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1、16CHAPTER1TheCrystalStructureofSolids(a)(b)Figure1.19
2、Representationof(a)hydrogenvalenceelectronsFigure1.20
3、Two-dimensionalrepresentationofand(b)covalentbondingina(a)siliconvalenceelectronsand(b)covalentbondinghydrogenmolecule.Inthesiliconcrystal.Tetrahedralconfiguration,aswasshowninFigure1
4、.15.,thetwo-dimensionalrepresentationwillbeusefulinmuchofthediscussionofsemiconductorbehavior.Asignificantdifferencebetweenthecovalentbondingofhydrogenandofsiliconisthat,whenthehydrogenmoleculeisformed,ithasnoadditionalelectronstoformadditionalcovalentbonds,whiletheoutersiliconatomsalwaysha
5、vevalenceelectronsavailableforadditionalcovalentbonding.Thesiliconarraymaythenbeformedintoaninfinitecrystal,witheachsiliconatomhavingfournearestneighborsandeightsharedelectrons.Thefournearestneighborsinsiliconformingthecovalentbondcorrespondtothetetrahedralstructureandthediamondlattice,whic
6、hwereshowninFigures1.15and1.14,respectively.Atomicbondingcrystalstructureareobviouslydirectlyrelated.Thethirdmajoratomicbondingschemeisreferredtoasmetallicbonding.GroupIelementshaveonevalenceelectron.Iftwosodiumatoms(Z=11),forexample,arebroughtintocloseproximity,thevalenceelectronsinteracti
7、nawaysimilartothatincovalentbonding.Whenathirdsodiumatomisbroughtintocloseproximitywiththefirsttwo,thevalenceelectronscanalsointeractandcontinuetoformabond.Solidsodiumhasabody-centeredcubicstructure,soeachatomhaseightnearestneighborswitheachatomsharingmanyvalenceelectrons.Wecanthinkofthepos
8、itivemetallicionsasbeingsurroundedbyaseaofnegativeelectrons,thesolidbeingheldtogetherbytheelectrostaticforces.Thisdescriptiongivesqualitativepictureofthemetallicbond.MetallicbondingAfourthtypeofatomicbond,calledtheVanderWaalsbond,istheweakestofthechemicalbonds.Ahydrogenfluoride(HF)molecule,
9、forexample,isformedbyanionicbond.Theeffectivecenterofthepositivechargeofthemoleculeisnotthesameastheeffectivecenterofthenegativecharge.ThisnonsymmetryinthechargedistributionresultsinasmallelectricdipolethatcaninteractwiththedipolesofotherHFmolecules.With
