资源描述：
《Valence Bond Theory 价键理论》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。
1、ValenceBondTheoryAnothertopicthatyou’llneedtobefamiliarwithfortheSATIIChemistrytestisthatofvalencebondtheory.Bynow,youareawarethattwoatomswillformabondwhenthereisorbitaloverlapbetweenthem,andamaximumoftwoelectronscanbepresentintheoverlappingorbitals. Sincethepair
2、ofelectronsisattractedtobothatomicnuclei,abondisformed,andastheextentofoverlapincreases,thestrengthofthebondincreases.Theelectronicenergydropsastheatomsapproacheachother,butitbeginstoincreaseagainwhentheybecometooclose.Thismeansthereisanoptimumdistance,theobserved
3、bonddistance,atwhichthetotalenergyisataminimum. Let’sdelvealittlemoredeeplyintosigmabondsnowanddescribetheminmoredetail.Asyouknow,sigma(s)bondsaresinglebonds.Theyresultfromtheoverlapoftwosorbitals,ansandaporbital,ortwohead-to-headporbitals.Theelectrondensityofasi
4、gmabondisgreatestalongtheaxisofthebond.Maximumoverlapformsthestrongest-possiblesigmabond,andthetwoatomswillarrangethemselvestogivethegreatest-possibleorbitaloverlap.Thisistrickywithporbitalssincetheyaredirectionalalongthex,y,andzaxes. Hybridorbitalsresultfromable
5、ndingofatomicorbitals(inotherwords,sandporbitals)tocreateorbitalsthathaveenergythat’sinbetweentheenergyoftheloneorbitals.Lookatthemethanemolecule,forexample:allfouroftheC—Hbondsare109.5oapart,whilenonbondedporbitalsareonly90oapart. Theorbitalsshownattheleftofthef
6、igureareforanonbondedcarbonatom,butoncethecarbonatombeginstobondwithotheratoms(inthiscasehydrogen),theatomicorbitalshybridize,andthischangestheirshapeconsiderably.Noticehowthefirstsetoffiguresformthesp3atomicorbital,thehybrid,andthisleadstofurtherhybridization. A
7、mmoniaalsohassp3hybridization,eventhoughithasalonepair. MultipleBonding Nowlet’slookmorecloselyatpibonds.Aswementionedearlierinthischapter,pi(p)bondsresultfromthesidewaysoverlapofporbitals,andpiorbitalsaredefinedbytheregionaboveandbelowanimaginarylineconnectingt
8、henucleiofthetwoatoms.Keepinmindthatpibondsneveroccurunlessasigmabondhasformedfirst,andtheymayformonlyifunhybridizedporbitalsremainonthebondedatoms.Also
