MolecularGeometryand
ChemicalBonding
Chapter10
Introduction
Eachmoleculeinthenaturehasa
definiteshape.BF3andNF3havethe
samegeneralformulaAX3,whereA
isthecentralatomtowhich
surroundingatoms,X,arebonded.
BF3istheplanarmoleculewitha
bondangle1200whereasNF3isa
nonplanarmoleculewithabond
angleof960.
ThegeometryofBF3istrigonal
planarandthatofNF3istrigonal
pyramidal.Thereisnorelation
betweentheformulaofacompound
andtheshapeofitsmolecules.
Howdoweexplainthegeometriesof
themoleculesintermsoftheir
electronicstructures?
TopicsofDiscussion
TheValenceShellElectron
Repulsion(VSEPR)Theory
DipoleMomentandMolecularGeometry
ValenceBondTheory
DescriptionofMultipleBonding
PrinciplesofMolecularOrbitalTheory
ElectronConfigurationsofDiatomicMolecules
oftheSecondRowElements
MolecularOrbitalsandDelocalizedBonding
MolecularGeometryand
DirectionalBonding
Moleculargeometryofa
moleculeisthearrange_
mentsofatomsinspace.
Electronpairgeometryisthe
arrangementsofelectron
pairsaroundthecentralatom.
Structuralformula(lewis
electrondot)indicateswhich
atomsarebondedtoeach
otherwithbondingepairsand
wherethenonbondinge
pairsarelocatedinmolecule.
TheValenceShellElectron
Repulsion(VSEPR)Model
Theelectronsareinpairsinthevalenceshell
ofatomsinamolecule.Thebondingepair
formsacovalentbondbutnonbonding(lone)
epairdoesnt.
VSEPRModelstatesthatvalenceshelle
pairsarearrangedabouteachatomsothate
pairsstayasfarawayfromanotheras
possible,thusminimizingepairrepulsions.
Thedirectioninspaceofthebondingpairs
givesthegeometryofthemolecule.
InVSEPRmodel,eachmultiplebondis
treatedasthoughitwereasinglebondinge
pair(exceptaslightdistortiononbondangles)
PredictionofGeometryby
VSEPR
Stepstobefollowedtopredictthegeometryofan
AXnmoleculebyVSEPRmodel.Aisthecentral
atom(CA)inthemolecule.
TwoElectronPairs
..
..
1. WritedowntheLewis
..
..
O=C=O
electrondotformula
: F Be F :
..
..
..
..
2. Determinethenumberof 2(!) bonding
2 bonding
epairsaroundtheCA
e - pairs
e - pairs
3. Arrangethegeometryof
epairsaroundtheCA
Linear
Linear
4. Decidethemolecular
geometryfromdirectionof Linear
Linear
bondingepairs
Eachmultiplebondistreatedasthoughitweresingle
bondingepair(exceptaslightdistortiononbondangles)
..
..
ThreeElectronPairs
O
||
H-C-H
WritedowntheLewis
electrondotformula
..
2. Determinethenumber 3(!) bonding
ofepairsaroundtheCA
3 bonding
e - pairs
3. Arrangethegeometryof
e - pairs
epairsaroundtheCA. Trigonal planar
Trigonal planar
4. Decidethemolecular
Trigonal planar
Trigonal planar geometryfromdirection
ofbondingepairs.
:F B F:
..
..
|
:F:
1.
Therearefourepairsinthevalenceshellofthecarbonatom
informaldehyde,twooftheseconstituteadoublebond
betweenCandOleavesthreeepairswithtrigonalplanar
arrangement.However,doublebonddomainislargerinsize
thanasinglebond.Hence,HCHbonddecreasesto1180
FourElectronPairs
H
..
1.Lewiselectron
|
HOH
dotformula
HNH
..
..
2.Numberofe
4 bonding 3 bonding 1 2 bonding 2
pairs
lone e - pairs lone e - pairs
e - pairs
3.Geometryofe
pairs
Tetrahedral Tetrahedral Tetrahedral
Trigonal
Bent
4.Molecular
Tetrahedral pyramidal
geometry
(Angular)
0
0
5.HAHAngle
0
109.5
105
107
H
|
HCH
|
H
Considermethane,CH4,ammonia,NH3,andwater,H2O.Allthree
havetetrahedralepairgeometriesbuttheirbondanglesdiffer
slightlyasshownabove.Duetoalonepairoccupiesmorespace
whencomparedtoabondingpair,theangleofHAHbond
decreasesasthenumberoflonepairsincreases.
FiveElectronPairs
Five electron pairs:
PCl5
Cl
Cl
Cl
P Cl
SF4
F
F
ClF3
F
S
F
F Cl
I3F
F
Cl
5 bonding
e - pairs
Trigonal
Trigonal
bipyramidal
bipyramidal
Trigonal
Trigonal
bipyramidal
bipyramidal
I
I
I
4 bonding 1
lone e - pairs
3 bonding 2
lone e - pairs
2 bonding 3
lone e - pairs
Trigonal
Trigonal
bipyramidal
bipyramidal
Trigonal
Trigonal
bipyramidal
bipyramidal
Trigonal
Trigonal
bipyramidal
bipyramidal
T-shaped
Linear
See-saw
saw
distorted tetrahedral
distorted
Important: The lone pairs can be placed at two different
sites in trigonal bipyramidal geometry.
SixElectronPairs
Note that all corners of octahedron are identical.
SF6
IF5
F
F
F
F
F
S
F
F
F
XeF4
F
I
F
F
F
F
Xe
F
F
6 bonding
e - pair
5 bonding 1
lone e - pair
4 bonding 2
lone e - pair
Octahedral
Octahedral
Octahedral
Octahedral
Square pyramidal
Square planar
DipoleMomentandMolecular
Geometry
Dipolemomentisaquantitative
measureofthedegreeofcharge
separationinamolecule.Abond(ora
molecule)issaidtobepolarifits
centersofpositiveornegativecharge
donotcoincide.
Thedipolemomentsofsubstances
canbeobtainedbythemeasurement
ofthecapacitanceofthecharged
platesseparatedbythissubstances.
TheyareusuallyreportedinDebye(D)
1Dequalsto3.34x1034C.m
Thepolarityofabondisdenotedbyanarrowpointingtothe
negativeendofthebond.Thepolarityamoleculeispredicted
bythesumofthebomddipolevectors.
Examples
E10.3BrF3hasanonzero
dipolemoment.Which
geometryisconsistent?
a) Trigonalplanar
b) Trigonalpyramidal
c) Tshaped
E10.4Whichofthe
followingwouldbeexpected
tohavedipolemomentof
zeroonthebasisof
geometry?
a)SOCl2b)SiF4c)OF2
a) 0
b)=0
c) 0
ValenceBondTheory
TheVSEPRmodelisasatisfactorymethodfor
predictingmoleculargeometries.ButLewisformula
doesnotexplainhowandwhycovalentbondsare
formed.Twotheories
ValenceBondTheory(VB):Individualatomseachwithits
ownorbitalsandelectronscometogethertoformthe
covalentbondsofmolecules.Bondisformedwhenone
orbitalfromeachoftheatomsoverlapsandapairof
electronswithoppositespinsissharedbetweentwoatoms.
Whentwoatomicorbitalsfromdifferentatomsharethe
samespace,theyoverlap.
MolecularOrbitalTheory(MO):Moleculeshavemolecular
orbitalswhicharepopulatedbyelectrons
OverlapofOrbitalsI
Justtwoelectronswithpairedspinscanbeshared
betweentwooverlappingatomicorbitals.Theepair
becomesconcentratedintheregionofoverlapand
helpstocementthenucleitogether.
Thestrengthofbondingdependsonorbitaloverlap.
Orbitalsbondonlyinthedirectioninwhichtheypointto
givemaximumoverlap.
sorbitalsbondineverydirection
duetotheirsphericalsymmetry.
porbitalsbondat900anglesto
eachotherduetotheir
orientationinspace.
OverlapofOrbitalsII
Example:PredictthebondangleinH2SinVB.
1H:1s116S:[Ne]3s23p4
Overlapof1sorbitalsoftwohydrogenatoms
withtwo3porbitalsofsulfuratomgivetwo
covalentbondsinH2Swithapredictedbond
angleof900(Exp920).
Example:PredictthebondangleinH2OinVB.
H:1s116O:[He]2s22p4
1
predictedbondangle:900(Exp1050).
Example:PredictthebondangleinCH4inVB.
H:1s16C:[He]2s22p2
1
Prediction:CH2isastablecompoundnotCH4.
0
HybridOrbitals
Whenatomsformbonds,theiratomicorbitalsmixtoformnew
orbitalswhicharecalledashybridorbitals.
Thenumberofhybridorbitalsformedalwaysequaltothenumberof
atomicorbitalsused.
sphybridorbitals:onesandonepatomicorbitals(linear)
sp2hybridorbitals:onesandtwopatomicorbitals(trigonalplanar)
sp3hybridorbitals:onesandthreepatomicorbitals(tetrahedral)
sp3dhybridorbitals:ones,threepandonedatomicorbitals(trigonalbipyr.)
sp3d2hybridorbitals:ones,threepandtwodatomicorbitals(octahedral)
Energeticsofsp3Hybridization
2p
2s
1s
C g.s.
sp3
1s
C hyb.(CH4)
1s
H g.s
Considerthecarbonatominthegroundstate.Ane
pairin2sandanelectronineachoftwo2patomic
orbitals.Asetofhybridorbitals(sp3)isconstructed
fromonesorbitalandthreeporbitalsofcarbon
atom.Thefoursp3hybridorbitalspointintetrahedral
directions.TheCHbondsinCH4aredescribedby
theoverlapofeachsp3hybridorbitalofthecarbon
atomwith1sorbitalsofhydrogenatoms.
DescriptionofBondinginVB
StepstoobtainthebondingdescriptioninVB
WritetheLewiselectrondotformulaofthemolecule
UseVSEPRmodeltoobtainthearrangementsofepairs
aboutthecentralatom.
Deducethetypeofhybridorbitals
__
Assigntheelectronsofthecentralatomtothehybrid
orbitalsoneatatime,pairingthemonlywhennecessary
Formbondstothiscentralatombyoverlapingsingly
occupiedorbitalsofotheratomswithasinglyoccupied
hybridorbitalsofthecentralatom.
E10.5DescribethebondinginNH3,accordingtoVB
H
|
H N H tetrahedral sp 3 ;
..
sp3
sp3
MultipleBondingI
Onehybridorbitalisneededfor
eachbondandforeachlonepair.
Twotypes
Sigma()bond:theelectrondensityis
concentratedbetweentwonucleiofthe
atomsalonganimaginaryinternuclear
axis.
Pi()bond:theelectrondensityis
dividedbetweentwoseparateregions
thatlieonoppositesidesofinternuclear
axis.
Inethylene,C2H4,eachsp2hybridC
orbitalsoverlapsa1sorbitalofaH
atomsoransp2hybridofanotherC
toformabonds.Thebonds
givethemolecularframework.The
remaining2porbitalsformbond.
There are geometric, or cis-trans isomers of 1,2-dichloroethene because
the formation of a bond locks all the atoms into a flat, rigid molecule.
MultipleBondingII
Inacetylene,HC CH,
eachCatomisbondedto
twootheratomswithsp
hybridorbitalswithlinear
arrangement.Remaining
two2porbitalsformtwo
bonds.Geometryislinear.
E10.7DescribethebondingontheCatominCO2usingVB.
..
..
: O = C = O : 2 bonds sp orbitals on C and sp 2 on O
Theremainingtwo2porbitalsofCatomformbondswith2p
atomicorbitaloneachOatom.Oneofsp2hybridorbitalson
oxygenatomformsasigmabondwithCandremainingtwo
areoccupiedbythelonepairsoneachOatom.
MolecularOrbitalTheory
VBpredictsanymoleculewithevennumberof
valenceelectronsshouldbediamagnetic.For
..
..
exampleO2: : O = O :
ButO2isaparamagneticsubstance.
MolecularOrbital(MO)Theory:Moleculeshave
molecularorbitalswhichmayspreadover
severalatomsorentiremoleculeandelectrons
aredistributedinthem.
EachMOhasadefiniteenergy
Forthegrondstate,electronsareputintoorbitalsof
lowestenergy,consistentwithPauliPrinciple(each
MOcantakemaximumoftwoelectronswith
oppositespins)andHundsRule(MOswithequal
energiesisfilledwithelectronstogiveminimum
numberofepairs)
MolecularOrbitalsI
2
1s
Bonding()orbitalsareMOstheirelectrondensities
areconcentratedbetweentwonuclei.(constructive
interferenceofs)
Antibonding(*)orbitalsareMOstheirelectron
densitiesareconcentratedinregionsotherthan
betweentwonuclei.(destructiveinterferenceof s)
MolecularOrbitalsII
Thestrengthofinteractionbetweentwoatomicorbitalsto
formMOisdetermined:
1.
2.
Theenergydifferencebetweentheinteractingatomicorbitals
Themagnitudeoftheiroverlap
Fortheinteractiontobestrong,theenergiesofthetwo
orbitalsmustbeapproximatelyequalandtheoverlapmust
belarge.
1s+1s1s+*1s,1s+2sNo,2s+2s2s+*2s,
2s+2pNo,2p+2p2p+*2p+2p+*2p+2p+*2p
ElectronConfigurationof
DiatomicMolecules
2sand2patomicorbitalsinteractwitheach
othertochangetheenergyorderingofMOs
ForB2,C2andN2smallerenergydifferencecauses
larger2s+2pinteractionandpuchestheenergyof
2pabovethatof2p.
ForO2,F2andNe22s+2pinteractionissmall,the
energyorderingofMOsisaspredictedbefore.
Bond
Bondorder=(nbna)/2
nbisthenumberof
electronsinthe
bondingorbitals,nais
thenumberof
electronsinthe
antibondingorbitals.
2
2
*2
H 2 : 1s BO = (2 - 0)/2 = 1; He 2 : 1s 1s BO = (2 - 2)/2 = 0
Bond length decreases as the bond order increases.
Bond energy increases as the bond order increases.
Molecules with BO = 0 are unstable. He2, Ne2.
Molecules with unpaired electrons in their electronic
configuration is paramagnetic.
DelocalizedBonding
InVB,twoormoreresonanceformulasarerequiredtoexplain
bonding.
InMO,thebondingisdescribedintermsofasingleelectron
configuration.Onesp2hybridorbitalonsideOsandtwoforthe
centerOformtwobondsbetweenatoms.Theremainingsp2
arefilledbyfiveloneepairs.Three2porbitalsoverlap
sidewaystoproducethreebondsofbonding,antibondingand
nonbonding.Thedelocalizedepairisinbonding,lonepairisin
2
ArrangementofElectronPairs
MolecularGeometriesI
MolecularGeometriesII
MolecularGeometriesIII