Chapter 7-Gaya antar Molekul

8/18/2019 Chapter 7-Gaya antar Molekul

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Chapter 7

Covalent Bonds and Molecular

Structure


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The Covalent Bond

• Covalent bond – formed by the sharing of

electrons between two nonmetal atoms

– Forces involved in the bond

• lectrostatic attraction between proton and

electron

• lectrostatic repulsion between electron and

electron

– !hen will a bond form"

http://1covalent_bond.rm/http://1covalent_bond.rm/


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Strengths of Bonds

• Bond dissociation energy – the amount of

energy re#uired to brea$ a bond

– nergy increases as the length of the bond gets

shorter


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%nternuclear distance

&bond length'

Covalent

radius


&bond length'

Covalent

radius


&bond length'

Covalent

radius


&bond length'

Covalent

radius

Bond length and covalent radius.


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Strengths of Bonds

– Single bonds ( double bonds ( triple bonds


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)roblem

• *rrange the following bonds in order of

increasing bond strength+

– *+ C,% - C,Br - C,Cl - C,F

– B+ C,F - C,Cl - C,Br - C,%

– C+ C,Br - C,% - C,Cl - C,F

– .+ C,% - C,Br - C,F- C,Cl

– + none of these orders is correct


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)roblem

• Select the strongest bond in the following

group+

– *+ C,S

– B+ C,/

– C+ C0C

– .+ C12

– + C,F


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lectron .ot Structures

• lectron dot symbols

– *id in understanding the formation of bonds between atomic nuclei

– lemental symbol represents the type of element and all core electrons3

the valence electrons are represented by dots around the symbol


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• * metal in an ionic loses its electrons to achievean octet or pseudo,octet &transition elements' inits outermost shell

• * nonmetal in an ionic compound gainselectrons to achieve an octet in its outermostshell

• )eriod 4 and 5 elements of a covalentcompound share enough electrons to achievean octet


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lectron,.ot Structures

• Ionic:

• Covalent:


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• 6sing electron dot symbols build

– 58 5/8 C98 /58 258 C28 C/5• :east electronegative atom is often central &e;cept

'


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2aming Binary Molecular

Compounds

• lectronegativity – indicates how well an

elements nuclei attract the electrons in a

covalent bond

http://electronegavity.avi/http://electronegavity.avi/


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The )eriodic Table and

lectronegativity


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• 6sing electron dot symbols build

– 58 5/8 C98 /58 258 C28 C/5• :east electronegative atom is often central &e;cept

'


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• Single bond: * covalent bond formed by

sharing one electron pair+

• Double bond: * covalent bond formed by

sharing two electron pairs+• Triple bond: * covalent bond formed by sharing

three electron pairs+

• Single bonds are longer &wea$er' than double bonds

• .ouble bonds are longer &wea$er' than triple bonds


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lectron,dot Structures

• Step 4< Count the total valence electrons+

• Step 5< %dentify the central atom

, /ften least electronegative

• Step =< )lace all other atoms around the central

atom

• Step 9< .raw a single bond between each

e;ternal atom and the central atom

subtracting 5 electrons for each bond

drawn from the total valence electrons+


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lectron,dot Structures

• Step >< .istribute remaining valence

electrons around the e;ternal

atoms giving the e;ternal atoms

an octet

• Step ?< %f valence electrons still remain8

place them on the central atom in pairs

• Step 7< @erify that each atom has an octetA ydrogen needs only 5 electrons

A Boron needs only ? electrons


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lectron,.ot Structures

• Step < %f the central atom does not have an

octet8 form a multiple bond by bringing a

pair of electrons in from the e;ternal

atom

• Step < Calculate formal charge and minimiDe

the formal charge if acceptable

A )eriod = elements and greater can have e;panded octets ifone is necessary to minimiDe formal charge

A Formal charge 0 E valence electrons for the atom – 4 for

every dot on the atom – 4 for every line around the atom


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)roblems

• BF=

• )F=

• C5?

• %=F

• 29F

• S/95,

• GCl/=


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lectron,dot Structures and

Hesonance

• ow is the double bond formed in /="

• The correct answer is that both are correct 8

but neither is correct by itself +

O O O

O O O

O O O

or

Or from thisoxygen?

Move lone pair from

this oxygen?


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lectron,.ot Structures and

Hesonance

• !hen multiple structures can be drawn8 the actual

structure is an average of all possibilities+

• The average is called a resonance hybrid + *

straight double,headed arrow indicates resonance+

O O O O O O


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)roblem

• S=

• )/9=,

• C/=5,

• 2/5


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Molecular Shapes< The @S)H

Theory

• The appro;imate shape of molecules

is given by

Valence-Shell Electron-Pair Repulsio

n (VSEPR)

.

http://01vsepr.rm/http://01vsepr.rm/http://01vsepr.rm/http://01vsepr.rm/http://01vsepr.rm/http://01vsepr.rm/


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Molecular Shapes< The @S)H

TheoryMolecular

formula

:ewis

structure

lectron,group

arrangement

Bond

angles

Molecular

shape

&*Imn'

Count all e, groups around central

atom &*' – Single8 .ouble and Triple

bonds are all counted as 4 e, group

2ote lone pairs and

double bonds

Count bonding

and nonbonding e,

groups separately+

Step 1

Step 2

Step 3

Step 4


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)roblem

• .etermine the shape of the molecules for

which :ewis Structures have been

developed+


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@alence Bond Theory

• %f8 in order for a bond to form8 a pair of

electrons must be shared8 then how does

C form molecules with 9 bonds"

• @alence Bond Theory – hybrid orbitals

http://01hybrid_orbitals.rm/http://01hybrid_orbitals.rm/http://01hybrid_orbitals.rm/http://01hybrid_orbitals.rm/


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@alence Bond Theory

Basic )rinciple

* covalent bond forms when the orbtials of two atoms overlap

and are occupied by a pair of electrons that have the highest

probability of being located between the nuclei+

Themes

* set of overlapping orbitals has a ma;imum of two electrons

that must have opposite spins+

The greater the orbital overlap8 the stronger &more stable' the

bond+

The valence atomic orbitals in a molecule are different from

those in isolated atoms+


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@alence Bond Theory

The number of hybrid orbitals obtained e#uals the number of

atomic orbitals mi;ed+

The type of hybrid orbitals obtained varies with the types of

atomic orbitals mi;ed+

Gey )oints

sp sp5 sp= sp=d sp=d5

Types of ybrid /rbitals


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@alence Bond Theory

The conceptual steps fro olecular forula to the h!brid orbitals

used in bonding.

"olecular

forula

#e$is

structure

"olecular shape

and e% group

arrangeent

&!brid

orbitals

Step 1 Step 2 Step 3


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)roblems

• Carbon uses JJJJJJ hybrid orbitals in

ClC2+

– *+ sp

– B+ sp5

– C+ sp=

– .+ sp=d

– + sp=d 5


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The bonds in ethane.

both C are sp= hybridiDeds,sp= overlaps to σ bonds

sp=,sp= overlap to form a σ bond

relatively even

distribution of electron

density over all σ bonds


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The and bonds in eth!lene 'C2&4(

overlap in one position , σ

p overlap , π

electron density


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The and bonds in acet!lene 'C2&2(

overlap in one position , σ

p overlap ,π


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)olar Covalent Bonds<

lectronegativity

• lectronegativity – represents the ability of anatom to attract a shared pair of electrons

• igher the 2 – the more the electrons in abond will be pulled toward the atom – Most electronegative atom is F

• 2 K down a group• 2L across a period from left to right w few

e;ceptions

http://electronegavity.avi/http://electronegavity.avi/


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lectronegativity


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)roblem

• !hich of the following elements is the

most electronegative"

– *+ S

– B+ Hu

– C+ Si

– .+ Te

– + Cs


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)roblem

• *rrange calcium8 rubidium8 sulfur8 andarsenic in order of decreasingelectronegativity+

– *+ S ( *s ( Hb ( Ca

– B+ S ( *s ( Ca ( Hb

– C+ *s ( S ( Hb ( Ca – .+ *s ( S ( Ca ( Hb

– + 2one of these orders is correct+


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lectronegativity

• ) Ionic Character: *s a general rule for two

atoms in a bond8 we can calculate an

electronegativity difference &N2 '< N2 0

2&O' – 2&I' for I–O bond+

%f N2 - P+> the bond is covalent+

%f N2 P+> , - 5+P the bond is polar covalent+ %f N2 ( 5+P the bond is ionic+


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)roblem

• Select the most polar bond amongst the

following+

– *+ C,/

– B+ Si,F

– C+ Cl,F

– .+ C,F

– + C,%


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Molecular /rbital Theory

• The molecular orbital (MO) model provides a

better e;planation of chemical and physical

properties than the valence bond (VB) model +

– *tomic /rbital


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• *dditive cobination of orbitals &σ' is

lower in energy than two isolated 4s orbitals

and is called a bonding molecular orbital +


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• Subtractive cobination of orbitals &σ∗' is

higher in energy than two isolated 4s

orbitals and is called an antibonding

molecular orbital +


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• "olecular +rbital Diagra for &2:


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• "olecular +rbital Diagras for &2 , and&e2:


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• *dditive and subtractive combination of p

orbitals leads to the formation of both sigma

and pi orbitals+


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• Second%-o$ "+ nerg! #evel Diagras:


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• Bond +rder is the number of electron pairs

shared between atoms+

• Bond +rder is obtained by subtracting the

number of antibonding electrons from the

number of bonding electrons and dividing by 5+

Chapter 7-Gaya antar Molekul

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