Kuliah 2b Kimia Kristal

8/11/2019 Kuliah 2b Kimia Kristal

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Lecture 3 (9/13/2006)

Crystal Chemistry

Part 2:

Bonding and Ionic Radii


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Chemical Bonding in Minerals

Bonding forces are electrical in nature (related tocharged particles)Bond strength controls most physical and

chemical properties of minerals

(in general, the stronger the bond, the harderthe crystal, higher the melting point, and thelower the coefficient of thermal expansion)

Five general types bonding types:

Ionic Covalent Metallicvan der Waals HydrogenCommonly different bond types occur in thesame mineral


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Ionic Bonding

Common between elements that will...

1) easily exchange electrons so as to stabilize their

outer shells (i.e. become more inert gas-like)2) create an electronically neutral bond between

cations and anions

Example: NaClNa (1s22s22p63s1)> Na+(1s22s22p6) + e-

Cl (1s22s22p63s23p5) + e-> Cl-(1s22s22p63s23p6)


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Properties of Ionic Bonds

Results in minerals displaying moderatedegrees of hardness and specific gravity,

moderately high melting points, highdegrees of symmetry, and are poorconductors of heat (due to ionic stability)

Strength of ionic bonds are related:

1) the spacing between ions

2) the charge of the ions


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Cation Bond Strengthf(IA distance, ionic charge)

+1 cations

+2 cations


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Covalent Bonding formed by sharing of outer shell

electrons

strongest of all chemical bonds

produces minerals that areinsoluble, high melting points,hard, nonconductive (due tolocalization of electrons), havelow symmetry (due to

directional bonding). common among elements with

high numbers of vacancies inthe outer shell (e.g. C, Si, Al, S)

Diamond


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Tendencies for Ionic vs. Covalent Pairing

Ionic Pairs

CovalentPairs


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Ionic-Covalent GradationThese bond types share characteristics of each other

The degree of ionic character (exchangerather than sharing) can be estimatedfrom the contrasting electronegativity(ability to attract electrons) of the

elements involved.


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Metallic Bonding

atomic nuclei and inner filled electronshells in a sea of electrons made up of

unbound valence electronsYields minerals with minerals that are soft,

ductile/malleable, highly conductive (dueto easily mobile electrons).

Non-directional bonding produces highsymmetry


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van der Waals (Residual) Bonding

created by weak bonding of oppositelydipolarized electron clouds

commonly occurs around covalently bondedelements

produces solids that are soft, very poorconductors, have low melting points, lowsymmetry crystals


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Hydrogen Bonding

Electrostaticbonding between anH+ ion with an anion

or anionic complexor with a polarizedmoleculesWeaker than ionic

or covalent;stronger than vander Waals

polarized H2Omolecule Ice

Close packing of

polarized molecules

Anions

H+


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Summary of Bonding Characteristics


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Multiple Bonding in Minerals

Graphitecovalently bondedsheets of C loosely bound byvan der Waals bonds.

Micastrongly bonded silica

tetrahedra sheets (mixedcovalent and ionic) bound byweak ionic and hydrogenbonds

Cleavage planes commonlycorrelate to planes of weakionic bonding in an otherwisetightly bound atomic structure


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Atomic Radii

Absolute radius of an atom based onlocation of the maximum density ofoutermost electron shell

Effective radius dependent on thecharge, type, size, and number ofneighboring atoms/ions

- in bonds between identical atoms, thisis half the interatomic distance

- in bonds between different ions, the

distance between the ions is controlledby the attractive and repulsive forcebetween the two ions and their charges

F = k [(q+

)(q-

)/d2

] Coulombs law

Ch d i C l


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Charge and Attractive Force Control onEffective Ionic Radii

Eff t f C di ti N b d


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Effect of Coordination Number andValence on Effective Ionic Radius

Metallic Ionic Radii (CN-12) (Table 3.10)Na1.91K2.50Ca1.97Rb2.50

IncreasingI

onic

radii

DecreasingIonic radii


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Control of CN

(# of nearestneighbors) onionic radius

Reflectsexpansion ofcations intolarger pore

spacesbetween anionneighbors


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Next Lecture

Crystal Chemistry III

Coordination of Ions

Paulings Rules

Crystal Structures

Read p. 69 - 90

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