene) using benzoyl peroxide asan initiator. It is used as both a ther-mal and electrical insulator and forpacking and decorative purposes.

polysulphides See sulphides.

polytetraÛuoroethene (PTFE) Athermosetting plastic with a highsoftening point (327°C) prepared by the polymerization of tetra-Ûuoroethene under pressure (45–50atmospheres). The reaction requiresan initiator, ammonium peroxosul-phate. The polymer has a low coefÜ-cient of friction and its ‘anti-stick’properties are probably due to its he-lical structure with the Ûuorineatoms on the surface of an inner ringof carbon atoms. It is used for coatingcooking utensils and nonlubricatedbearings.

polythene See polyethene.

polythionate A salt of a poly-thionic acid.

polythionic acids Oxo acids of sul-phur with the general formulaHO.SO2.Sn.SO2.OH, where n = 0–4. Seealso sulphuric acid.

polyurethane A polymer contain-ing the urethane group –NH.CO.O–,prepared by reacting di-isocyanateswith appropriate diols or triols. Awide range of polyurethanes can bemade, and they are used in adhe-sives, durable paints and varnishes,plastics, and rubbers. Addition ofwater to the polyurethane plasticsturns them into foams.

polyvinylacetate (PVA) A thermo-plastic polymer used in adhesivesand coatings. It is made by polymer-izing vinyl acetate (CH2:CHCOOCH3).

polyvinyl alcohol A water-solublepolymer made from polyvinylacetate

(PVA) by hydrolysis with sodium hy-droxide. It is used for making adhe-sives that are miscible with water, asa size for textiles, and for makingsynthetic Übres.

polyvinyl chloride See poly-chloroethene.

polyyne An unsaturated hydrocar-bon that contains two or more triplecarbon–carbon bonds in its molecule.

POP See persistent organic pollu-tant.

population inversion See laser.

porphyrin Any of a group of or-ganic pigments characterized by thepossession of a cyclic group of fourlinked nitrogen-containing rings (atetrapyrrole nucleus), the nitrogenatoms of which are often coordinatedto metal ions. Porphyrins differ inthe nature of their side-chain groups.They include the *chlorophylls,which contain magnesium; and*haem, which contains iron andforms the *prosthetic group ofhaemoglobin, myoglobin, and the cy-tochromes. Porphyrins are also usedin host–guest chemistry. Seesupramolecular chemistry.

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NH N

N NH

Porphyrin

positronium See exotic atom.

potash Any of a number of potas-sium compounds, such as the carbon-ate or the hydroxide.


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POLYMERS

Polymers are substances that have *macromolecules composed of many repeatingunits (known as ‘mers’). A large number of naturally occurring substances arepolymers including *rubber and many substances based on glucose, such as thepolysaccharides *cellulose and *starch (in plants) and *glycogen (in animals).*Proteins, nucleic acids, and inorganic macromolecular substances, such as*silicates, are other examples.

Synthetic polymersOne of the unique features of the chemistry of carbon is its ability to form longchains of atoms. This property is the basis of an important area of industrialchemistry concerned with the manufacture of polymeric materials with a varietyof properties (see plastics). The molecules in these materials are essentially longchains of atoms of various lengths. In some polymers, cross-linkage occursbetween the chains. Synthetic polymers are formed by chemical reactions inwhich individual molecules (monomers) join together to form larger units (seepolymerization). Two types of polymer, homopolymers and heteropolymers, canbe distinguished

HomopolymersThese are polymers formed from a single monomer. An example is *polyethene(polyethylene), which is made by polymerization of ethene (CH2:CH2). Typicallysuch polymers are formed by *addition reactions involving unsaturatedmolecules. Other similar examples are *polypropene (polypropylene),polystyrene, and *polytetrafluoroethene (PTFE). Homopolymers may also be madeby *condensation reactions (as in the case of *polyurethane).

H H

C C

H H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

Addition polymerization of ethene to form polyethene: a homopolymer

H2N (CH

2)6

NH2

O

C

HO

(CH2)4

O

C

OH

+

1,6-diaminohexane hexanedioic acid

H

N (CH2)6

(CH2)4

(CH2)6

n H2O+N C C N N

H O O H H

Condensation polymerization to form nylon: a heteropolymer

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HeteropolymersThese are also known as copolymers. They are made from two (or more)different monomers, which usually undergo a condensation reaction with theelimination of a simple molecule, such as water. A typical example is thecondensation of 1,6-diaminohexane (hexamethylenediamine) with hexanedioicacid (adipic acid) to form nylon 6,6. The reaction occurs between the aminegroups on the diaminohexane and the carboxyl groups on the hexanedioicacid, with elimination of water molecules (see diagram opposite).

The properties of a polymeric plastic can most easily be modified if it is acopolymer of two or more different monomers. A well-known example is ABS(acrylonitrile–butadiene–styrene) copolymer, commonly used for the bodyshells of computers and other electronic apparatus. Its properties can bepreselected by varying the proportions of the component monomers.

Stereospecific polymersIn both normal polyethene and nylon the polymer molecules take the form oflong chains of various lengths with no regular arrangement of the subunits.Such polymers are said to be atactic. If the constituent subunits repeat alongthe chain in a regular way, a stereospecific polymer may result. The polymermay be isotactic, with a particular group always along the same side of themain chain, or syndiotactic, with the group alternating from side to side of thechain. Stereospecific polymerization can be performed by use of certaincatalytic agents (see ziegler process).

alternating A B A B A B A B

random A A B A B B B A

block A A B B B B A A

graft A A A A A A A A A

B B B

B B B

Types of copolymer depending on the arrangement of the monomers A and B

isotactic syndiotactic

H H H

H H H

H H H

X X X

H H H

H H H

H

H

H

X

X

X

X

H

Types of stereospecific polymer

Structure of polymers


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potash alum See aluminium potas-sium sulphate; alums.

potash mica See muscovite.

potassamide See potassiummonoxide.

potassium Symbol K. A soft silverymetallic element belonging to group1 (formerly IA) of the periodic table(see alkali metals); a.n. 19; r.a.m.39.098; r.d. 0.86; m.p. 63.7°C; b.p.774°C. The element occurs in seawa-ter and in a number of minerals,such as sylvite (KCl), carnallite(KCl.MgCl2.6H2O), and kainite(MgSO4.KCl.3H2O). It is obtained byelectrolysis. The metal has few usesbut potassium salts are used for awide range of applications. Potas-sium is an *essential element for liv-ing organisms. The potassium ion,K+, is the most abundant cation inplant tissues, being absorbed throughthe roots and being used in suchprocesses as protein synthesis. In ani-mals the passage of potassium andsodium ions across the nerve-cellmembrane is responsible for thechanges of electrical potential thataccompany the transmission of im-pulses. Chemically, it is highly reac-tive, resembling sodium in itsbehaviour and compounds. It alsoforms an orange-coloured superox-ide, KO2, which contains the O2

– ion.Potassium was discovered by SirHumphry *Davy in 1807.A• Information from the WebElements site

potassium–argon dating A *dat-ing technique for certain rocks thatdepends on the decay of the radioiso-tope potassium–40 to argon–40, aprocess with a half-life of about 1.27× 1010 years. It assumes that all theargon–40 formed in the potassium-bearing mineral accumulates withinit and that all the argon present isformed by the decay of potas-

sium–40. The mass of argon–40 andpotassium–40 in the sample is esti-mated and the sample is then datedfrom the equation:

40Ar = 0.1102 40K(eλt – 1),where λ is the decay constant and t isthe time in years since the mineralcooled to about 300°C, when the 40Arbecame trapped in the crystal lattice.The method is effective for micas,feldspar, and some other minerals.

potassium bicarbonate Seepotassium hydrogencarbonate.

potassium bichromate See potas-sium dichromate.

potassium bromide A white orcolourless crystalline solid, KBr,slightly hygroscopic and soluble inwater and very slightly soluble inethanol; cubic; r.d. 2.75; m.p. 734°C;b.p. 1435°C. Potassium bromide maybe prepared by the action of bromineon hot potassium hydroxide solutionor by the action of iron(III) bromideor hydrogen bromide on potassiumcarbonate solution. It is used widelyin the photographic industry and isalso used as a sedative. Because of itsrange of transparency to infrared ra-diation, KBr is used both as a matrixfor solid samples and as a prism ma-terial in infrared spectroscopy.

potassium carbonate (pearl ash;potash) A translucent (granular) orwhite (powder) deliquescent solidknown in the anhydrous and hy-drated forms. K2CO3 (monoclinic; r.d.2.4; m.p. 891°C) decomposes withoutboiling. 2K2CO3.3H2O (monoclinic;r.d. 2.04) dehydrates to K2CO3.H2Oabove 100°C and to K2CO3 above130°C. It is prepared by the Engel–Precht process in which potas-sium chloride and magnesium oxide react with carbon dioxide togive the compound Engel’s salt,MgCO3.KHCO3.4H2O. This is decom-posed in solution to give the hydro-

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gencarbonate, which can then be cal-cined to K2CO3. Potassium carbonateis soluble in water (insoluble in alco-hol) with signiÜcant hydrolysis toproduce basic solutions. Industrialuses include glasses and glazes, themanufacture of soft soaps, and indyeing and wool Ünishing. It is usedin the laboratory as a drying agent.

potassium chlorate A colourlesscrystalline compound, KClO3, whichis soluble in water and moderatelysoluble in ethanol; monoclinic; r.d.2.32; m.p. 356°C; decomposes above400°C giving off oxygen. The indus-trial route to potassium chlorate in-volves the fractional crystallization ofa solution of potassium chloride andsodium chlorate but it may also beprepared by electrolysis of hot con-centrated solutions of potassiumchloride. It is a powerful oxidizingagent Ünding applications in weed-killers and disinfectants and, becauseof its ability to produce oxygen, it isused in explosives, pyrotechnics, andmatches.

potassium chloride A white crys-talline solid, KCl, which is soluble inwater and very slightly soluble inethanol; cubic; r.d. 1.98; m.p. 772°C;sublimes at 1500°C. Potassium chlo-ride occurs naturally as the mineralsylvite (KCl) and as carnallite(KCl.MgCl2.6H2O); it is produced in-dustrially by fractional crystallizationof these deposits or of solutions fromlake brines. It has the interestingproperty of being more soluble thansodium chloride in hot water but lesssoluble in cold. It is used as a fertil-izer, in photography, and as a sourceof other potassium salts, such as thechlorate and the hydroxide. It haslow toxicity.

potassium chromate A bright yel-low crystalline solid, K2CrO4, solublein water and insoluble in alcohol;rhombic; r.d. 2.73; m.p. 968.3°C; de-

composes without boiling. It is pro-duced industrially by roasting pow-dered chromite ore with potassiumhydroxide and limestone and leach-ing the resulting cinder with hotpotassium sulphate solution. Potas-sium chromate is used in leatherÜnishing, as a textile mordant, and inenamels and pigments. In the labora-tory it is used as an analyticalreagent and as an indicator. Likeother chromium(III) compounds it istoxic when ingested or inhaled.

potassium chromium sulphate(chrome alum) A violet or ruby-redcrystalline solid, K2SO4.Cr2(SO4)3.24H2O, that is soluble in water andinsoluble in ethanol; cubic or octahe-dral; r.d. 1.826; m.p. 89°C; loses10H2O at 100°C, 12H2O at 400°C. Sixwater molecules surround each ofthe chromium(III) ions and the re-maining ones are hydrogen bondedto the sulphate ions. Like all alums,the compound may be prepared bymixing equimolar quantities of theconstituent sulphates. See alums.

potassium cyanide (cyanide) Awhite crystalline or granular deli-quescent solid, KCN, soluble in waterand in ethanol and having a faintcharacteristic odour of almonds (dueto hydrolysis forming hydrogencyanide at the surface); cubic; r.d.1.52; m.p. 634°C. It is prepared indus-trially by the absorption of hydrogencyanide in potassium hydroxide. Thecompound is used in the extractionof silver and gold, in some metal-Ünishing processes and electroplat-ing, as an insecticide and fumigant(source of HCN), and in the prepara-tion of cyanogen derivatives. In thelaboratory it is used in analysis, as areducing agent, and as a stabilizing*ligand for low oxidation states. Thesalt itself is highly toxic and aqueoussolutions of potassium cyanide arestrongly hydrolysed to give rise to

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the slow release of equally toxic hy-drogen cyanide gas.

potassium dichromate (potas-sium bichromate) An orange-redcrystalline solid, K2Cr2O7, soluble inwater and insoluble in alcohol; mon-oclinic or triclinic; r.d. 2.68; mono-clinic changes to triclinic at 241.6°C;m.p. 396°C; decomposes above500°C. It is prepared by acidiÜcationof crude potassium chromate solu-tion (the addition of a base to solu-tions of potassium dichromatereverses this process). The compoundis used industrially as an oxidizingagent in the chemical industry and indyestuffs manufacture, in electroplat-ing, pyrotechnics, glass manufacture,glues, tanning, photography and lith-ography, and in ceramic products.Laboratory uses include applicationas an analytical reagent and as an ox-idizng agent. Potassium dichromateis toxic and considered a Üre risk onaccount of its oxidizing properties.

potassium dioxide See potassiumsuperoxide.

potassium hydride A white orgreyish white crystalline solid, KH;r.d. 1.43–1.47. It is prepared by pass-ing hydrogen over heated potassiumand marketed as a light grey powderdispersed in oil. The solid decom-poses on heating and in contact withmoisture and is an excellent reduc-ing agent. Potassium hydride is a Ürehazard because it produces hydrogenon reaction with water.

potassium hydrogencarbonate(potassium bicarbonate) A whitecrystalline solid, KHCO3, soluble inwater and insoluble in ethanol; r.d.2.17; decomposes about 120°C. It oc-curs naturally as calcinite and is pre-pared by passing carbon dioxide intosaturated potassium carbonate solu-tion. It is used in baking, soft-drinksmanufacture, and in CO2 Üre extin-

guishers. Because of its buffering ca-pacity, it is added to some detergentsand also used as a laboratory reagent.

potassium hydrogentartrate(cream of tartar) A white crystallineacid salt, HOOC(CHOH)2COOK. It isobtained from deposits on wine vats(argol) and used in baking powders.

potassium hydroxide (causticpotash; lye) A white deliquescentsolid, KOH, often sold as pellets,Ûakes, or sticks, soluble in water andin ethanol and very slightly solublein ether; rhombic; r.d. 2.044; m.p.360.4°C; b.p. 1320°C. It is preparedindustrially by the electrolysis of con-centrated potassium chloride solu-tion but it can also be made byheating potassium carbonate or sul-phate with slaked lime, Ca(OH)2. Itclosely resembles sodium hydroxidebut is more soluble and is thereforepreferred as an absorber for carbondioxide and sulphur dioxide. It is alsoused in the manufacture of soft soap,other potassium salts, and in Ni–Feand alkaline storage cells. Potassiumhydroxide is extremely corrosive tobody tissues and especially damagingto the eyes.

potassium iodate A white crys-talline solid, KIO3, soluble in waterand insoluble in ethanol; monoclinic;r.d. 3.9; m.p. 560°C. It may be pre-pared by the reaction of iodine withhot concentrated potassium hydrox-ide or by careful electrolysis ofpotassium iodide solution. It is anoxidizing agent and is used as ananalytical reagent. Some potassiumiodate is used as a food additive.

potassium iodide A white crys-talline solid, KI, with a strong bittertaste, soluble in water, ethanol, andacetone; cubic; r.d. 3.13; m.p. 681°C;b.p. 1330°C. It may be prepared bythe reaction of iodine with hot potas-sium hydroxide solution followed by

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separation from the iodate (which isalso formed) by fractional crystalliza-tion. In solution it has the interestingproperty of dissolving iodine to formthe triiodide ion I3–, which is brown.Potassium iodide is widely used as ananalytical reagent, in photography,and also as an additive to table salt toprevent goitre and other disordersdue to iodine deÜciency.

potassium manganate(VII)(potassium permanganate) A com-pound, KMnO4, forming purple crys-tals with a metallic sheen, soluble inwater (intense purple solution), ace-tone, and methanol, but decomposedby ethanol; r.d. 2.70; decompositionbegins slightly above 100°C and iscomplete at 240°C. The compound isprepared by fusing manganese(IV)oxide with potassium hydroxide toform the manganate and elec-trolysing the manganate solutionusing iron electrodes at about 60°C.An alternative route employs produc-tion of sodium manganate by a simi-lar fusion process, oxidation withchlorine and sulphuric acid, thentreatment with potassium chloride tocrystallize the required product.

Potassium manganate(VII) is widelyused as an oxidizing agent and as adisinfectant in a variety of applica-tions, and as an analytical reagent.

potassium monoxide A grey crys-talline solid, K2O; cubic; r.d. 2.32; de-composition occurs at 350°C. It maybe prepared by the oxidation ofpotassium metal with potassium ni-trate. It reacts with ethanol to formpotassium ethoxide (KOC2H5), andwith liquid ammonia to form potas-sium hydroxide and potassamide(KNH2).

potassium nitrate (saltpetre) Acolourless rhombohedral or trigonalsolid, KNO3, soluble in water, insolu-ble in alcohol; r.d. 2.109; transition totrigonal form at 129°C; m.p. 334°C;

decomposes at 400°C. It occurs natu-rally as nitre and may be prepared bythe reaction of sodium nitrate withpotassium chloride followed by frac-tional crystallization. It is a powerfuloxidizing agent (releases oxygen onheating) and is used in gunpowderand fertilizers.

potassium nitrite A white orslightly yellow deliquescent solid,KNO2, soluble in water and insolublein ethanol; r.d. 1.91; m.p. 440°C; mayexplode at 600°C. Potassium nitrite isprepared by the reduction of potas-sium nitrate. It reacts with cold di-lute mineral acids to give nitrousacid and is also able to behave as areducing agent (if oxidized to the ni-trate) or as an oxidizing agent (if re-duced to nitrogen). It is used inorganic synthesis because of its partin diazotization, and in detecting thepresence of the amino groups in or-ganic compounds.

potassium permanganate Seepotassium manganate(vii).

potassium sulphate A white crys-talline powder, K2SO4, soluble inwater and insoluble in ethanol;rhombic or hexagonal; r.d. 2.66; m.p.1069°C. It occurs naturally asschönite (Strassfurt deposits) and inlake brines, from which it is sepa-rated by fractional crystallization. Ithas also been produced by the Har-greaves process, which involves theoxidation of potassium chloride withsulphuric acid. In the laboratory itmay be obtained by the reaction ofeither potassium hydroxide or potas-sium carbonate with sulphuric acid.Potassium sulphate is used in ce-ments, in glass manufacture, as afood additive, and as a fertilizer(source of K+) for chloride-sensitiveplants, such as tobacco and citrus.

potassium sulphide A yellow-redor brown-red deliquescent solid, K2S,

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which is soluble in water and inethanol but insoluble in diethylether; cubic; r.d. 1.80; m.p. 840°C. Itis made industrially by reducingpotassium sulphate with carbon athigh temperatures in the absence ofair. In the laboratory it may be pre-pared by the reaction of hydrogensulphide with potassium hydroxide.The pentahydrate is obtained on crys-tallization. Solutions are strongly al-kaline due to hydrolysis. It is used asan analytical reagent and as a depila-tory. Potassium sulphide is generallyregarded as a hazardous chemicalwith a Üre risk; dusts of K2S havebeen known to explode.

potassium sulphite A white crys-talline solid, K2SO3, soluble in waterand very sparingly soluble inethanol; r.d. 1.51; decomposes onheating. It is a reducing agent and isused as such in photography and inthe food and brewing industries,where it prevents oxidation.

potassium superoxide (potassiumdioxide) A yellow paramagneticsolid, KO2, produced by burningpotassium in an excess of oxygen; itis very soluble (by reaction) in water,soluble in ethanol, and slightly solu-ble in diethyl ether; m.p. 380°C.When treated with cold water or di-lute mineral acids, hydrogen perox-ide is obtained. The compound is apowerful oxidizing agent and onstrong heating releases oxygen withthe formation of the monoxide, K2O.

potential barrier A region con-taining a maximum of potential thatprevents a particle on one side of itfrom passing to the other side. Ac-cording to classical theory a particlemust possess energy in excess of theheight of the potential barrier to passit. However, in quantum theory thereis a Ünite probability that a particlewith less energy will pass throughthe barrier (see tunnel effect). A po-

tential barrier surrounds the atomicnucleus and is important in nuclearphysics; a similar but much lowerbarrier exists at the interface be-tween semiconductors and metalsand between differently doped semi-conductors. These barriers are impor-tant in the design of electronicdevices.

potential energy See energy.

potential-energy curve A graphof the potential energy of electronsin a diatomic molecule, in which thepotential energy of an electronicstate is plotted vertically and the in-teratomic distance is plotted horizon-tally, with the minimum of the curvebeing the average internuclear dis-tance. The *Morse potential gives agood analytic description of a poten-tial-energy curve. The dissociationenergy and certain quantities of in-terest in vibrational spectroscopy canbe found from the potential-energycurve.

potential-energy surface A mul-tidimensional surface used in thetheory of electronic states of poly-atomic molecules and chemical reac-tions. A *potential-energy curve isthe simplest type of potential-energysurface. Each internuclear distance inthe molecule or reacting system isrepresented by one dimension, as isthe potential energy. This means thatif a nonlinear molecule has N atomsits potential-energy surface is a(3N – 6)-dimensional surface in a(3N – 5)-dimensional space. In thecase of a linear molecule the poten-tial-energy surface is a (3N – 5)-dimen-sional space. If the surface has aminimum then that electronic stateis stable. It is possible for there to bemore than one minimum. In poten-tial-energy surfaces for chemical re-actions the reactants and productsare frequently ‘valleys’ connected bya region of higher energy, which is

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associated with the *activation en-ergy of the reaction.

potentiometric titration A titra-tion in which the end point is foundby measuring the potential on anelectrode immersed in the reactionmixture.

Pourbaix diagram A diagramshowing how oxidation–reductionbehaviour for compounds of a givenelement depends on pH.

powder metallurgy A process inwhich powdered metals or alloys arepressed into a variety of shapes athigh temperatures. The processstarted with the pressing of pow-dered tungsten into incandescentlamp Ülaments in the Ürst decade ofthis century and is now widely usedfor making self-lubricating bearingsand cemented tungsten carbide cut-ting tools.

The powders are produced by at-omization of molten metals, chemi-cal decomposition of a compound ofthe metal, or crushing and grindingof the metal or alloy. The parts arepressed into moulds at pressuresranging from 140 × 106 Pa to 830 ×106 Pa after which they are heated ina controlled atmosphere to bond theparticles together (see sintering).

praseodymium Symbol Pr. A softsilvery metallic element belonging tothe *lanthanoids; a.n. 59; r.a.m.140.91; r.d. 6.773; m.p. 931°C; b.p.3512°C. It occurs in bastnasite andmonazite, from which it is recoveredby an ion-exchange process. The onlynaturally occurring isotope ispraseodymium–141, which is notradioactive; however, fourteen ra-dioisotopes have been produced. It isused in mischmetal, a rare-earthalloy containing 5% praseodymium,for use in lighter Ûints. Another rare-earth mixture containing 30%praseodymium is used as a catalyst in

cracking crude oil. The element wasdiscovered by C. A. von Welsbach in1885.


precessional motion A form ofmotion that occurs when a torque isapplied to a rotating body in such away that it tends to change the direc-tion of its axis of rotation. It arisesbecause the resultant of the angularvelocity of rotation and the incre-ment of angular velocity produced bythe torque is an angular velocityabout a new direction; this com-monly changes the axis of the ap-plied torque and leads to sustainedrotation of the original axis of rota-tion.

A spinning top, the axis of which isnot exactly vertical, has a torque act-ing on it as a result of gravity. Insteadof falling over, the top precessesabout a vertical line through thepivot. Precessional effects occur inatoms in magnetic Üelds.

precipitate A suspension of smallsolid particles produced in a liquid bychemical reaction.

precipitation 1. All liquid andsolid forms of water that are de-posited from the atmosphere; it in-cludes rain, drizzle, snow, hail, dew,and hoar frost. 2. The formation of aprecipitate.

precursor A compound that leadsto another compound in a series ofchemical reactions.

pressure The force acting normallyon unit area of a surface or the ratioof force to area. It is measured in*pascals in SI units. Absolute pres-sure is pressure measured on a gaugethat reads zero at zero pressurerather than at atmospheric pressure.Gauge pressure is measured on a

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gauge that reads zero at atmosphericpressure.

pressure gauge Any device usedto measure *pressure. Three basictypes are in use: the liquid-columngauge (e.g. the mercury barometerand the manometer), the expanding-element gauge (e.g. the Bourdongauge and the aneroid barometer),and the electrical transducer. In thelast category the strain gauge is anexample. Capacitor pressure gaugesalso come into this category. In thesedevices, the pressure to be measureddisplaces one plate of a capacitor andthus alters its capacitance.

presumptive test A simple testfor a given substance using a reagentthat changes colour when mixedwith the substance under investiga-tion. Presumptive tests are not deÜni-tive and further conÜrmatory testsare always required. They are usedextensively in forensic science. Ex-amples are the Duquenois–Levinetest for marijuana and Scott’s test forcocaine. In general analytical chem-istry, presumptive tests are oftencalled spot tests.

Priestley, Joseph (1733–1804)British chemist, who in 1755 becamea Presbyterian minister. In Leeds, in1767, he experimented with carbondioxide (‘Üxed air’) from a nearbybrewery; with it he invented sodawater. He moved to a ministry inBirmingham in 1780, and in 1791 hisrevolutionary views caused a mob toburn his house, as a result of whichhe emigrated to the USA in 1794. Inthe early 1770s he experimentedwith combustion and produced thegases hydrogen chloride, sulphurdioxide, and dinitrogen oxide (ni-trous oxide). In 1774 he isolated oxy-gen (see also lavoisier, antoine).

primary alcohol See alcohols.

primary amine See amines.

primary cell A *voltaic cell inwhich the chemical reaction produc-ing the e.m.f. is not satisfactorily re-versible and the cell cannot thereforebe recharged by the application of acharging current. See daniell cell;leclanché cell; weston cell; mer-cury cell. Compare secondary cell.

principal quantum number Seeatom.

prismane A saturated hydrocarbon,C6H6, in which the six carbon atomsare arranged at the corners of a trian-gular prism. The structure was sug-gested in 1869 by Albert Ladenburgas a possible structure for benzene(since referred to as Ladenburg ben-zene). The actual compound was syn-thesized by T.J. Katz and N. Acton in1973. Hexamethylprismane, in whichthe carbon atoms are linked tomethyl groups rather than hydrogenatoms, was synthesized in 1966.

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prochiral Denoting an organic mol-ecule lacking a chiral centre but con-taining one or more carbon atomsattached to two identical ligands andtwo other different ligands (Caabc).The carbon atom is said to be aprochiral centre. The name prochiralis used because if a is replaced by aligand d, different from either a, b, orc, there is a chiral centre in the mol-ecule Cabcd.

producer gas (air gas) A mixtureof carbon monoxide and nitrogenmade by passing air over very hotcarbon. Usually some steam is added


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to the air and the mixture containshydrogen. The gas is used as a fuel insome industrial processes.

product See chemical reaction.

progesterone A hormone, pro-duced primarily by the corpus lu-teum of the ovary but also by theplacenta, that prepares the inner lin-ing of the uterus for implantation ofa fertilized egg cell. If implantationfails, the corpus luteum degeneratesand progesterone production ceasesaccordingly. If implantation occurs,the corpus luteum continues to se-crete progesterone, under the in-Ûuence of luteinizing hormone andprolactin, for several months of preg-nancy, by which time the placentahas taken over this function. Duringpregnancy, progesterone maintainsthe constitution of the uterus andprevents further release of eggs fromthe ovary. Small amounts of proges-terone are produced by the testes. Seealso progestogen.

progestogen One of a group ofnaturally occurring or synthetic hor-mones that maintain the normalcourse of pregnancy. The best knownis *progesterone. In high dosesprogestogens inhibit secretion ofluteinizing hormone, thereby pre-venting ovulation, and alter the con-sistency of mucus in the vagina sothat conception tends not to occur.They are therefore used as majorconstituents of oral contraceptives.

projection formula A conven-tion for representing the three-dimensional structure of a moleculein two dimensions. See fischer pro-jection; newman projection;sawhorse projection.

prolactin (lactogenic hormone; lu-teotrophic hormone; luteotrophin) Ahormone produced by the anteriorpituitary gland. In mammals it stimu-lates the mammary glands to pro-

duce milk and the corpus luteum ofthe ovary to secrete the hormone*progesterone.

proline See amino acid.

promethium Symbol Pm. A soft sil-very metallic element belonging tothe *lanthanoids; a.n. 61; r.a.m. 145;r.d. 7.26 (20°C); m.p. 1080°C; b.p.2460°C. The only naturally occurringisotope, promethium–147, has a half-life of only 2.52 years. Eighteen otherradioisotopes have been produced,but they have very short half-lives.The only known source of the el-ement is nuclear-waste material.Promethium–147 is of interest as abeta-decay power source but thepromethium–146 and –148, whichemit penetrating gamma radiation,must Ürst be removed. It was discov-ered by J. A. Marinsky, L. E. Glen-denin, and C. D. Coryell in 1947.


promoter A substance added to acatalyst to increase its activity.

proof A measure of the amount ofalcohol (ethanol) in drinks. Proofspirit contains 49.28% ethanol byweight (about 57% by volume). De-grees of proof express the percentageof proof spirit present, so 70° proofspirit contains 0.7 × 57% alcohol.

1,2-propadiene (allene) A colour-less gas, CH2CCH2; r.d. 1.79; m.p.–136°C; b.p. –34.5°C. Propadiene maybe prepared from 1,3-dibromo-propane (CH2BrCHCH2Br) by the ac-tion of zinc dust. See also allenes.

propanal (propionaldehyde) Acolourless liquid *aldehyde,C2H5CHO; m.p. –81°C; b.p. 48.8°C.

propane A colourless gaseous hy-drocarbon, C3H8; m.p. –190°C; b.p.–42°C. It is the third member of the*alkane series and is obtained from

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petroleum. Its main use is as bottledgas for fuel.

propanedioic acid (malonic acid)A white crystalline dicarboxylic acid,HOOCCH2COOH; m.p. 132°C. It de-composes above its melting point toethanoic acid. Propanedioic acid isused in the synthesis of other dicar-boxylic acids.

propanoic acid (propionic acid) Acolourless liquid *carboxylic acid,CH3CH2COOH; r.d. 0.99; m.p.–20.8°C; b.p. 141°C. It is used tomake calcium propanate – an addi-tive in bread.

propanol Either of two *alcoholswith the formula C3H7OH. Propan-1-ol is CH3CH2CH2OH and propan-2-olis CH3CH(OH)CH3. Both are colour-less volatile liquids. Propan-2-ol isused in making propanone (acetone).

propanone (acetone) A colourlessÛammable volatile compound,CH3COCH3; r.d. 0.79; m.p. –95.4°C;b.p. 56.2°C. The simplest *ketone,propanone is miscible with water. Itis made by oxidation of propan-2-ol(see propanol) or is obtained as a by-product in the manufacture of phe-nol from cumene; it is used as asolvent and as a raw material formaking plastics.

propellant 1. A substance thatburns rapidly in a controlled way,used to propel a projectile (e.g. froma gun). In Ürearms, gunpowder andcordite are common examples. 2. Afuel used in a rocket engine. Usuallythe propellant is a fuel and an oxi-dizer; for example kerosene or liquidhydrogen with a liquid oxygen pro-pellant. 3. A substance used to pro-duce the spray in an aerosol can.Aerosol propellants are volatile sub-stances that can be liqueÜed underpressure and are able to dissolve theworking substance. When the pres-sure is released, the liquid vaporizes

producing the spray. FormerlychloroÛuorocarbons were used buttheir use has been discontinued be-cause of their effect on the ozonelayer. Most aerosol cans use liquidhydrocarbon mixtures as the propel-lant.

propenal (acrolein) A colourlesspungent liquid unsaturated aldehyde,CH2:CHCHO; r.d. 0.84; m.p. –87°C;b.p. 53°C. It is made from propeneand is used in producing polyesterand polyurethane resins.

propene (propylene) A colourlessgaseous hydrocarbon, CH3CH:CH2;m.p. –185.25°C; b.p. –47.4°C. It is an*alkene obtained from petroleum bycracking alkanes. Its main use is inthe manufacture of polypropene.

propenoate (acrylate) A salt orester of *propenoic acid.

propenoic acid (acrylic acid) Anunsaturated liquid *carboxylic acid,CH2:CHCOOH; m.p. 13°C; b.p.141.6°C. It readily polymerizes and itis used in the manufacture of*acrylic resins.

propenol (allyl alcohol) A pungent-smelling colourless unsaturated liq-uid alcohol, CH2=CHCH2OH; b.p.97°C. It is found in wood alcohol andcan be prepared in the laboratory byreducing propenal or by heating glyc-erol with oxalic acid. Industrially, itis made by the high-temperature cat-alytic reaction between propenal and2-propanol or by the hydrolysis of 2-chloropropene, which is made bychlorinating propene. Potassium per-manganate oxidizes propenol to glyc-erol and silver oxide converts it to amixture of propenoic acid and prope-nal.

propenonitrile (acrylonitrile; vinylcyanide) A colourless liquid,H2C:CHCN; r.d. 0.81; m.p. –83.5°C. Itis an unsaturated nitrile, made from

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propene and used to make acrylicresins.

propenyl group (allyl group) Theorganic group H2C=CHCH2–.

propionaldehyde See propanal.

propylene See propene.

propyl group The organic groupCH3CH2CH2–.

prostaglandin Any of a group oforganic compounds derived from*essential fatty acids and causing arange of physiological effects in ani-mals. Prostaglandins have been de-tected in most body tissues. They actat very low concentrations to causethe contraction of smooth muscle;natural and synthetic prostaglandinsare used to induce abortion or labourin humans and domestic animals.Two prostaglandin derivatives haveantagonistic effects on blood circula-tion: thromboxane A2 causes bloodclotting while prostacyclin causesblood vessels to dilate. InÛammationin allergic reactions and other dis-eases is also thought to involveprostaglandins.

prosthetic group A tightly boundnonpeptide inorganic or organiccomponent of a protein. Prostheticgroups may be lipids, carbohydrates,metal ions, phosphate groups, etc.Some *coenzymes are more correctlyregarded as prosthetic groups.

protactinium Symbol Pa. A radio-active metallic element belonging tothe *actinoids; a.n. 91; r.a.m.231.036; r.d. 15.37 (calculated); m.p.<1600°C (estimated). The most stableisotope, protactinium–231, has ahalf-life of 3.43 × 104 years; at leastten other radioisotopes are known.Protactinium–231 occurs in all ura-nium ores as it is derived from ura-nium–235. Protactinium has nopractical applications; it was discov-

ered by Lise Meitner and Otto Hahnin 1917.A• Information from the WebElements site

protamine Any of a group of pro-teins of relatively low molecularweight found in association with thechromosomal *DNA of vertebratesperm cells. They contain a singlepolypeptide chain comprising about67% arginine. Protamines are thoughtto protect and support the chromo-somes.

protease (peptidase; proteinase;proteolytic enzyme) Any enzymethat catalyses the hydrolysis of pro-teins into smaller *peptide fractionsand amino acids, a process known asproteolysis. Examples are *pepsinand *trypsin. Several proteases, act-ing sequentially, are normally re-quired for the complete digestion ofa protein to its constituent aminoacids.

protecting group A group used toprotect a certain functional group ina chemical synthesis. For example, a hydroxyl group (–OH) can beconverted into an acetyl group(–OOCCH3) to protect it taking part ina certain step of the synthesis. In thiscase, the acetyl is the protectinggroup. Later it can easily be changedback into the original hydroxylgroup.

protein Any of a large group of or-ganic compounds found in all livingorganisms. Proteins comprise carbon,hydrogen, oxygen, and nitrogen andmost also contain sulphur; molecularweights range from 6000 to severalmillion. Protein molecules consist ofone or several long chains (*polypep-tides) of *amino acids linked in acharacteristic sequence. This se-quence is called the primary structureof the protein. These polypeptidesmay undergo coiling or pleating, the

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nature and extent of which is de-scribed as the secondary structure.The three-dimensional shape of thecoiled or pleated polypeptides iscalled the tertiary structure. Quater-nary structure speciÜes the structuralrelationship of the componentpolypeptides.

Proteins may be broadly classiÜedinto globular proteins and Übrousproteins. Globular proteins havecompact rounded molecules and areusually water-soluble. Of prime im-portance are the *enzymes, proteinsthat catalyse biochemical reactions.Other globular proteins include theantibodies, which combine with for-eign substances in the body; the car-rier proteins, such as *haemoglobin;the storage proteins (e.g. casein inmilk and albumin in egg white), andcertain hormones (e.g. insulin). Fi-brous proteins are generally insolu-ble in water and consist of longcoiled strands or Ûat sheets, whichconfer strength and elasticity. In thiscategory are keratin and collagen.Actin and myosin are the principalÜbrous proteins of muscle, the inter-action of which brings about musclecontraction. Blood clotting involvesthe Übrous protein called Übrin.

When heated over 50°C or sub-jected to strong acids or alkalis, pro-teins lose their speciÜc tertiarystructure and may form insoluble co-agulates (e.g. egg white). This usuallyinactivates their biological proper-ties.

protein engineering The tech-niques used to alter the structure ofproteins (especially enzymes) inorder to improve their use to hu-mans. This involves artiÜcially modi-fying the DNA sequences that encodethem so that, for example, newamino acids are inserted into existingproteins. Synthesized lengths ofnovel DNA can be used to produce

new proteins by cells or other sys-tems containing the necessary fac-tors for transcription and translation.Alternatively, new proteins can besynthesized by solid state synthesis,in which polypeptide chains are as-sembled under the control of chemi-cals. One end of the chain isanchored to a solid support and thechemicals selectively determinewhich amino acids are added to thefree end. The appropriate chemicalscan be renewed during the process;when synthesized, the polypeptide isremoved and puriÜed. Protein engi-neering is used to synthesize en-zymes (so-called ‘designer enzymes’)used in biotechnology. The three-dimensional tertiary structure of pro-teins is crucially important for theirfunction, and this can be investigatedusing computer-aided modelling.

protein synthesis The process bywhich living cells manufacture pro-teins from their constituent aminoacids, in accordance with the geneticinformation carried in the DNA ofthe chromosomes. This informationis encoded in messenger *RNA,which is transcribed from DNA inthe nucleus of the cell: the sequenceof amino acids in a particular proteinis determined by the sequence of nu-cleotides in messenger RNA. At theribosomes the information carried bymessenger RNA is translated into thesequence of amino acids of the pro-tein in the process of translation.

proteolysis The enzymic splittingof proteins. See protease.

proteolytic enzyme See protease.

proton An elementary particle thatis stable, bears a positive chargeequal in magnitude to that of the*electron, and has a mass of1.672 614 × 10–27 kg, which is1836.12 times that of the electron.The proton is a hydrogen ion and oc-

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curs in all atomic nuclei. See also hy-dron.

protonic acid An *acid that formspositive hydrogen ions (or, strictly,oxonium ions) in aqueous solution.The term is used to distinguish ‘tradi-tional’ acids from Lewis acids or fromLowry–Brønsted acids in nonaqueoussolvents.

proton number See atomic num-ber.

Prout’s hypothesis The hypothe-sis put forward by the Britishchemist William Prout (1785–1850)in 1815 that all atomic weights areinteger multiples of the atomicweight of hydrogen and hence thatall atoms are made out of hydrogen.Subsequent work on atomic weightsin the 19th century showed that thishypothesis is incorrect (with chlorinehaving an atomic weight of 35.5being a glaring example of this). Theunderstanding of atomic structurethat emerged in the 20th century,with atomic number being the num-ber of protons in an atom and non-integer atomic weights being due tomixtures of isotopes, has vindicatedthe spirit of Prout’s hypothesis.

A• William Prout’s original paper

prussic acid See hydrogencyanide.

pseudoaromatic (antiaromatic) Acompound that has a ring of atomscontaining alternating double andsingle bonds, yet does not have thecharacteristic properties of *aromaticcompounds. Such compounds do notobey the Hückel rule. Cyclooctate-traene (C8H8), for instance, has a ringof eight carbon atoms with conju-gated double bonds, but the ring isnot planar and the compound actslike an alkene, undergoing additionreactions.

pseudo-axial See ring conforma-tions.

pseudoephedrine See ephedrine.

pseudo-equatorial See ring con-formations.

pseudohalogens A group of com-pounds, including cyanogen (CN)2and thiocyanogen (SCN)2, that havesome resemblance to the halogens.Thus, they form hydrogen acids(HCN and HSCN) and ionic salts con-taining such ions as CN– and SCN–.

pseudo order An order of a chemi-cal reaction that appears to be lessthan the true order because of theexperimental conditions used.Pseudo orders occur when one reac-tant is present in large excess. For ex-ample, a reaction of substance Aundergoing hydrolysis may appear tobe proportional only to [A] becausethe amount of water present is solarge.

psilocin See psilocybin.

psilocybin A hallucinogen, similarin effect to psi*mescaline, found incertain species of mushroom. It is ac-companied by a related, more active,compound psilocin. Both are clas-siÜed as class A drugs in the UK. Theycan be detected by the Marquis testor the Froehde test.

PTFE See polytetrafluoroethene.

ptyalin An enzyme that digests car-bohydrates (see amylase). It is presentin mammalian saliva and is responsi-ble for the initial stages of starch di-gestion.

p-type semiconductor See semi-conductor.

pullulan A water-soluble polysac-charide composed of glucose unitsthat are polymerized in such a wayas to make it viscous and imperme-able to oxygen. Pullulan is used in

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adhesives, food packaging, andmoulded articles. It is derived fromthe fungus Aureobasidium pullulans.

pumice A porous volcanic rock thatis light and full of cavities due to ex-panding gases that were liberatedfrom solution in the lava while it so-lidiÜed. Pumice is often light enoughto Ûoat on water. It is usually acid(siliceous) in composition, and is usedas an abrasive and for polishing.

pump A device that imparts energyto a Ûuid in order to move it fromone place or level to another or toraise its pressure (compare vacuumpump). Centrifugal pumps and tur-bines have rotating impellers, whichincrease the velocity of the Ûuid, partof the energy so acquired by the Ûuidthen being converted to pressure en-ergy. Displacement pumps act di-rectly on the Ûuid, forcing it to Ûowagainst a pressure. They include pis-ton, plunger, gear, screw, and campumps.

purine An organic nitrogenous base(see formula), sparingly soluble inwater, that gives rise to a group ofbiologically important derivatives,notably *adenine and *guanine,which occur in nucleotides and nu-cleic acids (DNA and RNA).

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CH2

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N

N

Pyrazine

NNH

Pyrazole

N

Pyridine

PVA See polyvinylacetate.

PVC See polychloroethene.

pyran A heterocyclic compoundhaving an oxygen atom and two dou-ble bonds in a six-membered ring,C5H6O. There are two isomers de-pending on the position of the CH2

group.

pyridine A colourless liquid with astrong unpleasant smell, C5H5N (seeformula); r.d. 0.98; m.p. –42°C; b.p.115°C. Pyridine is an aromatic hete-rocyclic compound present in coal

pyrazole An unsaturated hetero-cyclic compound having two nitro-gen atoms in a Üve-membered ring,C3H4N2; m.p. 66–70°C; b.p. 168–188°C. Pyrazine is a symmetricdiazine.

pyranose A *sugar having a six-membered ring containing Üve car-bon atoms and one oxygen atom.

pyrazine An unsaturated hetero-cyclic compound having two nitro-gen atoms in a six-membered ring,C4H4N2; r.d. 1.03; m.p. 52°C; b.p.115°C. Pyrazine is a symmetric di-azine.


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tar. It is used in making other or-ganic chemicals.

pyridoxine See vitamin b complex.

pyrimidine An organic nitrog-enous base (see formula), sparinglysoluble in water, that gives rise to agroup of biologically important deriv-atives, notably *uracil, *thymine,and *cytosine, which occur in *nu-cleotides and nucleic acids (DNA andRNA).

445 pyrometry

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Pyrimidine

pyrolusite See manganese(iv)oxide.

pyrolysis Chemical decompositionoccurring as a result of high tempera-ture.

pyrometric cones See segercones.

pyrometry The measurement ofhigh temperatures from the amountof radiation emitted, using a pyrom-eter. Modern narrow-band or spectralpyrometers use infrared-sensitivephotoelectric cells behind Ülters thatexclude visible light. In the opticalpyrometer (or disappearing Ülamentpyrometer) the image of the incan-descent source is focused in theplane of a tungsten Ülament that isheated electrically. A variable resistoris used to adjust the current throughthe Ülament until it blends into theimage of the source, when viewedthrough a red Ülter and an eyepiece.The temperature is then read from acalibrated ammeter or a calibrateddial on the variable resistor. In thetotal-radiation pyrometer radiationemitted by the source is focused by a

produces a polarization of some 10–5

C m–2.

pyrogallol 1,2,3-trihydroxyben-zene, C6H3(OH)3, a white crystallinesolid, m.p. 132°C. Alkaline solutionsturn dark brown on exposure to airthrough reaction with oxygen. It is apowerful reducing agent, employedin photographic developers. It is alsoused in volumetric gas analysis as anabsorber of oxygen.

pyrite (iron pyrites) A mineral formof iron(II) sulphide, FeS2. SuperÜciallyit resembles gold in appearance,hence it is also known as fool’s gold,but it is harder and more brittle thangold (which may be cut with a knife).Pyrite crystallizes in the cubic sys-tem, is brass yellow in colour, has ametallic lustre, and a hardness of6–6.5 on the Mohs’ scale. It is themost common and widespread of thesulphide minerals and is used as asource of sulphur for the productionof sulphuric acid. Sources include theRio Tinto mines in Spain.

pyro- PreÜx denoting an oxo acidthat could be obtained from a loweracid by dehydration of two mol-ecules. For example, pyrosulphuricacid is H2S2O7 (i.e. 2H2SO4 minusH2O).

pyroboric acid See boric acid.

pyroelectricity The property ofcertain crystals, such as tourmaline,of acquiring opposite electricalcharges on opposite faces whenheated. In tourmaline a rise in tem-perature of 1 K at room temperature


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concave mirror onto a blackened foilto which a thermopile is attached.From the e.m.f. produced by the ther-mopile the temperature of the sourcecan be calculated.

pyrones Compounds containing asix-membered ring system having anoxygen hetero atom and a carbonylgroup attached to the ring. There aretwo forms depending on whether thecarbonyl is in the 1 or 3 position. Thepyrone ring system occurs in manynaturally occurring compounds.

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Pyrones

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CH2

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CH2

Pyrrolidine

pyrophoric Igniting spontaneouslyin air. Pyrophoric alloys are alloysthat give sparks when struck. Seemisch metal.

pyrophosphoric acid See phos-phoric(v) acid.

pyrosilicate See silicate.

pyrosulphuric acid See disul-phuric(vi) acid.

pyroxenes A group of ferromagne-sian rock-forming silicate minerals.They are common in basic igneousrocks but may also be developed bymetamorphic processes in gneisses,schists, and marbles. Pyroxenes havea complex crystal chemistry; they arecomposed of continuous chains of sil-icon and oxygen atoms linked by avariety of other elements. They arerelated to the *amphiboles, fromwhich they differ in cleavage angles.

pyruvic acid (2-oxopropanoic acid)A colourless liquid organic acid,CH3COCOOH. Pyruvate is an impor-tant intermediate compound in me-tabolism, being produced during*glycolysis and converted to acetylcoenzyme A, required for the *Krebscycle. Under anaerobic conditionspyruvate is converted to lactate orethanol.

The general formula is X1–pY1+pZ2O6,where X = Ca,Na; Y = Mg,Fe2+,Mn,Li,Al,Fe3+,Ti; and Z = Si,Al.

Orthorhombic pyroxenes (ortho-pyroxenes), (Mg,Fe)2Si2O6, vary incomposition between the end-members enstatite (Mg2Si2O6) andorthoferrosilite (Fe2Si2O6). Mono-clinic pyroxenes (clinopyroxenes), the larger group, include:diopside, CaMgSi2O6;hedenbergite, CaFe2+Si2O6;johannsenite, CaMnSi2O6;augite, (Ca,Mg,Fe,Ti,Al)2(Si,Al)2O6;aegirine, NaFe3+Si2O6;jadeite (see jade);pigeonite (Mg,Fe2+,Ca)(Mg,Fe2+)Si2O6.

pyrrole An organic nitrogen-containing compound that formspart of the structure of *porphyrins.

pyrrolidine A saturated hetero-cyclic compound having one nitro-gen atom in a Üve-membered ring,C4H9N; r.d. 0.87; m.p. –63°C b.p.87°C. It is found in certain plants andthe ring structure is present in manyalkaloids.


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Q

QSAR See quantitativestructure–activity relationship.

QSMR See quantitativestructure–metabolism relation-ship.

quadrivalent Having a valency offour.

quadrupole A set of four pointcharges that has zero net charge anddipole moment. An example of aquadrupole is a carbon dioxide (CO2)molecule. Quadrupole interactionsare much smaller than dipole inter-actions. Transitions involvingquadrupole moments are muchweaker than transitions involving di-pole moments, but can allow transi-tions forbidden in dipole momenttransitions.

quadrupole mass spectrometerSee mass spectroscopy.

qualitative analysis See analysis.

quantitative analysis See analy-sis.

quantitative structure–activityrelationship (QSAR) A statistical al-gorithm that quantitatively deÜnesthe relationship between the chemi-cal structure of a drug and its effecton an organism. QSAR studies areoften used to predict the activity ortoxicity of new drugs. Similar meth-ods can be used to predict the metab-olism of new drugs (quantitativestructure–metabolism relationships).

quantitative structure–metabo-lism relationship (QSMR) Seequantitative structure–activityrelationship.

quantum (pl. quanta) The mini-mum amount by which certain prop-erties, such as energy or angularmomentum, of a system can change.Such properties do not, therefore,vary continuously, but in integralmultiples of the relevant quantum,and are described as quantized. Thisconcept forms the basis of the *quan-tum theory. In waves and Üelds thequantum can be regarded as an exci-tation, giving a particle-like interpre-tation to the wave or Üeld. Thus, thequantum of the electromagnetic Üeldis the *photon and the graviton isthe quantum of the gravitationalÜeld.

quantum chaos The *quantummechanics of systems for which thecorresponding classical system canexhibit *chaos. This subject was initi-ated by Einstein in 1917, whoshowed that the quantization condi-tions associated with the *Bohrtheory need to be modiÜed for sys-tems that show chaos in classical me-chanics. The subject of quantumchaos is an active Üeld of research inwhich many basic issues still requireclariÜcation. It appears that systemsexhibiting chaos in classical mechan-ics do not necessarily exhibit chaosin quantum mechanics.

quantum chemistry The applica-tion of quantum mechanics to chem-istry. Quantum chemistry mostlyused empirical methods at Ürst but ascomputing power has developed ithas been increasingly based on theÜrst principles of quantum mechan-ics.

quantum entanglement A phe-


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nomenon in quantum mechanics inwhich a particle or system does nothave a deÜnite state but exists as anintermediate form of two ‘entangled’states. One of these states is realizedwhen a ‘measurement’ is made.

quantum jump A change in a sys-tem (e.g. an atom or molecule) fromone quantum state to another.

quantum mechanics A system ofmechanics that was developed from*quantum theory and is used to ex-plain the properties of atoms andmolecules. Using the energy *quan-tum as a starting point it incorpo-rates Heisenberg’s *uncertaintyprinciple and the *de Broglie wave-length to establish the wave–particleduality on which *Schrödinger’sequation is based. This form of quan-tum mechanics is called wave me-chanics. An alternative but equivalentformalism, *matrix mechanics, isbased on mathematical operators.

quantum number See atom; spin.

quantum simulation The mathe-matical modelling of systems of largenumbers of atoms or molecules bycomputer studies of relatively smallclusters. It is possible to obtain infor-mation about solids and liquids inthis way and to study surface proper-ties and reactions.

quantum state The state of aquantized system as described by itsquantum numbers. For instance, thestate of a hydrogen *atom is de-scribed by the four quantum num-bers n, l, m, ms. In the ground statethey have values 1, 0, 0, and ½ re-spectively.

quantum statistics A statisticaldescription of a system of particlesthat obeys the rules of *quantummechanics rather than classical me-chanics. In quantum statistics, en-ergy states are considered to be

quantized. If the particles are treatedas indistinguishable, Bose–Einsteinstatistics apply if any number of par-ticles can occupy a given quantumstate. Such particles are calledbosons. All known bosons have anangular momentum nh, where n iszero or an integer and h is the Planckconstant. For identical bosons the*wave function is always symmetric.If only one particle may occupy eachquantum state, Fermi–Dirac statisticsapply and the particles are calledfermions. All known fermions have atotal angular momentum (n + ½)h/2πand any wave function that involvesidentical fermions is always antisym-metric.

quantum theory The theory de-vised by Max *Planck in 1900 to ac-count for the emission of theblack-body radiation from hot bodies.According to this theory energy isemitted in quanta (see quantum),each of which has an energy equal tohν, where h is the *Planck constantand ν is the frequency of the radia-tion. This theory led to the moderntheory of the interaction betweenmatter and radiation known as*quantum mechanics, which gener-alizes and replaces classical mechan-ics and Maxwell’s electromagnetictheory. In nonrelativistic quantumtheory particles are assumed to beneither created nor destroyed, tomove slowly relative to the speed oflight, and to have a mass that doesnot change with velocity. These as-sumptions apply to atomic and mo-lecular phenomena and to someaspects of nuclear physics. Relativis-tic quantum theory applies to parti-cles that have zero rest mass ortravel at or near the speed of light.

quantum theory of radiationThe theory that describes the emis-sion and absorption of electromag-netic radiation. Since it is a quantum

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theory the processes of emission andabsorption are described in terms ofthe creation and disappearance ofphotons, with photons being createdin emission processes and disappear-ing in absorption. Absorption occurswhen electromagnetic radiation thatimpinges on a quantum mechanicalsystem, such as an atom or mol-ecules, induces a transition from theground state of the system to an ex-cited state by virtue of a photon ofthe radiation being absorbed. Simi-larly, emission of a photon occurswhen an atom returns to the groundstate from an excited state, i.e.*spontaneous emission occurs. It isalso possible for an atom in an ex-cited state to emit a photon by *in-duced emission, a process that isused in *lasers.

quantum yield In a photochemi-cal reaction, the number of eventsoccurring per photon absorbed. It isgiven by the number of moles ofproduct formed (or reactant con-sumed) divided by the number ofmoles of photons absorbed.

quartz The most abundant andcommon mineral, consisting of crys-talline silica (silicon dioxide, SiO2),crystallizing in the trigonal system. Ithas a hardness of 7 on the Mohs’scale. Well-formed crystals of quartzare six-sided prisms terminating insix-sided pyramids. Quartz is ordinar-ily colourless and transparent, inwhich form it is known as rock crys-tal. Coloured varieties, a number ofwhich are used as gemstones, in-clude *amethyst, citrine quartz (yel-low), rose quartz (pink), milk quartz(white), smoky quartz (grey-brown),*chalcedony, *agate, and *jasper.Quartz occurs in many rocks, espe-cially igneous rocks such as graniteand quartzite (of which it is the chiefconstituent), metamorphic rockssuch as gneisses and schists, and sedi-

mentary rocks such as sandstone andlimestone. The mineral is piezoelec-tric and is used in oscillators. It isalso used in optical instruments andin glass, glaze, and abrasives.

quasicrystal A solid structure inwhich there is (1) long-range incom-mensurate translational order (see in-commensurate lattice) and (2)long-range orientational order with apoint group, which is not allowed in*crystallography. Condition (1) iscalled quasiperiodicity. In two dimen-sions, the Üvefold symmetry of a pen-tagon is an example of a pointsymmetry, which is not allowed incrystallography, but for which qua-sicrystals exist. In three dimensionsthe symmetry of an icosahedron isnot allowed in crystallography, butquasicrystals with this symmetryexist (e.g. AlMn). Diffraction patternsfor quasicrystals have Bragg peaks,with the density of Bragg peaks ineach plane being higher than wouldbe expected for a perfect periodiccrystal.

quasiperiodicity See quasicrystal.

quaternary ammonium com-pounds See amine salts.

quenching 1. (in metallurgy) Therapid cooling of a metal by immers-ing it in a bath of liquid in order toimprove its properties. Steels arequenched to make them harder but some nonferrous metals arequenched for other reasons (copper,for example, is made softer byquenching). 2. (in physics) Theprocess of inhibiting a continuousdischarge in a *Geiger counter sothat the incidence of further ionizingradiation can cause a new discharge.This is achieved by introducing aquenching vapour, such as methanemixed with argon or with neon, intothe tube.

quicklime See calcium oxide.

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O

O

OH

OH

+ 2H+

Quinhydrone electrode

Quinoline

quinhydrone electrode A *halfcell consisting of a platinum elec-trode in an equimolar solution ofquinone (cyclohexadiene-1,4-dione)and hydroquinone (benzene-1,4-diol).It depends on the oxidation–reduc-tion reaction

C6H4(OH)2 ˆ C6H4O2 + 2H+ + 2e.

quinine A white solid,C20H24N2O2.3H2O, m.p. 57°C. It is apoisonous alkaloid occurring in thebark of the South American cinchonatree, although it is now usually pro-duced synthetically. It forms saltsand is toxic to the malarial parasite,and so quinine and its salts are used

to treat malaria; in small doses itmay be prescribed for colds and in-Ûuenza. In dilute solutions it has apleasant astringent taste and is addedto some types of tonic water.

quinol See benzene-1,4-diol.

quinoline A hygroscopic unpleas-ant-smelling colourless oily liquid,C9H7N; b.p. 240°C. Its molecules con-sist of a benzene ring fused to a pyri-dine ring. It occurs in coal tar andbone oil, and is made from phenyl-amine and nitrobenzene. Quinolineis a basic compound, forming saltswith mineral acids and forming qua-ternary ammonium compounds withhaloalkanes. It is used for makingmedicines and dyes. In quinoline, thenitrogen atom is one atom awayfrom the position at which the ringsare fused. In an isomer, isoquinoline,the nitrogen atom is positioned twoatoms away from the fused ring.

quinone 1. See cyclohexadiene-1,4-dione. 2. Any similar compoundcontaining C=O groups in an unsatu-rated ring.


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R

racemate See racemic mixture.

racemic mixture (racemate) Amixture of equal quantities of the d-and l-forms of an optically activecompound. Racemic mixtures are de-noted by the preÜx dl- (e.g. dl-lacticacid). A racemic mixture shows no*optical activity.

racemization A chemical reactionin which an optically active com-pound is converted into a *racemicmixture.

rad See radiation units.

radial distribution function Theaverage number of atoms found at adistance r from a central atom. In thecase of a crystal, the radial distribu-tion function is a series of regularspikes, corresponding to the atomicpositions in the lattice. Calculatingthe radial distribution function is offundamental importance in thetheory of liquids.

radiation 1. Energy travelling inthe form of electromagnetic waves orphotons. 2. A stream of particles, es-pecially alpha- or beta-particles froma radioactive source or neutronsfrom a nuclear reactor.

radiationless decay Decay of anatom or molecule from an excitedstate to a lower energy state withoutthe emission of electromagnetic radi-ation. A common example of a radia-tionless process is the *Auger effect,in which an electron rather than aphoton is emitted as a result ofdecay.

radiation units Units of measure-ment used to express the activity of a

radionuclide and the dose of ionizingradiation. The units curie, roentgen,rad, and rem are not coherent withSI units but their temporary use withSI units has been approved while thederived SI units becquerel, gray, andsievert become familiar.

The becquerel (Bq), the SI unit ofactivity, is the activity of a radionu-clide decaying at a rate, on average,of one spontaneous nuclear transi-tion per second. Thus 1 Bq = 1 s–1.The former unit, the curie (Ci), isequal to 3.7 × 1010 Bq. The curie wasoriginally chosen to approximate theactivity of 1 gram of radium–226.

The gray (Gy), the SI unit of ab-sorbed dose, is the absorbed dosewhen the energy per unit mass im-parted to matter by ionizing radia-tion is 1 joule per kilogram. Theformer unit, the rad (rd), is equal to10–2 Gy.

The sievert (Sv), the SI unit of doseequivalent, is the dose equivalentwhen the absorbed dose of ionizingradiation multiplied by the stipulateddimensionless factors is 1 J kg–1. Asdifferent types of radiation cause dif-ferent effects in biological tissue aweighted absorbed dose, called thedose equivalent, is used in which theabsorbed dose is modiÜed by multi-plying it by dimensionless factorsstipulated by the International Com-mission on Radiological Protection.The former unit of dose equivalent,the rem (originally an acronym forroentgen equivalent man), is equal to10–2 Sv.

In SI units, exposure to ionizing ra-diation is expressed in coulombs perkilogram, the quantity of X- or


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gamma-radiation that produces ionpairs carrying 1 coulomb of charge ofeither sign in 1 kilogram of pure dryair. The former unit, the roentgen(R), is equal to 2.58 × 10–4 C kg–1.

radiative capture See capture.

radical A group of atoms, either ina compound or existing alone. Seefree radical; functional group.

A• Information about IUPAC nomenclature• List of radical names

radical ion A radical that has a pos-itive or negative charge. An exampleis the benzene radical cation C6H6

.+.In radical ions, the odd electron andthe charge are usually (but not neces-sarily) on the same atom.

radioactive age The age of an ar-chaeological or geological specimenas determined by a process that de-pends on a radioactive decay. See car-bon dating; fission-track dating;potassium–argon dating; rubidium–strontium dating; uranium–leaddating.

radioactive dating See radiomet-ric dating.

radioactive isotope See radioiso-tope.

radioactive nuclide See radionu-clide.

radioactive series A series ofradioactive nuclides in which eachmember of the series is formed bythe decay of the nuclide before it.The series ends with a stable nuclide.Three radioactive series occur natu-rally, those headed by thorium–232(thorium series), uranium–235 (ac-tinium series), and uranium–238 (ura-nium series). All three series end withan isotope of lead. The neptunium se-ries starts with the artiÜcial isotopeplutonium–241, which decays to nep-

tunium–237, and ends with bis-muth–209.

radioactive tracing See labelling.

radioactivity The spontaneous dis-integration of certain atomic nucleiaccompanied by the emission ofalpha-particles (helium nuclei), beta-particles (electrons or positrons), orgamma radiation (short-wavelengthelectromagnetic waves).

Natural radioactivity is the result ofthe spontaneous disintegration ofnaturally occurring radioisotopes.Many radioisotopes can be arrangedin three *radioactive series. The rateof disintegration is uninÛuenced bychemical changes or any normalchanges in their environment. How-ever, radioactivity can be induced inmany nuclides by bombarding themwith neutrons or other particles. Seealso decay; ionizing radiation; radi-ation units.

radiocarbon dating See carbondating.

radiochemistry The branch ofchemistry concerned with radio-active compounds and with ioni-zation. It includes the study ofcompounds of radioactive elementsand the preparation and use of com-pounds containing radioactive atoms.See labelling; radiolysis.

radiogenic Resulting from radio-active decay.

radioimmunoassay (RIA) A sensi-tive quantitative method for detect-ing trace amounts of a biomolecule,based on its capacity to displace a ra-dioactively labelled form of the mol-ecule from combination with itsantibody.

radioisotope (radioactive isotope)An isotope of an element that isradioactive. See labelling.

radiolysis The use of ionizing radi-

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ation to produce chemical reactions.The radiation used includes alphaparticles, electrons, neutrons, X-rays,and gamma rays from radioactivematerials or from accelerators. En-ergy transfer produces ions and ex-cited species, which undergo furtherreaction. A particular feature of radi-olysis is the formation of short-livedsolvated electrons in water and otherpolar solvents.

radiometric dating (radioactivedating) See dating techniques;radioactive age.

radionuclide (radioactive nuclide)A *nuclide that is radioactive.

radiopharmaceuticals Com-pounds used in medicine that have aradioactive atom in the molecule. Ra-diopharmaceuticals are both for diag-nostic purposes (as in radionuclideimaging) or for therapy (e.g. certaincancer treatments).

radium Symbol Ra. A radioactivemetallic element belonging to*group 2 (formerly IIA) of the peri-odic table; a.n. 88; r.a.m. 226.0254;r.d. ∼5; m.p. 700°C; b.p. 1140°C. It oc-curs in uranium ores (e.g. pitch-blende). The most stable isotope isradium–226 (half-life 1602 years),which decays to radon. It is used as aradioactive source in research and, tosome extent, in radiotherapy. The el-ement was isolated from pitchblendein 1898 by Marie and Pierre Curie.


radon Symbol Rn. A colourlessradioactive gaseous element belong-ing to group 18 of the periodic table(the *noble gases); a.n. 86; r.a.m. 222;d. 9.73 g dm–3; m.p. –71°C; b.p.–61.8°C. At least 20 isotopes areknown, the most stable beingradon–222 (half-life 3.8 days). It isformed by decay of radium–226 and

undergoes alpha decay. It is used inradiotherapy. Radon occurs naturally,particularly in areas underlain bygranite, where it is thought to be ahealth hazard. As a noble gas, radonis practically inert, although a fewcompounds, e.g. radon Ûuoride, can be made. It was Ürst isolated by William Ramsey and RobertWhytlaw-Gray (1877–1958) in 1908.


rafÜnate A liquid puriÜed by sol-vent extraction.

rafÜnose A white solid carbohy-drate, C18H32O16, m.p. 80°C. It is atrisaccharide (a type of *sugar) con-sisting of fructose, galactose and glu-cose. It occurs naturally in sugar-beetand cotton-seed residues.

r.a.m. See relative atomic mass.

Raman effect A type of scatteringof electromagnetic radiation inwhich light suffers a change in fre-quency and a change in phase as itpasses through a material medium.The intensity of Raman scattering isabout one-thousandth of that inRayleigh scattering in liquids; for thisreason it was not discovered until1928. However, it was not until thedevelopment of the laser that the ef-fect was put to use.

In Raman spectroscopy light from alaser is passed through a substanceand the scattering is analysed spec-troscopically. The new frequencies inthe Raman spectrum of monochro-matic light scattered by a substanceare characteristic of the substance.Both inelastic and superelastic scat-tering occurs. The technique is usedas a means of determining molecularstructure and as a tool in chemicalanalysis. The effect was discovered bythe Indian scientist Sir C. V. Raman(1888–1970).

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A• The original paper by Raman and

Krishnan

Ramsay, Sir William (1852–1916)British chemist, born in Glasgow.After working under Robert *Bun-sen, he returned to Glasgow beforetaking up professorships at Bristol(1880–87) and London (1887–1912). Inthe early 1890s he worked with Lord*Rayleigh on the gases in air and in1894 they discovered argon. In 1898,with Morris Travers (1872–1961), hediscovered neon, krypton, andxenon. Six years later he discoveredthe last of the noble gases, radon. Hewas awarded the Nobel Prize forchemistry in 1904, the year in whichRayleigh received the physics prize.

random alloy See disorderedsolid.

random walk The problem of de-termining the distance from a start-ing position made by a walker, whocan either move forward (toward +x)or backwards (toward –x) with thechoice being made randomly (e.g. bytossing a coin). The progress of thewalker is characterized by the netdistance DN travelled in N steps. AfterN steps the root mean square valueDrms, which is the average distanceaway from the starting position, isgiven by Drms = √N. Physical applica-tions of the random walk include dif-fusion and the related problem ofBrownian motion as well as prob-lems involving the structures of poly-mers and disordered solids.

Raney nickel A spongy form ofnickel made by the action of sodiumhydroxide on a nickel–aluminiumalloy. The sodium hydroxide dis-solves the aluminium leaving ahighly active form of nickel with alarge surface area. The material is ablack pyrophoric powder saturatedwith hydrogen. It is an extremely

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efÜcient catalyst, especially for hy-drogenation reactions at room tem-perature. It was discovered in 1927by the American chemist M. Raney(1885–1966).

ranksite A mineral consisting of a mixed sodium carbonate, sodiumsulphate, and potassium chloride,2Na2CO3.9Na2SO4.KCl.

Raoult’s law The partial vapourpressure of a solvent is proportionalto its mole fraction. If p is the vapourpressure of the solvent (with a sub-stance dissolved in it) and X the molefraction of solvent (number of molesof solvent divided by total number ofmoles) then p = p0X, where p0 is thevapour pressure of the pure solvent.A solution that obeys Raoult’s law issaid to be an ideal solution. In gen-eral the law holds only for dilute so-lutions, although some mixtures ofliquids obey it over a whole range ofconcentrations. Such solutions areperfect solutions and occur when theintermolecular forces between mol-ecules of the pure substances aresimilar to the forces between mol-ecules of one and molecules of theother. Deviations in Raoult’s law formixtures of liquids cause the forma-tion of *azeotropes. The law was dis-covered by the French chemistFrançois Raoult (1830–1901).

A• Raoult’s original paper

rapeseed methyl ester See bio-fuel.

rare-earth elements See lan-thanoids.

rarefaction A reduction in thepressure of a Ûuid and therefore ofits density.

rare gases See noble gases.

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(and phenol) by a gas-phase reactionbetween benzene vapour, hydrogenchloride, and oxygen (air) at 230°C:

2C6H6 + 2HCl + O2 → 2H2O +2C6H5Cl

The catalyst is copper(II) chloride.The chlorobenzene is mainly usedfor making phenol by the reaction

C6H5Cl + H2O → HCl + C6H5OHThis reaction proceeds at 430°C witha silicon catalyst. The process was in-vented by the German chemist FritzRaschig (1863–1928).

Raschig synthesis See hydrazine.

RasMol A commonly used programfor visualizing molecules. It can beused with a range of Üle formats andthe display can be choosen (e.g. wire-frame, ball-and-stick, space Ülling,etc.). The original version was writ-ten by Roger Sayle of Glaxo around1990. It is freely available.A• A downloadable version of RasMol from

the website of the University ofMassachusetts Amherst

rate constant (velocity constant)Symbol k. The constant in an expres-sion for the rate of a chemical reac-tion in terms of concentrations (oractivities). For instance, in a simpleunimolecular reaction A → B, therate is proportional to the concentra-tion of A, i.e. rate = k[A], where k isthe rate constant, which depends onthe temperature. The equation is therate equation of the reaction, and itsform depends on the reaction mecha-nism.

rate-determining step The slow-est step in a chemical reaction thatinvolves a number of steps. In suchreactions, there is often a single stepthat is appreciably slower than theother steps, and the rate of this de-termines the overall rate of the reac-tion.

rationalized Planck constant Seeplanck constant.

rationalized units A system ofunits in which the deÜning equationshave been made to conform to thegeometry of the system in a logicalway. Thus equations that involve cir-cular symmetry contain the factor2π, while those involving sphericalsymmetry contain the factor 4π. *SIunits are rationalized; c.g.s. units areunrationalized.

Rayleigh, Lord (John WilliamStrutt; 1842–1919) British physicist,who built a private laboratory afterworking at Cambridge University.His work in this laboratory includedthe discovery of *Rayleigh scatteringof electromagnetic radiation. He alsoworked in acoustics, electricity, andoptics, as well as collaborating withWilliam *Ramsay on the discovery ofargon. He was awarded the 1904Nobel Prize for physics.

Rayleigh scattering Scattering ofelectromagnetic radiation by mol-ecules in which the frequency of thescattered radiation is unchanged.This type of scattering was analysedby Lord *Rayleigh in his papers inthe late 19th century, which showedthat the blue colour of the sky is a re-sult of this type of light scattering,with molecules of the atmosphere ofthe earth scattering light from thesun.

rayon A textile made from cellu-lose. There are two types, both madefrom wood pulp. In the viscoseprocess, the pulp is dissolved in car-bon disulphide and sodium hydrox-ide to give a thick brown liquidcontaining cellulose xanthate. Theliquid is then forced through Ünenozzles into acid, where the xanthateis decomposed and a cellulose Üla-ment is produced. The product is vis-cose rayon. In the acetate process

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cellulose acetate is made and dis-solved in a solvent. The solution isforced through nozzles into air,where the solvent quickly evaporatesleaving a Ülament of acetate rayon.

RBS See rutherford backscatter-ing spectrometry.

RDX See cyclonite.

reactant See chemical reaction.

reaction See chemical reaction.

reactive dye See dyes.

reagent A substance reacting withanother substance. Laboratoryreagents are compounds, such as sul-phuric acid, hydrochloric acid,sodium hydroxide, etc., used inchemical analysis or experiments.

realgar A red mineral form of ar-senic(II) sulphide, As2S2.

real gas A gas that does not havethe properties assigned to an *idealgas. Its molecules have a Ünite sizeand there are forces between them(see equation of state).

rearrangement A type of chemicalreaction in which the atoms in amolecule rearrange to form a newmolecule. Examples are the *pinacolrearrangement and the *Wagner–Meerwein rearrangement.

reciprocal lattice A lattice for acrystal that can be deÜned from thelattice in real space. If the primitivetranslation vectors of the lattice inreal space are denoted by a, b, c thenthe primitive translations a′, b′, c′ inthe reciprocal lattice are deÜned bya′ = b × c[abc], b′ = c × a[abc], c′ =a × b[abc], where [abc] denotes thescalar triple product a.(b × c).

The reciprocal lattice is a funda-mental concept in the theory of *X-ray diffraction and energy bandswith a diffraction pattern beingmuch more closely related to the rec-

iprocal lattice than the real-space lat-tice.

reciprocal proportions See chemi-cal combination.

recombination process Theprocess in which a neutral atom ormolecule is formed by the combina-tion of a positive ion and a negativeion or electron; i.e. a process of thetype:

A+ + B– → ABor

A+ + e– → AIn recombination, the neutral

species formed is usually in an ex-cited state, from which it can decaywith emission of light or other elec-tromagnetic radiation.

recreational drug A drug used forrecreation, as opposed to medicaluse. Strictly, recreational drugs in-clude alcohol and nicotine, but theterm is often understood to meansubstances such as marijuana, co-caine, and amphetamines.

recrystallization A process of re-peated crystallization in order to pu-rify a substance or to obtain moreregular crystals of a puriÜed sub-stance.

rectiÜcation The process of purify-ing a liquid by *distillation. See frac-tional distillation.

rectiÜed spirit A constant-boilingmixture of *ethanol (95.6°) andwater; it is obtained by distillation.

red lead See dilead(ii) lead(iv)oxide.

redox See oxidation–reduction.

reducing agent (reductant) A sub-stance that brings about reduction inother substances. It achieves this bybeing itself oxidized. Reducingagents contain atoms with low oxida-tion numbers; that is the atoms have

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gained electrons. In reducing othersubstances, these atoms lose elec-trons.

reducing sugar A monosaccharideor disaccharide sugar that can donateelectrons to other molecules and cantherefore act as a reducing agent.The possession of a free ketone(–CO–) or aldehyde (–CHO) group en-ables most monosaccharides and dis-accharides to act as reducing sugars.Reducing sugars can be detected by*Benedict’s test. Compare nonreduc-ing sugar.

reductant See reducing agent.

reduction See oxidation–reduction.

reÜnery gas See petroleum.

reÜning The process of purifyingsubstances or extracting substancesfrom mixtures.

reÛuxing A laboratory techniquein which a liquid is boiled in a con-tainer attached to a condenser (reÛuxcondenser), so that the liquid contin-uously Ûows back into the container.It is used for carrying out reactionsover long periods in organic synthe-sis.

reforming The conversion ofstraight-chain alkanes into branched-chain alkanes by *cracking or bycatalytic reaction. It is used in pe-troleum reÜning to produce hydro-carbons suitable for use in gasoline.Benzene is also manufactured fromalkane hydrocarbons by catalytic re-forming. Steam reforming is aprocess used to convert methane(from natural gas) into a mixture ofcarbon monoxide and hydrogen,which is used to synthesize organicchemicals. The reaction

CH4 + H2O → CO + 3H2

occurs at about 900°C using a nickelcatalyst.

refractory 1. Having a high melt-ing point. Metal oxides, carbides, andsilicides tend to be refractory, andare extensively used for lining fur-naces. 2. A refractory material.

regioselectivity The effect inwhich certain positions in the mol-ecule are favoured over others in areaction. An example is the way inwhich certain groups on the benzenering direct further substituents to themeta position or to the ortho andpara positions. *Markovnikoff’s rulefor electrophilic additions to alkenesis another example.

Regnault’s method A techniquefor measuring gas density by evacuat-ing and weighing a glass bulb ofknown volume, admitting gas atknown pressure, and reweighing.The determination must be carriedout at constant known temperatureand the result corrected to standardtemperature and pressure. Themethod is named after the Frenchchemist Henri Victor Regnault(1810–78).

relative atomic mass (atomicweight; r.a.m.) Symbol Ar. The ratioof the average mass per atom of thenaturally occurring form of an el-ement to 1/12 of the mass of a car-bon–12 atom.

A• Current values for relative atomic

masses

relative density (r.d.) The ratio ofthe *density of a substance to thedensity of some reference substance.For liquids or solids it is the ratio ofthe density (usually at 20°C) to thedensity of water (at its maximumdensity). This quantity was formerlycalled speciÜc gravity. Sometimes rel-ative densities of gases are used; forexample, relative to dry air, bothgases being at s.t.p.

457 relative density

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relative molecular mass (molecu-lar weight) Symbol Mr. The ratio ofthe average mass per molecule of thenaturally occurring form of an el-ement or compound to 1/12 of themass of a carbon–12 atom. It is equalto the sum of the relative atomicmasses of all the atoms that com-prise a molecule.

relative permittivity See permit-tivity.

relativistic quantum mechanicsQuantum mechanics that is in accordwith special relativity theory. Themain equation of relativistic quan-tum mechanics is the *Dirac equa-tion. It is necessary to use relativisticquantum mechanics to describe theelectronic properties of heavy atomsand all the *Üne structure of atomicspectra. The colour of solid gold andmercury existing as a liquid are bothdue to relativistic effects in quantummechanics.

relativistic quantum theory Seequantum theory.

relaxation The return to the equi-librium state of a system after it hasexperienced a sudden change due toan external inÛuence. The time forrelaxation to take place is called therelaxation time. An example is theaverage time that a system remainsin the higher energy state before itfalls to the lower energy state in nu-clear magnetic resonance (NMR).In general, any process involvingdecay is assumed to have exponentialdecay, with the relaxation time beingthe time it takes for the variable tofall from its initial value to 1/e of itsinitial value. Another example of re-laxation time is the time needed fora gas to return to the Maxwell distri-bution of velocities, after it has beensuddenly disturbed from that state.

release agent A substance that isapplied to surfaces to prevent them

from sticking together. Releaseagents are used in many industrialprocesses, including the manufactureof food, glass, paper, and plastics.They include polyethene, polyte-traÛuoroethene (PTFE), polyvinyl al-cohol (PVA), silicones, and waxes, aswell as stearates and other glyc-erides. They are also known as abher-ents or parting agents.

rem See radiation units.

renaturation The reconstructionof a protein or nucleic acid that hasbeen denatured such that the mol-ecule resumes its original function.Some proteins can be renatured byreversing the conditions (of tempera-ture, pH, etc.) that brought about de-naturation.

renewable energy sourcesSources of energy that do not use upthe earth’s Ünite mineral resources.Nonrenewable energy sources are*fossil fuels and Üssion fuels. Variousrenewable energy sources are beingused or investigated, including geo-thermal energy, hydroelectric power,nuclear fusion, solar energy, tides,wind power, and wave power.

rennin An enzyme secreted by cellslining the stomach in mammals thatis responsible for clotting milk. Itacts on a soluble milk protein (ca-seinogen), which it converts to theinsoluble form casein. This ensuresthat milk remains in the stomachlong enough to be acted on by pro-tein-digesting enzymes.

Reppe processes A set of relatedindustrial reactions of acetylene toproduce vinyl compounds, such as

HC≡CH + ROH → H2C=CHR

HC≡CH + RCN → H2C=CR(CN)They take place at high pressureusing metal acelylide catalysts. Theprocesses are named after the Ger-man chemist Walter Reppe (1892–

relative molecular mass 458

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1969), who pioneered techniques forthe safe industrial use of acetylene.

reptation A motion describing thedynamics of a polymer in a highlyentangled state, such as a network.Regarding the entangled state as aset of chains between crosslinks it ispossible to regard the chain as beingin a ‘tube’, with the tube beingformed by topological constraints.The chain is longer than the tube sothat the ‘slack’ of the chain movesthrough the tube, which causes thetube itself to change with time. Thismotion was called reptation (fromthe Latin reptare, to creep), by theFrench physicist P. G. de Gennes,who postulated it in 1971. Many ex-periments indicate that reptationdominates the dynamics of polymerchains when they are entangled.

resin A synthetic or naturally occur-ring *polymer. Synthetic resins areused in making *plastics. Naturalresins are acidic chemicals secretedby many trees (especially conifers)into ducts or canals. They are foundeither as brittle glassy substances ordissolved in essential oils. Their func-tions are probably similar to those ofgums and mucilages.

resolution The process of separat-ing a racemic mixture into its opti-cally active constituents. In somecases the crystals of the two formshave a different appearance, and theseparation can be done by hand. Ingeneral, however, physical methods(distillation, crystallization, etc.) can-not be used because the optical iso-mers have identical physicalproperties. The most common tech-nique is to react the mixture with acompound that is itself optically ac-tive, and then separate the two. Forinstance, a racemic mixture of l-Aand d-A reacted with l-B, gives twocompounds AB that are not opticalisomers but diastereoisomers and can

be separated and reconverted intothe pure l-A and d-A. Biological tech-niques using bacteria that convertone form but not the other can alsobe used.

resonance The representation ofthe structure of a molecule by two ormore conventional formulae. For ex-ample, the formula of methanal canbe represented by a covalent struc-ture H2C=O, in which there is a dou-ble bond in the carbonyl group. It isknown that in such compounds theoxygen has some negative chargeand the carbon some positive charge.The true bonding in the molecule issomewhere between H2C=O and theionic compound H2C+O–. It is said tobe a resonance hybrid of the two, in-dicated by

H2C=O ↔ H2C+O–

The two possible structures are calledcanonical forms, and they need notcontribute equally to the actual form.Note that the double-headed arrowdoes not imply that the two formsare in equilibrium.

resonance effect See electroniceffects.

resonance ionization spec-troscopy (RIS) A spectroscopic tech-nique in which single atoms in a gasare detected using a laser to ionizethat atom. A sample containing theatoms to be excited is subjected tolight from a laser, tuned so that onlythat type of atom is excited by thelight. If the frequency of light atwhich the atom is excited is ν, theatoms in the excited state can be ion-ized if the ionization potential of theatom is less than 2ν. In contrast toother techniques of ionization, thistype of ionization only occurs foratoms that are ‘in tune’ with the fre-quency of light. Because RIS is veryselective in determining which atom

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is ionized for a given frequency it hasmany applications in chemistry.

resorcinol See 1,3-dihydroxyben-zene.

retinol See vitamin a.

retort 1. A laboratory apparatusconsisting of a glass bulb with a longneck. 2. A vessel used for reaction ordistillation in industrial chemicalprocesses.

retrosynthetic analysis A tech-nique for planning the series of reac-tions required to synthesize a givenmolecule. It involves working back-wards from the known structure. Themolecule to be synthesized (the tar-get molecule) is considered to be splitinto two parts (a process called dis-connection). The parts are not them-selves molecules, but are fragments(called synthons) that correspond toactual reagents. A trivial examplewould be the synthesis of the targetmolecule

R1–O–CO–R2

This is an ester. A suitable disconnec-tion is at the O–CO bond, giving twosynthons,

R1–O– and R2C(O)–.The convention is to write this in theform

R1–O–C(O)– R2 ⇒ R1–O– + R2–C(O)–.Here the symbol is a logic symbol for‘implies’; it is called the retrosyn-thetic arrow. In this example, suit-able reagents corresponding to thesesynthons would be an alcohol R1OHand an acyl chloride R2COCl. Notethat the disconnection is alwaysmade at a point corresponding to aknown reaction. In actual schemes,the target molecules are complexand a number of retrosynthetic stepsare needed.

reverberatory furnace A metal-lurgical furnace in which the charge

to be heated is kept separate fromthe fuel. It consists of a shallowhearth on which the charge is heatedby Ûames that pass over it and by ra-diation reÛected onto it from a lowroof.

reverse osmosis A method of ob-taining pure water from water con-taining a salt, as in *desalination.Pure water and the salt water areseparated by a semipermeable mem-brane and the pressure of the saltwater is raised above the osmoticpressure, causing water from thebrine to pass through the membraneinto the pure water. This process re-quires a pressure of some 25 atmos-pheres, which makes it difÜcult toapply on a large scale.

reversible process Any process inwhich the variables that deÜne thestate of the system can be made tochange in such a way that they passthrough the same values in the re-verse order when the process is re-versed. It is also a condition of areversible process that any ex-changes of energy, work, or matterwith the surroundings should be re-versed in direction and order whenthe process is reversed. Any processthat does not comply with these con-ditions when it is reversed is said tobe an irreversible process. All naturalprocesses are irreversible, althoughsome processes can be made to ap-proach closely to a reversible process.

RF value (in chromatography) Thedistance travelled by a given compo-nent divided by the distance trav-elled by the solvent front. For a givensystem at a known temperature, it isa characteristic of the componentand can be used to identify compo-nents.

rhe A unit of Ûuidity equal to thereciprocal of the *poise.

rhenium Symbol Re. A silvery-

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white metallic *transition element;a.n. 75; r.a.m. 186.2; r.d. 20.53; m.p.3180°C; b.p. 5627 (estimated)°C. Theelement is obtained as a by-productin reÜning molybdenum, and is usedin certain alloys (e.g. rhenium–molybdenum alloys are supercon-ducting). The element forms a num-ber of complexes with oxidationstates in the range 1–7. It was discov-ered by Walter Noddack (1893–1960)and Ida Tacke (1896–1978) in 1925.A• Information from the WebElements site

rheology The study of the defor-mation and Ûow of matter.

rheopexy The process by whichcertain thixotropic substances setmore rapidly when they are stirred,shaken, or tapped. Gypsum in wateris such a rheopectic substance.

rhodinol See aminophenol.

rhodium Symbol Rh. A silvery-white metallic *transition element;a.n. 45; r.a.m. 102.9; r.d. 12.4; m.p.1966°C; b.p. 3727°C. It occurs withplatinum and is used in certain plat-inum alloys (e.g. for thermocouples)and in plating jewellery and opticalreÛectors. Chemically, it is not at-tacked by acids (dissolves only slowlyin aqua regia) and reacts with non-metals (e.g. oxygen and chlorine) atred heat. Its main oxidation state is+3 although it also forms complexesin the +4 state. The element was dis-covered in 1803 by William Wolla-ston (1766–1828).A• Information from the WebElements site

RIA See radioimmunoassay.

riboÛavin See vitamin b complex.

ribonucleic acid See rna.

ribose A *monosaccharide,C5H10O5, rarely occurring free in na-ture but important as a component

of *RNA (ribonucleic acid). Its deriva-tive deoxyribose, C5H10O4, is equallyimportant as a constituent of *DNA(deoxyribonucleic acid), which car-ries the genetic code in chromo-somes.

ribulose A ketopentose sugar (seemonosaccharide), C5H11O5, that isinvolved in carbon dioxide Üxation inphotosynthesis as a component of*ribulose bisphosphate.

ribulose bisphosphate (RuBP) AÜve-carbon sugar that is combinedwith carbon dioxide to form twothree-carbon intermediates in theÜrst stage of the light-dependent re-actions of *photosynthesis (seecalvin cycle). The enzyme that me-diates the carboxylation of ribulosebisphosphate is ribulose bisphos-phate carboxylase.

Rice–Herzfeld mechanism Amechanism enabling complex chainreactions to give simple rate laws inchemical kinetics. An example of aRice–Herzfeld mechanism occurswith the pyrolysis of acetaldehyde,the mechanism consisting of initia-tion, propagation (in two steps), andtermination. This leads to the experi-mental result that the overall reac-tion is three-halves order in CH3CHO.To ascertain that such a mechanismis correct, either the steady-state ap-proximation is used or the differen-tial equations for the reaction ratesare solved numerically. It is pos-sible, as in this example, for theRice–Herzfeld mechanism to give acorrect description of the main partof the reaction but not to take ac-count of reactions that produce by-products. The Rice–Herzfeldmechanism is named after F. O. Riceand K. F. Herzfeld, who put forwardthe scheme in 1934.

Rice–Ramsperger–Kassel theory(RRK theory) A statistical theory used

461 Rice–Ramsperger–Kassel theory

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to describe unimolecular reactions.The RRK theory, put forward by O. K.Rice, H. C. Ramsperger, and indepen-dently by L. S. Kassel in the late1920s, assumes that a molecule is asystem of loosely coupled oscillators.Regarding the coupling of the oscilla-tors as loose enables calculations tobe made on the statistical distribu-tion of energy freely Ûowing betweenthe vibrational modes. In RRK theorythe rate constant for the decomposi-tion of a molecule increases with theenergy of the molecule. The RRKtheory has been applied with successto some chemical reactions but islimited in its predictive power. Theunsatisfactory features of the RRKtheory are improved by the *RRKMtheory (Rice–Ramsperger–Kassel–Marcus theory), in which the individ-ual vibrational modes of the systemare taken into account.

ricin A highly toxic protein presentin the castor oil plant (Ricinus commu-nis), in particular in the seeds of theplant (castor beans). It is probably themost poisonous substance present inplants – the fatal dose is about 1 mil-ligram per kilogram of body weight.

ring A closed chain of atoms in amolecule. In compounds, such asnaphthalene, in which two ringsshare a common side, the rings arefused rings. Ring closures are chemi-cal reactions in which one part of achain reacts with another to form aring, as in the formation of *lactamsand *lactones.

ring conformations Shapes thatcan be taken by nonplanar rings andcan be interconverted by formal rota-tions about single bonds. In cyclo-hexane, for example, the most stableconformation is the chair conforma-tion in which the 2, 3, 5 and 6 atomslie in a plane and the 1 and 4 carbonatoms lie on opposite sides of theplane. In this form, there is no ring

strain. In the boat conformation, the1 and 4 carbon atoms lie on the sameside of the plane. Other conformersare the twist conformation and thehalf chair conformation (see dia-gram). The energy difference be-tween the chair conformation andthe (least stable) half chair is 10.8kcal/mol and the forms interconvertquickly at normal temperatures(about 99% of molecules are in thechair form).

Other rings also have different con-formations. In a Üve-membered ring,such as that of cyclopentane, thetwist conformation has three adja-cent carbon atoms in a plane withone of the carbons above this planeand the other below it. The less sta-ble envelope conformation has fouratoms in a plane with one atom outof the plane. In an eight-memberedring, such as that of cyclooctane, thetub conformation has four atoms cor-responding to a pair of diametricallyopposite bonds in one plane, withthe other four atoms on the sameside of this plane. It is analogous tothe boat form of cyclohexane. Thecrown conformation has atoms alter-natively above and below the averageplane of the molecule.

Various names are given to posi-tions attached to atoms in differentring conformations. In the chair con-formation of cyclohexane, an axialbond is one that makes a large anglewith the average plane of the ring.An equatorial bond is one that makesa small angle. In a cyclopentane ring,the terms pseudo-axial and pseudo-equatorial are used. In the boat con-formation of cyclohexane the bondroughly parallel to the plane of fouratoms is the bowsprit, the otherbond being the Ûagpole.

RIS See resonance ionization spec-troscopy.

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463

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chairboat

twist half chair

flagpole

bowsprit

axial

equatorial

Conformations of a six-membered ring

twist (1, 2, 3, are coplanar) envelope (1, 2, 3, 5, are coplanar)

Conformations of a five -membered ring

1

5

2

4

31

5

2

3

4

crown tub

Conformations of an eight -membered ring

Conformations of a six-membered ring

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R-isomer See absolute configura-tion.

RME See biofuel.

RNA (ribonucleic acid) A complex or-ganic compound (a nucleic acid) inliving cells that is concerned with*protein synthesis. In some viruses,RNA is also the hereditary material.Most RNA is synthesized in the nu-

cleus and then distributed to variousparts of the cytoplasm. An RNA mol-ecule consists of a long chain of *nu-cleotides in which the sugar is*ribose and the bases are adenine,cytosine, guanine, and uracil (see il-lustration). Compare dna.

roasting The heating of a Ünelyground ore, especially a sulphide, inair prior to *smelting. The roasting

G

A U GA

A

G

U

U

G

AUC

AU

C

A

U

GG

AG

U

A

G

C

AA

G U A G C G C

bases

sugar–phosphate backbone

Single-stranded structure of RNA

P

O–

O–O O CH2

O

O OH

P

O–O O CH2

O

O OH

P

O–O O CH2

O

O OH

P

O–O O CH2

O

OH OH

nucleotide

ribose

basesbasesbases

Detail of molecular structure of sugar–phosphate backbone. Each ribose unit isattached to a phosphate group and a base,forming a nucleotide.

C

NH

C

NHHC

HC

O

O

C

NH

C

NHC

HC

NH2

O

C

NC

NHC

C

O

NH2

C

NCH

NC

C

NH2

N

NH

HC

N

NH

HC

adenine(A)

guanine(G)

cytosine(C)

uracil(U)

The four bases of RNA

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process expels moisture, chemicallycombined water, and volatile matter;in the case of sulphides, the sulphuris expelled as sulphur dioxide andthe ore is converted into an oxide.

Rochelle salt Potassium sodiumtartrate tetrahydrate, KNaC4H4O6.4H2O. A colourless crystalline saltused for its piezoelectric properties.

Rochon prism An optical deviceconsisting of two quartz prisms; theÜrst, cut parallel to the optic axis, re-ceives the light; the second, with theoptic axis at right angles, transmitsthe ordinary ray without deviationbut the extraordinary ray is deÛectedand can be absorbed by a screen. Thedevice can be used to produce plane-polarized light and it can also beused with ultraviolet radiation.

rock An aggregate of mineral parti-cles that makes up part of the earth’scrust. It may be consolidated or un-consolidated (e.g. sand, gravel, mud,shells, coral, and clay).

rock crystal See quartz.

rocking-chair cell See intercala-tion cell.

rock salt See halite.

rock salt structure (sodium chlo-ride structure) A type of ionic crystalstructure in which the cations have aface-centred cubic arrangement, withanions occupying all the octahedralholes. It can equally be described as afcc array of anions with cations inthe octahedral holes. Each type ofion has a coordination number of 6.Examples of compounds that havethe structure are NaCl, KBr, AgCl,AgBr, HgO, CaO, FeO, NiO, and SnAs.


roentgen The former unit of doseequivalent (see radiation units). It is

named after the discoverer of X-rays,W. K. Roentgen (1845–1923).

roentgenium Symbol Rg. A radio-active transactinide; a.n. 111. It wasmade by fusion of 209Bi with 64Ni.Only a few atoms have been de-tected.

A• Information from the WebElements

site

Rohypnol See flunitrazepam.

root-mean-square value (RMSvalue) 1. (in statistics) A typical valueof a number (n) of values of a quan-tity (x1,x2,x3…) equal to the squareroot of the sum of the squares of thevalues divided by n, i.e.

RMS value = √[(x12 + x2

2 + x32…)/n]

2. (in physics) A typical value of acontinuously varying quantity, suchas an alternating electric current, ob-tained similarly from many samplestaken at regular time intervals dur-ing a cycle. Theoretically this can beshown to be the effective value, i.e.the value of the equivalent direct cur-rent that would produce the samepower dissipation. For a sinusoidalcurrent this is equal to Im/√2, whereIm is the maximum value of the cur-rent.

ROSDAL A type of *line notationused in databases. It is based on a*connection table for the moleculewritten in a single text line. ROSDALis an acronym for Representation ofOrganic Structure DescriptionsArranged Linearly.

Rose’s metal An alloy of low melt-ing point (about 100°C) consisting of50% bismuth, 25–28% lead, and22–25% tin.

rosin A hard natural resin obtainedfrom pine tree oil and the wastesfrom processing wood pulp. It maybe colourless, yellow, brown, or

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black. It is used as a Ûotation agent,solder Ûux, sizing compound, and inlacquers and plasticizers. It is alsoused to provide ‘grip’ to violinists’bows (when it may be calledcolophony) and dancers’ and boxers’shoes.

rotamer One of a set of *conforma-tional isomers that differ from eachother by restricted rotation aboutone or more single bonds.

rotational spectroscopy Thespectroscopic study of the rotationalmotion of molecules. Rotational spec-troscopy gives information about in-teratomic distances. The transitionsbetween different rotational energylevels in molecules correspond to themicrowave and far-infrared regionsof the electromagnetic spectrum. It isonly possible for there to be transi-tions between rotational energy lev-els in pure rotational spectra if themolecule has a permanent dipolemoment. In the near-infrared regionrotational transitions are superim-posed on vibrational transitions, re-sulting in a vibrational–rotationalspectrum. This type of spectrum isconsiderably more complicated thana purely rotational spectrum. See vi-brational spectroscopy.

rotation group A group formed bythe set of all rotations about a point.The rotation group is associated withthe angular momentum of a systemand is important in the theory of therotational motion of molecules. Thegroup representations of the rotationgroup are closely associated with thequantum theory of angular momen-tum.

rotaxane A type of compound thathas a dumbbell-shaped moleculewith a cyclic molecule around itsaxis. The dumbbell has a chain withlarge groups at each end, these beinglarge enough to trap the ring. There

is no formal chemical bonding be-tween the dumbbell and the ring. Ro-taxanes are examples of compoundswith *mechanical bonding. A num-ber of natural peptide rotaxanes havebeen identiÜed. Synthesis of new ro-taxanes is a matter of interest be-cause of their possible use as‘molecular machines’ in nanotech-nology (e.g. as molecular switches orinformation storage units).

RRK theory See rice–ramsperger–kassel theory.

RRKM theory (Rice–Ramsperger–Kassel–Marcus theory) A theory ofunimolecular chemical reactions inwhich the *Rice–Ramsperger–Kasseltheory was improved by the USchemists O. K. Rice and R. A. Marcusin 1950; it was further improved sub-sequently in several papers by Mar-cus and associates by taking intoaccount the individual vibrationalfrequencies, rotations, and zero-pointenergies of the energized species andactivated complexes taking part. TheRRKM theory is very successful in ex-plaining the results of experimentsfor unimolecular reactions for a vari-ety of reactions.

R–S convention See absolute con-figuration.


rubber A polymeric substance ob-tained from the sap of the tree Heveabrasiliensis. Crude natural rubber isobtained by coagulating and dryingthe sap (latex), and is then modi-Üed by *vulcanization and com-pounding with Üllers. It is a polymerof *isoprene containing the unit–CH2C(CH3):CHCH2–. Various syn-thetic rubbers can also be made. See neoprene; nitrile rubber; sili-cones.

rubidium Symbol Rb. A soft silvery-

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white metallic element belonging to*group 1 (formerly IA) of the periodictable; a.n. 37; r.a.m. 85.47; r.d. 1.53;m.p. 38.89°C; b.p. 688°C. It is foundin a number of minerals (e.g. lepido-lite) and in certain brines. The metalis obtained by electrolysis of moltenrubidium chloride. The naturally oc-curring isotope rubidium–87 is radio-active (see rubidium–strontiumdating). The metal is highly reactive,with properties similar to those ofother group 1 elements, ignitingspontaneously in air. It was discov-ered spectroscopically by Robert Bun-sen and Gustav Kirchhoff in 1861.


rubidium–strontium dating Amethod of dating geological speci-mens based on the decay of the ra-dioisotope rubidium–87 into thestable isotope strontium–87. Naturalrubidium contains 27.85% of rubid-ium–87, which has a half-life of 4.7 ×1010 years. The ratio 87Rb/87Sr in aspecimen gives an estimate of its age(up to several thousand millionyears).

ruby The transparent red variety ofthe mineral *corundum, the colourbeing due to the presence of traces ofchromium. It is a valuable gemstone,more precious than diamonds. TheÜnest rubies are obtained fromMogok in Burma, where they occurin metamorphic limestones; SriLanka and Thailand are the onlyother important sources. Rubies havebeen produced synthetically by theVerneuil Ûame-fusion process. Indus-trial rubies are used in lasers,watches, and other precision instru-ments.

Russell–Saunders coupling (L–Scoupling) A type of coupling in sys-tems involving many *fermions.These systems include electrons in

atoms and nucleons in nuclei, inwhich the energies associated withelectrostatic repulsion are muchgreater than the energies associatedwith *spin–orbit coupling. *Multi-plets of many-electron atoms with alow atomic number are characterizedby Russell–Saunders coupling. It isnamed after the US physicists HenryNorris Russell (1877–1957) and Fred-erick Saunders (1875–1963) who pos-tulated this type of coupling toexplain the spectra of many-electronatoms with low atomic number in1925. The multiplets of heavy atomsand nuclei are better described by *j-j coupling or intermediate coupling,i.e. a coupling in which the energiesof electrostatic repulsion andspin–orbit coupling are similar insize.

rusting Corrosion of iron (or steel)to form a hydrated iron(III) oxideFe2O3.xH2O. Rusting occurs only inthe presence of both water and oxy-gen. It is an electrochemical processin which different parts of the ironsurface act as electrodes in a cell re-action. At the anode, iron atoms dis-solve as Fe2+ ions:

Fe(s) → Fe2+(aq) + 2eAt the cathode, hydroxide ions areformed:

O2(aq) + 2H2O(l) + 4e → 4OH–(aq)The Fe(OH)2 in solution is oxidized toFe2O3. Rusting is accelerated by im-purities in the iron and by the pres-ence of acids or other electrolytes inthe water.

ruthenium Symbol Ru. A hardwhite metallic *transition element;a.n. 44; r.a.m. 101.07; r.d. 12.3; m.p.2310°C; b.p. 3900°C. It is found asso-ciated with platinum and is used as acatalyst and in certain platinum al-loys. Chemically, it dissolves in fusedalkalis but is not attacked by acids. Itreacts with oxygen and halogens at

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high temperatures. It also formscomplexes with a range of oxidationstates. The element was isolated by K. K. Klaus in 1844.


Rutherford, Ernest, Lord (1871–1937) New Zealand-born Britishphysicist, who worked under Sir J. J.*Thomson at Cambridge University(1895–98). He then took up a profes-sorship at McGill University, Canada,and collaborated with Frederick*Soddy in studying radioactivity. In1899 he discovered *alpha particlesand beta particles, followed by thediscovery of *gamma radiation thefollowing year. In 1905, with Soddy,he announced that radioactive*decay involves a series of transfor-mations. He moved to ManchesterUniversity in 1907 and there, withHans Geiger and E. Marsden, devisedthe alpha-particle scattering experi-ment that led in 1911 to the discov-ery of the atomic nucleus Aftermoving to Cambridge in 1919 heachieved the artiÜcial splitting oflight atoms. In 1908 he was awardedthe Nobel Prize for chemistry.

Rutherford backscattering spec-trometry (RBS) A technique foranalysing samples of material byirradiation with a beam of alphaparticles and measurement of theenergies of the alpha particles afterthey have been scattered by thesample. This enables the elementspresent and their amounts to be de-termined because the energy of ascattered alpha particle depends onthe mass of the nucleus with whichit collides. RBS is used extensively inmedicine and industry.

rutherfordium Symbol Rf. A radio-active *transactinide element; a.n.104. It was Ürst reported in 1964 atDubna, near Moscow, and in 1969 it

was detected by A. Ghiorso and ateam at Berkeley, California. It canbe made by bombarding californium-249 nuclei with carbon-12 nuclei.


Rutherford model The model ofan atom put forward by Ernest*Rutherford in 1911 on the basis ofexperiments on the scattering ofalpha particles. The model consistedof a very dense positively charged nu-cleus, with electrons orbiting roundthe nucleus. This model presented aserious difÜculty for the classicaltheory of electricity and magnetism,which predicts that the electronshould spiral into the nucleus in afraction of a second, radiating elec-tromagnetic energy while doing so.This difÜculty led to the developmentof the *Bohr theory in 1913 and wasdeÜnitively solved by the develop-ment of quantum mechanics and itsapplication to atomic structure in themid-1920s.

rutile A mineral form oftitanium(IV) oxide, TiO2.

rutile structure A type of ioniccrystal structure in which the anionshave a hexagonal close packedarrangement with cations in half theoctahedral holes. The coordinationnumber of the anions is 6 and the co-ordination number of the cations is3. compounds with this structures in-clude TiO2, MnO2, SnO2, MgF2, andNiF2.


Ruybal test See scott’s test.

Rydberg constant Symbol R. Aconstant that occurs in the formulaefor atomic spectra and is related tothe binding energy between an elec-tron and a nucleon. It is connected toother constants by the relationship R

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= µ02me4c3/8h3, where µ0 is the mag-

netic constant, m and e are the massand charge of an electron, c is thespeed of light, and h is the *Planckconstant. It has the value 1.097 × 107

m–1. It is named after the Swedishphysicist Johannes Robert Rydberg(1854–1919), who developed a for-mula for the spectrum of hydrogen.

Rydberg spectrum An absorptionspectrum of a gas in the ultravioletregion, consisting of a series of linesthat become closer together towardsshorter wavelengths, merging into acontinuous absorption region. Theabsorption lines correspond to elec-tronic transitions to successivelyhigher energy levels. The onset ofthe continuum corresponds to pho-

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S

Sabatier–Senderens process Amethod of organic synthesis employ-ing hydrogenation and a heatednickel catalyst. It is employed com-mercially for hydrogenating unsatu-rated vegetable oils to makemargarine. It is named after PaulSabatier (1854–1941) and Jean-Bap-tiste Senderens (1856–1937).

saccharide See sugar.A• Information about IUPAC nomenclature

saccharin A white crystalline solid,C7H5NO3S, m.p. 224°C. It is madefrom a compound of toluene, derivedfrom petroleum or coal tar. It is awell-known artiÜcial sweetener,being some 500 times as sweet assugar (sucrose), and is usually mar-keted as its sodium salt. Because ofan association with cancer in labora-tory animals, its use is restricted insome countries.

SO2

NH

O

Saccharin

saccharose See sucrose.

Sachse reaction A reaction ofmethane at high temperature to pro-duce ethyne:

2CH4 → C2H2 + 3H2

The reaction occurs at about 1500°C,the high temperature being obtainedby burning part of the methane inair.

Sackur–Tetrode equation Anequation for the entropy of a perfectmonatomic gas. The entropy S isgiven by:

S = nRln(e5/2V/nNAΛ3),where Λ = h/(2πmkT)½, where n is theamount of the gas, R is the gas con-stant, e is the base of natural loga-rithms, V is the volume of thesystem, NA is the Avogadro constant,h is the Planck constant, m is themass of each atom, k is the Boltz-mann constant, and T is the thermo-dynamic temperature. To calculatethe molar entropy of the gas bothsides are divided by n. The Sackur–Tetrode equation can be used toshow that the entropy change ∆S,when a perfect gas expands isother-mally from Vi to Vf, is given by:

∆S = nRln(aVf) – nRln(aVi) =nRln(Vf/Vi),

where aV is the quantity inside thelogarithm bracket in the Sackur–Tetrode equation.

sacriÜcial protection (cathodicprotection) The protection of iron orsteel against corrosion (see rusting)by using a more reactive metal. Acommon form is galvanizing (see gal-vanized iron), in which the iron sur-face is coated with a layer of zinc.Even if the zinc layer is scratched,the iron does not rust because zincions are formed in solution in prefer-ence to iron ions. Pieces of magne-sium alloy are similarly used inprotecting pipelines, etc.

sal ammoniac See ammoniumchloride.

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combinations) A linear combinationof atomic orbitals (LCAO), which is a‘building block’ of the LCAO orbitalsmaking up a *molecular orbital. TheSALC are constructed by grouptheory appropriate for the symmetrygroup of the molecule. SALC are usedin the construction of molecular or-bitals.

salicylic acid (1-hydroxybenzoicacid) A naturally occurring car-boxylic acid, HOC6H4COOH, found incertain plants; r.d. 1.44; m.p. 159°C;sublimes at 211°C. It is used in thefoodstuffs and dyestuffs industries.See also aspirin.

saline Describing a chemical com-pound that is a salt, or a solutioncontaining a salt.

salinometer An instrument formeasuring the salinity of a solution.There are two main types: one is atype of hydrometer to measure den-sity; the other is an apparatus formeasuring the electrical conductivityof the solution.

sal soda Anhydrous *sodium car-bonate, Na2CO3.

salt A compound formed by reac-tion of an acid with a base, in whichthe hydrogen of the acid has been re-placed by metal or other positiveions. Typically, salts are crystallineionic compounds such as Na+Cl– andNH4

+NO3–. Covalent metal com-

pounds, such as TiCl4, are also oftenregarded as salts.

salt bridge An electrical connec-tion made between two half cells. Itusually consists of a glass U-tubeÜlled with agar jelly containing asalt, such as potassium chloride. Astrip of Ülter paper soaked in the saltsolution can also be used.

salt cake Industrial *sodium sul-phate.

salting in See salting out.

salting out The effect in which thesolubility of a substance in a certainsolvent is reduced by the presence ofa second solute dissolved in the sol-vent. For example, certain substancesdissolved in water can be precipi-tated (or evolved as a gas) by additionof an ionic salt. The substance ismore soluble in pure water than inthe salt solution. The opposite effectinvolving an increase in solubilitymay occur. This is known as saltingin.

saltpetre See nitre.

samarium Symbol Sm. A soft sil-very metallic element belonging tothe *lanthanoids; a.n. 62; r.a.m.150.35; r.d. 7.52 (20°C); m.p. 1077°C;b.p. 1791°C. It occurs in monaziteand bastnatite. There are seven natu-rally occurring isotopes, all of whichare stable except samarium–147,which is weakly radioactive (half-life2.5 × 1011 years). The metal is usedin special alloys for making nuclear-

reactor parts as it is a neutron ab-sorber. Samarium oxide (Sm2O3) isused in small quantities in special op-tical glasses. The largest use of the el-ement is in the ferromagnetic alloySmCo5, which produces permanentmagnets Üve times stronger than anyother material. The element was dis-covered by François Lecoq de Bois-baudran in 1879.


sand Particles of rock with diame-ters in the range 0.06–2.00 mm. Mostsands are composed chieÛy of parti-cles of quartz, which are derivedfrom the weathering of quartz-bearing rocks.

Sandmeyer reaction A reaction ofdiazonium salts used to preparechloro- or bromo-substituted aro-

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matic compounds. The method is todiazotize an aromatic amide at lowtemperature and add an equimolarsolution of the halogen acid and cop-per(I) halide. A complex of the diazo-nium salt and copper halide forms,which decomposes when the temper-ature is raised. The copper halide actsas a catalyst in the reaction of thehalide ions from the acid, for exam-ple

C6H5N2+(aq) + Cl–(aq) + CuCl(aq) →

C6H5Cl(l) + N2(g) + CuCl(aq)The reaction was discovered in 1884by the German chemist TraugottSandmeyer (1854–1922). See also gat-termann reaction.

sandwich compound A transi-tion-metal complex in which a metalatom or ion is ‘sandwiched’ betweentwo rings of atoms. *Ferrocene wasthe Ürst such compound to be pre-pared, having two parallel cyclopen-tadienyl rings with an iron ionbetween them. In such compounds(also known as metallocenes) themetal coordinates to the pi electronsystem of the ring, rather than to in-dividual atoms. A wide variety ofthese compounds are known, havingÜve-, six-, seven-, or eight-memberedrings and involving such metals asCr, Mn, Co, Ni, and Fe. Other similarcompounds are known. A multi-decker sandwich has three or moreparallel rings with metal atoms be-tween them. In a bent sandwich, therings are not parallel. A half sand-wich (or piano stool) has one ring,with single ligands on the other sideof the metal.

Sanger’s reagent 2,4-dinitro-Ûuorobenzene, C6H3F(NO2)2, used toidentify the end *amino acid in aprotein chain. It is named after Fred-erick Sanger (1918– ).

saponiÜcation The reaction of es-

ters with alkalis to give alcohols andsalts of carboxylic acids:

RCOOR′ + OH– → RCOO– + R′OHSee esterification; soap.

saponin A type of toxic *glycosidethat forms a frothy colloidal solutionon shaking with water. Saponinsoccur in many plants (such as horsechestnut). They break down redblood cells and have been used forpoisoning Üsh. On hydrolysis theyyield a variety of sugars.

sapphire Any of the gem varietiesof *corundum except ruby, especiallythe blue variety, but other colours ofsapphire include yellow, brown,green, pink, orange, and purple. Sap-phires are obtained from igneous andmetamorphic rocks and from alluvialdeposits. The chief sources are SriLanka, Kashmir, Burma, Thailand,East Africa, the USA, and Australia.Sapphires are used as gemstones andin record-player styluses and sometypes of laser. They are synthesizedby the Verneuil Ûame-fusion process.

sarin A highly toxic colourless liq-uid, C4H10FO2P; r.d. 1.09; m.p. –56°C;b.p. 158°C. It is an organophosphoruscompound, O-isopropyl methylphos-phonoÛuoridate. Sarin was discov-ered in 1938 and belongs to theG-series of *nerve agents (GB). It wasused by Iraq in the Iran–Iraq war(1980–88). In 1988, sarin was alsoused by Iraq in a poison gas attack onthe Kurd city of Halabja in the northof Iraq. About 5000 people died. In1994, a Japanese religious sect re-leased sarin in Matsumoto and in1995 released it in the Tokyo subway.

saturable laser See dye laser.

saturated 1. (of a compound) Con-sisting of molecules that have onlysingle bonds (i.e. no double or triplebonds). Saturated compounds can un-dergo substitution reactions but not

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addition reactions. Compare unsatu-rated. 2. (of a solution) Containingthe maximum equilibrium amountof solute at a given temperature. In asaturated solution the dissolved sub-stance is in equilibrium with undis-solved substance; i.e. the rate atwhich solute particles leave the solu-tion is exactly balanced by the rate atwhich they dissolve. A solution con-taining less than the equilibriumamount is said to be unsaturated.One containing more than the equi-librium amount is supersaturated.Supersaturated solutions can bemade by slowly cooling a saturatedsolution. Such solutions are meta-stable; if a small crystal seed is addedthe excess solute crystallizes out ofsolution. 3. (of a vapour) See vapourpressure.

saturation See supersaturation.

saturation spectroscopy A spec-troscopic technique using lasers to lo-cate absorption maxima with greatprecision. In saturation spectroscopythe laser beam is split into an intensesaturating beam and a less intensebeam that pass through the cavitycontaining the sample in almost op-posite directions. The saturatingbeam sometimes excites molecules,which are shifted to its frequency bythe Doppler effect. The other beamgives a modulated signal at the detec-tor if it is interacting with the sameDoppler-shifted molecules. Thesemolecules are not moving parallel tothe beams and have an extremelysmall Doppler shift, thus providingvery high resolution. See also lamb-dipspectroscopy.

sawhorse projection A type of *projection in which a three-dimensional view is drawn. See con-formation.

s-block elements The elements ofthe Ürst two groups of the *periodic

table; i.e. groups 1 (Li, Na, K, Rb, Cs,Fr) and 2 (Be, Mg, Ca, Sr, Ba, Ra). Theouter electronic conÜgurations ofthese elements all have inert-gasstructures plus outer ns1 (group 1) orns2 (group 2) electrons. The term thusexcludes elements with incompleteinner d-levels (transition metals) orwith incomplete inner f-levels (lan-thanoids and actinoids) even thoughthese often have outer ns2 or occa-sionally ns1 conÜgurations. Typically,the s-block elements are reactivemetals forming stable ionic com-pounds containing M+ or M2+ ions.See alkali metals; alkaline-earthmetals.

sc Synclinal. See torsion angle.

scandium Symbol Sc. A rare softsilvery metallic element belonging togroup 3 (formerly IIIA) of the peri-odic table; a.n. 21; r.a.m. 44.956; r.d.2.989 (alpha form), 3.19 (beta form);m.p. 1541°C; b.p. 2831°C. Scandiumoften occurs in *lanthanoid ores,from which it can be separated on ac-count of the greater solubility of itsthiocyanate in ether. The only nat-ural isotope, which is not radioactive,is scandium–45, and there are nineradioactive isotopes, all with rela-tively short half-lives. Because of themetal’s high reactivity and high costno substantial uses have been foundfor either the metal or its com-pounds. Predicted in 1869 by Dmitri*Mendeleev, and then called eka-boron, the oxide (called scandia) wasisolated by Lars Nilson (1840–99) in1879.A• Information from the WebElements site

scanning electron microscopeSee electron microscope.

scanning tunnelling microscope(STM) A type of electron microscopethat uses the quantum-mechanical*tunnel effect to study atomic struc-

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tures and observe single atoms. AÜne-pointed conducting tip near asurface of a sample causes electronsto tunnel from the surface to the tip.Its occurrence depends on the elec-tron density of the surface and thedistance between the tip and the sur-face itself. The electric current pro-duced is kept constant by moving thetip up or down as moves across thesurface.

scavenger A reagent that removesa trace component from a system orthat removes a reactive intermediatefrom a reaction.

SCF See self-consistent field.

Scheele, Karl Wilhelm (1742–86)Swedish chemist, who became anapothecary and in 1775 set up hisown pharmacy at Köping. He mademany chemical discoveries. In 1772 he prepared oxygen (see alsolavoisier, antoine; *Priestley,Joseph) and in 1774 he isolated chlo-rine. He also discovered manganese,glycerol, hydrocyanic (prussic) acid,citric acid, and many other sub-stances.

scheelite A mineral form of cal-cium tungstate, CaWO4, used as anore of tungsten. It occurs in contactmetamorphosed deposits and veindeposits as colourless or white tetrag-onal crystals.

Schiff base An imine with the gen-eral formula R1R2C = NR3, where R1,R2, and R3 are alkyl or aryl groups.

Schiff’s base A compound formedby a condensation reaction betweenan aromatic amine and an aldehydeor ketone, for example

RNH2 + R′CHO → RN:CHR′ + H2OThe compounds are often crystallineand are used in organic chemistry forcharacterizing aromatic amines (bypreparing the Schiff’s base and meas-uring the melting point). They are

named after the German chemistHugo Schiff (1834–1915).

Schiff’s reagent A reagent, de-vised by Hugo Schiff, used for testingfor aldehydes and ketones; it consistsof a solution of fuchsin dye that hasbeen decolorized by sulphur dioxide.Aliphatic aldehydes restore the pinkimmediately, whereas aromatic ke-tones have no effect on the reagent.Aromatic aldehydes and aliphatic ke-tones restore the colour slowly.

SchoenÛies system A system forcategorizing symmetries of mol-ecules. Cn groups contain only an n-fold rotation axis. Cnv groups, inaddition to the n-fold rotation axis,have a mirror plane that contains theaxis of rotation (and mirror planes as-sociated with the existence of the n-fold axis). Cnh groups, in addition tothe n-fold rotation axis, have a mirrorplane perpendicular to the axis. Sngroups have an n-fold rotation–reÛec-tion axis. Dn groups have an n-fold ro-tation axis and a two-fold axisperpendicular to the n-fold axis (andtwo-fold axes associated with the ex-istence of the n-fold axis). Dnh groupshave all the symmetry operations ofDn and also a mirror plane perpendic-ular to the n-fold axis. Dnd groups con-tain all the symmetry operations ofDn and also mirror planes that con-tain the n-fold axis and bisect the an-gles between the two-fold axes. Inthe SchoenÛies notation C stands for‘cyclic’, S stands for ‘spiegel’ (mirror),and D stands for ‘dihedral’. Thesubscripts h, v, and d stand for hori-zontal, vertical, and diagonal respec-tively, where these words refer to the position of the mirror planeswith respect to the n-fold axis (con-sidered to be vertical). In addition tothe noncubic groups referred to sofar, there are cubic groups, whichhave several rotation axes with thesame value of n. These are the tetra-

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hedral groups T, Th, and Td, the octa-hedral groups O and Oh, and theicosahedral group I. The SchoenÛiessystem is commonly used for isolatedmolecules, while the *Hermann–Mauguin system is commonly usedin crystallography.

schönite A mineral form of potas-sium sulphate, K2SO4.

Schottky defect See crystal de-fect.

Schrödinger, Erwin (1887–1961)Austrian physicist, who became pro-fessor of physics at Berlin Universityin 1927. He left for Oxford to escapethe Nazis in 1933, returned to Grazin Austria in 1936, and then leftagain in 1938 for Dublin’s Institute ofAdvanced Studies. He Ünally returnedto Austria in 1956. He is best knownfor the development of *wave me-chanics and the *Schrödinger equa-tion, work that earned him a share ofthe 1933 Nobel Prize for physics withPaul *Dirac.

Schrödinger equation An equa-tion used in wave mechanics (seequantum mechanics) for the wavefunction of a particle. The time-independent Schrödinger equation is:

∇2ψ + 8π2m(E – U)ψ/h2 = 0,where ψ is the wave function, ∇2 theLaplace operator, h the Planck con-stant, m the particle’s mass, E its totalenergy, and U its potential energy. Itcan also be written as:

Hψ = Eψ,where H is the *Hamiltonian opera-tor. It was devised by ErwinSchrödinger, who was mainly re-sponsible for wave mechanics.

Schweizer’s reagent A solutionmade by dissolving copper(II) hydrox-ide in concentrated ammonia solu-tion. It has a deep blue colour and isused as a solvent for cellulose in thecuprammonium process for making

rayon. When the cellulose solution isforced through spinnarets into anacid bath, Übres of cellulose are re-formed.

Scott’s test (cobalt thiocyanatetest; Ruybal test) A *presumptivetest for cocaine. Scott’s reagent hasan initial solution of 2% cobalt thio-cyanate (Co(SCN)2) in glycerine andwater. This is followed by concen-trated hydrochloric acid, and thenchloroform. A positive test is indi-cated by a blue colour in the chloro-form layer.

scrubber See absorption tower.

seaborgium Symbol Sg. A radio-active *transactinide element; a.n.106. It was Ürst detected in 1974 byAlbert Ghiorso and a team in Califor-nia. It can be produced by bombard-ing californium-249 nuclei withoxygen-18 nuclei. It is named afterthe US physicist Glenn Seaborg(1912–99).


sebacic acid See decanedioic acid.

secondary alcohol See alcohols.

secondary amine See amines.

secondary cell A *voltaic cell inwhich the chemical reaction produc-ing the e.m.f. is reversible and thecell can therefore be charged by pass-ing a current through it. See accumu-lator; intercalation cell. Compareprimary cell.

secondary emission The emissionof electrons from a surface as a resultof the impact of other charged parti-cles, especially as a result of bom-bardment with (primary) electrons.As the number of secondary elec-trons can exceed the number of pri-mary electrons, the process isimportant in photomultipliers. Seealso auger effect.

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secondary-ion mass spectrom-etry (SIMS) A technique for ana-lysing the chemical structure of asolid by bombarding it with ions. Asample of the solid to be analysed isbombarded with ions, referred to asprimary ions, having energies be-tween 5 and 20 keV. A number of dif-ferent types of entity are detachedfrom the surface of the solid, includ-ing neutral atoms and molecules,electrons, photons, and negative andpositive ions. The negative and posi-tive ions given off, called the sec-ondary ions, can be identiÜed usingmass spectrometry. Using this tech-nique solid samples can be character-ized with great accuracy.

second-order reaction See order.

sedimentation The settling of thesolid particles through a liquid eitherto produce a concentrated slurryfrom a dilute suspension or to clarifya liquid containing solid particles.Usually this relies on the force ofgravity, but if the particles are toosmall or the difference in density be-tween the solid and liquid phases istoo small, a *centrifuge may be used.In the simplest case the rate of sedi-mentation is determined by Stokes’slaw, but in practice the predictedrate is rarely reached. Measurementof the rate of sedimentation in an*ultracentrifuge can be used to esti-mate the size of macromolecules.

seed A crystal used to induce othercrystals to form from a gas, liquid, orsolution.

Seger cones (pyrometric cones) Aseries of cones used to indicate thetemperature inside a furnace or kiln.The cones are made from differentmixtures of clay, limestone, feld-spars, etc., and each one softens at adifferent temperature. The droopingof the vertex is an indication that theknown softening temperature has

been reached and thus the furnacetemperature can be estimated.

selection rules Rules that deter-mine which transitions between dif-ferent energy levels are possible in a system, such as an elementaryparticle, nucleus, atom, molecule, orcrystal, described by quantum me-chanics. Transitions cannot takeplace between any two energy levels.Group theory, associated with thesymmetry of the system, determineswhich transitions, called *allowedtransitions, can take place and which transitions, called *forbiddentransitions, cannot take place. Se-lection rules determined in this way are very useful in analysing thespectra of quantum-mechanical sys-tems.

selenides Binary compounds ofselenium with other more elec-tropositive elements. Selenides ofnonmetals are covalent (e.g. H2Se).Most metal selenides can be pre-pared by direct combination of theelements. Some are well-deÜnedionic compounds (containing Se2–),while others are nonstoichiometricinterstitial compounds (e.g. Pd4Se,PdSe2).

selenium Symbol Se. A metalloidelement belonging to group 16 (for-merly VIB) of the periodic table; a.n.34; r.a.m. 78.96; r.d. 4.81 (grey); m.p.217°C (grey); b.p. 684.9°C. There are anumber of allotropic forms, includ-ing grey, red, and black selenium. Itoccurs in sulphide ores of other met-als and is obtained as a by-product(e.g. from the anode sludge in elec-trolytic reÜning). The element is asemiconductor; the grey allotrope islight-sensitive and is used in photo-cells, xerography, and similar appli-cations. Chemically, it resemblessulphur, and forms compounds withselenium in the +2, +4, and +6 oxida-

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tion states. Selenium was discoveredin 1817 by Jöns Berzelius.


selenium cell Either of two typesof photoelectric cell; one type relieson the photoconductive effect, theother on the photovoltaic effect (seephotoelectric effect). In the photo-conductive selenium cell an externale.m.f. must be applied; as the sele-nium changes its resistance on expo-sure to light, the current produced isa measure of the light energy fallingon the selenium. In the photovoltaicselenium cell, the e.m.f. is generatedwithin the cell. In this type of cell, athin Ülm of vitreous or metallic sele-nium is applied to a metal surface, atransparent Ülm of another metal,usually gold or platinum, beingplaced over the selenium. Both typesof cell are used as light meters inphotography.

self-assembly See self-organiza-tion.

self-consistent Üeld (SCF) A con-cept used to Ünd approximate solu-tions to the many-body problem in*quantum mechanics. The procedurestarts with an approximate solutionfor a particle moving in a single-particle potential, which derivesfrom its average interaction with allthe other particles. This average in-teraction is determined by the *wavefunctions of all the other particles.The equation describing this averageinteraction is solved and the im-proved solution obtained is used inthe calculation of the interactionterm. This procedure is repeated forwave functions until the wave func-tions and associated energies are not signiÜcantly changed in thecycle, self-consistency having beenattained. In atomic theory the

*Hartree–Fock procedure makes useof self-consistent Üelds.

self-organization The sponta-neous order arising in a system whencertain parameters of the systemreach critical values. Self-organiza-tion occurs in many systems inphysics, chemistry, and biology. Itcan occur when a system is drivenfar from thermal *equilibrium. Sincea self-organizing system is open to itsenvironment, the second law of*thermodynamics is not violated bythe formation of an ordered phase, asentropy can be transferred to the en-vironment. Self-organization is re-lated to the concepts of brokensymmetry, complexity, nonlinearity,and *nonequilibrium statistical me-chanics. Many systems that undergotransitions to self-organization canalso undergo transitions to *chaos. Inchemistry, self-assembly is one of thefeatures of *supramolecular chem-istry.

Seliwanoff’s test A biochemicaltest to identify the presence of ke-tonic sugars, such as fructose, in so-lution. It was devised by the Russianchemist F. F. Seliwanoff. A few dropsof the reagent, consisting of resorci-nol crystals dissolved in equalamounts of water and hydrochloricacid, are heated with the test solu-tion and the formation of a red pre-cipitate indicates a positive result.

semicarbazones Organic com-pounds containing the unsaturatedgroup =C:N.NH.CO.NH2. They areformed when aldehydes or ke-tones react with a semicarbazide(H2N.NH.CO.NH2). Semicarbazonesare crystalline compounds with rela-tively high melting points. They areused to identify aldehydes and ke-tones in quantitative analysis: thesemicarbazone derivative is madeand identiÜed by its melting point.Semicarbazones are also used in sep-

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arating ketones from reaction mix-tures: the derivative is crystallizedout and hydrolysed to give the ke-tone.A• Information about IUPAC nomenclature

semiclassical approximation Anapproximation technique used to cal-culate quantities in quantum me-chanics. This technique is called thesemiclassical approximation becausethe *wave function is written as anasymptotic series with ascendingpowers of the Planck constant, h,with the Ürst term being purelyclassical. It is also known as theWentzel–Kramers–Brillouin (WKB)approximation, named after GregorWentzel (1898–1978), Hendrik AntonKramers (1894–1952), and Léon Bril-louin (1889–1969), who invented itindependently in 1926. The semi-classical approximation is particu-larly successful for calculationsinvolving the *tunnel effect, such asÜeld emission, and radioactive decayproducing alpha particles.

semiconductor A crystalline solidwith an electrical conductivity (typi-cally 105–10–7 siemens per metre)intermediate between that of aconductor (up to 109 S m–1) and an in-sulator (as low as 10–15 S m–1). Semi-conducting properties are a featureof *metalloid elements, such as sili-con and germanium. As the atoms ina crystalline solid are close together,the orbitals of their electrons overlapand their individual energy levels arespread out into energy bands. Con-duction occurs in semiconductors asthe result of a net movement, underthe inÛuence of an electric Üeld, ofelectrons in the conduction band andempty states, called holes, in the va-lence band. A hole behaves as if itwas an electron with a positivecharge. Electrons and holes areknown as the charge carriers in a

semiconductor. The type of chargecarrier that predominates in a partic-ular region or material is called themajority carrier and that with thelower concentration is the minoritycarrier. An intrinsic semiconductor isone in which the concentration ofcharge carriers is a characteristic ofthe material itself; electrons jump tothe conduction band from the va-lence band as a result of thermal ex-citation, each electron that makesthe jump leaving behind a hole inthe valence band. Therefore, in an in-trinsic semiconductor the charge car-riers are equally divided betweenelectrons and holes. In extrinsic semi-conductors the type of conductionthat predominates depends on thenumber and valence of the impurityatoms present. Germanium and sili-con atoms have a valence of four. Ifimpurity atoms with a valence ofÜve, such as arsenic, antimony, orphosphorus, are added to the lattice,there will be an extra electron peratom available for conduction, i.e.one that is not required to pair withthe four valence electrons of the ger-manium or silicon. Thus extrinsicsemiconductors doped with atoms ofvalence Üve give rise to crystals withelectrons as majority carriers, the so-called n-type conductors. Similarly, ifthe impurity atoms have a valence ofthree, such as boron, aluminium, in-dium, or gallium, one hole per atomis created by an unsatisÜed bond. Themajority carriers are therefore holes,i.e. p-type conductors.

semiconductor laser A type of*laser in which semiconductors pro-vide the excitation. The laser actionresults from electrons in the conduc-tion band (see energy bands) beingstimulated to recombine with holesin the valence band. When this oc-curs the electrons give up the energycorresponding to the band gap. Ma-

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terials, such as gallium arsenide, aresuitable for this purpose. A junctionbetween p-type and n-type semicon-ductors can be used with the lightpassing along the plane of the junc-tion. Mirrors for the laser action areprovided by the ends of the crystals.Semiconductor lasers can be as smallas 1 mm in length.

semi-empirical calculations Atechnique for calculating atomic andmolecular quantities in which thevalues of integrals are found usingquantities derived from experiment(such as ionization energies obtainedspectroscopically). Semi-empiricalcalculations were formerly widelyused, particularly for large moleculesinvolving extensive computation tocalculate all the integrals using *ab-initio calculations. As computingpower has increased, the propertiesof large molecules can be calculatedby ab-initio calculations, thereforethe use of semi-empirical calcula-tions has steadily declined, Ürst forsmall molecules and more recentlyfor large molecules.

semimetal See metalloid.

semipermeable membrane Amembrane that is permeable to mol-ecules of the solvent but not thesolute in *osmosis. Semipermeablemembranes can be made by support-ing a Ülm of material (e.g. cellulose)on a wire gauze or porous pot.

semipolar bond See chemicalbond.

Semtex A nitrogen-based stableodourless plastic *explosive.

septivalent (heptavalent) Having avalency of seven.

sequence rule See cip system.

sequestration The process offorming coordination complexes ofan ion in solution. Sequestration

often involves the formation ofchelate complexes, and is used toprevent the chemical effect of an ionwithout removing it from the solu-tion (e.g. the sequestration of Ca2+

ions in water softening). It is also away of supplying ions in a protectedform, e.g. sequestered iron solutionsfor plants in regions having alkalinesoil.

serine See amino acid.

serpentine Any of a group of hy-drous magnesium silicate mineralswith the general compositionMg3Si2O5(OH)4. Serpentine is mono-clinic and occurs in two main forms:chrysotile, which is Übrous and thechief source of *asbestos; and antig-orite, which occurs as platy masses. Itis generally green or white with amottled appearance, sometimes re-sembling a snakeskin – hence thename. It is formed through the meta-morphic alteration of ultrabasicrocks rich in olivine, pyroxene, andamphibole. Serpentinite is a rockconsisting mainly of serpentine; it isused as an ornamental stone.

sesqui- PreÜx indicating a ratio of2:3 in a chemical compound. For ex-ample, a sesquioxide has the formulaM2O3.

sesquiterpene See terpenes.

SEXAFS (surface-extended X-ray ab-sorption Üne-structure spectroscopy)A technique that makes use of the os-cillations in X-ray absorbance foundin the high-frequency side of the ab-sorption edge to investigate thestructure of surfaces. The physicalprinciple behind SEXAFS is quantum-mechanical interference between thewave function of a photoelectron andthe wave function associated withscattering by surrounding atoms. Inthe case of constructive interference,the photoelectron has a higher prob-ability of appearing, while in the case

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of destructive interference, the pho-toelectron has a lower probability ofappearing. The technique thereforeprovides information about the sur-roundings of an atom. Analysis of re-sults from SEXAFS has given a greatdeal of information about the struc-ture of surfaces and how they re-spond to adsorption.

sexivalent (hexavalent) Having avalency of six.

shale A form of *clay that occurs inthin layers. Shales are very commonsedimentary rocks. See also oil shale.

shell See atom.

shellac A hard resin produced as asecretion by a plant parasite, thesouth-east Asian lac insect Laciferlacca. It is used in sealing wax, var-nish (French polish), and electrical in-sulators.

sherardizing The process of coat-ing iron or steel with a zinc corro-sion-resistant layer by heating it incontact with zinc dust to a tempera-ture slightly below the melting pointof zinc. At a temperature of about371°C the two metals amalgamate toform internal layers of zinc–iron al-loys and an external layer of purezinc. The process was invented bySherard Cowper-Coles (d. 1935).

shock wave A very narrow regionof high pressure and temperatureformed in a Ûuid when the ÛuidÛows supersonically over a stationaryobject or a projectile Ûying superson-ically passes through a stationaryÛuid. A shock wave may also be gen-erated by violent disturbances in aÛuid, such as a lightning stroke or abomb blast. Shock waves are gener-ated experimentally to excite mol-ecules for spectroscopicinvestigations.

short period See periodic table.

sial The rocks that form the earth’scontinental crust. These are graniterock types rich in silica (SiO2) andaluminium (Al), hence the name.Compare sima.

side chain See chain.

side reaction A chemical reactionthat occurs at the same time as amain reaction but to a lesser extent,thus leading to other products mixedwith the main products.

siderite A brown or grey-greenmineral form of iron(II) carbonate,FeCO3, often with magnesium andmanganese substituting for the iron.It occurs in sedimentary deposits orin hydrothermal veins and is an im-portant iron ore. It is found in Eng-land, Greenland, Spain, N Africa, andthe USA.

siemens Symbol S. The SI unit ofelectrical conductance equal to theconductance of a circuit or elementthat has a resistance of 1 ohm. 1 S =10–1 Ω. The unit was formerly calledthe mho or reciprocal ohm. It isnamed after Ernst Werner vonSiemens (1816–92).

sievert The SI unit of dose equiva-lent (see radiation units). It isnamed after the Swedish physicistRolf Sievert (1896–1966).

sigma bond See orbital.

sigma electron An electron in asigma orbital. See orbital.

sigmatropic reaction A type ofrearrangement in which a sigmabond is formed between two non-linked atoms at the same time as anexisting sigma bond is broken. Sig-matropic rearrangements are a typeof *pericyclic reaction. An example isthe *Cope rearrangement.

silane (silicane) 1. A colourless gas,SiH4, which is insoluble in water; d.1.44 g dm–3; r.d. 0.68 (liquid); m.p.

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–185°C; b.p. –112°C. Silane is pro-duced by reduction of silicon withlithium tetrahydridoaluminate(III). Itis also formed by the reaction ofmagnesium silicide (Mg2Si) withacids, although other silicon hydridesare also produced at the same time.Silane itself is stable in the absenceof air but is spontaneously Ûamma-ble, even at low temperatures. It is areducing agent and has been used forthe removal of corrosion in inacces-sible plants (e.g. pipes in nuclear re-actors). 2. (or silicon hydride) Any ofa class of compounds of silicon andhydrogen. They have the general for-mula SinH2n+2. The Ürst three in theseries are silane itself (SiH4), disilane(Si2H6), and trisilane (Si3H8). The com-pounds are analogous to the alkanesbut are much less stable and only the lower members of the series canbe prepared in any quantity (up toSi6H14). No silicon hydrides contain-ing double or triple bonds exist (i.e.there are no analogues of the alkenesand alkynes).

silica See silicon(iv) oxide.

silica gel A rigid gel made by coag-ulating a sol of sodium silicate andheating to drive off water. It is usedas a support for catalysts and also asa drying agent because it readily ab-sorbs moisture from the air. The gelitself is colourless but, when used indesiccators, etc., a blue cobalt salt isadded. As moisture is taken up, thesalt turns pink, indicating that thegel needs to be regenerated (by heat-ing).

silicane See silane.

silicate Any of a group of sub-stances containing negative ionscomposed of silicon and oxygen. Thesilicates are a very extensive groupand natural silicates form the majorcomponent of most rocks (see sili-cate minerals). The basic structural

unit is the tetrahedral SiO4 group.This may occur as a simple discreteSiO4

4– anion as in the orthosilicates,e.g. phenacite (Be2SiO4) and willemite(Zn2SiO4). Many larger silicate speciesare also found (see illustration).These are composed of SiO4 tetrahe-dra linked by sharing oxygen atomsas in the pyrosilicates, Si2O7

6–, e.g.Sc2Si2O7. The linking can extend tosuch forms as bentonite, BaTiSi3O9,or alternatively inÜnite chain anions,which are single strand (*pyroxenes)or double strand (*amphiboles). Spo-dumene, LiAl(SiO3)2, is a pyroxeneand the asbestos minerals are amphi-boles. Large two-dimensional sheetsare also possible, as in the various*micas (see illustration), and the linking can extend to full three-dimensional framework structures,often with substituted trivalentatoms in the lattice. The *zeolites are examples of this.

silicate minerals A group of rock-forming minerals that make up thebulk of the earth’s outer crust (about90%) and constitute one-third of allminerals. All silicate minerals arebased on a fundamental structuralunit – the SiO4 tetrahedron (see sili-cate). They consist of a metal (e.g.calcium, magnesium, aluminium)combined with silicon and oxygen.The silicate minerals are classiÜed ona structural basis according to howthe tetrahedra are linked together.The six groups are: nesosilicates (e.g.olivine and *garnet); sorosilicates(e.g. hemimorphite); cyclosilicates(e.g. axinite, *beryl, and *tourma-line); inosilicates (e.g. *amphibolesand *pyroxenes); phyllosilicates (e.g.*micas, *clay minerals, and *talc);and tektosilicates (e.g. *feldspars and*feldspathoids). Many silicate miner-als are of economic importance.

silicide A compound of silicon witha more electropositive element. The

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silicon 482

s

silicides are structurally similar tothe interstitial carbides but the rangeencountered is more diverse. Theyreact with mineral acids to form arange of *silanes.

silicon Symbol Si. A metalloid el-

ement belonging to *group 14 (for-merly IVB) of the periodic table; a.n.14; r.a.m. 28.086; r.d. 2.33; m.p.1410°C; b.p. 2355°C. Silicon is thesecond most abundant element inthe earth’s crust (25.7% by weight) oc-

Si

O

O

OO

=

SiO4– as in Be2SiO

4 (phenacite)

Si2O2– as in Sc

2Si

2O

7(thortveitite)

Si3 O6– as in BaTiSi

3O

9 (bentonite) Si

6O12– as in Be

3Al

2Si

6O

18 (beryl)

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single chain : pyroxenes

double chain : amphiboles sheet : micas

Structure of some discrete silicate ions

Structure of some polymeric silicate ions

Silicate minerals

Structure of some polymeric silicate ions

Structure of some discrete silicate ions


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curring in various forms of silicon(IV)oxide (e.g. *quartz) and in *silicateminerals. The element is extractedby reducing the oxide with carbon inan electric furnace and is used exten-sively for its semiconductor proper-ties. It has a diamond-like crystalstructure; an amorphous form alsoexists. Chemically, silicon is less reac-tive than carbon. The element com-bines with oxygen at red heat and isalso dissolved by molten alkali. Thereis a large number of organosiliconcompounds (e.g. *siloxanes) althoughsilicon does not form the range ofsilicon–hydrogen compounds andderivatives that carbon does (seesilane). The element was identiÜedby Antoine *Lavoisier in 1787 andÜrst isolated in 1823 by Jöns*Berzelius.


silicon carbide (carborundum) Ablack solid compound, SiC, insolublein water and soluble in molten alkali;r.d. 3.217; m.p. c. 2700°C. Silicon car-bide is made by heating silicon(IV)oxide with carbon in an electric fur-nace (depending on the grade re-quired sand and coke may be used). Itis extremely hard and is widely usedas an abrasive. The solid exists inboth zinc blende and wurtzite struc-tures. There is a clear form known asmoissanite, used as a diamond substi-tute.

silicon dioxide See silicon(iv)oxide.

silicones Polymeric compoundscontaining chains of silicon atoms al-ternating with oxygen atoms, withthe silicon atoms linked to organicgroups. A variety of silicone ma-terials exist, including oils, waxes,and rubbers. They tend to be moreresistant to temperature and chemi-

cal attack than their carbon ana-logues.

silicon hydride See silane.

silicon(IV) oxide (silicon dioxide;silica) A colourless or white vitreoussolid, SiO2, insoluble in water andsoluble (by reaction) in hydroÛuoricacid and in strong alkali; m.p.1713°C; b.p. 2230°C. The followingforms occur naturally: cristobalite(cubic or tetragonal crystals; r.d.2.32); tridymite (rhombic; r.d. 2.26);*quartz (hexagonal; r.d. 2.63–2.66);lechatelierite (r.d. 2.19). Quartz hastwo modiÜcations: α-quartz below575°C and β-quartz above 575°C;above 870°C β-quartz is slowly trans-formed to tridymite and above1470°C this is slowly converted tocristobalite. Various forms ofsilicon(IV) oxide occur widely in theearth’s crust; yellow sand for exam-ple is quartz with iron(III) oxide im-purities and Ûint is essentiallyamorphous silica. The gemstonesamethyst, opal, and rock crystal arealso forms of quartz.

Silica is an important commercialmaterial in the form of silica brick, ahighly refractive furnace lining,which is also resistant to abrasionand to corrosion. Silicon(IV) oxide isalso the basis of both clear andopaque silica glass, which is used onaccount of its transparency to ultravi-olet radiation and its resistance toboth thermal and mechanical shock.A certain proportion of silicon(IV)oxide is also used in ordinary glassand in some glazes and enamels. Italso Ünds many applications as a dry-ing agent in the form of *silica gel.

siloxanes A group of compoundscontaining silicon atoms bound tooxygen atoms, with organic groupslinked to the silicon atoms, e.g.R3SiOSiR3, where R is an organicgroup. *Silicones are polymers ofsiloxanes.

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silver Symbol Ag. A white lustroussoft metallic *transition element; a.n.47; r.a.m. 107.87; r.d. 10.5; m.p.961.93°C; b.p. 2212°C. It occurs as theelement and as the minerals argen-tite (Ag2S) and horn silver (AgCl). It isalso present in ores of lead and cop-per, and is extracted as a by-productof smelting and reÜning these met-als. The element is used in jewellery,tableware, etc., and silver com-pounds are used in photography.Chemically, silver is less reactivethan copper. A dark silver sulphideforms when silver tarnishes in air be-cause of the presence of sulphurcompounds. Silver(I) ionic salts exist(e.g. AgNO3, AgCl) and there are anumber of silver(II) complexes.


silver(I) bromide A yellowishsolid compound, AgBr; r.d. 6.5; m.p.432°C. It can be precipitated from sil-ver(I) nitrate solution by adding a so-lution containing bromide ions. Itdissolves in concentrated ammoniasolutions (but, unlike the chloride,does not dissolve in dilute ammonia).The compound is used in photo-graphic emulsions.

silver(I) chloride A white solidcompound, AgCl; r.d. 5.6; m.p. 455°C;b.p. 1550°C. It can be precipitatedfrom silver(I) nitrate solution byadding a solution of chloride ions. Itdissolves in ammonia solution (dueto formation of the complex ion[Ag(NH3)2]+). The compound is used inphotographic emulsions.

silver(I) iodide A yellow solidcompound, AgI; r.d. 6.01; m.p. 558°C;b.p. 1506°C. It can be precipitatedfrom silver(I) nitrate solutions byadding a solution of iodide ions. Un-like the chloride and bromide, it doesnot dissolve in ammonia solutions.

silver-mirror test See tollensreagent.

silver(I) nitrate A colourless solid,AgNO3; r.d. 4.3; m.p. 212°C. It is animportant silver salt because it iswater-soluble. It is used in photogra-phy. In the laboratory, it is used as atest for chloride, bromide, and iodideions and in volumetric analysis ofchlorides using an *adsorption indi-cator.

silver(I) oxide A brown slightlywater-soluble amorphous powder,Ag2O; r.d. 7.14. It can be made byadding sodium hydroxide solution tosilver(I) nitrate solution. Silver(I)oxide is strongly basic and is also anoxidizing agent. It is used in certainreactions in preparative organicchemistry; for example, moistsilver(I) oxide converts haloalkanesinto alcohols; dry silver oxide converts haloalkanes into ethers. The compound decomposes to the elements at 300°C and can be reduced by hydrogen to silver. With ozone it gives the oxide AgO(which is diamagnetic and probablyAgIAgIIIO2).

sima The rocks that form theearth’s oceanic crust and underliethe upper crust. These are basalticrock types rich in silica (SiO2) andmagnesium (Mg), hence the name.The sima is denser and more plasticthan the *sial that forms the conti-nental crust.

Simmons–Smith reaction A reac-tion in which a cyclopropane ring isproduced form an alkene. It uses theSimmons–Smith reagent, which wasoriginally diiodomethane (CH2I2)with a Zn/Cu couple. Usually, diethylzinc is used rather than Zn/Cu. Themechanism involves the formation ofH2C(I)(ZnI) and *carbene transferfrom the zinc to the double bond ofthe alkene.

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Simon test A *presumptive test formethamphetamines. The Simonreagent consists of 1 gram of sodiumnitroprusside (Na2Fe(CN)5 NO) and 2ml of acetaldehyde (ethanal) in 50 mlof water. A 2% solution of sodiumcarbonate is then added. MDA gives aturquoise colour; MDMA gives an ini-tial turquoise, which changes to darkblue.

SIMS See secondary-ion mass spec-trometry.

single bond See chemical bond.

singlet An atomic state in whichtwo spin angular momenta of elec-trons cancel each other, resulting inzero net spin. A singlet state usuallyhas a higher energy than a *tripletstate because of the effect of correla-tions of spin on the Coulomb interac-tions between electrons (as in*Hund’s rules). This can lead to sub-stantial energy differences betweentriplet and singlet states.

sintered glass Porous glass madeby sintering powdered glass, used forÜltration of precipitates in gravimet-ric analysis.

sintering The process of heatingand compacting a powdered materialat a temperature below its meltingpoint in order to weld the particlestogether into a single rigid shape.Materials commonly sintered includemetals and alloys, glass, and ceramicoxides. Sintered magnetic materials,cooled in a magnetic Üeld, make es-pecially retentive permanent mag-nets.

S-isomer See absolute configura-tion.

SI units Système International d’Unités: the international system ofunits now recommended for all sci-entiÜc purposes. A coherent and ra-tionalized system of units derivedfrom the *m.k.s. units, SI units have

now replaced *c.g.s. units and *Impe-rial units. The system has seven baseunits and two dimensionless units(formerly called supplementaryunits), all other units being derivedfrom these nine units. There are 18derived units with special names.Each unit has an agreed symbol (acapital letter or an initial capital let-ter if it is named after a scientist,otherwise the symbol consists of oneor two lower-case letters). Decimalmultiples of the units are indicatedby a set of preÜxes; whenever possi-ble a preÜx representing 10 raised toa power that is a multiple of threeshould be used. See Appendix.

skeletal electrons See wade’srules.

skew See torsion angle.

slag Material produced during the*smelting or reÜning of metals by re-action of the Ûux with impurities(e.g. calcium silicate formed by reac-tion of calcium oxide Ûux with sili-con dioxide impurities). The liquidslag can be separated from the liquidmetal because it Ûoats on the surface.See also basic slag.

slaked lime See calcium hydrox-ide.

SLN See sybyl line notation.

SLUMO Second-lowest unoccupiedmolecular orbital. See subjacent or-bitals.

slurry A paste consisting of a sus-pension of a solid in a liquid.

SMARTS See smiles.

smectic See liquid crystal.

smelting The process of separatinga metal from its ore by heating theore to a high temperature in a suit-able furnace in the presence of a re-ducing agent, such as carbon, and aÛuxing agent, such as limestone. Iron

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ore is smelted in this way so that themetal melts and, being denser thanthe molten *slag, sinks below theslag, enabling it to be removed fromthe furnace separately.

SMILES SimpliÜed molecular inputline entry system. A type of line nota-tion for representing molecules andreactions. It uses standard atom sym-bols but hydrogen atoms are omitted.A double bond is represented by anequals sign and a triple bond by ahash sign. So ethane is CC, ethene isC=C, and ethyne is C#C.

Branched structures are givenusing brackets. For example, triethyl-amine ((C2H5)3N) is represented byCCN(CC)CC; ethanoic (acetic acid)(CH3COOC) is CC(=O)O. Ring struc-tures use numbers to show connec-tions. For instance, cyclohexane isC1CCCCC1, where the C1 atoms arethe ones joined to form the ring.SMILES notation for aromatic mol-ecules uses lower case letters. Ben-zene is c1ccccc1; chlorobenzenecould be c1c(Cl)cccc1. Rules exist forexpressing isotope and stereochem-istry information and there is a nota-tion for reactions. Rules also exist forchoosing a unique SMILES, which is adeÜnitive representation of the manypossible valid strings that may bewritten for a given compound. Anextension of SMILES to describe mo-lecular patterns (for substructuresearches) is called SMARTS.

A• A SMILES tutorial produced by Daylight

Chemical Information Systems Inc.

smoke A Üne suspension of solidparticles in a gas.

SN1 reaction See nucleophilic sub-stitution.

SN2 reaction See nucleophilic sub-stitution.

SNG Substitute (or synthetic) nat-

ural gas; a mixture of gaseous hydro-carbons produced from coal, petro-leum, etc., and suitable for use as afuel. Before the discovery of naturalgas *coal gas was widely used as a do-mestic and industrial fuel. This gaveway to natural gas in the early partof this century in the US and othercountries where natural gas wasplentiful. The replacement of coalgas occurred somewhat later in theUK and other parts of Europe. Morerecently, interest has developed inways of manufacturing hydrocarbongas fuels. The main sources are coaland the naphtha fraction of petro-leum. In the case of coal three meth-ods have been used: (1) pyrolysis – i.e.more efÜcient forms of destructivedistillation, often with further hydro-genation of the hydrocarbon prod-ucts; (2) heating the coal withhydrogen and catalysts to give hydro-carbons – a process known ashydroliquefaction (see also bergiusprocess); (3) producing carbonmonoxide and hydrogen and obtain-ing hydrocarbons by the *Fischer–Tropsch process. SNG from naptha ismade by steam *reforming. See crgprocess.

soap A substance made by boilinganimal fats with sodium hydroxide.The reaction involves the hydrolysisof *glyceride esters of fatty acids toglycerol and sodium salts of the acidspresent (mainly the stearate, oleate,and palmitate), giving a soft semi-solid with *detergent action. Potas-sium hydroxide gives a more liquidproduct (soft soap). By extension,other metal salts of long-chain fattyacids are also called soaps. See alsosaponification.

soda Any of a number of sodiumcompounds, such as caustic soda(NaOH) or, especially, washing soda(Na2CO3.10H2O).

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soda ash Anhydrous *sodium car-bonate, Na2CO3.

soda lime A mixed hydroxide ofsodium and calcium made by slakinglime with caustic soda solution (togive NaOH + Ca(OH)2) and recoveringgreyish white granules by evapora-tion. The material is produced largelyfor industrial adsorption of carbondioxide and water, but also Ündssome applications in pollution andefÛuent control. It is also used as alaboratory drying agent.

sodamide See sodium amide.

Soddy, Frederick (1877–1956)British chemist, who worked withErnest *Rutherford in Canada andWilliam *Ramsay in London beforeÜnally settling in Oxford in 1919. Hisannouncement in 1913 of the exis-tence of *isotopes won him the 1921Nobel Prize for physics.

sodium Symbol Na. A soft silveryreactive element belonging to group1 (formerly IA) of the periodic table(see alkali metals); a.n. 11; r.a.m.22.9898; r.d. 0.97; m.p. 97.8°C; b.p.882–889°C. Sodium occurs as thechloride in sea water and in the min-eral halite. It is extracted by electrol-ysis in a Downs cell. The metal isused as a reducing agent in certainreactions and liquid sodium is also acoolant in nuclear reactors. Chemi-cally, it is highly reactive, oxidizingin air and reacting violently withwater (it is kept under oil). It dis-solves in liquid ammonia to formblue solutions containing solvatedelectrons. Sodium is a major *essen-tial element required by living organ-isms. The element was Ürst isolatedby Humphry Davy in 1807.


sodium acetate See sodiumethanoate.

sodium aluminate A white solid,NaA1O2 or Na2Al2O4, which is in-soluble in ethanol and soluble inwater giving strongly alkaline solu-tions; m.p. 1800°C. It is manufac-tured by heating bauxite withsodium carbonate and extracting the residue with water, or it may beprepared in the laboratory by addingexcess aluminium to hot concen-trated sodium hydroxide. In solu-tion the ion Al(OH)4– predominates.Sodium aluminate is used as amordant, in the production of zeo-lites, in efÛuent treatment, in glassmanufacture, and in cleansing com-pounds.

sodium amide (sodamide) A whitecrystalline powder, NaNH2, which de-composes in water and in warmethanol, and has an odour of ammo-nia; m.p. 210°C; b.p. 400°C. It is pro-duced by passing dry ammonia overmetallic sodium at 350°C. It reactswith red-hot carbon to give sodiumcyanide and with nitrogen(I) oxide togive sodium azide.

sodium azide A white or colour-less crystalline solid, NaN3, soluble inwater and slightly soluble in alcohol;hexagonal; r.d. 1.846; decomposes onheating. It is made by the action ofnitrogen(I) oxide on hot sodamide(NaNH2) and is used as an organicreagent and in the manufacture ofdetonators.

sodium benzenecarboxylate(sodium benzoate) An either colour-less crystalline or white amorphouspowder, C6H5COONa, soluble inwater and slightly soluble in ethanol.It is made by the reaction of sodiumhydroxide with benzoic acid and isused in the dyestuffs industry and asa food preservative. It was formerlyused as an antiseptic.

sodium benzoate See sodium ben-zenecarboxylate.

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sodium bicarbonate See sodiumhydrogencarbonate.

sodium bisulphate See sodiumhydrogensulphate.

sodium bisulphite See sodium hy-drogensulphite.

sodium bromide A white crys-talline solid, NaBr, known chieÛy asthe dihydrate (monoclinic; r.d. 2.17),and as the anhydrous salt (cubic; r.d.3.20; m.p. 747°C; b.p. 1390°C). The di-hydrate loses water at about 52°Cand is very slightly soluble in alcohol.Sodium bromide is prepared by thereaction of bromine on hot sodiumhydroxide solution or of hydrogenbromide on sodium carbonate solu-tion. It is used in photographic pro-cessing and in analytical chemistry.

sodium carbonate Anhydroussodium carbonate (soda ash, sal soda)is a white powder, which cakes andaggregates on exposure to air due to the formation of hydrates. Themonohydrate, Na2CO3.H2O, is a whitecrystalline material, which is solublein water and insoluble in alcohol; r.d.2.532; loses water at 109°C; m.p.851°C.

The decahydrate, Na2CO3.10H2O(washing soda), is a translucentefÛorescent crystalline solid; r.d.1.44; loses water at 32–34°C to givethe monohydrate; m.p. 851°C.Sodium carbonate may be manufac-tured by the *Solvay process or bysuitable crystallization proceduresfrom any one of a number of naturaldeposits, such as:trona (Na2CO3.NaHCO3.2H2O),natron (Na2CO3.10H2O),ranksite (2Na2CO3.9Na2SO4.KCl),pirsonnite (Na2CO3.CaCO3.2H2O),gaylussite (Na2CO3.CaCO3.5H2O).The method of extraction is very sen-sitive to the relative energy costs andtransport costs in the region in-volved. Sodium carbonate is used in

photography, in cleaning, in pH con-trol of water, in textile treatment,glasses and glazes, and as a food addi-tive and volumetric reagent. See alsosodium sesquicarbonate.

sodium chlorate(V) A white crys-talline solid, NaClO3; cubic; r.d. 2.49;m.p. 250°C. It decomposes above itsmelting point to give oxygen andsodium chloride. The compound issoluble in water and in ethanol andis prepared by the reaction of chlo-rine on hot concentrated sodiumhydroxide. Sodium chlorate is a pow-erful oxidizing agent and is used inthe manufacture of matches and softexplosives, in calico printing, and asa garden weedkiller.

sodium chloride (common salt) Acolourless crystalline solid, NaCl, sol-uble in water and very slightly solu-ble in ethanol; cubic; r.d. 2.17; m.p.801°C; b.p. 1413°C. It occurs as themineral *halite (rock salt) and in nat-ural brines and sea water. It has theinteresting property of a solubility inwater that changes very little withtemperature. It is used industrially asthe starting point for a range ofsodium-based products (e.g. Solvayprocess for Na2CO3, Castner–Kellnerprocess for NaOH), and is known uni-versally as a preservative and sea-soner of foods. Sodium chloride has akey role in biological systems inmaintaining electrolyte balances.

sodium chloride structure Seerock salt structure.

sodium cyanide A white orcolourless crystalline solid, NaCN,deliquescent, soluble in water and inliquid ammonia, and slightly solublein ethanol; cubic; m.p. 564°C; b.p.1496°C. Sodium cyanide is now madeby absorbing hydrogen cyanide insodium hydroxide or sodium carbon-ate solution. The compound is ex-tremely poisonous because it reacts

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with the iron in haemoglobin in theblood, so preventing oxygen reachingthe tissues of the body. It is used inthe extraction of precious metals andin electroplating industries. Aqueoussolutions are alkaline due to salt hy-drolysis.

sodium dichromate A red crys-talline solid, Na2Cr2O7.2H2O, solublein water and insoluble in ethanol. Itis usually known as the dihydrate(r.d. 2.52), which starts to lose waterabove 100°C; the compound decom-poses above 400°C. It is made bymelting chrome iron ore with limeand soda ash and acidiÜcation of thechromate thus formed. Sodiumdichromate is cheaper than the cor-responding potassium compound buthas the disadvantage of being hygro-scopic. It is used as a mordant in dye-ing, as an oxidizing agent in organicchemistry, and in analytical chem-istry.

sodium dihydrogenorthophos-phate See sodium dihydrogen-phosphate(v).

sodium dihydrogenphos-phate(V) (sodium dihydrogen-orthophosphate) A colourlesscrystalline solid, NaH2PO4, which issoluble in water and insoluble in al-cohol, known as the monohydrate(r.d. 2.04) and the dihydrate (r.d.1.91). The dihydrate loses one watermolecule at 60°C and the secondmolecule of water at 100°C, followedby decomposition at 204°C. The com-pound may be prepared by treatingsodium carbonate with an equimolarquantity of phosphoric acid or byneutralizing phosphoric acid withsodium hydroxide. It is used in thepreparation of sodium phosphate(Na3PO4), in baking powders, as afood additive, and as a constituent ofbuffering systems. Both sodium dihy-drogenphosphate and trisodiumphosphate enriched in 32P have been

used to study phosphate participa-tion in metabolic processes.

sodium dioxide See sodium super-oxide.

sodium ethanoate (sodium ac-etate) A colourless crystalline com-pound, CH3COONa, which is knownas the anhydrous salt (r.d. 1.52; m.p.324°C) or the trihydrate (r.d. 1.45;loses water at 58°C). Both forms aresoluble in water and in ethoxy-ethane, and slightly soluble inethanol. The compound may be pre-pared by the reaction of ethanoicacid (acetic acid) with sodium carbon-ate or with sodium hydroxide. Be-cause it is a salt of a strong base anda weak acid, sodium ethanoate isused in buffers for pH control inmany laboratory applications, infoodstuffs, and in electroplating. It isalso used in dyeing, soaps, pharma-ceuticals, and in photography.

sodium Ûuoride A crystallinecompound, NaF, soluble in water andvery slightly soluble in ethanol;cubic; r.d. 2.56; m.p. 993°C; b.p.1695°C. It occurs naturally as villiau-mite and may be prepared by the re-action of sodium hydroxide or ofsodium carbonate with hydrogenÛuoride. The reaction of sodiumÛuoride with concentrated sulphuricacid may be used as a source of hy-drogen Ûuoride. The compound isused in ceramic enamels and as apreservative agent for fermentation.It is highly toxic but in very dilute so-lution (less than 1 part per million) itis used in the Ûuoridation of waterfor the prevention of tooth decay onaccount of its ability to replace OHgroups with F groups in the materialof dental enamel.

sodium formate See sodiummethanoate.

sodium hexaÛuoraluminate Acolourless monoclinic solid, Na3AlF6,

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very slightly soluble in water; r.d. 2.9;m.p. 1000°C. It changes to a cubicform at 580°C. The compound occursnaturally as the mineral *cryolite buta considerable amount is manufac-tured by the reaction of aluminiumÛuoride wth alumina and sodium hy-droxide or directly with sodium alu-minate. Its most important use is inthe manufacture of aluminium inthe *Hall–Heroult cell. It is also usedin the manufacture of enamels,opaque glasses, and ceramic glazes.

sodium hydride A white crys-talline solid, NaH; cubic; r.d. 0.92; de-composes above 300°C (slow);completely decomposed at 800°C.Sodium hydride is prepared by thereaction of pure dry hydrogen withsodium at 350°C. Electrolysis ofsodium hydride in molten LiCl/KClleads to the evolution of hydrogen;this is taken as evidence for the ionicnature of NaH and the presence ofthe hydride ion (H–). It reacts vio-lently with water to give sodium hy-droxide and hydrogen, with halogensto give the halide and appropriatehydrogen halide, and ignites sponta-neously with oxygen at 230°C. It is apowerful reducing agent with severallaboratory applications.

sodium hydrogencarbonate (bicarbonate of soda; sodium bicar-bonate) A white crystalline solid,NaHCO3, soluble in water andslightly soluble in ethanol; mono-clinic; r.d. 2.159; loses carbon dioxideabove 270°C. It is manufactured inthe *Solvay process and may be pre-pared in the laboratory by passingcarbon dioxide through sodium car-bonate or sodium hydroxide solution.Sodium hydrogencarbonate reactswith acids to give carbon dioxideand, as it does not have strongly cor-rosive or strongly basic properties it-self, it is employed in bulk for thetreatment of acid spillage and in

medicinal applications as an antacid.Sodium hydrogencarbonate is alsoused in baking powders (and isknown as baking soda), dry-powderÜre extinguishers, and in the textiles,tanning, paper, and ceramics indus-tries. The hydrogencarbonate ion hasan important biological role as an in-termediate between atmosphericCO2/H2CO3 and the carbonate ionCO3

2–. For water-living organisms thisis the most important and in somecases the only source of carbon.

sodium hydrogensulphate(sodium bisulphate) A colourlesssolid, NaHSO4, known in anhydrousand monohydrate forms. The anhy-drous solid is triclinic (r.d. 2.435; m.p.>315°C). The monohydrate is mono-clinic and deliquescent (r.d. 2.103;m.p. 59°C). Both forms are soluble inwater and slightly soluble in alcohol.Sodium hydrogensulphate was origi-nally made by the reaction betweensodium nitrate and sulphuric acid,hence its old name of nitre cake. Itmay be manufactured by the reac-tion of sodium hydroxide with sul-phuric acid, or by heating equimolarproportions of sodium chloride andconcentrated sulphuric acid. Solu-tions of sodium hydrogensulphateare acidic. On heating the compounddecomposes (via Na2S2O7) to give sul-phur trioxide. It is used in papermaking, glass making, and textileÜnishing.

sodium hydrogensulphite(sodium bisulphite) A white solid,NaHSO3, which is very soluble inwater (yellow in solution) andslightly soluble in ethanol; mono-clinic; r.d. 1.48. It decomposes onheating to give sodium sulphate, sul-phur dioxide, and sulphur. It isformed by saturating a solution ofsodium carbonate with sulphur diox-ide. The compound is used in thebrewing industry and in the steriliza-

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tion of wine casks. It is a general an-tiseptic and bleaching agent. See alsoaldehydes.

sodium hydroxide (caustic soda)A white transluscent deliquescentsolid, NaOH, soluble in water andethanol but insoluble in ether; r.d.2.13; m.p. 318°C; b.p. 1390°C. Hy-drates containing 7, 5, 4, 3.5, 3, 2,and 1 molecule of water are known.

Sodium hydroxide was formerlymade by the treatment of sodiumcarbonate with lime but its mainsource today is from the electrolysisof brine using mercury cells or any ofa variety of diaphragm cells. Theprincipal product demanded fromthese cells is chlorine (for use in plas-tics) and sodium hydroxide is almostreduced to the status of a by-product.It is strongly alkaline and Ünds manyapplications in the chemical indus-try, particularly in the production ofsoaps and paper. It is also used to ad-sorb acidic gases, such as carbondioxide and sulphur dioxide, and isused in the treatment of efÛuent forthe removal of heavy metals (as hy-droxides) and of acidity. Sodium hy-droxide solutions are extremelycorrosive to body tissue and are par-ticularly hazardous to the eyes.

sodium iodide A white crystallinesolid, NaI, very soluble in water andsoluble in both ethanol and ethanoicacid. It is known in both the anhy-drous form (cubic; r.d. 3.67; m.p.661°C; b.p. 1304°C) and as the dihy-drate (monoclinic; r.d. 2.45). It is pre-pared by the reaction of hydrogeniodide with sodium carbonate orsodium hydroxide in solution. Likepotassium iodide, sodium iodide inaqueous solution dissolves iodine toform a brown solution containingthe I3

– ion. It Ünds applications inphotography and is also used in med-icine as an expectorant and in the ad-ministration of radioactive iodine for

studies of thyroid function and fortreatment of diseases of the thyroid.

sodium methanoate (sodium for-mate) A colourless deliquescentsolid, HCOONa, soluble in water andslightly soluble in ethanol; mono-clinic; r.d. 1.92; m.p. 253°C; decom-poses on further heating. Themonohydrate is also known. Thecompound may be produced by thereaction of carbon monoxide withsolid sodium hydroxide at 200°C and10 atmospheres pressure; in the labo-ratory it can be conveniently pre-pared by the reaction of methanoicacid and sodium hydroxide. Its usesare in the production of oxalic acid(ethanedioic acid) and methanoicacid and in the laboratory it is a con-venient source of carbon monoxide.

sodium monoxide A whitish-greydeliquescent solid, Na2O; r.d. 2.27;sublimes at 1275°C. It is manufac-tured by oxidation of the metal in alimited supply of oxygen and puriÜedby sublimation. Reaction with waterproduces sodium hydroxide. Its com-mercial applications are similar tothose of sodium hydroxide.

sodium nitrate (Chile saltpetre) Awhite solid, NaNO3, soluble in waterand in ethanol; trigonal; r.d. 2.261;m.p. 306°C; decomposes at 380°C. Arhombohedral form is also known. Itis obtained from deposits of calicheor may be prepared by the reactionof nitric acid with sodium hydroxideor sodium carbonate. It was previ-ously used for the manufacture ofnitric acid by heating with concen-trated sulphuric acid. Its main use isin nitrate fertilizers.

sodium nitrite A yellow hygro-scopic crystalline compound, NaNO2,soluble in water, slightly soluble inether and in ethanol; rhombohedral;r.d. 2.17; m.p. 271°C; decomposesabove 320°C. It is formed by the ther-

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mal decomposition of sodium nitrateand is used in the preparation of ni-trous acid (reaction with cold dilutehydrochloric acid). Sodium nitrite isused in organic *diazotization and asa corrosion inhibitor.

sodium nitroprusside test Seesimon’s test.

sodium orthophosphate Seetrisodium phosphate(v).

sodium peroxide A whitish solid(yellow when hot), Na2O2, soluble inice-water and decomposed in warmwater or alcohol; r.d. 2.80; decom-poses at 460°C. A crystalline octa-hydrate (hexagonal) is obtained bycrystallization from ice-water. Thecompound is formed by the combus-tion of sodium metal in excess oxy-gen. At normal temperatures it reacts with water to give sodiumhydroxide and hydrogen peroxide. It is a powerful oxidizing agent react-ing with iodine vapour to give theiodate and periodate, with carbon at300°C to give the carbonate, andwith nitrogen(II) oxide to give thenitrate. It is used as a bleaching agent in wool and yarn processing, in the reÜning of oils and fats, and in the production of wood pulp.

sodium sesquicarbonate A whitecrystalline hydrated double salt,Na2CO3.NaHCO3.2H2O, soluble inwater but less alkaline than sodiumcarbonate; r.d. 2.12; decomposes onheating. It may be prepared by crys-tallizing equimolar quantities of theconstituent materials; it also occursnaturally as trona and in Searles Lake brines. It is widely used as adetergent and soap builder and,because of its mild alkaline proper-ties, as a water-softening agent andbath-salt base. See also sodium car-bonate.

sodium sulphate A white crys-talline compound, Na2SO4, usually

known as the anhydrous compound(orthorhombic; r.d. 2.67; m.p. 888°C)or the decahydrate (monoclinic; r.d.1.46; which loses water at 100°C).The decahydrate is known asGlauber’s salt. A metastable heptahy-drate (Na2SO4.7H2O) also exists. Allforms are soluble in water, dissolvingto give a neutral solution. The com-pound occurs naturally as

mirabilite (Na2SO4.10H2O),threnardite (Na2SO4), andglauberite (Na2SO4.CaSO4).

Sodium sulphate may be producedindustrially by the reaction of mag-nesium sulphate with sodium chlo-ride in solution followed bycrystallization, or by the reaction ofconcentrated sulphuric acid withsolid sodium chloride. The lattermethod was used in the *Leblancprocess for the production of alkaliand has given the name salt cake toimpure industrial sodium sulphate.Sodium sulphate is used in the man-ufacture of glass and soft glazes andin dyeing to promote an even Ünish.It also Ünds medicinal application asa purgative and in commercial aperi-ent salts.

sodium sulphide A yellow-redsolid, Na2S, formed by the reductionof sodium sulphate with carbon(coke) at elevated temperatures. It isa corrosive and readily oxidized ma-terial of variable composition andusually contains polysulphides of thetype Na2S2, Na2S3, and Na2S4, whichcause the variety of colours. It isknown in an anhydrous form (r.d.1.85; m.p. 1180°C) and as a nonahy-drate, Na2S.9H2O (r.d. 1.43; decom-poses at 920°C). Other hydrates ofsodium sulphide have been reported.The compound is deliquescent, solu-ble in water with extensive hydroly-sis, and slightly soluble in alcohol. Itis used in wood pulping, dyestuffsmanufacture, and metallurgy on ac-

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count of its reducing properties. Ithas also been used for the productionof sodium thiosulphate (for the pho-tographic industry) and as a depila-tory agent in leather preparation. Itis a strong skin irritant.

sodium sulphite A white solid,Na2SO3, existing in an anhydrousform (r.d. 2.63) and as a heptahydrate(r.d. 1.59). Sodium sulphite is solublein water and because it is readily oxi-dized it is widely used as a conve-nient reducing agent. It is preparedby reacting sulphur dioxide with ei-ther sodium carbonate or sodium hy-droxide. Dilute mineral acids reversethis process and release sulphur diox-ide. Sodium sulphite is used as ableaching agent in textiles and inpaper manufacture. Its use as an an-tioxidant in some canned foodstuffsgives rise to a slightly sulphuroussmell immediately on opening, butits use is prohibited in meats or foodsthat contain vitamin B1. Sodium sul-phite solutions are occasionally usedas biological preservatives.

sodium–sulphur cell A type of*secondary cell that has molten elec-trodes of sodium and sulphur sepa-rated by a solid electrolyte consistingof beta alumina (a crystalline form ofaluminium oxide). When the cell isproducing current, sodium ions Ûowthrough the alumina to the sulphur,where they form sodium polysul-phide. Electrons from the sodiumÛow in the external circuit. The op-posite process takes place duringcharging of the cell. Sodium–sulphurbatteries have been considered foruse in electric vehicles because oftheir high peak power levels and rel-atively low weight. However, some ofthe output has to be used to main-tain the operating temperature(about 370°C) and the cost of sodiumis high.

sodium superoxide (sodium diox-

ide) A whitish-yellow solid, NaO2,formed by the reaction of sodiumperoxide with excess oxygen at el-evated temperatures and pressures. Itreacts with water to form hydrogenperoxide and oxygen.

sodium thiosulphate (hypo) Acolourless efÛorescent solid, Na2S2O3,soluble in water but insoluble inethanol, commonly encountered asthe pentahydrate (monoclinic; r.d.1.73; m.p. 42°C), which loses water at100°C to give the anhydrous form(r.d. 1.66). It is prepared by the reac-tion of sulphur dioxide with a sus-pension of sulphur in boiling sodiumhydroxide solution. Aqueous solu-tions of sodium thiosulphate arereadily oxidized in the presence ofair to sodium tetrathionate andsodium sulphate. The reaction withdilute acids gives sulphur and sul-phur dioxide. It is used in the photo-graphic industry and in analyticalchemistry.

soft acid See hsab principle.

soft base See hsab principle.

soft soap See soap.

soft water See hardness of water.

sol A *colloid in which small solidparticles are dispersed in a liquidcontinuous phase.

solder An alloy used to join metalsurfaces. A soft solder melts at a tem-perature in the range 200–300°C andconsists of a tin–lead alloy. The tincontent varies between 80% for thelower end of the melting range and31% for the higher end. Hard solderscontain substantial quantities of sil-ver in the alloy. Brazing solders areusually alloys of copper and zinc,which melt at over 800°C.

solid A state of matter in whichthere is a three-dimensional regular-ity of structure, resulting from the

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proximity of the component atoms,ions, or molecules and the strengthof the forces between them. Truesolids are crystalline (see also amor-phous). If a crystalline solid isheated, the kinetic energy of thecomponents increases. At a speciÜctemperature, called the meltingpoint, the forces between the compo-nents become unable to containthem within the crystal structure. Atthis temperature, the lattice breaksdown and the solid becomes a liquid.

solid solution A crystalline ma-terial that is a mixture of two ormore components, with ions, atoms,or molecules of one component re-placing some of the ions, atoms, ormolecules of the other component inits normal crystal lattice. Solid solu-tions are found in certain alloys. Forexample, gold and copper form solidsolutions in which some of the cop-per atoms in the lattice are replacedby gold atoms. In general, the goldatoms are distributed at random, anda range of gold–copper compositionsis possible. At a certain composition,the gold and copper atoms can eachform regular individual lattices (re-ferred to as superlattices). Mixed crys-tals of double salts (such as alums)are also examples of solid solutions.Compounds can form solid solutionsif they are isomorphous (see isomor-phism).

solidus A line on a phase diagrambelow which a substance is solid.

solubility The quantity of solutethat dissolves in a given quantity ofsolvent to form a saturated solution.Solubility is measured in kilogramsper metre cubed, moles per kilogramof solvent, etc. The solubility of asubstance in a given solvent dependson the temperature. Generally, for asolid in a liquid, solubility increaseswith temperature; for a gas, solubil-ity decreases. See also concentration.

solubility product Symbol Ks. Theproduct of the concentrations of ionsin a saturated solution. For instance,if a compound AxBy is in equilibriumwith its solution

AxBy(s) ˆ xA+(aq) + yB–(aq)the equilibrium constant is

Kc = [A+]x[B–]y/[AxBy]Since the concentration of the undis-solved solid can be put equal to 1,the solubility product is given by

Ks = [A+]x[B–]y

The expression is only true for spar-ingly soluble salts. If the product ofionic concentrations in a solution ex-ceeds the solubility product, thenprecipitation occurs.

solute The substance dissolved in asolvent in forming a *solution.

solution A homogeneous mixtureof a liquid (the *solvent) with a gasor solid (the solute). In a solution, themolecules of the solute are discreteand mixed with the molecules of sol-vent. There is usually some interac-tion between the solvent and solutemolecules (see solvation). Two liq-uids that can mix on the molecularlevel are said to be miscible. In thiscase, the solvent is the major compo-nent and the solute the minor com-ponent. See also solid solution.

solvation The interaction of ions ofa solute with the molecules of sol-vent. For instance, when sodiumchloride is dissolved in water thesodium ions attract polar water mol-ecules, with the negative oxygenatoms pointing towards the positiveNa+ ion. Solvation of transition-metalions can also occur by formation ofcoordinate bonds, as in the hexaquo-copper(II) ion [Cu(H2O)6]2+. Solvationis the process that causes ionic solidsto dissolve, because the energy re-leased compensates for the energynecessary to break down the crystal

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lattice. It occurs only with polar sol-vents. Solvation in which the solventis water is called hydration.

Solvay process (ammonia–sodaprocess) An industrial method ofmaking sodium carbonate from cal-cium carbonate and sodium chloride.The calcium carbonate is Ürst heatedto give calcium oxide and carbondioxide, which is bubbled into a solu-tion of sodium chloride in ammonia.Sodium hydrogencarbonate is precip-itated:

H2O + CO2(g) + NaCl(aq) + NH3(aq)→ NaHCO3(s) + NH4Cl(aq)

The sodium hydrogencarbonate isheated to give sodium carbonate andcarbon dioxide. The ammonium chlo-ride is heated with calcium oxide(from the Ürst stage) to regeneratethe ammonia. The process waspatented in 1861 by the Belgianchemist Ernest Solvay (1838–1922).

solvent A liquid that dissolves an-other substance or substances toform a *solution. Polar solvents arecompounds such as water and liquidammonia, which have dipole mo-ments and consequently high dielec-tric constants. These solvents arecapable of dissolving ionic com-pounds or covalent compounds thationize (see solvation). Nonpolar sol-vents are compounds such asethoxyethane and benzene, which donot have permanent dipole mo-ments. These do not dissolve ioniccompounds but will dissolve nonpo-lar covalent compounds. Solvents canbe further categorized according totheir proton-donating and acceptingproperties. Amphiprotic solvents self-ionize and can therefore act both asproton donators and acceptors. A typ-ical example is water:

2H2O ˆ H3O+ + OH–

Aprotic solvents neither accept nor

donate protons; tetrachloromethane(carbon tetrachloride) is an example.

solvent extraction The process ofseparating one constituent from amixture by dissolving it in a solventin which it is soluble but in whichthe other constituents of the mixtureare not. The process is usually carriedout in the liquid phase, in which caseit is also known as liquid–liquid ex-traction. In liquid–liquid extraction,the solution containing the desiredconstituent must be immiscible withthe rest of the mixture. The processis widely used in extracting oil fromoil-bearing materials.

solvolysis A reaction between acompound and its solvent. See hy-drolysis.

soman A highly toxic colourlessvolatile liquid, C7H16FO2P; r.d. 1.02;m.p. –42°C; b.p. 198°C. It is anorganophosphorus compound, O-pinacolyl methylphosphonoÛuori-date. Soman was discovered in 1944and belongs to the G-series of *nerveagents (GD).

SOMO Singly occupied molecularorbital.

sonochemistry The study of chem-ical reactions in liquids subjected tohigh-intensity sound or ultrasound.This causes the formation, growth,and collapse of tiny bubbles withinthe liquid, generating localized cen-tres of very high temperature andpressure, with extremely rapid cool-ing rates. Such conditions are suit-able for studying novel reactions,decomposing polymers, and produc-ing amorphous materials.

sorbitol A polyhydric alcohol,CH2OH(CHOH)4CH2OH, derived fromglucose; it is isomeric with *manni-tol. It is found in rose hips and rowanberries and is manufactured by thecatalytic reduction of glucose with

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hydrogen. Sorbitol is used as a sweet-ener (in diabetic foods) and in themanufacture of vitamin C and vari-ous cosmetics, foodstuffs, and medi-cines.

sorption *Absorption of a gas by asolid.

sorption pump A type of vacuumpump in which gas is removed froma system by absorption on a solid(e.g. activated charcoal or a zeolite) atlow temperature.

Soxhlet apparatus An apparatusfor extracting components from asolid (e.g. extracting natural productsfrom plant material). The materialused is placed in a thimble made ofthick Ülter paper and this is held in aspecially designed reÛex condenserwith a suitable solvent. The chamberholding the thimble Ülls with warmsolvent and this is led back to thesource via a side arm. The apparatuscan be operated for long periods,with components concentrating inthe source vessel. It is named afterFranz Soxhlet, who devised it in1879.

sp Synperiplanar. See torsionangle.

space group A group of symmetryelements applying to a lattice. Com-pare point group.

species A chemical entity, such as aparticular atom, ion, or molecule.

speciÜc 1. Denoting that an exten-sive physical quantity so described isexpressed per unit mass. For exam-ple, the speciÜc latent heat of a bodyis its latent heat per unit mass. Whenthe extensive physical quantity is de-noted by a capital letter (e.g. L for la-tent heat), the speciÜc quantity isdenoted by the corresponding lower-case letter (e.g. l for speciÜc latentheat). 2. In some older physicalquantities the adjective ‘speciÜc’ was

added for other reasons (e.g. speciÜcgravity, speciÜc resistance). Thesenames are now no longer used.

speciÜc activity See activity.

speciÜc gravity See relative den-sity; specific.

speciÜc heat capacity See heatcapacity.

spectrochemical series A seriesof ligands arranged in the order inwhich they cause splitting of the en-ergy levels of d-orbitals in metal com-plexes (see crystal-field theory).The series for some common ligandshas the form:

CN–>NO2–>NH3>C5H5N>H2O>OH–>

F–>Cl–>Br–>I–

spectrograph See spectroscope.

spectrometer Any of various in-struments for producing a spectrumand measuring the wavelengths, en-ergies, etc., involved. A simple type,for visible radiation, is a spectroscopeequipped with a calibrated scale al-lowing wavelengths to be read off orcalculated. In the X-ray to infrared re-gion of the electromagnetic spec-trum, the spectrum is produced bydispersing the radiation with a prismor diffraction grating (or crystal, inthe case of hard X-rays). Some formof photoelectric detector is used, andthe spectrum can be obtained as agraphical plot, which shows how theintensity of the radiation varies withwavelength. Such instruments arealso called spectrophotometers. Spec-trometers also exist for investigatingthe gamma-ray region and the mi-crowave and radio-wave regions ofthe spectrum (see electron paramag-netic resonance; nuclear mag-netic resonance). Instruments forobtaining spectra of particle beamsare also called spectrometers (seespectrum; photoelectron spec-troscopy).

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spectrophotometer See spectrom-eter.

spectroscope An optical instru-ment that produces a *spectrum forvisual observation. The Ürst such in-strument was made by R. W. Bunsen;in its simplest form it consists of ahollow tube with a slit at one end bywhich the light enters and a collimat-ing lens at the other end to producea parallel beam, a prism to dispersethe light, and a telescope for viewingthe spectrum. In the spectrograph,the spectroscope is provided with acamera to record the spectrum.

For a broad range of spectroscopicwork, from the ultraviolet to the in-frared, a diffraction grating is usedinstead of a prism. See also spectrom-eter.

spectroscopy The study of meth-ods of producing and analysing*spectra using *spectroscopes, *spec-trometers, spectrographs, and spec-trophotometers. The interpretations

of the spectra so produced can beused for chemical analysis, examin-ing atomic and molecular energy lev-els and molecular structures, and fordetermining the composition andmotions of celestial bodies.

A• Original paper by Kirchoff and Bunsen

spectrum (pl. spectra) 1. A distribu-tion of entities or properties arrayedin order of increasing or decreasingmagnitude. For example, a beam ofions passed through a mass spectro-graph, in which they are deÛected ac-cording to their charge-to-massratios, will have a range of massescalled a mass spectrum. A soundspectrum is the distribution of en-ergy over a range of frequencies of aparticular source. 2. A range of elec-tromagnetic energies arrayed inorder of increasing or decreasingwavelength or frequency (see elec-tromagnetic spectrum). The emis-sion spectrum of a body or substance

X-raysX-raysX-rays ultravioletultravioletultraviolet microwavesmicrowavesmicrowaves

radioradioradiowaveswaveswaves

10-4 10210-2 10-1 106103 10410-3

wavelength (micrometres)

3 × 1018 3 × 1016 3 × 1014 3 × 1012 3 × 1010 3 × 108

frequency (hertz)

inner shellelectrons

outer shell(valence) electrons

molecular vibrations molecular rotations

1 10 105

infraredinfraredinfrared

visi

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Spectroscopy


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is the characteristic range of radia-tions it emits when it is heated, bom-barded by electron or ions, orabsorbs photons. The absorptionspectrum of a substance is producedby examining, through the substanceand through a spectroscope, a contin-uous spectrum of radiation. The ener-gies removed from the continuousspectrum by the absorbing mediumshow up as black lines or bands.With a substance capable of emittinga spectrum, these are in exactly thesame positions in the spectrum assome of the lines and bands in theemission spectrum.

Emission and absorption spectramay show a continuous spectrum, aline spectrum, or a band spectrum. Acontinuous spectrum contains an un-broken sequence of frequencies overa relatively wide range; it is producedby incandescent solids, liquids, andcompressed gases. Line spectra arediscontinuous lines produced by ex-cited atoms and ions as they fall backto a lower energy level. Band spectra(closely grouped bands of lines) arecharacteristic of molecular gases orchemical compounds. See also spec-troscopy.

speculum An alloy of copper andtin formerly used in reÛecting tele-scopes to make the main mirror as itcould be cast, ground, and polishedto make a highly reÛective surface. Ithas now been largely replaced by sil-vered glass for this purpose.

sphalerite (zinc blende) A mineralform of zinc sulphide, ZnS, crystalliz-ing in the cubic system; the principalore of zinc. It is usually yellow-brownto brownish-black in colour andoccurs, often with galena, in meta-somatic deposits and also in hydro-thermal veins and replacementdeposits. Sphalerite is mined onevery continent, the chief sources in-

cluding the USA, Canada, Mexico,Russia, Australia, Peru, and Poland.

sphalerite structure (zinc-blendestructure) A type of ionic crystalstructure in which the anions havean expanded face-centred cubicarrangement with the cations occu-pying one type of tetrahedral hole.The coordination number of eachtype of ion is 4. Examples of com-pounds with the sphalerite structureare ZnS, CuCl, CdS, and InAs.


spherical top See moment of iner-tia.

sphingolipid See phospholipid.

spiegel (spiegeleisen) A form of*pig iron containing 15–30% of man-ganese and 4–5% of carbon. It isadded to steel in a Bessemer con-verter as a deoxidizing agent and toraise the manganese content of steel.

spin (intrinsic angular momentum)Symbol s. The part of the total angu-lar momentum of a particle, atom,nucleus, etc., that can continue toexist even when the particle is appar-ently at rest, i.e. when its transla-tional motion is zero and thereforeits orbital angular momentum iszero. A molecule, atom, or nucleus ina speciÜed energy level, or a particu-lar elementary particle, has a particu-lar spin, just as it has a particularcharge or mass. According to *quan-tum theory, this is quantized and isrestricted to multiples of h/2π, whereh is the *Planck constant. Spin ischaracterized by a quantum numbers. For example, for an electron s =±½, implying a spin of + h/4π when itis spinning in one direction and –h/4πwhen it is spinning in the other. Be-cause of their spin, particles alsohave their own intrinsic magneticmoments and in a magnetic Üeld the

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spin of the particles lines up at anangle to the direction of the Üeld,precessing around this direction. Seealso nuclear magnetic resonance.

spinel A group of oxide mineralswith the general formula F2+R2

3+O4,where F2+ = Mg, Fe, Zn, Mn, or Ni andR3+ = Al, Fe, or Cr, crystallizing in thecubic system. The spinels are dividedinto three series: spinel (MgAl2O4),*magnetite, and *chromite. Theyoccur in high-temperature igneous ormetamorphic rocks.

spinel structure An ionic crystalstructure shown by compounds ofthe type AB2O4. There is a face-centred cubic arrangement of O2–

ions. The A cations occupy oneeighth of the tetrahedral holes andthe B cations occupy the octahedralholes. Example of the spinel struc-ture are MgAl2O4, Fe3O4, and Mn3O4.


spin glass An alloy of a smallamount of a magnetic metal(0.1–10%) with a nonmagnetic metal,in which the atoms of the magneticelement are randomly distributedthrough the crystal lattice of the non-magnetic element. Examples areAuFe and CuMn. Theories of themagnetic and other properties ofspin glasses are complicated by therandom distribution of the magneticatoms.

spin label A molecule or groupthat contains an unpaired electronand can be attached to another mol-ecule. The spin of the unpaired elec-tron can be detected by electronparamagnetic resonance. The tech-nique of spin labelling is used to in-vestigate proteins and biologicalsystems.

spin–lattice relaxation A processin which electrons in a crystal return

to the distribution for equilibriumstatistical mechanics after some per-turbation, such as magnetic reso-nance, has caused more electronspins to be in high-energy states. Theexcess energy of these high-energystates is taken up by vibrations of thelattice of the solid.

spinodal curve A curve that sepa-rates a metastable region from an un-stable region in the coexistenceregion of a binary Ûuid. Above thespinodal curve the process of movingtowards equilibrium occurs bydroplet nucleation, while below thespinodal curve there are periodicmodulations of the order parameter,which have a small amplitude at Ürst(see spinodal decomposition). Thespinodal curve is not a sharp bound-ary in real systems as a result of Ûuc-tuations.

spinodal decomposition Theprocess of moving towards equilib-rium in a part of a phase diagram inwhich the order parameter is con-served. Spinodal decomposition is ob-served in the quenching of binarymixtures. See also spinodal curve.

spin–orbit coupling An interac-tion between the orbital angular mo-mentum and the spin angularmomentum of an individual particle,such as an electron. For light atoms,spin–orbit coupling is small so that*multiplets of many-electron atomsare described by *Russell–Saunderscoupling. For heavy atoms, spin–orbitcoupling is large so that multiplets ofmany-electron atoms are describedby *j-j coupling. For medium-sizedatoms the sizes of the energies asso-ciated with spin–orbit coupling arecomparable to the sizes of energiesassociated with electrostatic repul-sion between the electrons, the mul-tiplets in this case being described ashaving intermediate coupling.

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Spin–orbit coupling is large in manynuclei, particularly heavy nuclei.

spirits of salt A name formerlygiven to hydrogen chloride becausethis compound can be made byadding sulphuric acid to commonsalt (sodium chloride).

spiro compound A type of com-pound in which there are two ringslinked through a single atom (thespiro atom). In most cases, the spiroatom is a carbon atom, and the ringsare not in the same plane. Somespiro compounds with substituentson both rings may have a chiralityelement and consequently show opti-cal activity.A• Information about IUPAC nomenclature• Alternative naming system for spiro

compounds

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Spiro compound

spontaneous combustion Com-bustion in which a substance pro-duces sufÜcient heat within itself,usually by a slow oxidation process,for ignition to take place without the need for an external high-temperature energy source.

spontaneous emission The emis-sion of a photon by an atom as itmakes a transition from an excitedstate to the ground state. Sponta-neous emission occurs independentlyof any external electromagnetic radi-ation; the transition is caused by in-teractions between atoms andvacuum Ûuctuations of the quantizedelectromagnetic Üeld. The process ofspontaneous emission, which cannotbe described by nonrelativistic*quantum mechanics, as given by

formulations such as the *Schrö-dinger equation, is responsible forthe limited lifetime of an excitedstate of an atom before it emits aphoton.

spot test A simple test for a givensubstance using a reagent thatchanges colour when mixed with thesubstance. In forensic science spottests are often called *presumptivetests.

sputtering The process by whichsome of the atoms of an electrode(usually a cathode) are ejected as a re-sult of bombardment by heavy posi-tive ions. Although the process isgenerally unwanted, it can be used toproduce a clean surface or to deposita uniform Ülm of a metal on an ob-ject in an evacuated enclosure.

squalene An intermediate com-pound formed in the synthesis ofcholesterol; it is a hydrocarbon con-taining 30 carbon atoms. The im-mediate oxidation of squalene tosqualene 2,3-epoxide is the last com-mon step in the synthesis of *sterolsin animals, plants, and fungi.

square-planar Describing a coordi-nation compound in which four lig-ands positioned at the corners of asquare coordinate to a metal ion atthe centre of the square. See complex.

stabilization energy The amountby which the energy of a delocalizedchemical structure is less than thetheoretical energy of a structure withlocalized bonds. It is obtained by sub-tracting the experimental heat of for-mation of the compound (in kJ mol–1)from that calculated on the basis of aclassical structure with localizedbonds.

stabilizer 1. A substance used to in-hibit a chemical reaction, i.e. a nega-tive catalyst. 2. A substance used toprevent a colloid from coagulating.


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stable equilibrium See equilib-rium.

staggered See conformation.

staggered conformation See con-formation.

stainless steel A form of *steelcontaining at least 11–12% ofchromium, a low percentage of car-bon, and often some other elements,notably nickel and molybdenum.Stainless steel does not rust or stainand therefore has a wide variety ofuses in industrial, chemical, and do-mestic environments. A particularlysuccessful alloy is the steel known as18–8, which contains 18% Cr, 8% Ni,and 0.08% C.

stalactites and stalagmites Ac-cretions of calcium carbonate inlimestone caves. Stalactites are taper-ing cones or pendants that hangdown from the roofs of caves; stalag-mites are upward projections fromthe cave Ûoor and tend to be broaderat their bases than stalactites. Bothare formed from drips of water con-taining calcium hydrogencarbonatein solution and may take thousandsof years to grow.

standard cell A *voltaic cell, suchas a *Clark cell, or *Weston cell,used as a standard of e.m.f.

standard electrode An electrode(a half cell) used in measuring elec-trode potential. See hydrogen halfcell.

standard electrode potential Seeelectrode potential.

standard solution A solution ofknown concentration for use in volu-metric analysis.

standard state A state of a systemused as a reference value in thermo-dynamic measurements. Standardstates involve a reference value ofpressure (usually one atmosphere,

101.325 kPa) or concentration (usu-ally 1 M). Thermodynamic functionsare designated as ‘standard’ whenthey refer to changes in which reac-tants and products are all in theirstandard and their normal physicalstate. For example, the standardmolar enthalpy of formation of waterat 298 K is the enthalpy change forthe reaction

H2(g) + ½O2(g) → H2O(l)∆HŠ298 = –285.83 kJ mol–1. Note thesuperscript Š is used to denote stan-dard state and the temperatureshould be indicated.

standard temperature and pres-sure See s.t.p.

stannane See tin(iv) hydride.

stannate A compound formed byreaction of tin oxides (or hydroxides)with alkali. Tin oxides are ampho-teric (weakly acidic) and react to givestannate ions. Tin(IV) oxide withmolten alkali gives the stannate(IV)ion

SnO2 + 2OH– → SnO32– + H2O

In fact, there are various ions presentin which the tin is bound to hydrox-ide groups, the main one being thehexahydroxostannate(IV) ion,Sn(OH)62–. This is the negative ionpresent in crystalline ‘trihydrates’ ofthe type K2Sn2O3.3H2O.

Tin(II) oxide gives the trihydroxo-stannate(II) ion in alkaline solutions

SnO(s) + OH–(aq) + H2O(l) →Sn(OH)3–(aq)

Stannate(IV) compounds were for-merly referred to as orthostannates(SnO4

4–) or metastannates (SnO32–).

Stannate(II) compounds were calledstannites.

stannic compounds Compoundsof tin in its higher (+4) oxidationstate; e.g. stannic chloride is tin(IV)chloride.

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stannite See stannate.

stannous compounds Com-pounds of tin in its lower (+2) oxida-tion state; e.g. stannous chloride istin(II) chloride.

starch A *polysaccharide consistingof various proportions of two glucosepolymers, *amylose and *amy-lopectin. It occurs widely in plants,especially in roots, tubers, seeds, andfruits, as a carbohydrate storageproduct and energy source. Starch istherefore a major energy source foranimals. When digested it ultimatelyyields glucose. Starch granules are in-soluble in cold water but disrupt ifheated to form a gelatinous solution.This gives an intense blue colourwith iodine solutions and starch isused as an *indicator in certain titra-tions.

Stark effect The splitting of linesin the *spectra of atoms due to thepresence of a strong electric Üeld. Itis named after the German physicistJohannes Stark (1874–1957), who dis-covered it in 1913. Like the normal*Zeeman effect, the Stark effect canbe understood in terms of the classi-cal electron theory of Lorentz. TheStark effect for hydrogen atoms wasalso described by the Bohr theory ofthe atom. In terms of quantum me-chanics, the Stark effect is describedby regarding the electric Üeld as aperturbation on the quantum statesand energy levels of an atom in theabsence of an electric Üeld. This ap-plication of perturbation theory wasits Ürst use in quantum mechanics.

Stark–Einstein law The law stat-ing that in a photochemical process(such as a photochemical reaction)one photon is absorbed by each mol-ecule causing the main photochemi-cal process. In some circumstances,one molecule, having absorbed aphoton, initiates a process involv-

ing several molecules. The Stark–Einstein law is named after JohannesStark and Albert *Einstein.

state of matter One of the threephysical states in which matter canexist, i.e. *solid, *liquid, or *gas.Plasma is sometimes regarded as thefourth state of matter.

stationary phase See chromatog-raphy.

stationary state A state of a sys-tem when it has an energy level per-mitted by *quantum mechanics.Transitions from one stationary stateto another can occur by the emissionor absorption of an appropriatequanta of energy (e.g. in the form ofphotons).

statistical mechanics The branchof physics in which statistical meth-ods are applied to the microscopicconstituents of a system in order topredict its macroscopic properties.The earliest application of thismethod was Boltzmann’s attempt toexplain the thermodynamic proper-ties of gases on the basis of the statis-tical properties of large assemblies ofmolecules.

In classical statistical mechanics,each particle is regarded as occupy-ing a point in phase space, i.e. tohave an exact position and momen-tum at any particular instant. Theprobability that this point will oc-cupy any small volume of the phasespace is taken to be proportional tothe volume. The Maxwell–Boltzmannlaw gives the most probable distribu-tion of the particles in phase space.

With the advent of quantumtheory, the exactness of thesepremises was disturbed (by theHeisenberg uncertainty principle). Inthe *quantum statistics that evolvedas a result, the phase space is dividedinto cells, each having a volume hf,where h is the Planck constant and f

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is the number of degrees of freedomof the particles. This new concept ledto Bose–Einstein statistics, and forparticles obeying the Pauli exclusionprinciple, to Fermi–Dirac statistics.

steam distillation A method ofdistilling liquids that are immisciblewith water by bubbling steamthrough them. It depends on the factthat the vapour pressure (and hencethe boiling point) of a mixture of twoimmiscible liquids is lower than thevapour pressure of either pure liquid.

steam point The temperature atwhich the maximum vapour pres-sure of water is equal to the standardatmospheric pressure (101 325 Pa).On the Celsius scale it has the value100°C.

stearate (octadecanoate) A salt orester of stearic acid.

stearic acid (octadecanoic acid)A solid saturated *fatty acid,CH3(CH2)16COOH; r.d. 0.94; m.p.71.5–72°C; b.p. 360°C (with decompo-sition). It occurs widely (as *glyc-erides) in animal and vegetable fats.

steel Any of a number of alloys con-sisting predominantly of iron withvarying proportions of carbon (up to 1.7%) and, in some cases, smallquantities of other elements (alloysteels), such as manganese, silicon,chromium, molybdenum, and nickel.Steels containing over 11–12% ofchromium are known as *stainlesssteels.

Carbon steels exist in three stablecrystalline phases: ferrite has a body-centred cubic crystal, austenite has aface-centred cubic crystal, and ce-mentite has an orthorhombic crystal.Pearlite is a mixture of ferrite and ce-mentite arranged in parallel plates.The phase diagram shows how thephases form at different tempera-tures and compositions.

Steels are manufactured by the

*basic-oxygen process (L–D process),which has largely replaced the*Bessemer process and the *open-hearth process, or in electrical fur-naces.

503 stereoisomerism

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920

880

840

800

760

720

680

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tem

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atu

re (

°C)

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

austeniteausteniteaustenite

austenite +austenite +austenite +cementitecementitecementite

cementite +cementite +cementite +pearlitepearlitepearlite

ferrite +ferrite +ferrite +pearlitepearlitepearlite

ferrite +ferrite +ferrite +austeniteausteniteaustenite

carbon (%)

Steel

step A single stage in a chemical re-action. For example, the addition ofhydrogen chloride to ethene involvesthree steps:

HCl → H+ + Cl–

H+ + C2H4 → CH3CH2+

CH3CH2+ + Cl– → CH3CH2Cl

steradian Symbol sr. The dimen-sionless (supplementary) *SI unit ofsolid angle equal to the solid anglethat encloses a surface on a sphereequal to the square of the radius ofthe sphere.

stere A unit of volume equal to1 m3. It is not now used for scientiÜcpurposes.

stereochemistry The branch ofchemistry concerned with the struc-ture of molecules and the way thearrangement of atoms and groups af-fects the chemical properties.

stereoisomerism See isomerism.


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stereoregular Describing a *poly-mer that has a regular pattern of sidegroups along its chain.

stereospeciÜc Describing chemi-cal reactions that give products witha particular arrangement of atoms inspace. An example of a stereospeciÜcreaction is the *Ziegler process formaking polyethene.

steric effect An effect in whichthe rate or path of a chemical reac-tion depends on the size or arrange-ment of groups in a molecule.

steric hindrance An effect inwhich a chemical reaction is sloweddown or prevented because largegroups on a reactant molecule hinderthe approach of another reactantmolecule.

Stern–Gerlach experiment Anexperiment demonstrating the spacequantization of rotating bodies, Ürstconducted by the German scientistsOtto Stern (1888–1969) and WaltherGerlach (1899–1979) in 1921. Theirexperiment involved passing a beamof silver atoms through an inhomo-geneous magnetic Üeld. The purposeof the experiment was to investigatethe interaction between a charged ro-tating body and the Üeld. A chargedrotating body acts like a magnet. Inclassical mechanics the orientation ofangular momentum can have anyvalue so that the magnet associatedwith it can take any orientation.However, in quantum mechanics an-

gular momentum is quantized,which means that the associatedmagnet can only lie in certain dis-crete orientations. As expected, sharpbands of atoms are observed, pro-vided that the beam has low inten-sity. The low intensity reduces theblurring effects caused by collisionsbetween atoms.

steroid Any of a group of lipids de-rived from saturated compoundcalled cyclopentanoperhydrophenan-threne, which has a nucleus of fourrings (see formula). Some of the mostimportant steroid derivatives are thesteroid alcohols, or sterols. Othersteroids include the bile acids, whichaid digestion of fats in the intestine;the sex hormones (androgens and oe-strogens); and the corticosteroid hor-mones, produced by the adrenalcortex. *Vitamin D is also based onthe steroid structure.


sterol Any of a group of *steroid-based alcohols having a hydrocarbonside-chain of 8–10 carbon atoms.Sterols exist either as free sterols oras esters of fatty acids. Animal sterols(zoosterols) include cholesterol andlanosterol. The major plant sterol(phytosterol) is beta-sitosterol, whilefungal sterols (mycosterols) includeergosterol.

stimulated emission See inducedemission; laser.

Steroid


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Stirling’s approximation An ap-proximation for the factorial, n!, of anumber n. The precise form of Stir-ling’s approximation is:

n! = (2π)½nn+½exp(–n)This approximation is valid for largevalues of n, being accurate for ngreater than about 10. A simpliÜedversion of Stirling’s expression islogen! = nlogen – n, which is an ap-proximation to the precise form, de-rived by dropping all those termsthat do not increase at least asquickly as n. An important applica-tion of Stirling’s approximation is inthe derivation of the Boltzmann dis-tribution (see boltzmann equation).In this application n is very muchgreater than 10, enabling thesimpliÜed version of Stirling’s ap-proximation to be used.

STM See scanning tunnellingmicroscope.

stochastic process Any process inwhich there is a random element.Stochastic processes are important in*nonequilibrium statistical mechan-ics and *disordered solids. In a time-dependent stochastic process, avariable that changes with time doesso in such a way that there is no cor-relation between different time inter-vals. An example of a stochasticprocess is *Brownian movement.Equations, such as the *Langevinequation and the *Fokker–Planckequation, that describe stochasticprocesses are called stochastic equa-tions. It is necessary to use statisticalmethods and the theory of probabil-ity to analyse stochastic processesand their equations.

Stockholm Convention See per-sistent organic pollutant.

stoichiometric Describing chemi-cal reactions in which the reactantscombine in simple whole-number ra-tios.

stoichiometric coefÜcient Seechemical equation.

stoichiometric compound Acompound in which atoms are com-bined in exact whole-number ratios.Compare nonstoichiometric com-pound.

stoichiometric mixture A mix-ture of substances that can react togive products with no excess reac-tant.

stoichiometric sum See chemicalequation.

stoichiometry The relative propor-tions in which elements form com-pounds or in which substances react.

stokes Symbol St. A c.g.s. unit ofkinematic viscosity equal to the ratioof the viscosity of a Ûuid in poises toits density in grams per cubic cen-timetre. 1 stokes = 10–4 m2 s–1. It isnamed after the British mathemati-cian and physicist Sir George GabrielStokes (1819–1903).

Stokes radiation The electromag-netic radiation that occurs in the*Raman effect when the frequencyof the scattered radiation is lowerthan the frequency of the unscat-tered radiation, i.e. energy is trans-ferred from the photon to thescattering molecule. The spectralRaman lines corresponding to thistype of radiation are called Stokeslines, with the set of Stokes linesbeing called the O-branch of theRaman spectrum.

Similarly, *anti-Stokes radiation isthe radiation when the frequency ofthe scattered electromagnetic radia-tion is higher than the frequency ofthe unscattered radiation, i.e. energyis transferred to the photon from thescattering molecule. The spectralRaman lines corresponding to thistype of radiation are called anti-Stokes lines, with the set of anti-

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Stokes lines being called the S-branchof the Raman spectrum.

Stokes radiation is named after theBritish physicist Sir George GabrielStokes (1819–1903).

stopped-Ûow technique A tech-nique for investigating fast reactionsin solution. The reactant solutionsare rapidly mixed and Ûow through atube. The composition or propertiesof the mixture are monitored atsome point in the tube (e.g. by pho-tometry). The Ûow is suddenlystopped and the change of signalwith time is used to elucidate the ki-netics of the process. The stopped-Ûow technique is useful for reactionsoccurring in the millisecond timerange.

s.t.p. Standard temperature andpressure, formerly known as N.T.P.(normal temperature and pressure).The standard conditions used as abasis for calculations involving quan-tities that vary with temperature andpressure. These conditions are usedwhen comparing the properties ofgases. They are 273.15 K (or 0°C) and101 325 Pa (or 760.0 mmHg).

straight chain See chain.

strange attractor See attractor.

streaming potential A potentialdifference that occurs when a liquidunder pressure is forced through anarrow opening or diaphragm. It canbe measured between two electrodesat each end of a capillary tube madeof the same material as the dia-phragm.

strong acid An *acid that is com-pletely dissociated in aqueous solu-tion.

strontia See strontium oxide.

strontianite A mineral form of*strontium carbonate, SrCO3.

strontium Symbol Sr. A soft yel-

lowish metallic element belonging togroup 2 (formerly IIA) of the periodictable (see alkaline-earth metals);a.n. 38; r.a.m. 87.62; r.d. 2.6; m.p.769°C; b.p. 1384°C. The element isfound in the minerals strontianite(SrCO3) and celestine (SrSO4). It canbe obtained by roasting the ore togive the oxide, followed by reductionwith aluminium (i.e. the *Gold-schmidt process). The element,which is highly reactive, is used incertain alloys and as a vacuum getter.The isotope strontium–90 is presentin radioactive fallout (half-life 28years), and can be metabolized withcalcium so that it collects in bone.Strontium was discovered by MartinKlaproth (1743–1817) and ThomasHope (1766–1844) in 1798 and iso-lated by Humphry Davy in 1808.


strontium bicarbonate See stron-tium hydrogencarbonate.

strontium carbonate A whitesolid, SrCO3; orthorhombic; r.d. 3.7;decomposes at 1340°C. It occurs nat-urally as the mineral strontianite andis prepared industrially by boiling ce-lestine (strontium sulphate) with am-monium carbonate. It can also beprepared by passing carbon dioxideover strontium oxide or hydroxide orby passing the gas through a solutionof strontium salt. It is a phosphor,used to coat the glass of cathode-rayscreens, and is also used in the reÜn-ing of sugar, as a slagging agent incertain metal furnaces, and to pro-vide a red Ûame in Üreworks.

strontium chloride A white com-pound, SrCl2. The anhydrous salt(cubic; r.d. 3.05; m.p. 872°C; b.p.1250°C) can be prepared by passingchlorine over heated strontium. It isdeliquescent and readily forms thehexahydrate, SrCl2.6H2O (r.d. 2.67).

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This can be made by neutralizinghydrochloric acid with strontiumcarbonate, oxide, or hydroxide.Strontium chloride is used for mili-tary Ûares.

strontium hydrogencarbonate(strontium bicarbonate) A com-pound, Sr(HCO3)2, which is stableonly in solution. It is formed by theaction of carbon dioxide on a suspen-sion of strontium carbonate in water.On heating, this process is reversed.

strontium oxide (strontia) A whitecompound, SrO; r.d. 4.7; m.p. 2430°C,b.p. 3000°C. It can be prepared by thedecomposition of heated strontiumcarbonate, hydroxide, or nitrate, andis used in the manufacture of otherstrontium salts, in pigments, soapsand greases, and as a drying agent.

strontium sulphate A white solid,SrSO4; r.d. 3.96; m.p. 1605°C. It canbe made by dissolving strontiumoxide, hydroxide, or carbonate in sul-phuric acid. It is used as a pigment inpaints and ceramic glazes and to pro-vide a red colour in Üreworks.

structural formula See formula.

structural isomerism See iso-merism.

structure factor A quantity de-noted Fhkl, where h, k, and l are theMiller indices of the crystal, whichoccurs in *X-ray crystallography andother experiments involving scatter-ing in crystals. Fhkl is deÜned by theequation:

Fhkl = ∑i

fiexp[2πi(hxi + kyi + lzi)],

where the sum is over all atoms ofthe unit cell and fi is the scatteringfactor for atom i deÜned by:

fi = 4π0∞(ρsinkr/kr)r2dr

Here k = 4πsinθ/λ, where θ is theBragg angle (see bragg’s law), λ is thewavelength of the X-rays, and ρ is theelectron density distribution of the

atom i. The structure factor is used inPatterson synthesis.

strychnine A colourless poisonouscrystalline alkaloid found in certainplants.

styrene See phenylethene.

subjacent orbitals The next-to-highest occupied molecular orbital(NHOMO) and the second-lowest un-occupied molecular orbital (SLUMO).In certain cases these subjacent or-bitals are signiÜcant in *frontier-orbital theory.

sublimate A solid formed by subli-mation.

sublimation A direct change ofstate from solid to gas.

submillimetre waves Electromag-netic radiation with wavelengthsbelow one millimetre (and thereforefrequencies greater than 300 giga-hertz), extending to radiation of thefar infrared. A source of submilli-metre radiation is a medium pres-sure mercury lamp in quartz. Sub-millimetre waves can be detected bya *Golay cell.

subshell See atom.

substantive dye See dyes.

substantivity The afÜnity of a dyefor its substrate.

substituent 1. An atom or groupthat replaces another in a substitu-tion reaction. 2. An atom or groupregarded as having replaced a hydro-gen atom in a chemical derivative.For example, dibromobenzene(C6H4Br2) is a derivative of benzenewith bromine substituents.

substitute natural gas See sng.

substitution reaction (displace-ment reaction) A reaction in whichone atom or molecule is replaced byanother atom or molecule. See elec-

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trophilic substitution; nucleo-philic substitution.

substrate 1. The substance that isaffected by the action of a catalyst;for example, the substance uponwhich an *enzyme acts in a biochem-ical reaction. 2. The substance onwhich some other substance is ad-sorbed or in which it is absorbed. Ex-amples include the material to whicha dye is attached, the porous solid ab-sorbing a gas, and the *matrix trap-ping isolated atoms, radicals, etc.

succinic acid See butanedioic acid.

sucrose (cane sugar; beet sugar; sac-charose) A sugar comprising onemolecule of glucose linked to a fruc-tose molecule. It occurs widely inplants and is particularly abundant insugar cane and sugar beet (15–20%),from which it is extracted andreÜned for table sugar. If heated to200°C, sucrose becomes caramel.

sugar (saccharide) Any of a group ofwater-soluble *carbohydrates of rela-tively low molecular weight and typi-cally having a sweet taste. The simplesugars are called *monosaccharides.More complex sugars comprise be-tween two and ten monosaccharideslinked together: *disaccharides con-tain two, trisaccharides three, and soon. The name is often used to referspeciÜcally to *sucrose (table sugar).


sugar of lead See lead(ii)ethanoate.

sulpha drugs See sulphonamides.

sulphamic acid A colourless crys-talline solid, NH2SO2OH, which is ex-tremely soluble in water andnormally exists as the *zwitterionH3N+.SO3

–. It is a strong acid, readilyforming sulphamate salts. It is usedin electroplating, hard-water scale re-

movers, herbicides, and artiÜcialsweeteners.

sulphanes Compounds of hydro-gen and sulphur containing chains of sulphur atoms. They have thegeneral formula H2Sn. The simplest is hydrogen sulphide, H2S; othermembers of the series are H2S2, H2S3, H2S4, etc. See sulphides.


sulphanilic acid (4-aminobenzenesulphonic acid) A colourless crys-talline solid, H2NC6H4SO2OH, madeby prolonged heating of *phenyl-amine (aniline) sulphate. It readilyforms *diazo compounds and is usedto make dyes and sulpha drugs.

sulphate A salt or ester of sul-phuric(VI) acid. Organic sulphateshave the formula R2SO4, where R isan organic group. Sulphate salts con-tain the ion SO4

2–.

sulphides 1. Inorganic compoundsof sulphur with more electropositiveelements. Compounds of sulphurwith nonmetals are covalent com-pounds, e.g. hydrogen sulphide (H2S).Metals form ionic sulphides contain-ing the S2– ion; these are salts of hy-drogen sulphide. Polysulphides canalso be produced containing the poly-meric ion Sx

2–. 2. (or thio ethers) Or-ganic compounds that contain thegroup –S– linked to two hydrocarbongroups. Organic sulphides are namedfrom the linking groups, e.g. di-methyl sulphide (CH3SCH3), ethylmethyl sulphide (C2H5SCH3). Theyare analogues of ethers in which theoxygen is replaced by sulphur (hencethe alternative name) but are gener-ally more reactive than ethers. Thusthey react with halogen compoundsto form *sulphonium compoundsand can be oxidized to *sulphoxides.

sulphinate (dithionite; hyposul-

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phite) A salt that contains the nega-tive ion S2O4

2–, usually formed by thereduction of sulphites with excessSO2. Solutions are not very stable anddecompose to give thiosulphate andhydrogensulphite ions. The structureis –O2S-SO2

–.

sulphinic acid (dithionous acid; hy-posulphurous acid) An unstable acid,H2S2O4, known in the form of itssalts (sulphinates). See also sulphuricacid.

sulphite A salt or ester derivedfrom sulphurous acid. The salts con-tain the trioxosulphate(IV) ion SO3

2–.Sulphites generally have reducingproperties.

sulphonamides Organic com-pounds containing the group–SO2.NH2. The sulphonamides areamides of sulphonic acids. Manyhave antibacterial action and are alsoknown as sulpha drugs, includingsulphadiazine, NH2C6H4SO2-NHC4H3N2, sulphathiazole,NH2C6H4SO2NHC5H2NS, and several others. They act by pre-venting bacteria from reproducingand are used to treat a variety ofbacterial infections, especially of thegut and urinary system.

sulphonate A salt or ester of a sul-phonic acid.

sulphonation A type of chemicalreaction in which a –SO3H group issubstituted on a benzene ring toform a *sulphonic acid. The reactionis carried out by reÛuxing with con-centrated sulphuric(VI) acid for along period. It can also occur withcold disulphuric(VI) acid (H2S2O7).Sulphonation is an example of elec-trophilic substitution in which theelectrophile is a sulphur trioxidemolecule, SO3.

sulphonic acids Organic com-pounds containing the –SO2.OH

group. Sulphonic acids are formed byreaction of aromatic hydrocarbonswith concentrated sulphuric acid.They are strong acids, ionizing com-pletely in solution to form thesulphonate ion, –SO2.O–.


sulphonium compounds Com-pounds containing the ion R3S+ (sul-phonium ion), where R is any organicgroup. Sulphonium compounds canbe formed by reaction of organic sul-phides with halogen compounds. For example, diethyl sulphide,C2H5SC2H5, reacts with chloro-methane, CH3Cl, to give diethyl-methylsulphonium chloride,(C2H5)2.CH3.S+Cl–.

sulphoxides Organic compoundscontaining the group =S=O (sulphox-ide group) linked to two othergroups, e.g. dimethyl sulphoxide,(CH3)2SO.


sulphur Symbol S. A yellow non-metallic element belonging to*group 16 (formerly VIB) of the peri-odic table; a.n. 16; r.a.m. 32.06; r.d.2.07 (rhombic); m.p. 112.8°C; b.p.444.674°C. The element occurs inmany sulphide and sulphate miner-als and native sulphur is also foundin Sicily and the USA (obtained bythe *Frasch process). It can also beobtained from hydrogen sulphide bythe *Claus process.

Sulphur has various allotropicforms. Below 95.6°C the stable crys-tal form is rhombic; above this tem-perature the element transforms intoa triclinic form. These crystallineforms both contain cyclic S8 mol-ecules. At temperatures just above itsmelting point, molten sulphur is ayellow liquid containing S8 rings (asin the solid form). At about 160°C,

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the sulphur atoms form chains andthe liquid becomes more viscous anddark brown. If the molten sulphur iscooled quickly from this temperature(e.g. by pouring into cold water) areddish-brown solid known as plasticsulphur is obtained. Above 200°C theviscosity decreases. Sulphur vapourcontains a mixture of S2, S4, S6, andS8 molecules. Flowers of sulphur is ayellow powder obtained by sublim-ing the vapour. It is used as a plantfungicide. The element is also used toproduce sulphuric acid and other sul-phur compounds.

Sulphur is an *essential element inliving organisms, occurring in theamino acids cysteine and methionineand therefore in many proteins. It isalso a constituent of various cellmetabolites, e.g. coenzyme A. Sul-phur is absorbed by plants from thesoil as the sulphate ion (SO4

2–). Seesulphur cycle.


sulphur cycle The cycling of sul-phur between the biotic (living) andabiotic (nonliving) components of theenvironment. Most of the sulphur inthe abiotic environment is found inrocks, although a small amount ispresent in the atmosphere as sulphur

dioxide (SO2), produced by combus-tion of fossil fuels. Sulphate (SO4

2–),derived from the weathering and oxi-dation of rocks, is taken up by plantsand incorporated into sulphur-con-taining proteins. In this form sulphuris passed along food chains to ani-mals. Decomposition of dead organicmatter and faeces by anaerobic sul-phate-reducing bacteria returns sul-phur to the abiotic environment inthe form of hydrogen sulphide (H2S).Hydrogen sulphide can be convertedback to sulphate or to elemental sul-phur by the action of different groupsof photosynthetic and sulphide-oxi-dizing bacteria. Elemental sulphurbecomes incorporated into rocks.

sulphur dichloride See disulphurdichloride.

sulphur dichloride dioxide (sul-phuryl chloride) A colourless liquid,SO2Cl2; r.d. 1.67; m.p. –54.1°C; b.p.69°C. It decomposes in water but issoluble in benzene. The compound isformed by the action of chlorine onsulphur dioxide in the presence of aniron(III) chloride catalyst or sunlight.It is used as a chlorinating agent anda source of the related Ûuoride,SO2F2.

sulphur dichloride oxide (thionylchloride) A colourless fuming liquid,

sulphide (thio ether)

sulphonium ion

thiol (mercaptan)

sulphoxide

sulphonic acid

sulphonate ion

sulphonamide

R S R

R RS

R

+

R S H

R

RS O

R S OH

O

O

R S O–

O

O

R S NH2

O

O

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SOCl2; m.p. –105°C; b.p. 78.8°C. It hy-drolyses rapidly in water but is solu-ble in benzene. It may be preparedby the direct action of sulphur onchlorine monoxide or, more com-monly, by the reaction of phospho-rus(V) chloride with sulphur dioxide.It is used as a chlorinating agent insynthetic organic chemistry (replac-ing –OH groups with Cl).

sulphur dioxide (sulphur(IV) oxide)A colourless liquid or pungent gas,SO2, formed by sulphur burning inair; r.d. 1.43 (liquid); m.p. –72.7°C;b.p. –10°C. It can be made by heatingiron sulphide (pyrites) in air. Thecompound is a reducing agent and isused in bleaching and as a fumigantand food preservative. Large quanti-ties are also used in the *contactprocess for manufacturing sulphuricacid. It dissolves in water to give amixture of sulphuric and sulphurousacids. See also acid rain.

sulphuretted hydrogen See hy-drogen sulphide.

sulphuric acid (oil of vitriol) Acolourless oily liquid, H2SO4; r.d.1.84; m.p. 10.36°C; b.p. 338°C. Thepure acid is rarely used; it is com-monly available as a 96–98% solution(m.p. 3.0°C). The compound also

forms a range of hydrates: H2SO4.H2O(m.p. 8.62°C); H2SO4.2H2O (m.p.–38/39°C); H2SO4.6H2O (m.p. –54°C);H2SO4.8H2O (m.p. –62°C). Its full sys-tematic name is tetraoxosulphuric(VI)acid.

Until the 1930s, sulphuric acid wasmanufactured by the *lead-chamberprocess, but this has now been re-placed by the *contact process (cat-alytic oxidation of sulphur dioxide).More sulphuric acid is made in theUK than any other chemical product;production levels (UK) are commonly12 000 to 13 000 tonnes per day. It isextensively used in industry, themain applications being fertilizers(32%), chemicals (16%), paints andpigments (15%), detergents (11%), andÜbres (9%).

In concentrated sulphuric acidthere is extensive hydrogen bondingand several competing equilibria, togive species such as H3O+, HSO4

–,H3SO4

+, and H2S2O7. Apart frombeing a powerful protonating agent(it protonates chlorides and nitratesproducing hydrogen chloride and ni-tric acid), the compound is a moder-ately strong oxidizing agent. Thus, itwill dissolve copper:

Cu(s) + H2SO4(l) → CuO(s) + H2O(l) +SO2(g)

511 sulphuric acid

sHO S OH

O H2SO3

sulphurous acid(in sulphites)

HO S OH

O

O

H2SO4

sulphuric(VI) acid

H2S2O3

thiosulphuric acid

O

HO S OH

S

H2S2O7

disulphuric(VI) acid(pyrosulphuric)HO S O

O

O

OH

O

O

S

H2S2O4

sulphinic acid(dithionous orhyposulphurous)

HO S S

O O

OH

H2Sn+2O6

polythionic acidsHO S (S)n

O O

OH

O O

S

HO S S

O O

OH

O O

H2S2O6

dithionic acid

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CuO(s) + H2SO4(l) → CuSO4(aq) +H2O(l)

It is also a powerful dehydratingagent, capable of removing H2O frommany organic compounds (as in theproduction of acid *anhydrides). Indilute solution it is a strong dibasicacid forming two series of salts, thesulphates and the hydrogensul-phates.

sulphuric(IV) acid See sulphurousacid.

sulphur monochloride See disul-phur dichloride.

sulphur mustard A chemical war-fare agent, C4H8Cl2S; r.d. 1.27; m.p.14.4°C; b.p. 217°C. It is a potent blis-tering agent, Ürst used in 1915 byGermany in World War I. It is oftenknown simply as mustard gas, al-though it is an oily liquid, which canbe dispersed as an aerosol. It was oneof the early chemicals used in chemo-therapy treatment of cancer. The sys-tematic name is bis(2-chlorethyl)sulphide. See also nitrogen mustard.

sulphurous acid (sulphuric(IV) acid)A weak dibasic acid, H2SO3, known inthe form of its salts: the sulphitesand hydrogensulphites. It is consid-ered to be formed (along with sul-phuric acid) when sulphur dioxide isdissolved in water. It is probable,however, that the molecule H2SO3 isnot present and that the solutioncontains hydrated SO2. It is a reduc-ing agent. The systematic name istrioxosulphuric(IV) acid. See also sul-phuric acid.

sulphur(IV) oxide See sulphurdioxide.

sulphur(VI) oxide See sulphurtrioxide.

sulphur trioxide (sulphur(VI)oxide) A colourless fuming solid,SO3, which has three crystallinemodiÜcations. In decreasing order ofstability these are: α, r.d. 1.97; m.p.16.83°C; b.p. 44.8°C; β, m.p. 16.24°C;sublimes at 50°C; r.d. 2.29; γ, m.p.16.8°C; b.p. 44.8°C. All are polymeric,with linked SO4 tetrahedra: the γ-form has an icelike structure and isobtained by rapid quenching of thevapour; the β-form has inÜnite heli-cal chains; and the α-form has in-Ünite chains with some cross-linkingof the SO4 tetrahedra. Even in thevapour, there are polymeric species,and not discrete sulphur trioxidemolecules (hence the compound ismore correctly called by its system-atic name sulphur(VI) oxide).

Sulphur trioxide is prepared by theoxidation of sulphur dioxide withoxygen in the presence of a vana-dium(V) oxide catalyst. It may be pre-pared in the laboratory by distilling amixture of concentrated sulphuricacid and phosphorus(V) oxide. It re-acts violently with water to give sul-phuric(VI) acid and is an importantintermediate in the preparation ofsulphuric acid and oleum.

sulphuryl chloride See sulphurdichloride dioxide.

sulphuryl group The group =SO2,as in *sulphur dichloride oxide.

O

O

S

Cl

Cl

sulphurylgroup

sulphur (VI) dichloride dioxide O

Cl

Cl

sulphur (IV) dichloride oxide

S

thionylgroup

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sulphydryl group See thiols.

superacid An *acid that has a pro-ton-donating ability equal to orgreater than that of anhydrous sul-phuric acid. A superacid is a particu-lar type of Brønsted acid. Those thatare much stronger than sulphuricacids can be made by adding certainpentaÛuorides and their derivativesto such acids as Ûuorosulphuric acid(HSO3F) or hydrogen Ûuoride (HF).The pentaÛuorides, such as antimonypentaÛuoride (SbF5), are very strongLewis acids. The mixtures HF-SbF5and HSO3F-SbF5 are among thestrongest acids known; their applica-tions include the protonation of veryweak bases in organic chemistry andthe abstraction of hydrogen from sat-urated hydrocarbons to produce car-bonium ions. An equimolar mixtureof HSO3F and S6F5 is known by thetradename Magic acid. Very strongbases, such as lithium diisopropyl-amide, are sometimes known as su-perbases.

superbase See superacid.

superconductivity The propertypossessed by some substances belowa certain temperature, the transitionpoint, of zero resistance. Until re-cently, the known superconductingmaterials had very low transitionpoints. However, synthetic organicconductors and certain metal oxideceramics have now been producedthat become superconducting atmuch higher temperatures. For ex-ample, much work has been done onytterbium–barium–copper oxides,which have transition temperaturesof about 100 K.

supercooling 1. The cooling of aliquid to below its freezing pointwithout a change from the liquid tosolid state taking place. In thismetastable state the particles of theliquid lose energy but do not fall into

place in the lattice of the solid crys-tal. If the liquid is seeded (see seed)crystallization usually takes placeand the liquid returns to its normalfreezing point. Crystallization canalso be induced by the presence ofparticles of dust, by mechanical vi-bration, or by rough surfaces. 2. Theanalogous cooling of a vapour tomake it supersaturated.

superÛuidity The exhibition by aÛuid at extremely low temperatures,e.g. liquid helium at 2.186 K, of veryhigh thermal conductivity and fric-tionless Ûow. The temperature atwhich superÛuidity occurs is calledthe lambda point.

superheating The heating of a liq-uid to above its normal boiling pointby increasing the pressure.

superionic conductor An ionicsolid in which the electrical conduc-tivity due to the motion of ions issimilar to that of a molten salt, i.e. amuch higher conductivity than isusually observed in ionic solids.

superlattice See solid solution.

supernatant liquid The clear liq-uid remaining when a precipitate hassettled.

superoxides A group of inorganiccompounds that contain the O2

– ion.They are formed in signiÜcant quan-tities only for sodium, potassium, ru-bidium, and caesium. They are verypowerful oxidizing agents and reactvigorously with water to give oxygengas and OH– ions. The superoxide ionhas an unpaired electron and is para-magnetic and coloured (orange).

superphosphate A commercialphosphate mixture consisting mainlyof monocalcium phosphate. Single-superphosphate is made by treatingphosphate rock with sulphuric acid;the product contains 16–20% ‘avail-able’ P2O5:

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Ca10(PO4)6F2 + 7H2SO4 =3Ca(H2PO4)2 + 7CaSO4 + 2HF

Triple-superphosphate is made byusing phosphoric(V) acid in place ofsulphuric acid; the product contains45–50% ‘available’ P2O5:

Ca10(PO4)6F2 + 14H3PO4 =10Ca(H2PO4)2 + 2HF

superplasticity The ability ofsome metals and alloys to stretchuniformly by several thousand per-cent at high temperatures, unlikenormal alloys, which fail after beingstretched 100% or less. Since 1962,when this property was discovered inan alloy of zinc and aluminium (22%),many alloys and ceramics have beenshown to possess this property. Forsuperplasticity to occur, the metalgrain must be small and rounded andthe alloy must have a slow rate of de-formation.

superradiant Denoting a *laser inwhich the efÜciency of the transitionof stimulated emission is sufÜcientlylarge for the radiation to be producedby a single pulse of electromagneticradiation without using mirrors. Anexample of a superradiant laser is thenitrogen laser based on the ultravio-let transition C3Πu → B3Πg.

supersaturated solution See sat-urated.

supersaturation 1. The state ofthe atmosphere in which the relativehumidity is over 100%. This occurs inpure air where no condensation nu-clei are available. Supersaturation isusually prevented in the atmosphereby the abundance of condensationnuclei (e.g. dust, sea salt, and smokeparticles). 2. The state of any vapourwhose pressure exceeds that atwhich condensation usually occurs(at the prevailing temperature).

supplementary units See si units.

supramolecular chemistry AÜeld of chemical research concernedwith the formation and properties oflarge assemblies of molecules heldtogether by intramolecular forces(hydrogen bonds, van der Waals’forces, etc.). One feature of supramol-ecular chemistry is that of self-assembly (see self-organization), inwhich the structure forms sponta-neously as a consequence of the na-ture of the molecules. The molecularunits are sometimes known as syn-thons. Another aspect is the study ofvery large molecules able to be usedin complex chemical reactions in afashion similar to, for example, theactions of the naturally occurringhaemoglobin and nucleic acid mol-ecules. Such molecules have great po-tential in such areas as medicine,electronics, and optics. The *helicateand *texaphyrin molecules and *den-drimers are typical examples of com-pounds of interest in this Üeld. TheÜeld also includes host–guest chem-istry, which is concerned with mol-ecules speciÜcally designed to acceptother molecules. Examples include*crown ethers, *cryptands, and *cal-ixarenes.

surface-extended X-ray absorp-tion Üne-structure spectroscopySee sexafs.

surface tension Symbol γ. Theproperty of a liquid that makes it be-have as if its surface is enclosed in anelastic skin. The property resultsfrom intermolecular forces: a mol-ecule in the interior of a liquid expe-riences interactions from othermolecules equally from all sides,whereas a molecule at the surface isonly affected by molecules below itin the liquid. The surface tension isdeÜned as the force acting over thesurface per unit length of surfaceperpendicular to the force. It is meas-ured in newtons per metre. It can

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equally be deÜned as the energy re-quired to increase the surface areaisothermally by one square metre,i.e. it can be measured in joules permetre squared (which is equivalentto N m–1).

The property of surface tension isresponsible for the formation of liq-uid drops, soap bubbles, and menis-cuses, as well as the rise of liquids ina capillary tube (capillarity), the ab-sorption of liquids by porous sub-stances, and the ability of liquids towet a surface.

surfactant (surface active agent) Asubstance, such as a *detergent,added to a liquid to increase itsspreading or wetting properties byreducing its *surface tension.

suspension A mixture in whichsmall solid or liquid particles are sus-pended in a liquid or gas.

SYBYL line notation A type ofline notation, similar to *SMILES butwith some additional features. InSLN, hydrogens are not omitted, soethane is CH3CH3. Wild-card atomsand groups can be speciÜed forsearching.

sylvite (sylvine) A mineral form of*potassium chloride, KCl.

symmetric top See moment of in-ertia.

symmetry The property of an ob-ject that enables it to undergo cer-tain manipulations, called symmetryoperations, such that its new state isindistinguishable from its originalstate. Examples include inversionthrough a point, reÛection through aplane, and rotation about an axis.The geometrical feature of the objectwith respect to which the symmetryoperation is carried out is termed asymmetry element. In the above ex-amples, a point through which inver-sion can be performed, a plane of

reÛection, and an axis of rotation arethe symmetry elements. See also mo-lecular symmetry.

symmetry-adapted linear com-binations See salc.

syn See torsion angle.

synchrotron radiation Electro-magnetic radiation produced whencharged particles travel in a curvedpath. In a generator of synchrotronradiation, electrons are accelerated ina synchrotron and injected into astorage ring where they circulate at aconstant energy. Radiation can betaken from the storage ring tangen-tially at various points around thering. The advantage of this type ofsource is that it gives a narrow in-tense beam that is tunable to a par-ticular wavelength over a wholerange from infrared radiation to hardX-rays. Synchrotron radiation is usedfor a variety of studies including crys-tallography, X-ray scattering, and ion-ization experiments.

synclinal See torsion angle.

syndiotactic See polymer.

syneresis The spontaneous separa-tion of the solid and liquid compo-nents of a gel on standing.

synperiplanar See torsion angle.

synthesis The formation of chemi-cal compounds from more simplecompounds.

synthesis gas See haber process.

synthetic Describing a substancethat has been made artiÜcially; i.e.one that does not come from a nat-ural source.

synthon A fragment obtained bydisconnection in *retrosyntheticanalysis. By analogy, units buildingup structures by self-assembly in*supramolecular chemistry are some-

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time known as supramolecular syn-thons.

Système International d’UnitésSee si units.

Szilard–Chalmers effect An ef-fect discovered by L. Szilard (1898–1964) and T. A. Chalmers in 1934; ithas been used to separate radioactiveproducts in a nuclear reaction involv-ing the absorption of a neutron andthe emission of gamma rays. If a ma-terial absorbs a neutron and subse-quently emits a gamma ray, theemission of the gamma ray causesthe nucleus to recoil. Frequently, therecoil energy is sufÜcient to break

the chemical bond between the atomand the molecule of which it formspart. Thus, although the atom thathas absorbed the neutron is an iso-tope of the original atom it is in a dif-ferent form chemically, enablingseparation to take place. For exam-ple, if an aqueous solution of sodiumchlorate (NaClO3) is subjected tobombardment by slow neutrons, theCl37 is converted to Cl38, with manyof the Cl38 atoms breaking from thechlorate and moving into the solu-tion in the form of chloride ions.This is an example of a ‘hot atom’ re-action. The Cl38 can be precipitatedout using silver nitrate.

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T

2,4,5-T 2,4,5-trichlorophenoxyaceticacid (2,4,5-trichlorophenoxyethanoicacid): a synthetic auxin formerlywidely used as a herbicide and de-foliant. It is now banned in manycountries as it tends to become con-taminated with the toxic chemicaldioxin.

tabun A highly toxic colourless orbrown liquid, C5H11N2O2P; r.d. 1.09;m.p. –50°C; b.p. 247.5°C. It is anorganophosphorus compound, ethylN,N-dimethylphosphoramido-cyanidate. Tabun was discovered in1936 and belongs to the G-series of*nerve agents (GA). It was used byIraq in the Iran–Iraq war (1980–88).

tactic polymer See polymer.

tactosol A colloidal sol containingnonspherical particles capable of ori-entating themselves. An example of atactosol is given by an aged vana-dium pentoxide sol, in which theparticles are rod-shaped. When a tac-tosol is placed in a magnetic Üeld itsparticles arrange themselves alongthe lines of force of the Üeld.

Tafel plot The graph of the loga-rithm of the current density j againstthe overpotential η in electrochem-istry in the high overpotential limit.This limit can occur in electrolysis ei-ther when the overpotential is largeand positive, the electrode being theanode, or when the overpotential islarge and negative, the electrodebeing the cathode. In the case of pos-itive overpotential, the second expo-nential in the *Butler–Volmerequation can be ignored, as it ismuch smaller than the Ürst exponen-tial, and thus

j = j0exp[(1 – α)fη]In the case of a negative overpoten-tial, the Ürst exponential can be ig-nored, which gives j = j0exp(–αfη), sothat ln(–j) = lnj0 – αfη. Thus, in theTafel plot the value of α can be ob-tained from the slope and the valueof j0 can be obtained from the inter-cept at η = 0.

Takayama test See haemochro-mogen test.

talc A white or pale-green mineralform of magnesium silicate,Mg3Si4O10(OH)2, crystallizing in thetriclinic system. It forms as a sec-ondary mineral by alteration of mag-nesium-rich olivines, pyroxenes, andamphiboles of ultrabasic rocks. It issoaplike to touch and very soft, hav-ing a hardness of 1 on the Mohs’scale. Massive Üne-grained talc isknown as soapstone or steatite. Talcin powdered form is used as a lubri-cant, as a Üller in paper, paints, andrubber, and in cosmetics, ceramics,and French chalk. It occurs chieÛy inthe USA, Russia, France, and Japan.

tannic acid A yellowish complexorganic compound present in certainplants. It is used in dyeing as a mor-dant.

tannin One of a group of complexorganic chemicals (see depsides) com-monly found in leaves, unripe fruits,and the bark of trees. Their functionis uncertain though the unpleasanttaste may discourage grazing ani-mals. Some tannins have commercialuses, notably in the production ofleather and ink.

tantalum Symbol Ta. A heavy blue-


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grey metallic *transition element;a.n. 73; r.a.m. 180.948; r.d. 16.63;m.p. 2996°C; b.p. 5427°C. It is foundwith niobium in the ore columbite–tantalite (Fe,Mn)(Ta,Nb)2O6. It is ex-tracted by dissolving in hydroÛuoricacid, separating the tantalum andniobium Ûuorides to give K2TaF7, andreduction of this with sodium. Theelement contains the stable isotopetantalum–181 and the long-livedradioactive isotope tantalum–180(0.012%; half-life >107 years). Thereare several other short-lived isotopes.The element is used in certain alloysand in electronic components. Tanta-lum parts are also used in surgery be-cause of the unreactive nature of themetal (e.g. in pins to join bones).Chemically, the metal forms a pas-sive oxide layer in air. It forms com-plexes in the +2, +3, +4, and +5oxidation states. Tantalum wasidentiÜed in 1802 by Anders Ekeberg(1767–1813) and Ürst isolated in 1820by *Berzelius.


tar Any of various black semisolidmixtures of hydrocarbons and freecarbon, produced by destructive dis-tillation of *coal or by *petroleumreÜning.

target molecule See retrosyn-thetic analysis.

tar sand See oil sand.

tartaric acid A crystalline natu-rally occurring carboxylic acid,(CHOH)2(COOH)2; r.d. 1.8; m.p.171–174°C. It can be obtained fromtartar (potassium hydrogen tartrate)deposits from wine vats, and is usedin baking powders and as a food-stuffs additive. The compound isoptically active (see optical activity).The systematic name is 2,3-dihydroxybutanedioic acid.

tartrate A salt or ester of *tartaricacid.

tartrazine A food additive (E102)that gives foods a yellow colour. Tar-trazine can cause a toxic response inthe immune system and is banned insome countries.

TATP See triacetone triperoxide.

tautomerism A type of *iso-merism in which the two isomers(tautomers) are in equilibrium. Seeketo–enol tautomerism.

TCA cycle See krebs cycle.

technetium Symbol Tc. A radio-active metallic *transition element;a.n. 43; m.p. 2172°C; b.p. 4877°C. Theelement can be detected in certainstars and is present in the Üssionproducts of uranium. It was Ürstmade by Carlo Perrier and EmilioSegré (1905–89) by bombardingmolybdenum with deuterons to givetechnetium–97. The most stable iso-tope is technetium–99 (half-life 2.6 ×106 years); this is used to some extentin labelling for medical diagnosis.There are sixteen known isotopes.Chemically, the metal has propertiesintermediate between manganeseand rhenium.


TeÛon Tradename for a form of*polytetraÛuoroethene.

Teichmann test See haematintest.

tele-substitution A type of substi-tution reaction in which the enteringgroup takes a position on an atomthat is more than one atom awayfrom the atom to which the leavinggroup is attached. See also cine-substitution.

tellurides Binary compounds of tel-lurium with other more electroposi-

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tive elements. Compounds of tel-lurium with nonmetals are covalent(e.g. H2Te). Metal tellurides can bemade by direct combination of the el-ements and are ionic (containingTe2–) or nonstoichiometric interstitialcompounds (e.g. Pd4Te, PdTe2).

tellurium Symbol Te. A silvery met-alloid element belonging to *group16 (formerly VIB) of the periodictable; a.n. 52; r.a.m. 127.60; r.d. 6.24(crystalline); m.p. 449.5°C; b.p.989.8°C. It occurs mainly as *tel-lurides in ores of gold, silver, copper,and nickel and it is obtained as a by-product in copper reÜning. There areeight natural isotopes and nine radio-active isotopes. The element is usedin semiconductors and smallamounts are added to certain steels.Tellurium is also added in smallquantities to lead. Its chemistry issimilar to that of sulphur. It was dis-covered by Franz Müller (1740–1825)in 1782.A• Information from the WebElements site

temperature The property of abody or region of space that deter-mines whether or not there will be anet Ûow of heat into it or out of itfrom a neighbouring body or regionand in which direction (if any) theheat will Ûow. If there is no heat Ûowthe bodies or regions are said to be inthermodynamic equilibrium and atthe same temperature. If there is aÛow of heat, the direction of the Ûowis from the body or region of highertemperature. Broadly, there are twomethods of quantifying this prop-erty. The empirical method is to taketwo or more reproducible tempera-ture-dependent events and assignÜxed points on a scale of values tothese events. For example, the Cel-sius temperature scale uses the freez-ing point and boiling point of wateras the two Üxed points, assigns the

values 0 and 100 to them, respec-tively, and divides the scale betweenthem into 100 degrees. This methodis serviceable for many practical pur-poses (see temperature scales), butlacking a theoretical basis it is awk-ward to use in many scientiÜc con-texts. In the 19th century, Lord*Kelvin proposed a thermodynamicmethod to specify temperature,based on the measurement of thequantity of heat Ûowing betweenbodies at different temperatures.This concept relies on an absolutescale of temperature with an *ab-solute zero of temperature, at whichno body can give up heat. He alsoused Sadi Carnot’s concept of anideal frictionless perfectly efÜcientheat engine (see carnot cycle). ThisCarnot engine takes in a quantity ofheat q1 at a temperature T, and ex-hausts heat q2 at T2, so that T1/T2 =q1/q2. If T2 has a value Üxed by deÜni-tion, a Carnot engine can be run be-tween this Üxed temperature and any unknown temperature T1, en-abling T1 to be calculated by measur-ing the values of q1 and q2. Thisconcept remains the basis for deÜn-ing thermodynamic temperature,quite independently of the nature ofthe working substance. The unit inwhich thermodynamic temperatureis expressed is the *kelvin. In prac-tice thermodynamic temperaturescannot be measured directly; theyare usually inferred from measure-ments with a gas thermometer con-taining a nearly ideal gas. This ispossible because another aspect ofthermodynamic temperature is its re-lationship to the *internal energy ofa given amount of substance. Thiscan be shown most simply in thecase of an ideal monatomic gas, inwhich the internal energy per mole(U) is equal to the total kinetic energyof translation of the atoms in onemole of the gas (a monatomic gas has

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no rotational or vibrational energy).According to *kinetic theory, thethermodynamic temperature of sucha gas is given by T = 2U/3R, where R isthe universal *gas constant.

temperature-independent para-magnetism (TIP) Orbital paramag-netism that is independent oftemperature. In some substances TIPcan make the molecules paramag-netic, although all the electrons inthe ground state are paired, if thereare low-lying excited states to whichelectrons can readily move.

temperature scales A number ofempirical scales of *temperaturehave been in use: the *Celsius scaleis widely used for many purposes andin certain countries the *Fahrenheitscale is still used. These scales bothrely on the use of Üxed points, suchas the freezing point and the boilingpoint of water, and the division ofthe fundamental interval betweenthese two points into units of tem-perature (100 degrees in the case ofthe Celsius scale and 180 degrees inthe Fahrenheit scale).

However, for scientiÜc purposesthe scale in use is the International

Practical Temperature Scale (1968),which is designed to conform asclosely as possible to thermodynamictemperature and is expressed in theunit of thermodynamic temperature,the *kelvin. The eleven Üxed pointsof the scale are given in the table,with the instruments speciÜed for in-terpolating between them. Above thefreezing point of gold, a radiation py-rometer is used, based on Planck’slaw of radiation. The scale is ex-pected to be reÜned in the late 1980s.

tempering The process of increas-ing the toughness of an alloy, such assteel, by heating it to a predeter-mined temperature, maintaining it atthis temperature for a predeterminedtime, and cooling it to room temper-ature at a predetermined rate. Insteel, the purpose of the process is toheat the alloy to a temperature thatwill enable the excess carbide to pre-cipitate out of the supersaturatedsolid solution of *martensite andthen to cool the saturated solutionfast enough to prevent further pre-cipitation or grain growth. For thisreason steel is quenched rapidly bydipping into cold water.

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triple point of equilibrium hydrogen

temperature of equilibrium hydrogen when its vapour pressure is 25/76 standard atmosphere

b.p. of equilibrium hydrogen

b.p. of neon

triple point of oxygen

b.p. of oxygen

triple point of water

b.p. of water

f.p. of zinc

f.p. of silver

f.p. of gold

13.81

17.042

20.28

27.102

54.361

90.188

273.16

373.15

692.73

1235.08

1337.58

–259.34

–256.108

–252.87

–246.048

–218.789

–182.962

0.01

100

419.58

961.93

1064.43

T/K t/°C

Temperature scales


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temporary hardness See hard-ness of water.

tera- Symbol T. A preÜx used in themetric system to denote one millionmillion times. For example, 1012 volts= 1 teravolt (TV).

terbium Symbol Tb. A silverymetallic element belonging to the*lanthanoids; a.n. 65; r.a.m. 158.92;r.d. 8.23 (20°C); m.p. 1356°C; b.p.3123°C. It occurs in apatite and xeno-time, from which it is obtained by anion-exchange process. There is onlyone natural isotope, terbium–159,which is stable. Seventeen artiÜcialisotopes have been identiÜed. It isused as a dopant in semiconductingdevices. It was discovered by CarlMosander (1797–1858) in 1843.A• Information from the WebElements site

terephthalic acid (1,4-benzenedi-carboxylic acid) A colourless crys-talline solid, C6H4(COOH)2, m.p.300°C. It is made by oxidizing p-xylene (1,4-dimethylebenzene) andused for making polyesters, such asTerylene.

term An electronic energy level inan atom. A term is characterized by aterm symbol, which is given by a cap-ital letter indicating the total orbitalangular momentum L of the atom,with a left superscript that gives thevalue of 2S + 1, where S is the totalspin angular momentum of theatom. Analogously to the letters usedfor single electron angular momen-tum the letters S, P, D, F correspondto L = 0, 1, 2, 3 respectively. For ex-ample, a term for which L = 1 and S =1 has the term symbol 3P.

In the absence of *spin–orbit cou-pling the degeneracy of a term is(2L + 1)(2S + 1). In the presence ofspin–orbit coupling a term splits upinto several closely spaced energylevels, with this set of closely spaced

energy levels being called a multiplet.The multiplicity of a multiplet isgiven by (2S + 1) since the number ofatomic energy levels a term splitsinto because of spin–orbit coupling is(2S + 1). This is the case because thetotal electronic angular momentum Jof an atom can have the (2S + 1) pos-sible values: L + S, L + S –1, etc. Eachlevel in a multiplet has the value of Jwritten as a right subscript to theterm symbol. The degeneracy of alevel in a multiplet is 2J + 1. Thenames given to multiplets with 2S + 1= 1, 2, 3, 4 are singlet, doublet,triplet, quartet, respectively, to corre-spond to the number of atomic levelsin the multiplet. For example, 3P1 isthe J = 1 level in a P triplet.

ternary compound A chemicalcompound containing three differentelements.

terpenes A group of unsaturatedhydrocarbons present in plants (seeessential oil). Terpenes consist ofisoprene units, CH2:C(CH3)CH:CH2.Monoterpenes have two units,C10H16, sesquiterpenes three units,C15H24, diterpenes four units, C20H32,etc. Terpenoids, which are derivativesof terpenes, include abscisic acid andgibberellin (plant growth substances)and the *carotenoid and *chloro-phyll pigments.


terpinene A cyclic *terpene,C10H16, used in perfumery and as aÛavouring. It exists in three isomericforms. α-Terpinene is an oily liquidthat smells of lemons, b.p. 182°C. Itoccurs in the herbs cardamom, co-riander and marjoram although it isusually extracted from the essentialoils of other plants. β-Terpinene, b.p.174°C, is generally synthesized fromoil of savin. γ-Terpinene, b.p. 182°C,occurs in various plant oils, including

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those of coriander, cumin, lemon,and samphire. It can be made by theaction of an alcoholic sulphuric acidsolution on pinene (the major com-ponent of turpentine).

tertiary alcohol See alcohols.

tertiary amine See amines.

tervalent (trivalent) Having a va-lency of three.

Terylene Tradename for a type of*polyester used in synthetic Übres.

tesla Symbol T. The SI unit of mag-netic Ûux density equal to one weberof magnetic Ûux per square metre,i.e. 1 T = 1 Wb m–2. It is named afterNikola Tesla (1870–1943), Croatian-born US electrical engineer.

tetrachloroethene A colourlessnonÛammable volatile liquid,CCl2:CCl2; r.d. 1.6; m.p. –22°C; b.p.121°C. It is used as a solvent.

tetrachloromethane (carbontetrachloride) A colourless volatileliquid with a characteristic odour,virtually insoluble in water but misci-ble with many organic liquids, suchas ethanol and benzene; r.d. 1.586;m.p. –23°C; b.p. 76.54°C. It is madeby the chlorination of methane (pre-viously by chlorination of carbondisulphide). The compound is a goodsolvent for waxes, lacquers, and rub-bers and the main industrial use is asa solvent, but safer substances (e.g.1,1,1-trichloroethane) are increas-ingly being used. Moist carbon tetra-chloride is partly decomposed tophosgene and hydrogen chloride andthis provides a further restriction onits use.

tetradecanoic acid (myristic acid)A saturated carboxylic acid CH3(CH2)12COOH; r.d. 0.86; m.p. 58.8°C;b.p. 250.5°C (100 mmHg). Its glyc-erides are found in nutmeg, palm oil,and butter fat. Compounds are used

in cosmetics and skin-care prepara-tions. The name comes from theLatin name for nutmeg, Myristica fra-grans.

tetraethyl lead See lead(iv)tetraethyl.

tetragonal See crystal system.

tetrahedral angle The angle be-tween the bonds in a *tetrahedralcompound (approximately 109° for aregular tetrahedron).

tetrahedral compound A com-pound in which four atoms or groupssituated at the corners of a tetrahe-dron are linked (by covalent or coor-dinate bonds) to an atom at thecentre of the tetrahedron. See alsocomplex.

tetrahedron A polyhedron withfour triangular faces. In a regulartetrahedron all four triangles are con-gruent equilateral triangles. It consti-tutes a regular triangular pyramid.

tetrahydrate A crystalline hydratecontaining four moles of water permole of compound.

tetrahydrocannabinol (THC) Themain *cannabinoid found in theplant Cannabis sativa, and the com-pound largely responsible for its psy-choactive effects. There have beenreports that use of THC has beenlinked to the onset of psychoticepisodes in both healthy users and,in particular, in schizophrenics. It

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O (CH2)4CH3CH3

CH3

CH3

H

HOH

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has been used as a possible treat-ment for a number of medical condi-tions. It is used illegally in the formof *cannabis, which is a class C drugin the UK. Tetrahydrocannabinol canbe detected by the *Duquenois–Levine test.

tetrahydrofuran (THF) A colour-less volatile liquid, C4H8O; r.d. 0.89;m.p. –65°C; b.p. 67°C. It is made bythe acid hydrolysis of polysaccha-rides in oat husks, and is widely usedas a solvent.

tetrahydroxomonoxodiboric(III)acid See boric acid.

tetraoxophosphoric(V) acid Seephosphoric(v) acid.

tetraoxosulphuric(VI) acid Seesulphuric acid.

tetravalent (quadrivalent) Havinga valency of four.

texaphyrin A synthetic moleculesimilar to a porphyrin but containingÜve central nitrogen atoms ratherthan four, thus increasing the size ofthe central ‘hole’ and enabling largercations, such as cadmium, to bebound stably. See also supramolecu-lar chemistry.

thallium Symbol Tl. A greyishmetallic element belonging to*group 13 (formerly IIIB) of the peri-odic table; a.n. 81; r.a.m. 204.39; r.d.11.85 (20°C); m.p. 303.5°C; b.p.1457±10°C. It occurs in zinc blendeand some iron ores and is recoveredin small quantities from lead andzinc concentrates. The naturally oc-curring isotopes are thallium–203and thallium–205; eleven radioiso-topes have been identiÜed. It has fewuses – experimental alloys for specialpurposes and some minor uses inelectronics. The sulphate has beenused as a rodenticide. Thallium(I)compounds resemble those of the al-kali metals. Thallium(III) compounds

are easily reduced to the thallium(I)state and are therefore strong oxidiz-ing agents. The element was discov-ered by Sir William *Crookes in1861.


theobromine (3,7-dimethylxan-thine) An alkaloid, C7H8N4O2, struc-turally related to caffeine. It is foundin small quantities in tea and largerquantities in the cacao tree, hence itspresence in cocoa and chocolate. Liketheophylline it is used in treating cer-tain respiratory conditions. Note thatthe compound contains no bromine.The name comes from the name ofthe cacao tree, Theobroma cacao. Seealso methylxanthines.

theophylline (1,3-dimethylxan-thine) An alkaloid, C7 H8 N4 O2,structurally related to caffein. It isfound in small quantities in tea andis used medicinally for certain respi-ratory conditions. See also methylxan-thines.

theory See laws, theories, and hy-potheses.

thermal analysis A technique forchemical analysis and the investiga-tion of the products formed by heat-ing a substance. In differentialthermal analysis (DTA) a sample isheated, usually in an inert atmos-phere, and a plot of weight againsttemperature made. In differentialscanning calorimetry (DSC) heat isadded to or removed from a sampleelectrically as the temperature is in-creased, thus allowing the enthalpychanges due to thermal decomposi-tion to be studied.

thermal capacity See heat capac-ity.

thermal equilibrium See equilib-rium.

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thermite A stoichiometric pow-dered mixture of iron(III) oxide andaluminium for the reaction:

2Al + Fe2O3 → Al2O3 + 2FeThe reaction is highly exothermicand the increase in temperature issufÜcient to melt the iron produced.It has been used for localized weld-ing of steel objects (e.g. railway lines)in the Thermit process. Thermite isalso used in incendiary bombs.

thermochemistry The branch ofphysical chemistry concerned withheats of chemical reaction, heats offormation of chemical compounds,etc.

thermodynamics The study of thelaws that govern the conversion ofenergy from one form to another,the direction in which heat will Ûow,and the availability of energy to dowork. It is based on the concept thatin an isolated system anywhere inthe universe there is a measurablequantity of energy called the *inter-nal energy (U) of the system. This isthe total kinetic and potential energyof the atoms and molecules of thesystem of all kinds that can be trans-ferred directly as heat; it thereforeexcludes chemical and nuclear en-ergy. The value of U can only bechanged if the system ceases to beisolated. In these circumstances Ucan change by the transfer of mass toor from the system, the transfer ofheat (Q) to or from the system, or bythe work (W) being done on or by thesystem. For an adiabatic (Q = 0) sys-tem of constant mass, ∆U = W. Byconvention, W is taken to be positiveif work is done on the system andnegative if work is done by the sys-tem. For nonadiabatic systems ofconstant mass, ∆U = Q + W. Thisstatement, which is equivalent to thelaw of conservation of energy, is

known as the Ürst law of thermody-namics.

All natural processes conform tothis law, but not all processes con-forming to it can occur in nature.Most natural processes are irre-versible, i.e. they will only proceed inone direction (see reversibleprocess). The direction that a naturalprocess can take is the subject of thesecond law of thermodynamics,which can be stated in a variety ofways. R. Clausius (1822–88) stated thelaw in two ways: “heat cannot betransferred from one body to a sec-ond body at a higher temperaturewithout producing some other ef-fect” and “the entropy of a closed sys-tem increases with time”. Thesestatements introduce the thermody-namic concepts of *temperature (T)and *entropy (S), both of which areparameters determining the direc-tion in which an irreversible processcan go. The temperature of a body orsystem determines whether heat willÛow into it or out of it; its entropy isa measure of the unavailability of itsenergy to do work. Thus T and S de-termine the relationship between Qand W in the statement of the Ürstlaw. This is usually presented by stat-ing the second law in the form ∆U =T∆S – W.

The second law is concerned withchanges in entropy (∆S). The thirdlaw of thermodynamics provides anabsolute scale of values for entropyby stating that for changes involvingonly perfect crystalline solids at *ab-solute zero, the change of the totalentropy is zero. This law enables ab-solute values to be stated for en-tropies.

One other law is used in thermody-namics. Because it is fundamental to,and assumed by, the other laws ofthermodynamics it is usually knownas the zeroth law of thermodynamics.This states that if two bodies are each

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in thermal equilibrium with a thirdbody, then all three bodies are inthermal equilibrium with each other.See also enthalpy; free energy.

A• A tutorial from the Division of Chemical

Education, Purdue University, Indiana• Thermodynamic properties from the

committee on Data for Science andTechnology

thermodynamic temperatureSee temperature.

thermoluminescence *Lumines-cence produced in a solid when itstemperature is raised. It arises whenfree electrons and holes, trapped in asolid as a result of exposure to ioniz-ing radiation, unite and emit photonsof light. The process is made use ofin thermoluminescent dating, whichassumes that the number of elec-trons and holes trapped in a sampleof pottery is related to the length oftime that has elapsed since the pot-tery was Üred. By comparing the lu-minescence produced by heating apiece of pottery of unknown agewith the luminescence produced byheating similar materials of knownage, a fairly accurate estimate of theage of an object can be made.

thermoluminescent dating Seethermoluminescence.

thermolysis (pyrolysis) The chemi-cal decomposition of a substance byheat. It is an important process inchemical manufacture, such as thethermal *cracking of hydrocarbonsin the petroleum industry.

thermometer An instrument usedfor measuring the *temperature of asubstance. A number of techniquesand forms are used in thermometersdepending on such factors as the de-gree of accuracy required and therange of temperatures to be meas-ured, but they all measure tempera-

ture by making use of some propertyof a substance that varies with tem-perature. For example, liquid-in-glassthermometers depend on the expan-sion of a liquid, usually mercury oralcohol coloured with dye. These con-sist of a liquid-Ülled glass bulb at-tached to a partially Ülled capillarytube. In the bimetallic thermometerthe unequal expansion of two dissim-ilar metals that have been bonded to-gether into a narrow strip and coiledis used to move a pointer round adial. The gas thermometer, which ismore accurate than the liquid-in-glass thermometer, measures thevariation in the pressure of a gaskept at constant volume. The resis-tance thermometer is based on thechange in resistance of conductors orsemiconductors with temperaturechange. Platinum, nickel, and copperare the metals most commonly usedin resistance thermometers.

thermoplastic See plastics.

thermosetting See plastics.

thermostat A device that controlsthe heating or cooling of a substancein order to maintain it at a constanttemperature. It consists of a tempera-ture-sensing instrument connected toa switching device. When the tem-perature reaches a predeterminedlevel the sensor switches the heatingor cooling source on or off accordingto a predetermined program. Thesensing thermometer is often abimetallic strip that triggers a simpleelectrical switch. Thermostats areused for space-heating controls, inwater heaters and refrigerators, andto maintain the environment of a sci-entiÜc experiment at a constant tem-perature.

THF See tetrahydrofuran.

thiamin(e) See vitamin b complex.

thiazole A heterocyclic compound

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containing a Üve-membered ringwith sulphur and nitrogen heteroatoms, C3SNH3. A range of thiazoledyes are manufactured containingthis ring system.

thienyl ring 526

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S

N

Thiazole

S

Thiophene

thienyl ring See thiophene.

thin-layer chromatography Atechnique for the analysis of liquidmixtures using *chromatography.The stationary phase is a thin layer ofan absorbing solid (e.g. alumina) pre-pared by spreading a slurry of thesolid on a plate (usually glass) anddrying it in an oven. A spot of themixture to be analysed is placed nearone edge and the plate is stood up-right in a solvent. The solvent risesthrough the layer by capillary actioncarrying the components up theplate at different rates (depending onthe extent to which they are ab-sorbed by the solid). After a giventime, the plate is dried and the loca-tion of spots noted. It is possible toidentify constituents of the mixtureby the distance moved in a giventime. The technique needs carefulcontrol of the thickness of the layerand of the temperature. See also rfvalue.

thiocyanate A salt or ester of thio-cyanic acid.

thiocyanic acid An unstable gas,HSCN.

thio ethers See sulphides.

thiol group See thiols.

thiols (mercaptans; thio alcohols)Organic compounds that contain thegroup –SH (called the thiol group,mercapto group, or sulphydryl

group). Thiols are analogues of alco-hols in which the oxygen atom is re-placed by a sulphur atom. They arenamed according to the parent hy-drocarbon; e.g. ethane thiol (C2H5SH).A characteristic property is theirstrong disagreeable odour. For exam-ple the odour of garlic is produced byethane thiol. Unlike alcohols they areacidic, reacting with alkalis and cer-tain metals to form saltlike com-pounds. The older name, mercaptan,comes from their ability to react with(‘seize’) mercury.

thionyl chloride See sulphurdichloride oxide.

thionyl group The group =SO, asin *sulphur dichloride oxide.

thiophene A colourless liquid com-pound, C4H4S; m.p. –38°C; b.p. 84°C.The compound is present in commer-cial benzene. The ring system is alsoknown as a thienyl ring.

thiosulphate A salt containing theion S2O3

2– formally derived fromthiosulphuric acid. Thiosulphatesreadily decompose in acid solution togive elemental sulphur and hydro-gensulphite (HSO3

–) ions.

thiosulphuric acid An unstableacid, H2S2O3, formed by the reactionof sulphur trioxide with hydrogensulphide. See also sulphuric acid.

thiourea A white crystalline solid,(NH2)2CS; r.d. 1.4; m.p. 182°C. It isused as a Üxer in photography.

thixotropy See newtonian fluid.

Thomson, Sir Joseph John(1856–1940) British physicist, who be-


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came a professor at Cambridge Uni-versity in 1884. He is best known forhis work on cathode rays, which ledto his discovery of the electron in1897. He went on to study the con-duction of electricity through gases,and it is for this work that he wasawarded the Nobel Prize for physicsin 1906. His son, Sir George PagetThomson (1892–1975), discovered*electron diffraction, for which heshared the 1937 Nobel Prize forphysics with Clinton J. Davisson(1881–1958), who independentlymade the same discovery.

Thomson, Sir William See kelvin,lord.

thoria See thorium.

thorium Symbol Th. A grey radio-active metallic element belonging tothe *actinoids; a.n. 90; r.a.m.232.038; r.d. 11.5–11.9 (17°C); m.p.1740–1760°C; b.p. 4780–4800°C. It oc-curs in monazite sand in Brazil,India, and USA. The isotopes of tho-rium have mass numbers from 223to 234 inclusive; the most stable iso-tope, thorium–232, has a half-life of1.39 × 1010 years. It has an oxidationstate of (+4) and its chemistry resem-bles that of the other actinoids. It canbe used as a nuclear fuel for breederreactors as thorium–232 capturesslow neutrons to breed uranium–233.Thorium dioxide (thoria, ThO2) isused on gas mantles and in specialrefractories. The element was discov-ered by J. J. *Berzelius in 1829.


thorium series See radioactive se-ries.

three-body problem See many-body problem.

threnardite A mineral form of*sodium sulphate, Na2SO4.

threonine See amino acid.

thulium Symbol Tm. A soft greymetallic element belonging to the*lanthanoids; a.n. 69; r.a.m. 168.934;r.d. 9.321 (20°C); m.p. 1545°C; b.p.1947°C. It occurs in apatite and xeno-time. There is one natural isotope,thulium–169, and seventeen artiÜcialisotopes have been produced. Thereare no uses for the element, whichwas discovered by Per Cleve (1840–1905) in 1879. A• Information from the WebElements site

thymidine A nucleoside consistingof one thymine molecule linked to ad-doxyribose sugar molecule.

thymine A *pyrimidine derivativeand one of the major componentbases of *nucleotides and the nucleicacid *DNA.

527 thymol

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thymol A pungent-smelling colour-

N

N

O

O

CH3

Thymine

OH

O

CH2

OH

N

NH

O

O

CH3

Thymidine


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less crystalline compound, C10H14O;m.p. 51°C. It occurs in various essen-tial oils, particularly oil of thyme,and can be made by using iron(III)chloride to oxidize piperitone (itselfextracted from eucalyptus oil). Itsantiseptic properties are exploited ingargles and mouthwashes.

tie line A horizontal line in a liq-uid–vapour phase diagram that isperpendicular to an *isopleth. Thestarting point is the Ürst point atwhich the liquid can coexist with itsvapour as the pressure is reduced;the end point is the point beyondwhich only vapour is present.

time-of-Ûight mass spectrom-eter See mass spectroscopy.

tin Symbol Sn. A silvery malleablemetallic element belonging to*group 14 (formerly IVB) of the peri-odic table; a.n. 50; r.a.m. 118.69; r.d.7.28; m.p. 231.88°C; b.p. 2260°C. It isfound as tin(IV) oxide in ores, such ascassiterite, and is extracted by reduc-tion with carbon. The metal (calledwhite tin) has a powdery nonmetallicallotrope grey tin, into which itchanges below 18°C. The formationof this allotrope is called tin plague;it can be reversed by heating to100°C. The natural element has 21isotopes (the largest number of anyelement); Üve radioactive isotopes arealso known. The metal is used as athin protective coating for steel plateand is a constituent of a number of

alloys (e.g. phosphor bronze, gunmetal, solder, Babbitt metal, andpewter). Chemically it is reactive. Itcombines directly with chlorine andoxygen and displaces hydrogen fromdilute acids. It also dissolves in alkalisto form *stannates. There are two se-ries of compounds with tin in the +2and +4 oxidation states.

A• Information from the WebElements

site

tin(II) chloride A white solid,SnCl2, soluble in water and ethanol.It exists in the anhydrous form(rhombic; r.d. 3.95; m.p. 246°C; b.p. 652°C) and as a dihydrate,SnCl2.2H2O (monoclinic; r.d. 2.71;m.p. 37.7°C). The compound is madeby dissolving metallic tin in hy-drochloric acid and is partially hy-drolysed in solution.

Sn2+ + H2O ˆ SnOH+ + H+

Excess acid must be present to pre-vent the precipitation of basic salts.In the presence of additional chlorideions the pyramidal ion [SnCl3]– isformed; in the gas phase the SnCl2molecule is bent. It is a reducingagent in acid solutions and oxidizesslowly in air:

Sn2+ → Sn4+ + 2e

tin(IV) chloride A colourless fum-ing liquid, SnCl4, hydrolysed in coldwater, decomposed by hot water, andsoluble in ethers; r.d. 2.226; m.p.–33°C; b.p. 114°C. Tin(IV) chloride isa covalent compound, which may beprepared directly from the elements.It dissolves sulphur, phosphorus,bromine, and iodine, and there is evi-dence for the presence of speciessuch as SnCl2I2. In hydrochloric acidand in chloride solutions the coordi-nation is extended from four to sixby the formation of the SnCl62– ion.

tie line 528

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OH

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tincture A solution with alcohol asthe solvent (e.g. tincture of iodine).

tin(IV) hydride (stannane) Ahighly reactive and volatile gas (b.p.–53°), SnH4, which decomposes onmoderate heating (150°C). It is pre-pared by the reduction of tin chlo-rides using lithium tetrahydrido-aluminate(III) and is used in thesynthesis of some organo-tin com-pounds. The compound has reducingproperties.

tin(IV) oxide (tin dioxide) A whitesolid, SnO2, insoluble in water; tetra-hedral; r.d. 6.95; m.p. 1127°C; sub-limes between 1800°C and 1900°C.Tin(IV) oxide is trimorphic: the com-mon form, which occurs naturally asthe ore *cassiterite, has a rutile lat-tice but hexagonal and rhombicforms are also known. There are alsotwo so-called dihydrates, SnO2.2H2O,known as α- and β-stannic acid.These are essentially tin hydroxides.Tin(IV) oxide is amphoteric, dissolv-ing in molten alkalis to form *stan-nates; in the presence of sulphur,thiostannates are produced.

tin plague See tin.

tin(II) sulphide A grey-black cubicor monoclinic solid, SnS, virtually in-soluble in water; r.d. 5.22; m.p.882°C; b.p. 1230°C. It has a layerstructure similar to that of blackphosphorus. Its heat of formation islow and it can be made by heatingthe elements together. Above 265°Cit slowly decomposes (disproportion-ates) to tin(IV) sulphide and tinmetal. The compound reacts with hy-drochloric acid to give tin(II) chlorideand hydrogen sulphide.

tin(IV) sulphide (mosaic gold) Abronze or golden yellow crystallinecompound, SnS2, insoluble in waterand in ethanol; hexagonal; r.d. 4.5;decomposes at 600°C. It is preparedby the reaction of hydrogen sulphide

with a soluble tin(IV) salt or by theaction of heat on thiostannic acid,H2SnS3. The golden-yellow form usedfor producing a gilded effect on woodis prepared by heating tin, sulphur,and ammonium chloride.

TIP See temperature-independentparamagnetism.

titania See titanium(iv) oxide.

titanic chloride See titanium(iv)chloride.

titanic tetrachloride See tita-nium(iv) chloride.

titanium Symbol Ti. A white metal-lic *transition element; a.n. 22; r.a.m.47.9; r.d. 4.5; m.p. 1660±10°C; b.p.3287°C. The main sources are rutile(TiO2) and, to a lesser extent, il-menite (FeTiO3). The element also oc-curs in numerous other minerals. Itis obtained by heating the oxide withcarbon and chlorine to give TiCl4,which is reduced by the *Krollprocess. The main use is in a largenumber of strong light corrosion-resistant alloys for aircraft, ships,chemical plant, etc. The elementforms a passive oxide coating in air.At higher temperatures it reacts withoxygen, nitrogen, chlorine, and othernonmetals. It dissolves in diluteacids. The main compounds are tita-nium(IV) salts and complexes; tita-nium(II) and titanium(III) compoundsare also known. The element wasÜrst discovered by William Gregor(1761–1817) in 1789.


titanium(IV) chloride (titanic chlo-ride; titanium tetrachloride) Acolourless volatile liquid, TiCl4, madeby strongly heating titanium(IV)oxide and carbon in a stream of drychorine gas. It fumes in moist air toproduce titanium oxychlorides. It is

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used to produce other titanium com-pounds and pure titanium metal.

titanium dioxide See titanium(iv)oxide.

titanium(IV) oxide (titania; tita-nium dioxide) A white oxide, TiO2,occurring naturally in various forms,particularly the mineral rutile. It isused as a white pigment and as aÜller for plastics, rubber, etc.

titration A method of volumetricanalysis in which a volume of onereagent (the titrant) is added to aknown volume of another reagentslowly from a burette until an endpoint is reached (see indicator). Thevolume added before the end point isreached is noted. If one of the solu-tions has a known concentration,that of the other can be calculated.

TNT See trinitrotoluene.

tocopherol See vitamin e.

Tollens reagent A reagent used intesting for aldehydes. It is made byadding sodium hydroxide to silver ni-trate to give silver(I) oxide, which isdissolved in aqueous ammonia (giv-ing the complex ion [Ag(NH3)2]+). Thesample is warmed with the reagentin a test tube. Aldehydes reduce thecomplex Ag+ ion to metallic silver,forming a bright silver mirror on theinside of the tube (hence the namesilver-mirror test). Ketones give a neg-ative result. It is named after Bern-hard Tollens (1841–1918).

toluene See methylbenzene.

topaz A variably coloured alu-minium silicate mineral,Al2(SiO4)(OH,F)2, that forms or-thorhombic crystals. It occurs chieÛyin acid igneous rocks, such as gran-ites and pegmatites. Topaz is valuedas a gemstone because of its trans-parency, variety of colours (the wine-yellow variety being most highly

prized), and great hardness (8 on theMohs’ scale). When heated, yellow orbrownish topaz often becomes arose-pink colour. The main sources oftopaz are Brazil, Russia, and the USA.

torr A unit of pressure, used inhigh-vacuum technology, deÜned as1 mmHg. 1 torr is equal to 133.322pascals. The unit is named afterEvangelista Torricelli (1609–47).

torsion angle In a nonlinear chainof atoms A–B–C–D, the angle be-tween the plane containing atomsABC and the plane containing BCD.The torsion angle can have any valuefrom 0° to 180°. If the chain isviewed along the line BC, the torsionangle is positive if the bond ABwould have to be rotated in a clock-wise sense (less than 180°) to eclipse(i.e. align with) the bond CD. If therotation of AB has to be in an anti-clockwise sense, the torsion anglewould be negative.

There is a terminology relating tothe steric arrangements of atomsbased on the size of the torsion angle(see also conformation). The synarrangement is one in which the sizeof the torsion angle is ±90°. The antiarrangement is one with a torsion

titanium dioxide 530

t

synperiplanar

antiperiplanar

+synclinal

+anticlinal

–synclinal

–anticlinal

–30°0°

+30°

sp sp

+sc

+90°

+ac

+150°

ap

–150°

–ac

–90°

–sc

Torsion angle


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angle between ±90° and 180°. An-other distinction is between clinalarrangements (value between 30°and 150° or –30° and –150°) and peri-planar arrangements (value between0 and ±30° or ±150° and 180°). Com-bining the two gives four ranges oftorsion angle in an arrangement:

synperiplanar (sp) 0° to ±30°;synclinal (sc) 30° to 90° or –30° to–90°;anticlinal (ac) 90° to 150° or –90° to–150°;antiperiplanar (ap) ±150° to 180°.The antiperiplanar conformation is

also called a trans conformation; thesynperiplanar conformation corre-sponds to a cis conformation. Thesynclinal conformation is also calleda gauche or skew conformation.

total-radiation pyrometer See py-rometry.

tourmaline A group of mineralscomposed of complex cyclosilicatescontaining boron with the generalformula NaR3

2+Al6B3Si6O27(H,F)4,where R = Fe2+, Mg, or (Al + Li). Thecrystals are trigonal, elongated, andvariably coloured, the two ends ofthe crystals often having differentcolours. Tourmaline is used as a gem-stone and because of its double re-fraction and piezoelectric propertiesis also used in polarizers and somepressure gauges.

town gas Any manufactured gassupplied as a fuel gas to domesticand industrial users. Examples in-clude *coal gas, substitute naturalgas (*SNG), and *natural gas itself.Most town gases are based on hydro-gen or methane, although coal gasalso contains up to 8% carbonmonoxide.

trace element See essential el-ement.

tracing (radioactive tracing) See la-belling.

trans See isomerism; torsionangle.

transactinide elements Elementswith an atomic number greater than103, i.e. elements above lawrenciumin the *periodic table. So far, el-ements up to 116 have been de-tected. Because of the highlyradioactive and transient nature ofthese elements, there has been muchdispute about priority of discoveryand, consequently, naming of the el-ements. The International Union ofPure and Applied Chemistry (IUPAC)introduced a set of systematic tempo-rary names based on afÜxes, asshown in the table.

531 transactinide elements

t

Affix Number Symbol

nil 0 nun 1 ubi 2 btri 3 tquad 4 qpent 5 phex 6 hsept 7 soct 8 oenn 9 e

All these element names end in -ium. So, for example, element 109 inthis system is called un + nil + enn +ium, i.e. unnilennium, and given thesymbol u+n+e, i.e. Une.

One long-standing dispute wasabout the element 104 (ruther-fordium), which has also been calledkurchatovium (Ku). There have alsobeen disputes between IUPAC andthe American Chemical Union aboutelement names.

In 1994 IUPAC suggested the fol-lowing list:

mendelevium (Md, 101)nobelium (No, 102)lawrencium (Lr, 103)dubnium (Db, 104)


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joliotium (Jl, 105)rutherfordium (Rf, 106)bohrium (Bh, 107)hahnium (Hn, 108)meitnerium (Mt, 109)

The ACU favoured a different set ofnames:

mendelevium (Md, 101)nobelium (No, 102)lawrencium (Lr, 103)rutherfordium (Rf, 104)hahnium (Ha, 105)seaborgium (Sg, 106)nielsbohrium (Ns, 107)hassium (Hs, 108)meitnerium (Mt, 109)

A compromise list was adopted byIUPAC in 1997 and is generally ac-cepted:

mendelevium (Md, 101)nobelium (No, 102)lawrencium (Lr, 103)rutherfordium (Rf, 104)dubnium (Db, 105)seaborgium (Sg, 106)bohrium (Bh, 107)hassium (Hs, 108)meitnerium (Mt, 109)

Element 110 was named as darm-stadtium in 2003 and element 111was named roentgenium in 2004. Sofar elements 112 (ununbium, Uub),113 (ununtrium, Uut), 114 (ununqua-dium, Uuq), 115 (ununpentium,Uup), and 116 (ununhexium, Uuh)are not ofÜcially named. All these el-ements are unstable and have veryshort half-lives.

transamination A biochemical re-action in amino acid metabolism inwhich an amine group is transferredfrom an amino acid to a keto acid toform a new amino acid and ketoacid. The coenzyme required for thisreaction is pyridoxal phosphate.

trans effect An effect in the substi-tution of inorganic square-planarcomplexes, in which certain ligandsin the original complex are able to

direct the incoming ligand into thetrans position. The order of ligandsin decreasing trans-directing poweris:

CN– > NO2 > I– > Br– > Cl– > NH3 >H2O.

transfer coefÜcient Symbol α. Aquantity used in the analysis ofprocesses in electrochemistry; it isthe fraction of the potential differ-ence at the surface of an electrodethat assists charge difference in onedirection but discourages it in theother direction. Its value lies between0 and 1, frequently being about ½. Itis related to the slope of a graph ofthe logarithm of the current againstthe potential.

transformation The conversion ofa compound into a particular prod-uct, irrespective of the reagents ormechanism involved.

transient species A short-lived in-termediate in a chemical reaction.

transition A change of a systemfrom one quantum state to another.

transition elements A set of el-ements in the *periodic table inwhich Ülling of electrons in an innerd- or f-level occurs. With increasingproton number, electrons Üll atomic levels up to argon, which has the electron conÜguration1s22s22p63s23p6. In this shell, thereare 5 d-orbitals, which can each con-tain 2 electrons. However, at thispoint the subshell of lowest energy isnot the 3d but the 4s. The next twoelements, potassium and calcium,have the conÜgurations [Ar]4s1 and[Ar]4s2 respectively. For the next el-ement, scandium, the 3d level is oflower energy than the 4p level, andscandium has the conÜguration[Ar]3d14s2. This Ülling of the inner d-level continues up to zinc [Ar]3d104s2,giving the Ürst transition series.

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There is a further series of this typein the next period of the table: be-tween yttrium ([Kr]4d5s2) and cad-mium ([Kr]4d105s2). This is the secondtransition series. In the next periodof the table the situation is rathermore complicated. Lanthanum hasthe conÜguration [Xe]5d16s2. Thelevel of lowest energy then becomesthe 4f level and the next element,cerium, has the conÜguration[Xe]4f15d16s2. There are 7 of these f-orbitals, each of which can contain 2 electrons, and Ülling of the f-levels continues up to lutetium([Xe]4f145d16s2). Then the Ülling of the5d levels continues from hafnium tomercury. The series of 14 elementsfrom cerium to lutetium is a ‘serieswithin a series’ called an inner transi-tion series. This one is the *lan-thanoid series. In the next periodthere is a similar inner transition se-ries, the *actinoid series, from tho-rium to lawrencium. Then Ülling ofthe d-level continues from element104 onwards.

In fact, the classiÜcation of chemi-cal elements is valuable only in so faras it illustrates chemical behaviour,and it is conventional to use the term‘transition elements’ in a more re-stricted sense. The elements in theinner transition series from cerium(58) to lutetium (71) are called thelanthanoids; those in the series fromthorium (90) to lawrencium (103) arethe actinoids. These two series to-gether make up the f-block in the pe-riodic table. It is also common toinclude scandium, yttrium, and lan-thanum with the lanthanoids (be-cause of chemical similarity) and toinclude actinium with the actinoids.Of the remaining transition el-ements, it is usual to speak of threemain transition series: from titaniumto copper; from zirconium to silver;and from hafnium to gold. All theseelements have similar chemical prop-

erties that result from the presenceof unÜlled d-orbitals in the elementor (in the case of copper, silver, andgold) in the ions. The elements from104 to 109 and the undiscovered el-ements 110 and 111 make up afourth transition series. The el-ements zinc, cadmium, and mercuryhave Ülled d-orbitals both in the el-ements and in compounds, and areusually regarded as nontransition el-ements forming group 12 of the peri-odic table.

The elements of the three maintransition series are all typical metals(in the nonchemical sense), i.e. mostare strong hard materials that aregood conductors of heat and elec-tricity and have high melting andboiling points. Chemically, their be-haviour depends on the existence ofunÜlled d-orbitals. They exhibit vari-able valency, have coloured com-pounds, and form *coordinationcompounds. Many of their com-pounds are paramagnetic as a resultof the presence of unpaired elec-trons. Many of them are good cata-lysts. They are less reactive than thes- and p-block metals.

transition point (transition tem-perature) 1. The temperature atwhich one crystalline form of a sub-stance changes to another form. 2. The temperature at which a sub-stance changes phase. 3. The tem-perature at which a substancebecomes superconducting. 4. Thetemperature at which some otherchange, such as a change of magneticproperties, takes place.

transition state (activated com-plex) The association of atoms ofhighest energy formed during achemical reaction. The transitionstate can be regarded as a short-livedintermediate that breaks down togive the products. For example, in aSN2 substitution reaction, one atom

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or group approaches the molecule asthe other leaves. The transition stateis an intermediate state in whichboth attacking and leaving groupsare partly bound to the molecule, e.g.

B + RA → B—-R—-A → BR + AIn the theory of reaction rates, thereactants are assumed to be in equi-librium with this activated complex,which decomposes to the products.

transmission electron micro-scope See electron microscope.

transmutation The transforma-tion of one element into another bybombardment of nuclei with parti-cles. For example, plutonium is ob-tained by the neutron bombardmentof uranium.

transport coefÜcients Quantitiesthat characterize transport in a sys-tem. Examples of transport coefÜ-cients include electrical and thermalconductivity. One of the main pur-poses of *nonequilibrium statisticalmechanics is to calculate such coefÜ-cients from Ürst principles. It isdifÜcult to calculate transport coefÜ-cients exactly for noninteracting sys-tems and it is therefore necessary touse approximation techniques. Atransport coefÜcient gives a measurefor Ûow in a system. An inversetransport coefÜcient gives a measureof resistance to Ûow in a system.

transport number Symbol t. Thefraction of the total charge carried bya particular type of ion in the con-duction of electricity through elec-trolytes.

transuranic elements Elementswith an atomic number greater than92, i.e. elements above uranium inthe *periodic table. Most of these el-ements are unstable and have shorthalf-lives. See also transactinide el-ements.

triacetone triperoxide (TATP) An

organic peroxide, C9H18O6, derivedfrom acetone; r.d. 1.18; m.p. 91°C. Itis a highly explosive white crystallinesubstance, very sensitive to heat andshock. It is favoured by some terror-ist groups because it can easily bemade from commonly available com-pounds. For example, it is probablethat it was used in the London bomb-ings of 7 July 2005.

triacylglycerol See triglyceride.

triatomic molecule A moleculeformed from three atoms (e.g. H2O orCO2).

triazine See azine.

transmission electron microscope 534

t

O

O

O

OO

O

CH3 CH3

CH3

CH3

CH3

CH3

Triacetone triperoxide

Triazine

tribology The study of friction, lu-brication, and lubricants.

triboluminescence *Lumines-cence caused by friction; for exam-ple, some crystalline substances emitlight when they are crushed as a re-sult of static electric charges gener-ated by the friction.

tribromomethane (bromoform) Acolourless liquid *haloform, CHBr3;r.d. 2.9; m.p. 8°C; b.p. 150°C.

tricarbon dioxide (carbon subox-ide) A colourless gas, C3O2, with an


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unpleasant odour; r.d. 1.114 (liquid at0°C); m.p. –111.3°C; b.p. 7°C. It is theacid anhydride of malonic acid, fromwhich it can be prepared by dehydra-tion using phosphorus(V) oxide. Themolecule is linear (O:C:C:C:O).

tricarboxylic acid cycle See krebscycle.

trichloroethanal (chloral) A liquidaldehyde, CCl3CHO; r.d. 1.51; m.p.–57.5°C; b.p. 97.8°C. It is made bychlorinating ethanal and used inmaking DDT. See also 2,2,2-trichloroethanediol.

2,2,2-trichloroethanediol (chloralhydrate) A colourless crystallinesolid, CCl3CH(OH)2; r.d. 1.91; m.p.57°C; b.p. 96.3°C. It is made by thehydrolysis of trichloroethanal and isunusual in having two –OH groupson the same carbon atom. Gem diolsof this type are usually unstable; inthis case the compound is stabilizedby the presence of the three Clatoms. It is used as a sedative.

trichloroethene (trichlorethylene)A colourless liquid, CCl2=CHCl, b.p.87°C. It is toxic and nonÛammable,with a smell resembling that of chlo-roform (trichloromethane). It iswidely used as a solvent in dry clean-ing and degreasing. It is also used toextract oils from nuts and fruit, as an anaesthetic, and as a Üre extin-guisher.

trichloromethane (chloroform) Acolourless volatile sweet-smelling liq-uid *haloform, CHCl3; r.d. 1.48; m.p.–63.5°C; b.p. 61.7°C. It can be madeby chlorination of methane (followedby separation of the mixture of prod-ucts) or by the haloform reaction. Itis an effective anaesthetic but cancause liver damage and it has nowbeen replaced by other halogenatedhydrocarbons. Chloroform is used asa solvent and raw material for mak-ing other compounds.

triclinic See crystal system.

tridymite A mineral form of *sili-con(IV) oxide, SiO2.

triethanolamine See ethanol-amine.

triglyceride (triacylglycerol) Anester of glycerol (propane-1,2,3-triol)in which all three hydroxyl groupsare esteriÜed with a fatty acid.Triglycerides are the major con-stituent of fats and oils and provide aconcentrated food energy store in liv-ing organisms as well as cooking fatsand oils, margarines, etc. Their physi-cal and chemical properties dependon the nature of their constituentfatty acids. In simple triglycerides allthree fatty acids are identical; inmixed triglycerides two or three dif-ferent fatty acids are present.

trigonal bipyramid See illustra-tion at complex.

trihydrate A crystalline hydratethat contains three moles of waterper mole of compound.

trihydric alcohol See triol.

3,4,5-trihydroxybenzoic acid Seegallic acid.

triiodomethane (iodoform) A yel-low volatile solid sweet-smelling*haloform, CHI3; r.d. 4.1; m.p. 115°C.It is made by the haloform reaction.

triiron tetroxide (ferrosoferricoxide) A black magnetic oxide,Fe3O4; r.d. 5.2. It is formed when ironis heated in steam and also occursnaturally as the mineral *magnetite.The oxide dissolves in acids to give amixture of iron(II) and iron(III) salts.

trimethylaluminium (aluminiumtrimethyl) A colourless liquid,Al(CH3)3, which ignites in air and re-acts with water to give aluminiumhydroxide and methane, usually withextreme vigour; r.d. 0.752; m.p. 0°C;

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2,4,6-trinitrophenol 536

t

b.p. 130°C. Like other aluminiumalkyls it may be prepared by reactinga Grignard reagent with aluminiumtrichloride. Aluminium alkyls areused in the *Ziegler process for themanufacture of high-density poly-ethene (polythene).

2,4,6-trinitrophenol See picricacid.

trinitrotoluene (TNT) A yellowhighly explosive crystalline solid,CH3C6H2(NO2)3; r.d. 1.65; m.p. 82°C. Itis made by nitrating toluene (methyl-benzene), the systematic name being1-methyl-2,4,6-trinitrobenzene.

triol (trihydric alcohol) An *alcoholcontaining three hydroxyl groups permolecule.

triose A sugar molecule that con-tains three carbon atoms. See mono-saccharide.

trioxoboric(III) acid See boricacid.

trioxosulphuric(IV) acid See sul-phurous acid.

trioxygen See ozone.

triphenylmethyl An unusual aro-matic compound that can exist as an anion, cation or free radical,(C6H5)3C. Treatment of triphenyl-carbinol in alcoholic solution withmineral acids produces triphenyl-methyl salts, containing the cation(C6H5)3C+, which crystallize as or-ange-red solids. The action of sodiumon a solution of triphenylmethylchloride in ether produces the yellow(C6H5)3C– anion. The free radicalform is made by treating triphenyl-methyl chloride with mercury orzinc in the absence of oxygen.

triple bond See chemical bond.

triple point The temperature andpressure at which the vapour, liquid,and solid phases of a substance are inequilibrium. For water the triplepoint occurs at 273.16 K and 611.2 Pa.This value forms the basis of thedeÜnition of the *kelvin and thethermodynamic *temperature scale.

triplet An atomic state in whichtwo spin angular momenta of elec-trons combine together to give atotal nonzero spin. A triplet state

solid

ice line

triple point

hoarfrostline

steamline

vapour

611.2

liquid

temperature/K

pressure/Pa

273.16

Triple point


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usually has a lower energy than a*singlet state because of the effect ofcorrelations of spin on the Coulombinteractions between electrons (as in*Hund’s rules). This can lead to sub-stantial energy differences betweentriplet and singlet states.

trisilane See silane.

trisodium phosphate(V) (sodiumorthophosphate) A colourless crys-talline compound, Na3PO4, soluble inwater and insoluble in ethanol. It isknown both as the decahydrate (oc-tagonal; r.d. 2.54) and the dodeca-hydrate (trigonal; r.d. 1.62) Thedodecahydrate loses water at about76°C and the decahydrate melts at100°C. Trisodium phosphate may beprepared by boiling sodium carbon-ate with the stoichiometric amountof phosphoric acid and subsequentlyadding sodium hydroxide to the dis-odium salt thus formed. It is useful asan additive for high-pressure boilerfeed water (for removal of calciumand magnesium as phosphates), inemulsiÜers, as a water-softeningagent, and as a component in deter-gents and cleaning agents. Sodiumphosphate labelled with the radio-active isotope 32P is used in the studyof the role of phosphate in biologicalprocesses and is also used (intra-venously) in the treatment of poly-cythaemia.

tritiated compound Seelabelling.

tritium Symbol T. An isotope of hy-drogen with mass number 3; i.e. thenucleus contains 2 neutrons and 1proton. It is radioactive (half-life 12.3years), undergoing beta decay to helium–3. Tritium is used in *la-belling.

triton A nucleus of a tritium atom,consisting of a proton and two neu-trons bound together; the ion T+

formed by ionization of a tritiumatom. See also hydron.

trivalent (tervalent) Having a va-lency of three.

trona A mineral form of sodiumsesquicarbonate, consisting of amixed hydrated sodium carbonateand sodium hydrogencarbonate,Na2CO3.NaHCO3.2H2O.

tropylium ion The positive ionC7H7

+, having a ring of seven carbonatoms. The ion is symmetrical andhas characteristic properties of *aro-matic compounds.

trypsin An enzyme that digests pro-teins (see protease). It is secreted inan inactive form (trypsinogen) by thepancreas into the duodenum. There,trypsinogen is acted on by an en-zyme (enterokinase) produced in theduodenum to yield trypsin. The ac-tive enzyme plays an important rolein the digestion of proteins in the an-terior portion of the small intestine.It also activates other proteases inthe pancreatic juice.

trypsinogen See trypsin.

tryptamine A naturally occurringalkaloid, C10H12N2, having an indolering system with a –CH2–CH2–NH2side chain in the 3-position of thenitrogen-containing ring. Deriva-tives of tryptamines have hallucino-genic effects. An example in *psilo-cybin.

tryptophan See amino acid.

tub See ring conformations.

tuneable laser See dye laser.

tungsten Symbol W. A white orgrey metallic *transition element(formerly called wolfram); a.n. 74;r.a.m. 183.85; r.d. 19.3; m.p. 3410°C;b.p. 5660°C. It is found in a numberof ores, including the oxides wol-framite, (Fe,Mn)WO4, and scheelite,

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CaWO4. The ore is heated with con-centrated sodium hydroxide solutionto form a soluble tungstate. Theoxide WO3 is precipitated from thisby adding acid, and is reduced to themetal using hydrogen. It is used invarious alloys, especially high-speedsteels (for cutting tools) and in lampÜlaments. Tungsten forms a protec-tive oxide in air and can be oxidizedat high temperature. It does not dis-solve in dilute acids. It forms com-pounds in which the oxidation stateranges from +2 to +6. The metal wasÜrst isolated by Juan d’Elhuyer andFausto d’Elhuyer (1755–1833) in1783.A• Information from the WebElements site

tungsten carbide A black powder,WC, made by heating powderedtungsten metal with lamp black at1600°C. It is extremely hard (9.5 onMohs’ scale) and is used in dies andcutting tools. A ditungsten carbide,W2C, also exists.

tunnel effect An effect in whichelectrons are able to tunnel througha narrow potential barrier that wouldconstitute a forbidden region if theelectrons were treated as classicalparticles. That there is a Ünite proba-bility of an electron tunnelling fromone classically allowed region to an-other arises as a consequence of*quantum mechanics. The effect ismade use of in the tunnel diode.

turpentine An oily liquid extractedfrom pine resin. It contains pinene,C10H16, and other terpenes and ismainly used as a solvent.

turquoise A mineral consisting of ahydrated phosphate of aluminiumand copper, CuAl6(PO4)4(OH)8.4H2O),that is prized as a semipreciousstone. It crystallizes in the triclinicsystem and is generally blue in col-our, the ‘robin’s egg’ blue variety

being the most sought after. It usu-ally occurs in veinlets and as massesand is formed by the action of sur-face waters on aluminium-rich rocks.The Ünest specimens are obtainedfrom Iran.

twinning The growth of crystals insuch a way that two distinct crystalsform sharing a common plane ofatoms.

twist See ring conformations.

Tyndall effect The scattering oflight as it passes through a mediumcontaining small particles. If a poly-chromatic beam of light is passedthrough a medium containing parti-cles with diameters less than aboutone-twentieth of the wavlength oflight, the scattered light appearsblue. This accounts for the blue ap-pearance of tobacco smoke. Athigher particle diameters, the scat-tered light remains polychromatic.The effect is seen in suspensions andcertain colloids. It is named afterJohn Tyndall (1820–93).

type A and B metals A classiÜca-tion of metal ions according to thestability of their complexes for agiven ligand. Type A metals cationsinclude the ions of group 1 (Li+ toCs+), the ions of group 2 (Be2+ toBa2+), and ions of lighter transitionmetals in high oxidation states (e.g.Co3+, Ti4+, Fe3+). The type B metalcations are those of heavier transi-tion metals in lower oxidation states(e.g. Ag+, Cu+, Ni2+, Pd2+, Pt2+). Certainligands tend to form more stablecomplexes with type A metals; othersform more stable complexes withtype B. For example, the tendency ofhalide anions to complex with type Ametals is in the sequence

F– > Cl– > Br– > I–.Their tendency to complex with typeB metals is the opposite sequence.This led to a classiÜcation of ligands

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into type A ligands (e.g. F–) which tendto complex with type A metals andtype B ligands (e.g. I–), which tend tocomplex with type B metals. TheclassiÜcation was introduced by Ahr-land in 1958. See also hsab principle.

type A metal See type a and bmetals.

type B metal See type a and bmetals.

tyrosine See amino acid.

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U

ubiquinone Any of a group of re-lated quinone-derived compoundsthat serve as electron carriers in the*electron transport chain reactionsof cellular respiration. Coenzyme Qmolecules have side chains of differ-ent lengths in different types of or-ganisms but function in similar ways.

ultracentrifuge A high-speed cen-trifuge used to measure the rate ofsedimentation of colloidal particlesor to separate macromolecules, suchas proteins or nucleic acids, from so-lutions. Ultracentrifuges are electri-cally driven and are capable ofspeeds up to 60 000 rpm.

ultrahigh frequency (UHF) Aradio frequency in the range 3 ×109– 0.3 × 109 Hz; i.e. having a wave-length in the range 10 cm to 1 m.

ultramicroscope A form of micro-scope that uses the Tyndall effect toreveal the presence of particles thatcannot be seen with a normal opticalmicroscope. Colloidal particles,smoke particles, etc., are suspendedin a liquid or gas in a cell with ablack background and illuminated byan intense cone of light that entersthe cell from the side and has itsapex in the Üeld of view. The parti-cles then produce diffraction-ring sys-tems, appearing as bright specks onthe dark background.

ultrasonics The study and use ofpressure waves that have a frequencyin excess of 20 000 Hz and are there-fore inaudible to the human ear.Ultrasonics are used in medical diag-nosis, particularly in conditions suchas pregnancy, in which X-rays couldhave a harmful effect. Ultrasonic

techniques are also used industriallyto test for Ûaws in metals, to cleansurfaces, to test the thickness ofparts, and to form colloids.

ultraviolet radiation (UV) Electro-magnetic radiation having wave-lengths between that of violet lightand long X-rays, i.e. between 400nanometres and 4 nm. In the range400–300 nm the radiation is knownas the near ultraviolet. In the range300–200 nm it is known as the far ul-traviolet. Below 200 nm it is knownas the extreme ultraviolet or the vac-uum ultraviolet, as absorption by theoxygen in the air makes the use ofevacuated apparatus essential. Thesun is a strong emitter of UV radia-tion but only the near UV reachesthe surface of the earth as the*ozone layer of the atmosphere ab-sorbs all wavelengths below 290 nm.Ultraviolet radiation is classiÜed inthree ranges according to its effecton the skin. The ranges are:

UV-A (320–400 nm);UV-B (290–320 nm);UV-C (230–290 nm).

The longest-wavelength range, UV-A,is not harmful in normal doses and isused clinically in the treatment ofcertain skin complaints, such as pso-riasis. It is also used to induce *vita-min D formation in patients that areallergic to vitamin D preparations.UV-B causes reddening of the skinfollowed by pigmentation (tanning).Excessive exposure can cause severeblistering. UV-C, with the shortestwavelengths, is particularly dam-aging. It is thought that short-wave-length ultraviolet radiation causesskin cancer and that the risk of con-


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tracting this has been increased bythe depletion of the ozone layer.

Most UV radiation for practical useis produced by various types of mer-cury-vapour lamps. Ordinary glass ab-sorbs UV radiation and thereforelenses and prisms for use in the UVare made from quartz.

ultraviolet–visible spectroscopy(UV–visible spectroscopy) A tech-nique for chemical analysis and thedetermination of structure. It isbased on the principle that electronictransitions in molecules occur in thevisible and ultraviolet regions of theelectromagnetic spectrum, and that agiven transition occurs at a character-istic wavelength.

uncertainty principle (Heisenberguncertainty principle; principle of in-determinism) The principle that it isnot possible to know with unlimitedaccuracy both the position and mo-mentum of a particle. This principle,discovered in 1927 by Werner*Heisenberg, is usually stated in theform: ΔxΔpx ≥ h/4π, where Δx is theuncertainty in the x-coordinate of theparticle, Δpx is the uncertainty in the x-component of the particle’smomentum, and h is the *Planckconstant. An explanation of the un-certainty is that in order to locate aparticle exactly, an observer must beable to bounce off it a photon of radi-ation; this act of location itself altersthe position of the particle in an un-predictable way. To locate the posi-tion accurately, photons of shortwavelength would have to be used.These would have associated largemomenta and cause a large effect onthe position. On the other hand, usinglong-wavelength photons would haveless effect on the particle’s position,but would be less accurate because ofthe longer wavelength. The principlehas had a profound effect on scienti-Üc thought as it appears to upset the

classical relationship between causeand effect at the atomic level.

undetermined multipliers See la-grange multipliers.

UNFO Urea nitrate–fuel oil. An ex-plosive based in the fertilizer urea ni-trate mixed with fuel oil, similar inits action to ammonium nitrate–fueloil (ANFO). It has been used in anumber of terrorist attacks, most no-tably a car bombing in the basementof the World Trade Centre, NewYork, on 26 February 1993.

ungerade See gerade.

uniaxial crystal A double-refract-ing crystal (see double refraction)having only one optic axis.

unimolecular reaction A chemi-cal reaction or step involving onlyone molecule. An example is the de-composition of dinitrogen tetroxide:

N2O4 → 2NO2

Molecules colliding with other mol-ecules acquire sufÜcient activationenergy to react, and the activatedcomplex only involves the atoms of asingle molecule.

unit A speciÜed measure of a physi-cal quantity, such as length, mass,time, etc., speciÜed multiples ofwhich are used to express magni-tudes of that physical quantity. ForscientiÜc purposes previous systemsof units have now been replaced by*SI units.

unit cell The group of particles(atoms, ions, or molecules) in a crys-tal that is repeated in three dimen-sions in the *crystal lattice. See alsocrystal system.

univalent (monovalent) Having avalency of one.

universal constants See funda-mental constants.

universal indicator A mixture of

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acid–base *indicators that changescolour (e.g. red-yellow-orange-green-blue) over a range of pH.

unnil- See transactinide elements.

unsaturated 1. (of a compound)Having double or triple bonds in itsmolecules. Unsaturated compoundscan undergo addition reactions as wellas substitution. Compare saturated.2. (of a solution) See saturated.

unstable equilibrium See equilib-rium.

UPS Ultraviolet photoelectron spec-troscopy. See photoelectron spec-troscopy.

UPVC Unplasticized PVC: a toughhardwearing form of PVC used forwindow frames and similar applica-tions.

uracil A *pyrimidine derivative andone of the major component bases of*nucleotides and the nucleic acid*RNA.

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uraninite A mineral form of ura-nium(IV) oxide, containing minuteamounts of radium, thorium, polo-nium, lead, and helium. When urani-nite occurs in a massive form with apitchy lustre it is known as pitch-blende, the chief ore of uranium.Uraninite occurs in Saxony (Ger-many), Romania, Norway, the UK(Cornwall), E Africa (Democratic Re-public of Congo), USA, and Canada(Great Bear Lake).

uranium Symbol U. A white radio-active metallic element belonging to

the *actinoids; a.n. 92; r.a.m. 238.03;r.d. 19.05 (20°C); m.p. 1132±1°C; b.p.3818°C. It occurs as *uraninite, fromwhich the metal is extracted by anion-exchange process. Three isotopesare found in nature: uranium–238(99.28%), uranium–235 (0.71%), anduranium–234 (0.006%). As ura-nium–235 undergoes nuclear Üssionwith slow neutrons it is the fuel usedin nuclear reactors and nuclearweapons; uranium has therefore as-sumed enormous technical and polit-ical importance since their invention.It was discovered by Martin Klaproth(1747–1817) in 1789.A• Information from the WebElements site

uranium(VI) Ûuoride (uraniumhexaÛuoride) A volatile white solid,UF6; r.d. 4.68; m.p. 64.5°C. It is usedin the separation of uranium isotopesby gas diffusion.

uranium hexaÛuoride See ura-nium(vi) fluoride.

uranium–lead dating A group ofmethods of dating certain rocks thatdepends on the decay of the radioiso-tope uranium–238 to lead–206 (half-life 4.5 × 109 years) or the decay ofuranium–235 to lead–207 (half-life7.1 × 108 years). One form of ura-nium–lead dating depends on meas-uring the ratio of the amount ofhelium trapped in the rock to theamount of uranium present (sincethe decay 238U → 206Pb releases eightalpha-particles). Another method ofcalculating the age of the rocks is tomeasure the ratio of radiogenic lead(206Pb, 207Pb, and 208Pb) present tononradiogenic lead (204Pb). Thesemethods give reliable results for agesof the order 107–109 years.

uranium(IV) oxide A black solid,UO2; r.d. 10.9; m.p. 3000°C. It occursnaturally as *uraninite and is used innuclear reactors.


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uranium series See radioactiveseries.

urea (carbamide) A white crystallinesolid, CO(NH2)2; r.d. 1.3; m.p. 135°C.It is soluble in water but insoluble incertain organic solvents. Urea is themajor end product of nitrogen excre-tion in mammals, being synthesizedby the *urea cycle. Urea is synthe-sized industrially from ammonia andcarbon dioxide for use in *urea–formaldehyde resins and pharmaceu-ticals, as a source of nonprotein ni-trogen for ruminant livestock, and asa nitrogen fertilizer.

urea cycle (ornithine cycle) The se-ries of biochemical reactions thatconverts ammonia to *urea duringthe excretion of metabolic nitrogen.Urea formation occurs in mammalsand, to a lesser extent, in some otheranimals. The liver converts ammoniato the much less toxic urea, which isexcreted in solution in urine.

urea–formaldehyde resins Syn-thetic resins made by copolymerizingurea with formaldehyde (methanal).They are used as adhesives or ther-mosetting plastics.

urea nitrate See unfo.

urethane resins (polyurethanes)Synthetic resins containing the re-peating group –NH–CO–O–. There arenumerous types made by copolymer-izing isocyanate esters with polyhy-dric alcohols. They have a variety ofuses in plastics, paints, and solidfoams.

Urey, Harold Clayton (1894–1981)US physical chemist, who became aprofessor at the University of Califor-nia in 1958. His best-known workwas the discovery of *deuterium(heavy hydrogen) in 1932, for which

he was awarded the 1939 Nobel Prizefor physics.

uric acid The end product of purinebreakdown in most primates, birds,terrestrial reptiles, and insects andalso (except in primates) the majorform in which metabolic nitrogen isexcreted. Being fairly insoluble, uricacid can be expelled in solid form,which conserves valuable water inarid environments. The accumula-tion of uric acid in the synovial Ûuidof joints causes gout.

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O

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uridine A nucleoside consisting of one uracil molecule linked to a d-ribose sugar molecule. The derivedmucleotide uridine diphosphate(UDP) is important in carbohydratemetabolism.

UV See ultraviolet radiation.

UV–visible spectroscopy See ul-traviolet–visible spectroscopy.


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V

vacancy See crystal defect.

vacuum A space in which there is alow pressure of gas, i.e. relatively fewatoms or molecules. A perfect vac-uum would contain no atoms or mol-ecules, but this is unobtainable as allthe materials that surround such aspace have a Ünite *vapour pressure.In a soft (or low) vacuum the pres-sure is reduced to about 10–2 pascal,whereas a hard (or high) vacuum hasa pressure of 10–2–10–7 pascal. Below10–7 pascal is known as an ultrahighvacuum. See also vacuum pump.

vacuum distillation Distillationunder reduced pressure. The depres-sion in the boiling point of the sub-stance distilled means that thetemperature is lower, which mayprevent the substance from decom-posing.

vacuum pump A pump used to re-duce the gas pressure in a container.The normal laboratory rotary oil-sealpump can maintain a pressure of10–1 Pa. For pressures down to 10–7 Paa *diffusion pump is required. *Ionpumps can achieve a pressure of10–9 Pa and a *cryogenic pump com-bined with a diffusion pump canreach 10–13 Pa.

valence See valency.

valence band See energy bands.

valence-bond theory A methodof computational chemistry in whichthe electrons are considered as be-longing to deÜnite bonds consistingof pairs of electrons associated withpairs of atoms in the molecule. Theactual state of the molecule can beregarded as the result of a set of

canonical forms (see resonance). Seealso density-function theory; mo-lecular-orbital theory.

valence electron An electron inone of the outer shells of an atomthat takes part in forming chemicalbonds.

valency (valence) The combiningpower of an atom or radical, equal tothe number of hydrogen atoms thatthe atom could combine with or dis-place in a chemical compound (hy-drogen has a valency of 1). It is equalto the ionic charge in ionic com-pounds; for example, in Na2S,sodium has a valency of 1 (Na+) andsulphur a valency of 2 (S2–). In cova-lent compounds it is equal to thenumber of bonds formed; in CO2 oxy-gen has a valency of 2 and carbonhas a valency of 4.

valine See amino acid.

Valium See diazepam.

vanadium Symbol V. A silvery-white metallic *transition element;a.n. 23; r.a.m. 50.94; r.d. 5.96; m.p.1890°C; b.p. 3380°C. It occurs in anumber of complex ores, includingvanadinite (Pb5Cl(VO4)3) and carnotite(K2(ClO2)2(VO4)2). The pure metal canbe obtained by reducing the oxidewith calcium. The element is used ina large number of alloy steels. Chem-ically, it reacts with nonmetals athigh temperatures but is not affectedby hydrochloric acid or alkalis. Itforms a range of complexes with oxi-dation states from +2 to +5. Vana-dium was discovered in 1801 byAndrés del Rio (1764–1849), who al-lowed himself to be persuaded that


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what he had discovered was an im-pure form of chromium. The el-ement was rediscovered and namedby Nils Sefström (1787–1854) in 1880.


vanadium(V) oxide (vanadiumpentoxide) A crystalline compound,V2O5, used extensively as a catalyst inindustrial gas-phase oxidationprocesses.

vanadium pentoxide See vana-dium(v) oxide.

van der Waals’ equation Seeequation of state.

van der Waals’ force An attrac-tive force between atoms or mol-ecules, named after Johannes van derWaals (1837–1923). The force ac-counts for the term a/V2 in van derWaals’ equation (see equation ofstate). These forces are much weakerthan those arising from valencebonds and are inversely proportionalto the seventh power of the distancebetween the atoms or molecules.They are the forces responsible fornonideal behaviour of gases and forthe lattice energy of molecular crys-tals. There are three factors causingsuch forces: (1) dipole–dipole interac-tion, i.e. electrostatic attractionsbetween two molecules with per-manent dipole moments; (2) dipole-induced dipole interactions, in whichthe dipole of one molecule polarizesa neighbouring molecule; (3) disper-sion forces arising because of smallinstantaneous dipoles in atoms.

van’t Hoff, Jacobus See hoff.

van’t Hoff factor Symbol i. A fac-tor appearing in equations for *col-ligative properties, equal to the ratioof the number of actual particles pre-sent to the number of undissociatedparticles.

van’t Hoff’s isochore An equationformulated by van’t Hoff for the vari-ation of equilibrium constant withtemperature:

(d loge K)/dT = ∆H/RT2,where K is the equilibrium constant,R is the gas constant, T is the thermo-dynamic temperature, and ∆H the en-thalpy of the reaction.

Van Urk’s reagent See p-dimethyl-aminobenzaldehyde.

vapour density The density of agas or vapour relative to hydrogen,oxygen, or air. Taking hydrogen asthe reference substance, the vapourdensity is the ratio of the mass of aparticular volume of a gas to themass of an equal volume of hydrogenunder identical conditions of pres-sure and temperature. Taking thedensity of hydrogen as 1, this ratio isequal to half the relative molecularmass of the gas.

vapour pressure The pressure ex-erted by a vapour. All solids and liq-uids give off vapours, consisting ofatoms or molecules of the substancesthat have evaporated from the con-densed forms. These atoms or mol-ecules exert a vapour pressure. If thesubstance is in an enclosed space, thevapour pressure will reach an equi-librium value that depends only onthe nature of the substance and thetemperature. This equilibrium valueoccurs when there is a dynamic equi-librium between the atoms or mol-ecules escaping from the liquid orsolid and those that strike the sur-face of the liquid or solid and returnto it. The vapour is then said to be asaturated vapour and the pressure itexerts is the saturated vapour pres-sure.

variational principle In calcula-tions in quantum mechanics theprinciple that if a trial *wave func-tion is used to calculate the energy of

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a system, then the energy calculatedcannot be lower than the actualenergy of the ground state of thesystem. To use variational principleseffectively in quantum mechanics a trial wave function contains para-meters that can be varied, with these parameters being changeduntil the lowest possible energy isfound.

Vaseline See petroleum jelly.

verdigris A green patina of basiccopper salts formed on copper. Thecomposition of verdigris variesdepending on the atmosphericconditions, but includes the basiccarbonate CuCO3.Cu(OH)2, the basicsulphate CuSO4.Cu(OH)2.H2O, andsometimes the basic chlorideCuCl2.Cu(OH)2.

vermiculite See clay minerals.

very high frequency (VHF) Aradio frequency in the range 3 ×108– 0.3 × 108 Hz, i.e. having a wave-length in the range 1–10 m.

very low frequency (VLF) A radiofrequency in the range 3 × 104– 0.3 ×104 Hz, i.e. having a wavelength inthe range 10–100 km.

vibrational relaxation A processin which a polyatomic molecule inan excited vibrational state returns toa lower vibrational state in the sameelectronic state by colliding withother molecules.

vibrational spectroscopy Thespectroscopic investigation of the vi-brational energy levels of molecules.In the infrared region of the electro-magnetic spectrum vibrational tran-sitions are accompanied by rotationaltransitions. Infrared spectra of mol-ecules are series of bands, with eachband being associated with a vibra-tional transition and every line inthat band being associated with a ro-

tational transition that accompaniesthe vibrational transition.

Some features of vibrational spec-tra can be analysed by regarding thevibrations as simple harmonic mo-tion, but a realistic account of mo-lecular vibrations requires thatanharmonicity is taken into account.

A diatomic molecule can only havea vibrational–rotational spectrum if ithas a permanent dipole moment. Apolyatomic molecule can only have avibrational–rotational spectrum ifthe normal modes of vibration causethe molecule to have an oscillatingdipole moment.

vicinal (vic) Designating a moleculein which two atoms or groups arelinked to adjacent atoms. For exam-ple, 1,2-dichloroethane (CH2ClCH2Cl)is a vicinal (or vic) dihalide and canbe named vic-dichloroethane.

Victor Meyer’s method Amethod of measuring vapour density,devised by Victor Meyer (1848–97). Aweighed sample in a small tube isdropped into a heated bulb with along neck. The sample vaporizes anddisplaces air, which is collected overwater and the volume measured. Thevapour density can then be calcu-lated.

villiaumite A mineral form ofsodium Ûuoride, NaF.

vinegar A dilute solution of*ethanoic acid (up to 6%), used as aÛavouring and pickling medium. Nat-ural vinegar is made by the contin-ued fermentation of alcoholicliquors, usually by Acetobacter species,which oxidize ethanol to ethanoicacid. Vinegar is also made by dilutingsynthetic ethanoic acid.

vinyl acetate See ethenylethanoate.

vinylation The catalysed reaction

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between ethyne and a compoundwith an active hydrogen atom, suchas alcohol, amine, or carboxylic acid.Addition takes place across the triplebond of ethyne to produce an eth-enyl (vinyl) compound, containingthe group CH2=CH–.

vinyl chloride See chloroethene.

vinyl group The organic groupCH2:CH–.

virial coefÜcients See virial equa-tion.

virial equation A gas law that at-tempts to account for the behaviourof real gases, as opposed to an idealgas. It takes the form

pV = RT + Bp + Cp2 + Dp3 + …,where B, C, and D are known as virialcoefÜcients.

viscose process See rayon.

viscosity A measure of the resis-tance to Ûow that a Ûuid offers whenit is subjected to shear stress. For aNewtonian Ûuid, the force, F, neededto maintain a velocity gradient,dv/dx, between adjacent planes of aÛuid of area A is given by: F =ηA(dv/dx), where η is a constant,called the coefÜcient of viscosity. InSI units it has the unit pascal second(in the c.g.s. system it was measuredin *poise). Non-Newtonian Ûuids,such as clays, do not conform to thissimple model. See also kinematic vis-cosity.

visible spectrum The *spectrum

of electromagnetic radiations towhich the human eye is sensitive.

vitamin One of a number of or-ganic compounds required by livingorganisms in relatively smallamounts to maintain normal health.There are some 14 generally recog-nized major vitamins: the water-soluble *vitamin B complex (contain-ing 9) and *vitamin C and the fat-soluble *vitamin A, *vitamin D,*vitamin E, and *vitamin K. Most Bvitamins and vitamin C occur inplants, animals, and microorganisms;they function typically as *coen-zymes. Vitamins A, D, E, and K occuronly in animals, especially verte-brates, and perform a variety ofmetabolic roles. Animals are unableto manufacture many vitamins them-selves and must have adequateamounts in the diet. Foods may con-tain vitamin precursors (called provi-tamins) that are chemically changedto the actual vitamin on entering thebody. Many vitamins are destroyedby light and heat, e.g. during cooking.

vitamin A (retinol) A fat-solublevitamin that cannot be synthesizedby mammals and other vertebratesand must be provided in the diet.Green plants contain precursors ofthe vitamin, notably carotenes, thatare converted to vitamin A in the in-testinal wall and liver. The aldehydederivative of vitamin A, retinal, is aconstituent of the visual pigmentrhodopsin. DeÜciency affects theeyes, causing night blindness, xe-rophthalmia, and eventually total

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blindness. The role of vitamin A inother aspects of metabolism is lessclear.A• Information about IUPAC nomenclature

vitamin B complex A group ofwater-soluble vitamins that charac-teristically serve as components of*coenzymes. Plants and many mi-croorganisms can manufacture Bvitamins but dietary sources are es-sential for most animals. Heat andlight tend to destroy B vitamins.

Vitamin B1 (thiamin(e)) is a precur-sor of the coenzyme thiamine py-rophosphate, which functions incarbohydrate metabolism. DeÜciencyleads to beriberi in humans and topolyneuritis in birds. Good sourcesinclude brewer’s yeast, wheatgerm,beans, peas, and green vegetables.

Vitamin B2 (riboÛavin) occurs ingreen vegetables, yeast, liver, andmilk. It is a constituent of the coen-

zymes *FAD and FMN, which havean important role in the metabolismof all major nutrients as well as inthe oxidative phosphorylation reac-tions of the *electron transportchain. DeÜciency of B2 causes inÛam-mation of the tongue and lips andmouth sores.

Vitamin B6 (pyridoxine) is widelydistributed in cereal grains, yeast,liver, milk, etc. It is a constituent of acoenzyme (pyridoxal phosphate) in-volved in amino acid metabolism.DeÜciency causes retarded growth,dermatitis, convulsions, and othersymptoms.

N

N

CH2

S

NCH2

CH2 OHCH3 NH2

CH3

Vitamin B1

N

N

N

NH

O

O

CH3

CH3

CH2

CH2OH

OH

OHOH

Vitamin B2

N

CH3

OH

HOH2C

CH2OH

Vitamin B6

Vitamin B12 (cyanocobalamin;cobalamin) is manufactured only bymicroorganisms and natural sourcesare entirely of animal origin. Liver isespecially rich in it. One form of B12

functions as a coenzyme in a numberof reactions, including the oxidationof fatty acids and the synthesis ofDNA. It also works in conjunctionwith *folic acid (another B vitamin)in the synthesis of the amino acidmethionine and it is required for nor-mal production of red blood cells.Vitamin B12 can only be absorbed


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from the gut in the presence of a gly-coprotein called intrinsic factor; lackof this factor or deÜciency of B12 re-sults in pernicious anaemia.

Other vitamins in the B complexinclude *nicotinic acid, *pantothenicacid, *biotin, and *lipoic acid. See alsocholine.A• Information about IUPAC nomenclature

vitamin C (ascorbic acid) A colour-less crystalline water-soluble vitaminfound especially in citrus fruits andgreen vegetables. Most organismssynthesize it from glucose but manand other primates and various other

species must obtain it from theirdiet. It is required for the mainte-nance of healthy connective tissue;deÜciency leads to scurvy. Vitamin Cis readily destroyed by heat and light.A• Information about IUPAC nomenclature

vitamin D A fat-soluble vitamin oc-curring in the form of two steroid de-rivatives: vitamin D2 (ergocalciferol,or calciferol), found in yeast; andvitamin D3 (cholecalciferol), which oc-curs in animals. Vitamin D2 is formedfrom a steroid by the action of ultra-violet light and D3 is produced by theaction of sunlight on a cholesterolderivative in the skin. Fish-liver oilsare the major dietary source. The ac-tive form of vitamin D is manufac-tured in response to the secretion ofparathyroid hormone, which occurswhen blood calcium levels are low. Itcauses increased uptake of calciumfrom the gut, which increases thesupply of calcium for bone synthesis.Vitamin D deÜciency causes ricketsin growing animals and osteomalacia

549 vitamin D

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Vitamin D

O O

OHOH

CH2

OH

OH

Vitamin C


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vitamin E 550

v

in mature animals. Both conditionsare characterized by weak deformedbones.


vitamin E (tocopherol) A fat-solublevitamin consisting of several closelyrelated compounds, deÜciency ofwhich leads to a range of disorders indifferent species, including musculardystrophy, liver damage, and infertil-ity. Good sources are cereal grainsand green vegetables. Vitamin E pre-vents the oxidation of unsaturatedfatty acids in cell membranes, somaintaining their structure.


vitamin K A fat-soluble vitaminconsisting of several related com-pounds that act as coenzymes in thesynthesis of several proteins neces-sary for blood clotting. DeÜciency ofvitamin K, which leads to extensivebleeding, is rare because a form ofthe vitamin is manufactured by in-testinal bacteria. Green vegetablesand egg yolk are good sources.

vitreous Having a glasslike appear-ance or structure.

volt Symbol V. The SI unit of elec-tric potential, potential difference, ore.m.f. deÜned as the difference of po-tential between two points on a con-ductor carrying a constant current ofone ampere when the power dissi-

pated between the points is one watt.It is named after Alessandro Volta.

Volta, Alessandro Giuseppe An-tonio Anastasio (1745–1827) Ital-ian physicist. In 1774 he beganteaching in Como and in that year in-vented the electrophorus. He movedto Pavia University in 1778. In 1800he made the *voltaic cell, thus pro-viding the Ürst practical source ofelectric current. The SI unit of poten-tial difference is named after him.

voltaic cell (galvanic cell) A devicethat produces an e.m.f. as a result ofchemical reactions that take placewithin it. These reactions occur atthe surfaces of two electrodes, eachof which dips into an electrolyte. TheÜrst voltaic cell, devised by Alessan-dro Volta, had electrodes of two dif-ferent metals dipping into brine. Seeprimary cell; secondary cell.

voltaic pile An early form of bat-tery, devised by Alessandro Volta,consisting of a number of Ûat*voltaic cells joined in series. The liq-uid electrolyte was absorbed intopaper or leather discs.

voltameter (coulometer) 1. Anelectrolytic cell formerly used tomeasure quantity of electric charge.The increase in mass (m) of the cath-ode of the cell as a result of the depo-sition on it of a metal from a solutionof its salt enables the charge (Q) to bedetermined from the relationship Q =m/z, where z is the electrochemical

CH3O

CH3

OH

CH3

CH3

CH3

CH3 CH3 CH3

Vitamin E


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equivalent of the metal. 2. Anyother type of electrolytic cell used formeasurement.

volume Symbol V. The space occu-pied by a body or mass of Ûuid.

volumetric analysis A method ofquantitative analysis using measure-ment of volumes. For gases, the maintechnique is in reacting or absorbinggases in graduated containers overmercury, and measuring the volumechanges. For liquids, it involves*titrations.

von Baeyer nomenclature A sys-tem of naming polycyclic compoundsoriginally introduced by Adolf vonBaeyer in 1900 and since extended.

A• Information about the IUPAC system

VSEPR theory Valence-shell elec-tron-pair repulsion theory; a theoryfor predicting the shapes of mol-ecules. See lone pair.

V-series See nerve agents.

vulcanite (ebonite) A hard black in-sulating material made by the vul-canization of rubber with a highproportion of sulphur (up to 30%).

vulcanization A process for hard-ening rubber by heating it with sul-phur or sulphur compounds.

VX A highly toxic colourless oily liq-uid, C11H26NO2PS; r.d. 1.01; m.p.–50°C; b.p. 298°C. It is an organo-phosphorus compound and is proba-bly one of the most toxic of all*nerve agents. VX was discovered in1952 at the Porton Down researchstation in Wiltshire. VX is the bestknown of the V-series of nerve agents(the V denotes very high persistencein the environment). Other, lessknown, members of the series in-clude VG and VM, which are bothorganophosphorus compounds re-lated to VX.

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W

Wacker process A process for themanufacture of ethanal by the air ox-idation of ethene. A mixture of airand ethene is bubbled through a so-lution containing palladium(II) chlo-ride and copper(II) chloride. The Pd2+

ions form a complex with the ethenein which the ion is bound to the pielectrons in the C=C bond. This de-creases the electron density in thebond, making it susceptible to nu-cleophilic attack by water molecules.The complex formed breaks down toethanal and palladium metal. TheCu2+ ions oxidize the palladium backto Pd2+, being reduced to Cu+ ions inthe process. The air present oxidizesCu+ back to Cu2+. Thus the copper(II)and palladium(II) ions effectively actas catalysts in the process, which isnow the main source of ethanal and,by further oxidation, ethanoic acid. Itcan also be applied to other alkenes.It is named after Alexander vonWacker (1846–1922).

Wade’s rules A set of rules for pre-dicting the structure of a clustercompound based in the number ofelectrons in the framework countedin a particular way. The electronscounted are known as skeletal elec-trons. The rules apply to polyhedrathat have triangular faces (known asdeltahedra). They were originally ap-plied to the boron hydrides. Electronpairs in bonding between two boronatoms are counted as skeletal elec-trons but the pairs in B–H units areignored. However, if a boron atom isconnected to two hydrogens (BH2),the second bond is counted with theskeletal electrons. According to therules, if the formula is [BnHn]2– and

there are n+1 skeletal electron pairs,then the structure is closo. If the for-mula is

BnHn+4

and these are n+2 skeletal electronpairs, the structure is nido. If the for-mula is

BnHn+6

and these are n+3 skeletal electronpairs, the structure is arachno. Therules are named after the Britishchemist Kenneth Wade, who Ürst for-mulated them in the early 1970s.

Wagenaar test See acetone–chlor–haemin test.

Wagner-Meerwein rearrange-ment A rearrangement in which analkyl group moves from a carbonatom to an adjacent carbon atomduring a reaction. It often occurs tostabilize a carbocation formed as anintermediate during the reaction. Re-arrangements of this type have beenextensively studied in terpene chem-istry.

WAHUHA A pulse sequence in *nu-clear magnetic resonance (NMR) usedto reduce linewidth. The WAHUHAsequence (named after its inventorsWaugh, Huber, and Haberlen) effectsan averaging procedure by twistingthe magnetization vector in variousdirections.

Walden’s rule An empirical rulesuggested by P. Walden (1863–1957)concerning ions in solutions, statingthat the product of the molar con-ductivity, Λm, and the viscosity, η, isapproximately constant for the sameions in different solvents. Some


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justiÜcation for Walden’s rule is pro-vided by the proportional relation-ship between Λm and the diffusioncoefÜcient, D; as D is inversely pro-portional to the viscosity, Λm is in-versely proportional to η, which is inaccordance with Walden’s rule. How-ever, different solvents hydrate thesame ions differently, so that boththe radius and the viscosity changewhen the solvent is changed. It isthis fact that limits the validity of therule.

warfarin 3-(alpha-acetonylbenzyl)-4-hydroxycoumarin: a synthetic anti-coagulant used both therapeuticallyin clinical medicine and, in lethaldoses, as a rodenticide (see pesticide).

washing soda *Sodium carbonatedecahydrate, Na2CO3.10H2O.

water A colourless liquid, H2O; r.d.1.000 (4°C); m.p. 0.000°C; b.p.100.000°C. In the gas phase waterconsists of single H2O molecules inwhich the H–O–H angle is 105°. Thestructure of liquid water is still con-troversial; hydrogen bonding of thetype H2O…H–O–H imposes a high de-gree of structure and current modelssupported by X-ray scattering studieshave short-range ordered regions,which are constantly disintegratingand re-forming. This ordering of theliquid state is sufÜcient to make thedensity of water at about 0°C higherthan that of the relatively open-structured ice; the maximum densityoccurs at 3.98°C. This accounts forthe well-known phenomenon of iceÛoating on water and the contractionof water below ice, a fact of enor-mous biological signiÜcance for allaquatic organisms.

Ice has nine distinct structuralmodiÜcations of which ordinary ice,or ice I, has an open structure builtof puckered six-membered rings in

which each H2O unit is tetrahedrallysurrounded by four other H2O units.

Because of its angular shape thewater molecule has a permanent di-pole moment and in addition it isstrongly hydrogen bonded and has ahigh dielectric constant. These prop-erties combine to make water a pow-erful solvent for both polar and ioniccompounds. Species in solution arefrequently strongly hydrated and in fact ions frequently written as, for example, Cu2+ are essentially[Cu(H2O)6]2+. Crystalline *hydratesare also common for inorganic sub-stances; polar organic compounds,particularly those with O–H and N–Hbonds, also form hydrates.

Pure liquid water is very weaklydissociated into H3O+ and OH– ionsby self ionization:

H2O ˆ H+ + OH–

(see ionic product) and consequentlyany species that increases the con-centration of the positive species,H3O+, is acidic and species increasingthe concentration of the negativespecies, OH–, are basic (see acid). Thephenomena of ion transport in waterand the division of materials into hy-drophilic (water loving) and hy-drophobic (water hating) substancesare central features of almost all bio-logical chemistry. A further propertyof water that is of fundamental im-portance to the whole planet is itsstrong absorption in the infraredrange of the spectrum and its trans-parency to visible and near ultravio-let radiation. This allows solarradiation to reach the earth duringhours of daylight but restricts rapidheat loss at night. Thus atmosphericwater prevents violent diurnal oscil-lations in the earth’s ambient tem-perature.

water gas A mixture of carbonmonoxide and hydrogen produced bypassing steam over hot carbon (coke):

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H2O(g) + C(s) → CO(g) + H2(g)The reaction is strongly endothermicbut the reaction can be used in con-junction with that for *producer gasfor making fuel gas. The main use ofwater gas before World War II was inproducing hydrogen for the *Haberprocess. Here the above reaction wascombined with the water-gas shiftreaction to increase the amount ofhydrogen:

CO + H2O ˆ CO2 + H2

Most hydrogen for the Haber processis now made from natural gas bysteam *reforming.

water glass A viscous colloidal so-lution of sodium silicates in water,used to make silica gel and as a sizeand preservative.

water of crystallization Waterpresent in crystalline compounds indeÜnite proportions. Many crystallinesalts form hydrates containing 1, 2,3, or more moles of water per moleof compound, and the water may beheld in the crystal in various ways.Thus, the water molecules may sim-ply occupy lattice positions in thecrystal, or they may form bonds withthe anions or the cations present. Inthe pentahydrate of copper sulphate(CuSO4.5H2O), for instance, each cop-per ion is coordinated to four watermolecules through the lone pairs onthe oxygen to form the *complex[Cu(H2O)4]2+. Each sulphate ion hasone water molecule held by hydro-gen bonding. The difference betweenthe two types of bonding is demon-strated by the fact that the pentahy-drate converts to the monohydrate at100°C and only becomes anhydrousabove 250°C. Water of constitution isan obsolete term for water combinedin a compound (as in a metal hydrox-ide M(OH)2 regarded as a hydratedoxide MO.H2O).

water softening See hardness ofwater.

watt Symbol W. The SI unit ofpower, deÜned as a power of onejoule per second. In electrical con-texts it is equal to the rate of energytransformation by an electric currentof one ampere Ûowing through aconductor the ends of which aremaintained at a potential differenceof one volt. The unit is named afterJames Watt (1736–1819).

wave equation A partial differen-tial equation of the form:

∇2u = (1/c2)∂2u/∂t2,where

∇2 = ∂2/∂x2 + ∂2/∂y2 + ∂2/∂z2

is the Laplace operator. It representsthe propagation of a wave, where u isthe displacement and c the speed ofpropagation. See also schrödingerequation.

wave function A function ψ(x,y,z)appearing in *Schrödinger’s equationin wave mechanics. The wave func-tion is a mathematical expression in-volving the coordinates of a particlein space. If the Schrödinger equationcan be solved for a particle in a givensystem (e.g. an electron in an atom)then, depending on the boundaryconditions, the solution is a set ofallowed wave functions (eigen-functions) of the particle, each cor-responding to an allowed energylevel (eigenvalue). The physicalsigniÜcance of the wave function isthat the square of its absolute value,|ψ|2, at a point is proportional to theprobability of Ünding the particle in asmall element of volume, dxdydz, atthat point. For an electron in anatom, this gives rise to the idea ofatomic and molecular *orbitals.

wave mechanics A formulation of*quantum mechanics in which thedual wave–particle nature of such en-

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tities as electrons is described by the*Schrödinger equation. Schrödingerput forward this formulation ofquantum mechanics in 1926 and inthe same year showed that it wasequivalent to matrix mechanics. Tak-ing into account the *de Brogliewavelength, Schrödinger postulateda wave mechanics that bears thesame relation to Newtonian mechan-ics as physical optics does to geomet-rical optics.

wave number Symbol k. The num-ber of cycles of a wave in unit length.It is the reciprocal of the wavelength.

wave packet A superposition ofwaves with one predominant *wavenumber k, but with several otherwave numbers near k. Wave packetsare useful for the analysis of scatter-ing in *quantum mechanics. Concen-trated packets of waves can be usedto describe localized particles of bothmatter and *photons. The Heisen-berg *uncertainty principle can bederived from a wave-packet descrip-tion of entities in quantum mechan-ics. The motion of a wave packet is inaccord with the motion of the corre-sponding classical particle, if the po-tential energy change across thedimensions of the packet is verysmall. This proposition is known asEhrenfest’s theorem, named after theDutch physicist Paul Ehrenfest(1880–1933), who proved it in 1927.

wave–particle duality The con-cept that waves carrying energy mayhave a corpuscular aspect and thatparticles may have a wave aspect;which of the two models is the moreappropriate will depend on the prop-erties the model is seeking to ex-plain. For example, waves ofelectromagnetic radiation need to bevisualized as particles, called *pho-tons, to explain the photoelectric ef-fect while electrons need to be

thought of as de Broglie waves in*electron diffraction.

wave vector A vector k associatedwith a *wave number k. In the caseof free electrons the wave vector k isrelated to the momentum p in*quantum mechanics by p = k,where is the rationalized *Planckconstant. In the case of *Bloch’s theo-rem, the wave vector k can only havecertain values and can be thought ofas a quantum number associatedwith the translational symmetry ofthe crystal.

wax Any of various solid or semi-solid substances. There are two maintypes. Mineral waxes are mixtures ofhydrocarbons with high molecularweights. ParafÜn wax, obtained from*petroleum, is an example. Waxessecreted by plants or animals aremainly esters of fatty acids and usu-ally have a protective function.

weak acid An *acid that is onlypartially dissociated in aqueous solu-tion.

weber Symbol Wb. The SI unit ofmagnetic Ûux equal to the Ûux that,linking a circuit of one turn, pro-duces in it an e.m.f. of one volt as itis reduced to zero at a uniform ratein one second. It is named after Wil-helm Weber (1804–91).

Weissenberg technique A tech-nique used to overcome the problemof overlapping reÛections in the iden-tiÜcation of the symmetry and the di-mensions of a unit cell in *X-raycrystallography. In this technique, ascreen is placed in front of the Ülmallowing only one set of reÛections tobe exposed. The Weissenberg tech-nique produces distorted photo-graphs, but this can be overcome byhaving a coupling between the mo-tions of the crystal and the Ülm.Using the precession camera tech-

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nique undistorted photographs canbe obtained.

Weston cell (cadmium cell) A typeof primary *voltaic cell devised byEdward Weston (1850–1936), whichis used as a standard; it produces aconstant e.m.f. of 1.0186 volts at20°C. The cell is usually made in anH-shaped glass vessel with a mercuryanode covered with a paste of cad-mium sulphate and mercury(I) sul-phate in one leg and a cadmiumamalgam cathode covered with cad-mium sulphate in the other leg. Theelectrolyte, which connects the twoelectrodes by means of the bar of theH, is a saturated solution of cadmiumsulphate. In some cells sulphuric acidis added to prevent the hydrolysis ofmercury sulphate.

white arsenic See arsenic(iii)oxide.

white mica See muscovite.

white spirit A liquid mixture ofhydrocarbons obtained from petro-leum, used as a solvent for paint(‘turpentine substitute’).

Wigner–Seitz cell A polyhedron ina crystal that is bounded by planesformed by perpendicular bisectors ofbonds between lattice sites. TheWigner–Seitz cell was used by Hun-garian-born US physicist EugeneWigner (1902–95) and Frederick Seitz(1911–2008) in 1933 in the course oftheir analysis of the cohesion of met-als. The concept has been used exten-sively in the theory of solids.

Wigner–Witmer rules A set of re-sults found by applying group theorythat states which molecular elec-tronic states can exist, starting fromthe electronic states of the isolatedatoms. The rules were stated for di-atomic molecules by Hungarian-born US physicist Eugene Wigner(1902–95) and E. E. Witmer in 1928

and were subsequently extended topolyatomic molecules. The Wigner–Witmer rules are also known ascorrelation rules since they involvethe correlation between atomic elec-tronic states and molecular elec-tronic states. They are useful inanalysing the *electronic spectra ofmolecules.

Williamson’s synthesis Either oftwo methods of producing ethers,both named after the British chemistAlexander Williamson (1824–1904).1. The dehydration of alcohols usingconcentrated sulphuric acid. Theoverall reaction can be written

2ROH → H2O + RORThe method is used for makingethoxyethane (C2H5OC2H5) fromethanol by heating at 140°C with ex-cess of alcohol (excess acid at 170°Cgives ethene). Although the steps inthe reaction are all reversible, theether is distilled off so the reactioncan proceed to completion. This isWilliamson’s continuous process. Ingeneral, there are two possible mech-anisms for this synthesis. In the Ürst(favoured by primary alcohols), analkylhydrogen sulphate is formed

ROH + H2SO4 ˆ ROSO3H + H2OThis reacts with another alcohol mol-ecule to give an oxonium ion

ROH + ROSO3H → ROHR+

This loses a proton to give ROR.The second mechanism (favoured

by tertiary alcohols) is formation of acarbonium ion

ROH + H+ → H2O + R+

This is attacked by the lone pair onthe other alcohol molecule

R+ + ROH → ROHR+

and the oxonium ion formed againgives the product by loss of a proton.

The method can be used for mak-ing symmetric ethers (i.e. havingboth R groups the same). It can suc-

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cessfully be used for mixed ethersonly when one alcohol is primaryand the other tertiary (otherwise amixture of the three possible prod-ucts results). 2. A method of prepar-ing ethers by reacting a haloalkanewith an alkoxide. The reaction, dis-covered in 1850, is a nucleophilicsubstitution in which the negativealkoxide ion displaces a halide ion;for example:

RI + –OR′ → ROR′ + I–

A mixture of the reagents is reÛuxedin ethanol. The method is particu-larly useful for preparing mixedethers, although a possible side reac-tion under some conditions is anelimination to give an alcohol and analkene.

Wiswesser line notation (WLN)An early *line notation for chemicalstructures. The symbols used are theupper-case letters of the alphabet(A–Z), the numerals (0–9), with threeother symbols: the ampersand (&),the hyphen (-), and the oblique stroke(/), and a blank space. Atomic sym-bols with one letter, such as B and F,are unchanged. Frequently occurringelements with more than one letterand functional groups are also as-signed one letter; for example, Gstands for chlorine, Q for hydroxyl,and Z for NH2. The numerals indicatethe number of carbon atoms in anunbranched internally saturatedalkyl chain. For example, CH3 is de-noted 1 and CH3CH2 is denoted 2. Toestablish the notation of a compoundthe characters for the fragments aregiven in an established order. For ex-ample, the notation for C2H5OH isQ2. Rules for structures withbranched chains and fused rings arealso given. WLN provides a short andunambiguous notation, which is suit-able for database searches.

witherite A mineral form of *bar-ium carbonate, BaCO3.

WLN See wiswesser line notation.

Wöhler, Friedrich (1800–82) Ger-man physician and chemist, who be-came a professor of chemistry atGöttingen. In 1828 he made his best-known discovery, the synthesis ofurea (an organic compound) fromammonium cyanate (an inorganicsalt). This Ünally disproved the asser-tion that organic substances can beformed only in living things. Wöhleralso isolated aluminium (1827), beryl-lium (1828), and yttrium (1828).

Wöhler’s synthesis A synthesis ofurea performed by Friedrich Wöhlerin 1828. He discovered that urea(CO(NH2)2) was formed when a solu-tion of ammonium isocyanate(NH4NCO) was evaporated. At thetime it was believed that organic sub-stances such as urea could be madeonly by living organisms, and its pro-duction from an inorganic com-pound was a notable discovery. It issometimes (erroneously) cited as end-ing the belief in vitalism.

wolfram See tungsten.

wolframite (iron manganese tung-sten) A mineral consisting of amixed iron–manganese tungstate,(FeMn)WO4, crystallizing in themonoclinic system; the principal oreof tungsten. It commonly occurs asblackish or brownish tabular crystalgroups. It is found chieÛy in quartzveins associated with granitic rocks.China is the major producer of wol-framite.

wood alcohol See methanol.

Wood’s metal A low-melting(71°C) alloy of bismuth (50%), lead(25%), tin (12.5%), and cadmium(12.5%). It is used for fusible links inautomatic sprinkler systems. Themelting point can be changed by

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varying the composition. It is namedafter William Wood (1671–1730).

Woodward, Robert Burns(1917–79) US organic chemist whoworked at Harvard. He is remem-bered for his work in organic synthe-sis, producing many organiccompounds including quinine, cho-lesterol, cortisone, lysergic acid,strychnine, chlorophyll, and vitaminB12. In 1965 he formulated the Wood-ward–Hoffmann rules for certaintypes of addition reactions. Wood-ward was awarded the 1965 NobelPrize for chemistry.

Woodward–Hoffmann rulesRules governing the formation ofproducts during certain types of or-ganic concerted reactions. The theoryof such reactions was put forward in1969 by Woodward and Roald Hoff-mann (1937– ), and is concernedwith the way that orbitals of the re-actants change continuously into or-bitals of the products during reactionand with conservation of orbital sym-metry during this process. It is some-times known as frontier-orbitaltheory.

work function A quantity thatdetermines the extent to whichthermionic or photoelectric emissionwill occur according to the Richard-son equation or Einstein’s photo-electric equation. It is sometimesexpressed as a potential difference(symbol φ) in volts and sometimes asthe energy required to remove anelectron (symbol W) in electronvoltsor joules. The former has been calledthe work function potential and thelatter the work function energy.

work hardening An increase inthe hardness of metals as a result ofworking them cold. It causes a per-manent distortion of the crystal

structure and is particularly apparentwith iron, copper, aluminium, etc.,whereas with lead and zinc it doesnot occur as these metals are capableof recrystallizing at room tempera-ture.

wrought iron A highly reÜnedform of iron containing 1–3% of slag(mostly iron silicate), which is evenlydistributed throughout the materialin threads and Übres so that the prod-uct has a Übrous structure quite dis-similar to that of crystalline cast iron.Wrought iron rusts less readily thanother forms of metallic iron and itwelds and works more easily. It isused for chains, hooks, tubes, etc.

wurtzite structure A type of ioniccrystal structure in which the anionshave a hexagonal close packedarrangement with the cations occu-pying one type of tetrahedral hole.Each type of ion has a coordinationnumber of 4. Examples of this struc-ture are found in ZnS, ZnO, AlN, SiC,and NH4F.


Wurtz reaction A reaction to pre-pare alkanes by reacting ahaloalkane with sodium:

2RX + 2Na → 2NaX + RRThe haloalkane is reÛuxed withsodium in dry ether. The method isnamed after the French chemistCharles-Adolphe Wurtz (1817–84).The analogous reaction using ahaloalkane and a haloarene, for ex-ample:

C6H5Cl + CH3Cl + 2Na → 2NaCl +C6H5CH3

is called the Fittig reaction after theGerman chemist Rudolph Fittig(1835–1910).

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X

xanthates Salts or esters contain-ing the group –SCS(OR), where R isan organic group. Cellulose xanthateis an intermediate in the manufac-ture of *rayon by the viscose process.

xanthene A heterocyclic com-pound having three fused rings withone oxygen atom, C13H10O; m.p.101–102°C b.p. 310–312°C. The ringstructure is present in a class of xan-thene dyes.

xanthine A purine base, C5H4N4O2,found in many organisms. Seemethylxanthines.

xanthone (dibenzo-4-pyrone) Acolourless crystalline compound,

C13H8O2; m.p. 174°C. The ring systemis present in xanthone dyes.

xenobiotic Any substance foreignto living systems. Xenobiotics includedrugs, pesticides, and carcinogens.DetoxiÜcation of such substances oc-curs mainly in the liver.

xenon Symbol Xe. A colourlessodourless gas belonging to group 18of the periodic table (see noblegases); a.n. 54; r.a.m. 131.30; d. 5.887g dm–3; m.p. –111.9°C; b.p. –107.1°C.It is present in the atmosphere(0.00087%) from which it is extractedby distillation of liquid air. There arenine natural isotopes with massnumbers 124, 126, 128–132, 134, and136. Seven radioactive isotopes arealso known. The element is used inÛuorescent lamps and bubble cham-bers. Liquid xenon in a supercriticalstate at high temperatures is used asa solvent for infrared spectroscopyand for chemical reactions. The com-pound Xe+PtF6

– was the Ürst noble-gas compound to be synthesized.Several other compounds of xenonare known, including XeF2, XeF4,XeSiF6, XeO2F2, and XeO3. Recently,compounds have been isolated thatcontain xenon–carbon bonds, such as[C6H5Xe][B(C6H5)3F] (pentaÛuoro-phenylxenon Ûuoroborate), which isstable under normal conditions. Theelement was discovered in 1898 byRamsey and Travers.


XPS X-ray photoelectron spec-troscopy. See photoelectron spec-troscopy.

O

O

Xanthone

NH

NH

N

NH

O

O

Xanthine

O

Xanthene


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X-ray crystallography The use of*X-ray diffraction to determine thestructure of crystals or molecules.The technique involves directing abeam of X-rays at a crystalline sam-ple and recording the diffracted X-rays on a photographic plate. Thediffraction pattern consists of a pat-tern of spots on the plate, and thecrystal structure can be worked outfrom the positions and intensities ofthe diffraction spots. X-rays are dif-fracted by the electrons in the mol-ecules and if molecular crystals of acompound are used, the electrondensity distribution in the moleculecan be determined.

X-ray diffraction The diffractionof X-rays by a crystal. The wave-lengths of X-rays are comparable insize to the distances between atomsin most crystals, and the repeatedpattern of the crystal lattice acts likea diffraction grating for X-rays. Thus,a crystal of suitable type can be usedto disperse X-rays in a spectrometer.X-ray diffraction is also the basis of X-ray crystallography.

X-ray Ûuorescence The emissionof *X-rays from excited atoms pro-duced by the impact of high-energyelectrons, other particles, or a pri-mary beam of other X-rays. Thewavelengths of the Ûuorescent X-rayscan be measured by an X-ray spec-trometer as a means of chemicalanalysis. X-ray Ûuorescence is used insuch techniques as electron-probemicroanalysis.

X-rays Electromagnetic radiation ofshorter wavelength than ultravioletradiation produced by bombardmentof atoms by high-quantum-energyparticles. The range of wavelengths is10–11 m to 10–9 m. Atoms of all the el-ements emit a characteristic X-rayspectrum when they are bombardedby electrons. The X-ray photons areemitted when the incident electronsknock an inner orbital electron outof an atom. When this happens anouter electron falls into the innershell to replace it, losing potentialenergy (∆E) in doing so. The wave-length λ of the emitted photon willthen be given by λ = ch/∆E, where c isthe speed of light and h is the Planckconstant.

X-rays can pass through manyforms of matter and they are there-fore used medically and industriallyto examine internal structures. X-raysare produced for these purposes byan X-ray tube.

X-ray spectrum See x-rays.

xylenes See dimethylbenzenes.

xylenol (hydroxydimethylbenzene)Any of six isomeric solid aromaticcompounds, C6H3(CH3)2OH. An im-pure mixture of isomers is a liquidmade from coal tar and employed asa solvent. The pure substances re-semble *phenol in their reactions.They are used to make thermosettingpolymer resins, and the chloro- derivative of 1,2,5-xylenol is an in-dustrial disinfectant.

X-ray crystallography 560

x


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Y

yeasts A group of unicellular fungiof the class Hemiascomycetae andphylum Ascomycota. They occur assingle cells or as groups or chains ofcells; yeasts reproduce asexually bybudding and sexually by producingascospores. Yeasts of the genus Sac-charomyces ferment sugars and areused in the baking and brewing in-dustries.

ylide A chemical species derivedfrom an onium ion by loss of a hy-dron. For example, the phosphorusylide (C6H5)2P=CR2, derived from(C6H5)2PCHR2

+ by loss of H+.

yocto- Symbol y. A preÜx used inthe metric system to indicate 10–24.For example, 10–24 second = 1 yocto-second (ys).

yotta- Symbol Y. A preÜx used inthe metric system to indicate 1024.For example, 1024 metres = 1 yotta-metre (Ym).

ytterbium Symbol Yb. A silverymetallic element belonging to the*lanthanoids; a.n. 70; r.a.m. 173.04;r.d. 6.965 (20°C); m.p. 819°C; b.p.1194°C. It occurs in gadolinite, mon-azite, and xenotime. There are sevennatural isotopes and ten artiÜcial iso-

topes are known. It is used in certainsteels. The element was discoveredby Jean de Marignac (1817–94) in1878.


yttrium Symbol Y. A silvery-greymetallic element belonging to group3 (formerly IIIA) of the periodic table;a.n. 39; r.a.m. 88.905; r.d. 4.469(20°C); m.p. 1522°C; b.p. 3338°C. Itoccurs in uranium ores and in *lan-thanoid ores, from which it can beextracted by an ion exchangeprocess. The natural isotope is yt-trium–89, and there are 14 known ar-tiÜcial isotopes. The metal is used insuperconducting alloys and in alloysfor strong permanent magnets (inboth cases, with cobalt). The oxide(Y2O3) is used in colour-televisionphosphors, neodymium-doped lasers,and microwave components. Chemi-cally it resembles the lanthanoids,forming ionic compounds containingY3+ ions. The metal is stable in airbelow 400°C. It was discovered in1828 by Friedrich Wöhler.



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Z

Zeeman effect The splitting of thelines in a spectrum when the sourceof the spectrum is exposed to a mag-netic Üeld. It was discovered in 1896by Pieter Zeeman (1865–1943). In thenormal Zeeman effect a single line issplit into three if the Üeld is perpen-dicular to the light path or two linesif the Üeld is parallel to the lightpath. This effect can be explained byclassical electromagnetic principlesin terms of the speeding up and slow-ing down of orbital electrons in thesource as a result of the applied Üeld.The anomalous Zeeman effect is acomplicated splitting of the lines intoseveral closely spaced lines, so calledbecause it does not agree with classi-cal predictions. This effect is ex-plained by quantum mechanics interms of electron spin.A• Pieter Zeeman’s original paper

Zeisel reaction A method of deter-mining the number of methoxy(–OCH3) groups in an organic com-pound. The compound is heated wihexcess hydriodic acid, forming an al-cohol and iodomethane:

R–O–CH3 + HI → ROH + CH3IThe iodomethane is distilled off andled into an alcoholic solution of sil-ver nitrate, where it precipitates sil-ver iodide. This is Ültered andweighed, and the number of iodineatoms and hence methoxy groupscan be calculated. The method wasdeveloped by S. Ziesel in 1886.

Zeise’s salt A complex of platinumand ethene, PtCl3 (CH2CH2), in whichthe Pt coordinates to the pi bond ofthe ethene. It was the Ürst example

of an enyl complex, synthesized byW.C. Zeise in 1827.

zeolite A natural or synthetic hy-drated aluminosilicate with an openthree-dimensional crystal structure,in which water molecules are held incavities in the lattice. The water canbe driven off by heating and the zeo-lite can then absorb other moleculesof suitable size. Zeolites are used forseparating mixtures by selective ab-sorption – for this reason they areoften called molecular sieves. Theyare also used in sorption pumps forvacuum systems and certain types(e.g. Permutit) are used in ion-exchange (e.g. water-softening).

zepto- Symbol z. A preÜx used inthe metric system to indicate 10–21.For example, 10–21 second = 1 zepto-second (zs).

zero order See order.

zero-point energy The energy re-maining in a substance at the *ab-solute zero of temperature (0 K). Thisis in accordance with quantumtheory, in which a particle oscillatingwith simple harmonic motion doesnot have a stationary state of zero ki-netic energy. Moreover, the *uncer-tainty principle does not allow such aparticle to be at rest at exactly thecentrepoint of its oscillations.

zeroth law of thermodynamicsSee thermodynamics.

zetta- Symbol Z. A preÜx used inthe metric system to indicate 1021.For example, 1021 metres = 1 zetta-metre (Zm).

Ziegler process An industrial


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process for the manufacture of high-density polyethene using catalysts oftitanium(IV) chloride (TiCl4) and alu-minium alkyls (e.g. triethylalu-minium, Al(C2H5)3). The process wasintroduced in 1953 by the Germanchemist Karl Ziegler (1898–1973). Itallowed the manufacture of poly-thene at lower temperatures (about60°C) and pressures (about 1 atm.)than used in the original process.Moreover, the polyethene producedhad more straight-chain molecules,giving the product more rigidity anda higher melting point than the ear-lier low-density polyethene. The reac-tion involves the formation of atitanium alkyl in which the titaniumcan coordinate directly to the pibond in ethene.

In 1954 the process was developedfurther by the Italian chemist GiulioNatta (1903–79), who extended theuse of Ziegler’s catalysts (and similarcatalysts) to other alkenes. In particu-lar he showed how to produce stereo-speciÜc polymers of propene.

zinc Symbol Zn. A blue-white metal-lic element; a.n. 30; r.a.m. 65.38; r.d.7.1; m.p. 419.88°C; b.p. 907°C. It oc-curs in sphalerite (or zinc blende,ZnS), which is found associated withthe lead sulphide, and in smithsonite(ZnCO3). Ores are roasted to give theoxide and this is reduced with carbon(coke) at high temperature, the zincvapour being condensed. Alterna-tively, the oxide is dissolved in sul-phuric acid and the zinc obtained byelectrolysis. There are Üve stable iso-topes (mass numbers 64, 66, 67, 68,and 70) and six radioactive isotopesare known. The metal is used in gal-vanizing and in a number of alloys(brass, bronze, etc.). Chemically it is areactive metal, combining with oxy-gen and other nonmetals and react-ing with dilute acids to releasehydrogen. It also dissolves in alkalis

to give *zincates. Most of its com-pounds contain the Zn2+ ion.


zincate A salt formed in solution bydissolving zinc or zinc oxide in alkali.The formula is often written ZnO2

2–

although in aqueous solution theions present are probably complexions in which the Zn2+ is coordinatedto OH– ions. ZnO2

2– ions may exist inmolten sodium zincate, but mostsolid ‘zincates’ are mixed oxides.

zinc blende A mineral form of*zinc sulphide, ZnS, the principal oreof zinc (see sphalerite).

zinc-blende structure See spha-lerite structure.

zinc chloride A white crystallinecompound, ZnCl2. The anhydroussalt, which is deliquescent, can bemade by the action of hydrogen chlo-ride gas on hot zinc; r.d. 2.9; m.p.283°C; b.p. 732°C. It has a relativelylow melting point and sublimes eas-ily, indicating that it is a molecularcompound rather than ionic. Varioushydrates also exist. Zinc chloride isused as a catalyst, dehydrating agent,and Ûux for hard solder. It was onceknown as butter of zinc.

zinc chloride cell See dry cell.

zinc group The group of elementsin the periodic table consisting ofzinc (Zn), cadmium (Cd), and mer-cury (Hg). See group 2 elements.

zincite A mineral form of *zincoxide, ZnO.

zinc oxide A powder, white whencold and yellow when hot, ZnO; r.d.5.606; m.p. 1975°C. It occurs natu-rally as a reddish orange ore zincite,and can also be made by oxidizinghot zinc in air. It is amphoteric,forming *zincates with bases. It isused as a pigment (Chinese white)

563 zinc oxide

z


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and a mild antiseptic in zinc oint-ments. An archaic name is philoso-pher’s wool.

zinc sulphate A white crystallinewater-soluble compound made byheating zinc sulphide ore in air anddissolving out and recrystallizing thesulphate. The common form is theheptahydrate, ZnSO4.7H2O; r.d. 1.9.This loses water above 30°C to givethe hexahydrate and more water islost above 70°C to form the monohy-drate. The anhydrous salt forms at280°C and this decomposes above500°C. The compound, which wasformerly called white vitriol, is usedas a mordant and as a styptic (tocheck bleeding).

zinc sulphide A yellow-whitewater-soluble solid, ZnS. It occursnaturally as *sphalerite (see also zincblende) and wurtzite. The compoundsublimes at 1180°C. It is used as apigment and phosphor.

zircon A naturally occurring silicateof zirconium, ZrSiO4, used as a gem-stone. The colour depends in smallamounts of other metals and may bered, brown, yellow, or green. Redgem-quality zircon is sometimescalled jacinth; gem-quality zirconswith other colours are called jar-goons. There is also a naturally oc-curring colourless variety. Zircongems can be given other colours, ormade colourless, by heat treatment.The colourless varieties (either nat-ural or treated) are sometimes calledMatura diamonds (after Matura in SriLanka). The name ‘zircon’ is often er-roneously applied to a synthetic formof the oxide *cubic zircona, which isused as a diamond substitute.

zirconia See zirconium.

zirconium Symbol Zr. A grey-whitemetallic *transition element; a.n. 40;r.a.m. 91.22; r.d. 6.49; m.p. 1852°C;b.p. 4377°C. It is found in zircon

(ZrSiO4; the main source) and in bad-deleyite (ZnO2). Extraction is by chlo-rination to give ZrCl4 which ispuriÜed by solvent extraction and re-duced with magnesium (Krollprocess). There are Üve natural iso-topes (mass numbers 90, 91, 92, 94,and 96) and six radioactive isotopesare known. The element is used innuclear reactors (it is an effectiveneutron absorber) and in certain al-loys. The metal forms a passive layerof oxide in air and burns at 500°C.Most of its compounds are complexesof zirconium(IV). Zirconium(IV) oxide(zirconia) is used as an electrolyte infuel cells. The element was identiÜedin 1789 by Klaproth and was Ürst iso-lated by Berzelius in 1824.A• Information from the WebElements site

zirconium(IV) oxide See zirco-nium.

Z-isomer See e–z convention.

zone reÜning A technique used toreduce the level of impurities in cer-tain metals, alloys, semiconductors,and other materials. It is based onthe observation that the solubility ofan impurity may be different in theliquid and solid phases of a material.To take advantage of this observa-tion, a narrow molten zone is movedalong the length of a specimen of thematerial, with the result that the im-purities are segregated at one end ofthe bar and the pure material at theother. In general, if the impuritieslower the melting point of the ma-terial they are moved in the same di-rection as the molten zone moves,and vice versa.

zwitterion (ampholyte ion) An ionthat has a positive and negativecharge on the same group of atoms.Zwitterions can be formed from com-pounds that contain both acid groupsand basic groups in their molecules.

zinc sulphate 564

z


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For example, aminoethanoic acid(the amino acid glycine) has the for-mula H2N.CH2.COOH. However,under neutral conditions, it exists inthe different form of the zwitterion+H3N.CH2.COO–, which can be re-garded as having been produced byan internal neutralization reaction(transfer of a proton from the car-boxyl group to the amino group).

Aminoethanoic acid, as a conse-quence, has some properties charac-teristic of ionic compounds; e.g. ahigh melting point and solubility inwater. In acid solutions, the positiveion +H3NCH2COOH is formed. Inbasic solutions, the negative ionH2NCH2COO– predominates. Thename comes from the German zwei,two.

565 zwitterion

z


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Appendices


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Appendix 1. The Greek alphabet

Letters Name Letters NameAA

A alpha nu

beta xi

gamma omicron

delta pi

epsilon rho

zeta sigma

eta tau

theta upsilon

! " iota # $ phi

% & kappa ' ( chi

) * lambda + , psi

- . mu / 0 omega

Appendix 2. Fundamental constants

Constant Symbol Value in SI unitsAAAAAAAJJ

acceleration of free fall g 9.806 65 m s–2

Avogadro constant L, NA 6.022 141 79(30) × 1023 mol–1

Boltzmann constant k = R/NA 1.380 6504(24) × 10–23 J K–1

electric constant ε0 8.854 187 817 × 10–12 F m–1

electronic charge e 1.602 176 487(40) × 10–19 C

electronic rest mass me 9.109 382 15(45) × 10–31 kg

Faraday constant F 9.648 3399(24) × 104 C mol–1

gas constant R 8.314 472(15) J K–1 mol–1

gravitational constant G 6.674 28(67) × 10–11 m3 kg–1 s–2

Loschmidt’s constant NL 2.686 7774(47) × 1025 m–3

magnetic constant µ0 4π × 10–7 H m–1

neutron rest mass mn 1.674 927 211(84) × 10–27 kg

Planck constant h 6.626 068 96(33) × 10–34 J s

proton rest mass mp 1.672 621 637(83) × 10–27 kg

speed of light c 2.997 924 58 × 108 m s–1

Stefan–Boltzmann constant σ 5.670 400(40) × 10–8 W m–2 K–4

Appendix 1569


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Dictionary of Chemistry [6th Ed.]

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