Chemical Bonding I Lewis Theory：化学我刘易斯理论结合

ChemicalBondingI:

LewisTheoryKimShihPh.D.KimShihWhyDoAtomsBond?ChemicalbondsformbecausetheylowerthepotentialenergybetweenthechargedparticlesthatcomposeatomsAchemicalbondformswhenthepotentialenergyofthebondedatomsislessthanthepotentialenergyoftheseparateatomsTocalculatethispotentialenergy,youneedtoconsiderthefollowinginteractions:nucleus–to–nucleusrepulsionselectron–to–electronrepulsionsnucleus–to–electronattractionsKimShihLewisBondingTheoryThesimplestbondingtheoriesiscalledLewisTheoryArepresentationofanatom’svalenceelectronsbyusingdotsandindicatesbytheplacementofdotshowthevalenceelectronsaredistributedinthemolecule.Lewisstructuresallowustopredictmanypropertiesofmoleculessuchasmolecularstability,shape,size,polarityKimShihKimShihTypesofBondingCovalentBond:AtomsshareelectronsIonicBond:Electronstransferfromoneatomtoanother.Product:MoleculeIoniccompoundMadeof….NonmetalNonmetal+MetalExample: H2,NH3,O2NaCl,MgO,KBrTypesofBondsTypesofAtomsTypeofBondBondCharacteristicmetalstononmetalsIonicelectronstransferrednonmetalstononmetalsCovalentelectronssharedmetalstometalsMetallicelectronspooledWecanclassifybondsbasedonthekindsofatomsthatarebondedtogetherKimShihIonicBondsWhenametalatomloseselectronsitbecomesacationmetalshavelowionizationenergy,makingitrelativelyeasytoremoveelectronsfromthemWhenanonmetalatomgainselectronsitbecomesananion

nonmetalshavehighelectronaffinities,makingitadvantageoustoaddelectronstotheseatomsTheoppositelychargedionsarethenattractedtoeachother,resultinginanionicbondKimShihCovalentBondsNonmetalatomshaverelativelyhighionizationenergies,soitisdifficulttoremoveelectronsfromthemWhennonmetalsbondtogether,itisbetterintermsofpotentialenergyfortheatomstosharevalenceelectronspotentialenergylowestwhentheelectronsarebetweenthenucleiSharedelectronsholdtheatomstogetherbyattractingnucleiofbothatomsKimShihMetallicBondsTherelativelylowionizationenergyofmetalsallowsthemtoloseelectronseasilyMetallicbondinginvolvesthemetalatomsreleasingtheirvalenceelectronstobesharedasapoolbyalltheatoms/ionsinthemetalanorganizationofmetalcationislandsinaseaofelectronselectronsdelocalizedthroughoutthemetalstructureBondingresultsfromattractionofcationforthedelocalizedelectronsKimShihLewisStructuresofAtomsInaLewisstructure,werepresentthevalenceelectronsofmain-groupelementsasdotssurroundingthesymbolfortheelementakaelectrondotstructuresWeusethesymbolofelementtorepresentnucleusandinnerelectronsAndweusedotsaroundthesymboltorepresentvalenceelectronspairfirsttwodotsforthesorbitalelectronsputonedotoneachopensideforfirstthreepelectronsthenpairrestofdotsfortheremainingpelectronsLiBeBCNOFNeKimShihLewisStructuresofIonsCationshaveLewissymbolswithoutvalenceelectronslostinthecationformationAnionshaveLewissymbolswitheightvalenceelectronselectronsgainedintheformationoftheanionKimShihLewisBondingTheoryAtomsbondbecauseitresultsinamorestableelectronconfiguration.morestable=lowerpotentialenergyAtomsbondtogetherbyeithertransferringorsharingelectronsUsuallythisresultsinallatomsobtaininganoutershellwitheightelectronsoctetruletherearesomeexceptions:H,Li,Be,BattainanelectronconfigurationlikeHeKimShihLewisTheoryandIonicBondingLewissymbolscanbeusedtorepresentthetransferofelectronsfrommetalatomtononmetalatom,resultinginionsthatareattractedtoeachotherandthereforebondKimShih+F[He]2s22p5•••••••PredictingIonicFormulas

UsingLewisSymbolsElectronsaretransferreduntilthemetallosesallitsvalenceelectronsandthenonmetalhasanoctetNumbersofatomsareadjustedsotheelectrontransfercomesoutevenLi2OKimShihUsingLewistheorytopredictchemicalformulasofioniccompoundsPredicttheformulaofthecompoundthatformsbetweencalciumandchlorine.DrawtheLewisdotsymbolsoftheelements.Ca∙∙Cl∙∙∙∙∙∙∙Transferallthevalenceelectronsfromthemetaltothenonmetal,addingmoreofeachatomasyougo,untilallelectronsarelostfromthemetalatomsandallnonmetalatomshaveeightelectrons.Ca∙∙Cl∙∙∙∙∙∙∙Cl∙∙∙∙∙∙∙Ca2+CaCl2KimShihUseLewissymbolstopredicttheformulaofanioniccompoundmadefromreactingametal,M,thathastwovalenceelectronswithanonmetal,X,thathasfivevalenceelectronsM3X2KimShihIonicBonding&theCrystalLatticeTheextraenergythatisreleasedcomesfromtheformationofastructureinwhicheverycationissurroundedbyanions,andviceversaThisstructureiscalledacrystallatticeThecrystallatticeisheldtogetherbytheelectrostaticattractionofthecationsforallthesurroundinganionsThecrystallatticemaximizestheattractionsbetweencationsandanions,leadingtothemoststablearrangementKimShihCrystalLatticeElectrostaticattractionisnondirectional!!nodirectanion–cationpairTherefore,thereis

noionicmoleculethechemicalformulaisanempiricalformula,simplygivingtheratioofionsbasedonchargebalanceKimShihLatticeEnergyTheextrastabilitythataccompaniestheformationofthecrystallatticeismeasuredasthelatticeenergyThelatticeenergyistheenergyreleasedwhenthesolidcrystalformsfromseparateionsinthegasstatealwaysexothermichardtomeasuredirectly,butcanbecalculatedfromknowledgeofotherprocessesLatticeenergydependsdirectlyonsizeofchargesandinverselyondistancebetweenionsKimShihDeterminingLatticeEnergy

TheBorn–HaberCycleTheBorn–HaberCycleisahypotheticalseriesofreactionsthatrepresentstheformationofanioniccompoundfromitsconstituentelementsThereactionsarechosensothatthechangeinenthalpyofeachreactionisknownexceptforthelastone,whichisthelatticeenergyKimShihBorn–HaberCycleforNaClNa(s)+½Cl2(g)-------------NaCl(s)Born–HaberCycleforNaClDH°f(NaCl,s)=DH°f(Naatoms,g)+DH°f(Clatoms,g)+DH°f(Na+,g)+DH°f(Cl−,g)+DH°(NaCllattice)DH°f(NaCl,s)=DH°f(Naatoms,g)+DH°f(Cl–Clbondenergy)+Na1stIonizationEnergy+ClElectronAffinity+NaClLatticeEnergyNa(s)→Na(g) DHf(Na,g)½Cl2(g)→Cl(g) DHf(Cl,g)Na(g)→Na+(g) DHf(Na+,g)Cl

(g)→Cl−(g) DHf(Cl−,g)Na+(g)+Cl−(g)→NaCl(s) DH(NaCllattice)Na(s)+½Cl2(g)→NaCl(s) DHf(NaCl,s)Na(s)→Na(g) DHf(Na,g)½Cl2(g)→Cl(g) DHf(Cl,g)Na(g)→Na+(g) DHf(Na+,g)Cl

(g)→Cl−(g) DHf(Cl−,g)Na+(g)+Cl−(g)→NaCl(s) DH(NaCllattice)Na(s)→Na(g) +108kJ½Cl2(g)→Cl(g) +½(244kJ)Na(g)→Na+(g) +496kJCl

(g)→Cl−(g) −349kJNa+(g)+Cl−(g)→NaCl(s) DH(NaCllattice)Na(s)+½Cl2(g)→NaCl(s) −411kJNaClLatticeEnergy=DH°f(NaCl,s)−[DH°f(Naatoms,g)+DH°f(Cl–Clbondenergy)+Na1stIonizationEnergy+ClElectronAffinity]NaClLatticeEnergy=(−411kJ)−[(+108kJ)+(+122kJ)+(+496kJ)+(−349kJ)]=−788kJKimShihKimShihHeatofsublimationforMg(s)=147.7kJ/mol FirstEiforMg(g)=737.7kJ/mol BonddissociationenergyforCl2(g)=121.5kJ/mol SecondEiforMg(g)=1450.7kJ/molNetreaction=-642kJ/mol EeaforCl=-348.6kJ/molGiventheinformationbelow,determinethelatticeenergyofMgCl2Giventheinformationbelow,determinethelatticeenergyofMgCl2Mg(s)®Mg(g)

DH1°f=+147.1kJ/mol2{½Cl2(g)®Cl(g)} 2

DH2°f=2(+122kJ/mol)Mg(g)®Mg+(g)+e-

DH3°f=+738kJ/molMg+(g)®Mg2+(g)+e-

DH4°f=+1450kJ/mol2{Cl(g)+e-

®Cl−(g)} 2

DH5°f=2(−349kJ/mol)Mg2+(g)+2Cl−(g)®MgCl2(s) DH°latticeenergy=?kJ/molMg(s)+Cl2(g)®MgCl2(s) DH6°f=−641kJ/molKimShihTrendsinLatticeEnergy

IonSizeTheforceofattractionbetweenchargedparticlesisinverselyproportionaltothedistancebetweenthemLargerionsmeanthecenterofpositivecharge(nucleusofthecation)isfartherawayfromthenegativecharge(electronsoftheanion)largerion=weakerattractionweakerattraction=smallerlatticeenergyKimShihLatticeEnergyvs.

IonSizeKimShihTrendsinLatticeEnergy

IonChargeTheforceofattractionbetweenoppositelychargedparticlesisdirectlyproportionaltotheproductofthechargesLargerchargemeanstheionsaremorestronglyattractedlargercharge=strongerattractionstrongerattraction=largerlatticeenergyOfthetwofactors,ionchargeisgenerallymoreimportantLatticeEnergy=−910kJ/molLatticeEnergy=−3414kJ/molKimShihOrderthefollowingioniccompoundsinorderofincreasingmagnitudeoflatticeenergy:

CaO,KBr,KCl,SrOFirstexaminetheionchargesandorderbysumofthechargesCa2+&O2-,K+&Br─,K+&Cl─,Sr2+&O2─(KBr,KCl)<(CaO,SrO)Thenexaminetheionsizesofeachgroupandorderbyradius;larger<smaller(KBr,KCl)samecation,Br─>Cl─(sameGroup)KBr<KCl<(CaO,SrO)(CaO,SrO)sameanion,Sr2+>Ca2+(sameGroup)KBr<KCl<SrO<CaOKimShihOrderthefollowingioniccompoundsinorderofincreasingmagnitudeoflatticeenergy:

MgS,NaBr,LiBr,SrSFirstexaminetheionchargesandorderbysumofthechargesMg2+&S2-,Na+&Br─,Li+&Br─,Sr2+&S2─(NaBr,LiBr)<(MgS,SrS)Thenexaminetheionsizesofeachgroupandorderbyradius;larger<smaller(NaBr,LiBr)sameanion,Na+>Li+(sameGroup)NaBr<LiBr<(MgS,SrS)(MgS,SrS)sameanion,Sr2+>Mg2+(sameGroup)NaBr<LiBr<SrS<MgSKimShihIonicBonding

Modelvs.RealityKimShihIonicBonding

Modelvs.RealityAttractionsbetweenionsarestrongthestrongertheattraction(largerthelatticeenergy),thehigherthemeltingpointMPgenerally>300°CallioniccompoundsaresolidsatroomtemperatureLewistheoryimpliesthatthepositionsoftheionsinthecrystallatticearecriticaltothestabilityofthestructureIonicsolidsarerelativelyhardKimShihWhichioniccompoundbelowhasthehighestmeltingpoint?KBr(734ºC)CaCl2(772ºC)MgF2

(1261ºC)KBrCaCl2MgF2KimShihIonicBonding

Modelvs.RealityLewistheoryimpliesthatiftheionsaredisplacedfromtheirpositioninthecrystallattice,thatrepulsiveforcesshouldoccurThispredictsthecrystalwillbecomeunstableandbreakapart.Lewistheorypredictsionicsolidswillbebrittle.Ionicsolidsarebrittle.Whenstrucktheyshatter.+-++++++++--------+-++++++++--------KimShihIonicBonding

Modelvs.RealityToconductelectricity,amaterialmusthavechargedparticlesthatareabletoflowthroughthematerialLewistheoryimpliesthat,intheionicsolid,theionsarelockedinpositionandcannotmovearoundLewistheorypredictsthationicsolidsshouldnotconductelectricityIonicsolidsdonotconductelectricityKimShihIonicBonding

Modelvs.RealityLewistheoryimpliesthat,intheliquidstateorwhendissolvedinwater,theionswillhavetheabilitytomovearoundLewistheorypredictsthatbothaliquidioniccompoundandanioniccompounddissolvedinwatershouldconductelectricityIoniccompoundsconductelectricityintheliquidstateorwhendissolvedinwaterKimShihConductivityofNaClinNaCl(aq),theionsareseparatedandallowedtomovetothechargedrodsinNaCl(s),theionsarestuckinpositionandnotallowedtomovetothechargedrodsKimShihLewisTheoryof

CovalentBondingLewistheoryimpliesthatanotherwayatomscanachieveanoctetofvalenceelectronsistosharetheirvalenceelectronswithotheratomsThesharedelectronswouldthencounttowardeachatom’soctetThesharingofvalenceelectronsiscalledcovalentbondingKimShih....OSO..............CovalentBonding:

BondingandLonePairElectronsBondingpairsLonepairsElectronsthataresharedbyatomsarecalledbondingpairsElectronsthatarenotsharedbyatomsbutbelongtoaparticularatomarecalledlonepairsakanonbondingpairsKimShihSingleCovalentBondsF•••••••F•••••••F••••••••••F••••HHO••••••••H•H•O••••••FFWhentwoatomsshareonepairofelectronsitiscalledasinglecovalentbond2electronsOneatommayusemorethanonesinglebondtofulfillitsoctettodifferentatomsHonlyduet(only2e-)KimShihDoubleCovalentBondWhentwoatomssharetwopairsofelectronstheresultiscalledadoublecovalentbondfourelectronsO••••O••••••••O••••••O••••••KimShihTripleCovalentBondWhentwoatomssharethreepairsofelectronstheresultiscalledatriplecovalentbondsixelectronsN•••••N•••••N••••••••••NKimShihCovalentBonding

Modelvs.RealityLewistheoryimpliesthatsomecombinationsshouldbestable,whereasothersshouldnotbecausethestablecombinationsresultin“octets”UsingtheseideasofLewistheoryallowsustopredicttheformulasofmoleculesofcovalentlybondedsubstancesHydrogenismorestablewhenitissinglybondedtoanotheratom++H2HClKimShihCovalentBonding

Modelvs.RealityCovalentbondingimpliesthattheattractionsbetweenatomsaredirectionalCompoundsofnonmetalsaremadeofindividualmoleculeunitsLewistheorypredictsthemeltingandboilingpointsofmolecularcompoundsshouldberelativelylowinvolvesbreakingtheattractionsbetweenthemolecules,butnotthebondsbetweentheatomsthecovalentbondsarestrong,buttheattractionsbetweenthemoleculesaregenerallyweakMPgenerally<300°CmolecularcompoundsarefoundinallthreestatesatroomtemperatureKimShihIntermolecularAttractions

vs.BondingKimShihCovalentBonding

Modelvs.RealityLewistheorypredictsthatthehardnessandbrittlenessofmolecularcompoundsshouldvarydependingonthestrengthofintermolecularattractiveforcesLewistheorypredictsthatneithermolecularsolidsnorliquidsshouldconductelectricitytherearenochargedparticlesaroundtoallowthematerialtoconductMolecularacidsconductelectricitywhendissolvedinwater,butnotinthesolidorliquidstate,duetothembeingionizedbythewaterKimShihCovalentBonding

Modelvs.RealityLewistheorypredictsthatthemoreelectronstwoatomsshare,theshorterthebondshouldbeLewistheorypredictsthatthemoreelectronstwoatomsshare,thestrongerthebondshouldbeLewistheorywouldpredictdoublebondsaretwiceasstrongassinglebonds,buttherealityistheyarelessthantwiceasstrongKimShihPolarCovalentBondingCovalentbondingbetweenunlikeatomsresultsinunequalsharingoftheelectronsoneatompullstheelectronsinthebondclosertoitssideoneendofthebondhaslargerelectrondensitythantheotherTheresultisapolarcovalentbondbondpolaritytheendwiththelargerelectrondensitygetsapartialnegativechargetheendthatiselectrondeficientgetsapartialpositivechargeKimShihHFHF••d+d-EN2.1EN4.0KimShihBondPolarityMostbondshavesomedegreeofsharingandsomedegreeofionformationtothemBondsareclassifiedascovalentiftheamountofelectrontransferisinsufficientforthematerialtodisplaytheclassicpropertiesofioniccompoundsIfthesharingisunequalenoughtoproduceadipoleinthebond,thebondisclassifiedaspolarcovalentKimShihElectronegativityTheabilityofanatomtoattractbondingelectronstoitselfIncreasesacrossperiod(lefttoright)Decreasesdowngroup(toptobottom)noblegasatomsarenotassignedvaluesoppositeofatomicsizetrendThelargerthedifferenceare,themorepolarthebondnegativeendtowardmoreelectronegativeatomKimShihElectronegativityDifferenceandBondTypeIfdifferenceinelectronegativitybetweenbondedatomsis0,thebondispurecovalentequalsharingIfdifferenceinelectronegativitybetweenbondedatomsis0.1to0.4,thebondisnonpolarcovalentIfdifferenceinelectronegativitybetweenbondedatomsis0.5to1.9,thebondispolarcovalentIfdifferenceinelectronegativitybetweenbondedatomsislargerthanorequalto2.0,thebondisionic“100%”00.42.04.04%51%PercentIonicCharacterElectronegativityDifferenceKimShihBondPolarityENCl=3.03.0−3.0=0PureCovalentENCl=3.0ENH=2.13.0–2.1=0.9PolarCovalentENCl=3.0ENNa=0.93.0–0.9=2.1IonicKimShihWater–APolarMoleculestreamofwaterattractedtoachargedglassrodstreamofhexanenotattractedtoachargedglassrodKimShihPolarMoleculeInteractionsPolar-ionPolar-polarKimShihBondDipoleMomentsDipolemoment,m,isameasureofbondpolarityadipoleisamaterialwitha+and−enditisdirectlyproportionaltothesizeofthepartialchargesanddirectlyproportionaltothedistancebetweenthemm=(q)(r)measuredinDebyes,1D=3.34x10-30C.mGenerally,themoreelectronstwoatomsshareandthelargertheatomsare,thelargerthedipolemoment.KimShihDipoleMomentsKimShihPercentIonicCharacterThepercentioniccharacteristhepercentageofabond’smeasureddipolemomentcomparedtowhatitwouldbeiftheelectronswerecompletelytransferredThepercentioniccharacterindicatesthedegreetowhichtheelectronistransferredKimShihQuestion:Whatisthedipolemomentthatresultfromseparatingaprotonandanelectronbydistanceof130pm?m=(q)(r)m=(1.6x10-19C)x(130x10-12m)=2.1x10-29C.m=6.2DQ=1.6x10-19CR=130pm=130x10-12mHint:Supposeadiatomicmoleculewithabondlengthof130pmhasadipolemomentof3.5D.Whatisthepercentioniccharacterofthebond?Percentioniccharacter=3.5D/6.2Dx100%=56%KimShihDeterminewhetheranN―Obondisionic,covalent,orpolarcovalentDeterminetheelectronegativityofeachelementN=3.0;O=3.5Findthedifferenceofelectronegativities(3.5)−(3.0)=0.5Ifthedifferenceis2.0orlarger,thenthebondisionic;otherwiseit’scovalentdifference(0.5)islessthan2.0,thereforecovalentIfthedifferenceis0.5to1.9,thenthebondispolarcovalent;otherwiseit’scovalentdifference(0.5)is0.5to1.9,thereforepolarcovalentKimShihLewisStructuresofMoleculesUnderstandthedistributionofvalenceelectronsinamoleculePredictthebondinginmanycompoundsWecanstudyshapes,propertiesofmoleculesandhowtheywillinteracttogetherKimShihLewisStructuresofMoleculesStep1:ValenceElectronsCountthetotalnumberofvalenceelectronsforallatomsinthemolecule.Addoneadditionalelectronforeachnegativechargeinananionorsubtractoneforeachpositivechargeinacation.KimShihStep2:Determinethecenteratom,puttheothersonthesidesUsuallytheatomwiththemostavailablevalencee-HydrogenalwaysonthesideHalogenmostofthetimesonthesideunlessinvolvedinhigherperiodofatomsLewisStructuresofMoleculesKimShihStep4:AssignElectronstotheCentralAtomIfunassignedelectronsremainafterstep3,placethemonthecentralatom.Step3:AssignElectronstotheTerminalAtomsSubtractthenumberofelectronsusedforbondingonthesideatomsfromthetotalnumberdeterminedinstep1.Completeeachterminalatom’soctet(exceptforhydrogen).LewisStructuresofMoleculesStep5:MultipleBondsIfnounassignedelectronsremainafterstep3butthecentralatomdoesnotyethaveanoctet,useoneormorelonepairsofelectronsfromaneighboringatomtoformamultiplebond(eitheradoubleoratriple).KimShihDrawanelectron-dotstructureforH3O1+.3(1)+6-1=8valenceelectronsStep4:Step1:Step2:IdentifythecenteratomHOHHHOHH1+HydrogenisalwaysonthesidesStep3:OHHHKimShihDrawanelectron-dotstructureforCH2O.4+2(1)+6=12valenceelectronsStep3:Step1:Step2:HCOHHCOHStep5:HCOHHCOHKimShihElectron-DotStructuresandResonanceDrawanelectron-dotstructureforO3.Step1:3(6)=18valenceelectronsStep2:OOOOOOStep3:Step4:OOOStep5:OOOStep6:OOOMovealonepairfromthisoxygen?Or,movealonepairfromthisoxygen?ResonanceOOOKimShihExerciseKimShihDrawLewisStructuresoftheFollowingCO2SeOF2NO2−H3PO4SO32−P2H4KimShihCO2SeOF2NO2−H3PO4SO32−P2H4LewisStructures16e−26e−18e−26e−32e−14e−KimShihFormalChargesFormalCharge#ofvalencee-infreeatom-21-#ofnonbondinge-#ofbondinge-=CalculatetheformalchargeoneachatominO3.OOO6-(2)-6=-1126-(4)-4=0126-(6)-2=+112BetterstructureshavesmallerformalchargesBetterstructureshavethenegativeformalchargeonthemoreelectronegativeatomKimShihKimShihWhataretheformalchargesforthefollowingstructure?Whichstructureisthemoststablestructure?#ofvalencee-#ofnonbondinge-½bondinge-645645645602 404206[OCN]..:..:[OCN]..::..[OCN]..::..143242341FormalCharge-10000-110-2OhasstrongerelectronegativitythenCandNThefirststructureismorestable.KimShihExerciseKimShihNNO..::..NNO..::..NNO..::..Whataretheformalchargesforthefollowingstructure?Whichstructureisthemoststablestructure?a.b.c.ResonanceLewistheorylocalizestheelectronsbetweentheatomsthatarebondingtogetherExtensionsofLewistheorysuggestthatthereissomedegreeofdelocalizationoftheelectrons–wecallthisconceptresonanceDelocalizationofchargehelpstostabilizethemoleculeKimShihResonanceStructures....OSO..................OSO..............WhenthereismorethanoneLewisstructureforamoleculethatdifferonlyinthepositionoftheelectrons,theyarecalledresonancestructuresTheactualmoleculeisacombinationoftheresonanceforms–aresonancehybridthemoleculedoesnotresonatebetweenthetwoforms,thoughweoftendrawitthatwayLookformultiplebondsorlonepairsKimShihResonanceKimShihRulesofResonanceStructuresResonancestructuresmusthavethesameconnectivityonlyelectronpositionscanchangeResonancestructuresmusthavethesamenumberofelectronsSecondrowelementshaveamaximumofeightelectronsbondingandnonbondingthirdrowcanhaveexpandedoctetFormalchargesmusttotalsameKimShihDrawingResonanceStructuresDrawfirstLewisstructurethatmaximizesoctetsAssignformalchargesMoveelectronpairsfromatomswith(−)formalchargetowardatomswith(+)formalchargeIf(+)fcatom2ndrow,onlymoveinelectronsifyoucanmoveoutelectronpairsfrommultiplebondIf(+)fcatom3rdroworbelow,keepbringinginelectronpairstoreducetheformalcharge,evenifgetexpandedoctet+1−1−1+1−1−1KimShihDrawingResonanceStructuresDrawfirstLewisstructurethatmaximizesoctetsAssignformalchargesMoveelectronpairsfromatomswith(−)formalchargetowardatomswith(+)formalchargeIf(+)fcatom2ndrow,onlymoveinelectronsifyoucanmoveoutelectronpairsfrommultiplebondIf(+)fcatom3rdroworbelow,keepbringinginelectronpairstoreducetheformalcharge,evenifgetexpandedoctet−1−1+2KimShihBondEnergiesChemicalreactionsinvolvebreakingbondsinreactantmoleculesandmakingnewbondstocreatetheproductsTheDH°reactioncanbeestimatedbycomparingthecostofbreakingoldbondstotheincomefrommakingnewbondsTheamountofenergyittakestobreakonemoleofabondinacompoundiscalledthebondenergyinthegasstateKimShihTrendsinBondEnergiesIngeneral,themoreelectronstwoatomsshare,thestrongerthecovalentbondmustbecomparingbondsbetweenlikeatomsC≡C(837kJ)>C=C(611kJ)>C−C(347kJ)C≡N(891kJ)>C=N(615kJ)>C−N(305kJ)Ingeneral,theshorterthecovalentbond,thestrongerthebondmustbecomparingsimilartypesofbondsBr−F(237kJ)>Br−Cl(218kJ)>Br−Br(193kJ)bondsgetweakerdownthecolumnbondsgetstrongeracrosstheperiodKimShihUsingBondEnergiestoEstimateDH°rxnTheactualbondenergydependsonthesurroundingatomsandotherfactorsWeoftenuseaveragebondenergiestoestimatetheDHrxnworksbestwhenallreactantsandproductsingasstateBondbreakingisendothermic,DH(breaking)=+Bondmakingisexothermic,DH(making)=−DHrxn=∑(DH(bondsbroken))+∑(DH(bondsformed))KimShihEstimatetheenthalpyofthefollowingreactionBondbreaking1moleC─H +414kJ1moleCl─Cl