Molecular Structure

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1、MolecularStructureI.ValenceBondTheoryA.General1.Wewillconsiderthecovalentcompoundsformedbytheinteractionsofnonmetals.2.Theyinteractby“sharingelectrons”betweenthem.3.Twotheories,thevalencebondtheoryandthemolecularorbitaltheory,havebeendevelopedtoanalyzetheseinteractionsandrationalizesomeimportant

2、propertiesofcovalentcompounds.B.Orbitaloverlapandthecovalentbond.TheH2molecule1.ConsidertwohydrogenatomsforminganH2molecule.OriginallyeachHhasanelectroninasphericallysymmetric1sorbital.ConsiderwhathappensasthetwoHatomsapproachoneanother.Overlapregion11HA1sAHB1sBH2LewisdiagramH••HH:HorH–H2.Whenth

3、etwoatomsgetcloseenough,theirelectroncloudscaninterpenetrateoneanother,thatis,theorbitalscanoverlapsothattherearecommonregionswhereboth1swavefunctionsaresizable.a.Whenanelectronentersthisregionofoverlap,itwillloseitsidentifyandcouldjustaswellmoveaboutnucleusAorB.Inthiswaythetwoelectronsareshared

4、bybothnuclei.b.Thissharingresultsinaloweringoftheenergiesoftheelectrons,because1)thepotentialenergiesdecreasebecausetheelectronscanbeclosetotwonuclearcenters.2)thekineticenergiesoftheelectronsdecreasebecausetheycanoccupylargerregionsinspace.c.Theenergyloweringgivesrisetoadirectionalforceofattrac

5、tionbetweenthetwoatoms,calledacovalentbondoranelectronpairbond.3.Wecanthinkofacovalentbondbeingformedbytheoverlapoftwohalf-filledorbitals,oneoneachatom.Itisalsopossibletoformacovalentbondbytheoverlapofavacantorbitalononeatomwithafilledorbitalonanother,suchacovalentbondiscalleda1coordinatecovalen

6、tbond.Ineithercase,thecovalentbondinvolvestoworbitalsanttwoelectrons.4.Sincethetwoelectronsinabondcanoccupythesameorbital,theirspinsmustbepaired.C.ConsidertwoFatomsforminganF2molecule.2251.TheelectronconfigurationofFis1s2s2p.Thevalenceelectronsaredistributedas↑↓↑↓↑↓↑2s2p2.Thetwohalf-filledporbit

7、alscanoverlaptoformaF–Fbond.FFFFFFFForF–F––––––3.IntheF2molecule,eachFatomwillhavefourelectronpairsassociatedwithit,onesharedpairandthreeotherpairthatarenotshared.4.Theunsharedpairsarecalledlonepairs.D.Summary.1.Consideronly