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ValenceBondTheory价键理论.doc

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ValenceBondTheory Anothertopicthatyou’llneedtobefamiliarwithfortheSATIIChemistrytestisthatofvalencebondtheory.Bynow,youareawarethattwoatomswillformabondwhenthereisorbitaloverlapbetweenthem,andamaximumoftwoelectronscanbepresentintheoverlappingorbitals.Sincethepairofelectronsisattractedtobothatomicnuclei,abondisformed,andastheextentofoverlapincreases,thestrengthofthebondincreases.Theelectronicenergydropsastheatomsapproacheachother,butitbeginstoincreaseagainwhentheybecometooclose.Thismeansthereisanoptimumdistance,theobservedbonddistance,atwhichthetotalenergyisataminimum.Let’sdelvealittlemoredeeplyintosigmabondsnowanddescribetheminmoredetail.Asyouknow,sigma(s)bondsaresinglebonds.Theyresultfromtheoverlapoftwosorbitals,ansandaporbital,ortwohead-to-headporbitals.Theelectrondensityofasigmabondisgreatestalongtheaxisofthebond.Maximumoverlapformsthestrongest-possiblesigmabond,andthetwoatomswillarrangethemselvestogivethegreatest-possibleorbitaloverlap.Thisistrickywithporbitalssincetheyaredirectionalalongthex,y,andzaxes.Hybridorbitalsresultfromablendingofatomicorbitals(inotherwords,sandporbitals)tocreateorbitalsthathaveenergythat’sinbetweentheenergyoftheloneorbitals.Lookatthemethanemolecule,forexample:allfouroftheC—Hbondsare109.5oapart,whilenonbondedporbitalsareonly90oapart. Theorbitalsshownattheleftofthefigureareforanonbondedcarbonatom,butoncethecarbonatombeginstobondwithotheratoms(inthiscasehydrogen),theatomicorbitalshybridize,andthischangestheirshapeconsiderably.Noticehowthefirstsetoffiguresformthesp3atomicorbital,thehybrid,andthisleadstofurtherhybridization. Ammoniaalsohassp3hybridization,eventhoughithasalonepair. MultipleBondingNowlet’slookmorecloselyatpibonds.Aswementionedearlierinthischapter,pi(p)bondsresultfromthesidewaysoverlapofporbitals,andpiorbitalsaredefinedbytheregionaboveandbelowanimaginarylineconnectingthenucleiofthetwoatoms.Keepinmindthatpibondsneveroccurunlessasigmabondhasformedfirst,andtheymayformonlyifunhybridizedporbitalsremainonthebondedatoms.Also,theyoc