物质例图表分子结构化学键

18,20Oct97BondingandStructure1ChemicalBonding

and

MolecularStructure

(Chapter9)Ionicvs.covalentbondingMolecularorbitalsandthecovalentbond(Ch.10)ValenceelectronLewisdotstructures octetvs.non-octet resonancestructures formalchargesVSEPR-predictingshapesofmoleculesBondproperties

polarity,bondorder,bondstrength18,20Oct97BondingandStructure2ChemicalBondingProblemsandquestions—Howisamoleculeorpolyatomicionheldtogether?Whyareatomsdistributedatstrangeangles?Whyaremoleculesnotflat?Canwepredictthestructure?Howisstructurerelatedtochemicalandphysicalproperties?18,20Oct97BondingandStructure3Mostbondsaresomewhereinbetween.FormsofChemicalBondsThereare2extremeformsofconnectingorbondingatoms:Ionic—completetransferofelectronsfromoneatomtoanotherCovalent—electronssharedbetweenatoms18,20Oct97BondingandStructure4Ionic

BondsIoniccompounds-essentiallycompleteelectrontransferfromanelementoflowIE(metal)toanelementofhighelectronaffinity(EA)(nonmetal)Na(s)+1/2Cl2(g)

Na++Cl-

NaCl(s)-NON-DIRECTIONALbondingviaCoulomb(charge)interaction-primarilybetweenmetals

(Grps1A,2Aandtransitionmetals)

andnonmetals

(espOandhalogens)18,20Oct97BondingandStructure5CovalentBondingCovalentbondisthesharingoftheVALENCEELECTRONSofeachatominabondRecall:Electronsaredividedbetweencoreandvalenceelectrons.ATOM core valenceNa1s22s22p63s1 [Ne] 3s1Br[Ar]3d104s24p5 [Ar]3d10 4s24p5BrBr18,20Oct97BondingandStructure6ValenceElectrons1A2A3A4A5A6A7A8ANumberofvalenceelectronsisequaltotheGroupnumber.18,20Oct97BondingandStructure7CovalentBondingThebondarisesfromthemutualattractionof2nucleiforthesameelectrons.Acovalentbondisabalanceofattractiveandrepulsiveforces.6_H2bond.mov18,20Oct97BondingandStructure8BondFormationAbondcanresultfroma“head-to-head”overlap

ofatomicorbitalsonneighboringatoms.HH

Cl••••••

Cl••••••+OverlapofH(1s)andCl(2p)ThistypeofoverlapplacesbondingelectronsinaMOLECULARORBITALalongthelinebetweenthetwoatomsandformsaSIGMABOND(s).18,20Oct97BondingandStructure9SigmaBondFormationbyOrbitalOverlapTwosAtomicOrbitals(A.O.s)overlaptoformans(sigma)MolecularOrbital(M.O.)6_H2pot.mov18,20Oct97BondingandStructure10SigmaBondFormationbyOrbitalOverlapTwosA.O.soverlaptofromansM.O.Similarly,twopA.O.scanoverlapend-ontofromapM.O.e.g.F218,20Oct97BondingandStructure11ElectronDistributioninMoleculesElectrondistributionisdepictedwithLewiselectrondotstructuresElectronsaredistributedas:sharedorBONDPAIRSandunsharedorLONEPAIRS.G.N.Lewis1875-194618,20Oct97BondingandStructure12BondandLonePairsElectronsaredistributedassharedorBONDPAIRSandunsharedorLONEPAIRS.••H

Cl••••ThisisaLEWISELECTRONDOTstructure.sharedorbondpairUnsharedorlonepair(LP)18,20Oct97BondingandStructure13ThisobservationiscalledtheOCTETRULERulesofLewisStructures•No.ofvalenceelectronsofanatom=GroupnumberExceptforH(andatomsof3rdandhigherperiods),

#BondPairs+#LonePairs=4ForGroups5A-7A(N-F),no.ofBONDPAIRS=8-groupNo.ForGroups1A-4A(Li-C),no.ofBONDPAIRS=groupnumber18,20Oct97BondingandStructure142.Countvalenceelectrons H=1andN=5 Total=(3x1)+5 =8electronsor1.Decideonthecentralatom;neverH.Centralatomisatomoflowestaffinityforelectrons.Inammonia,NiscentralBuildingaDotStructureAmmonia,NH34pairs18,20Oct97BondingandStructure154.RemainingelectronsformLONEPAIRStocompleteoctetasneeded.3.Formasigmabondbetweenthecentralatomandsurroundingatoms.BuildingaDotStructureH••HHN3BONDPAIRSand1LONEPAIR.NotethatNhasasharein4pairs(8electrons),whileeachHshares1pair.18,20Oct97BondingandStructure16Step2.Countvalenceelectrons S=6 3xO=3x6=18 Negativecharge=2 TOTAL=6+18+2=26e- or13pairsStep1.Centralatom=S10pairsofelectronsareleft.Sulfiteion,SO32-Step3.Formsigmabonds18,20Oct97BondingandStructure17Remainingpairselonepairs, firstonoutsideatoms thenoncentralatom.Sulfiteion,SO32-(2)Eachatomissurroundedbyanoctetofelectrons.••OOOS••••••••••••••••••NOTE-mustaddformalcharges(O-,S+)forcompletedotdiagram18,20Oct97BondingandStructure18CarbonDioxide,CO21.Centralatom=__C____2.Valenceelectrons=_16_or_8_pairs3.Formsigmabonds. Thisleaves__6__pairs.4.Placelonepairsonouteratoms.18,20Oct97BondingandStructure19CarbonDioxide,CO2(2)4.Placelonepairsonouteratoms.Thesecondbondingpairformsapi(p)

bond.5.TogiveCanoctet,formDOUBLEBONDSbetweenCandO.18,20Oct97BondingandStructure20SO3H2CODoubleandeventriplebondsarecommonlyobservedforC,N,P,O,andSC2F418,20Oct97BondingandStructure21SulfurDioxide,SO21.Centralatom=S2.Valenceelectrons=6+2*6=18electrons or9pairs3.Formpi(

)bondsothatShasanoctet—notethattherearetwowaysofdoingthis.18,20Oct97BondingandStructure22SulfurDioxide,SO2

Equivalentstructurescalled:RESONANCESTRUCTURES

TheproperLewisstructureisaHYBRIDofthetwo.ABETTERrepresentationofSO2

ismadebyforming2doublebondsO=S=OEachatomhas-OCTET-formalcharge=018,20Oct97BondingandStructure23Urea(NH2)2CO1.Numberofvalenceelectrons=24e-2.Drawsigmabonds.4.Placeremainingelectronpairsonoxygen3.CompleteCatomoctetwithdoublebond.Leaves24-14=10e-pairs.andnitrogenatoms.18,20Oct97BondingandStructure24ViolationsoftheOctetRuleUsuallyoccurswith: Boron BF3SF4elementsofhigherperiods.18,20Oct97BondingandStructure25BoronTrifluorideCentralatom=BValenceelectrons=3+3*7=24orelectronpairs=12AssembledotstructureTheBatomhasashareinonly6electrons(or3pairs).Batominmanymoleculesiselectrondeficient.18,20Oct97BondingandStructure26SulfurTetrafluoride,SF4Centralatom=SValenceelectrons=6+4*7=34e- or17pairs.Formsigmabondsanddistributeelectronpairs.5pairsaroundtheSatom.Acommonoccurrenceoutsidethe2ndperiod.18,20Oct97BondingandStructure27Formalcharge=Groupno. -1/2(no.bondelectrons) -(no.ofLPelectrons)FormalAtomChargesAtomsinmoleculesoftenbearacharge(+or-).Themostimportantdominantresonancestructureofamoleculeistheonewithformalchargesascloseto0aspossible.18,20Oct97BondingandStructure2804

-

(1/2)(8)

-

0

=

6

-

(1/2)(4)

-

4

=

0CarbonDioxide,CO2AtOXYGENOCO•••

••••

•AtCARBON18,20Oct97BondingandStructure29Catomchargeis0.6

-

(1/2)(6)

-

2

=

+16

-

(1/2)(2)

-

6

=

-1CarbonDioxide,CO2(2)OCO•••

••••

•AnalternateLewisstructureis:ANDthecorrespondingresonanceform+OCO•••

••••

•+18,20Oct97BondingandStructure30REALITY:PartialchargescalculatedbyCAChemolecularmodelingsystem(onCD-ROM).+1.46-0.73-0.73CarbonDioxide,CO2(3)Whichisthepredominantreson