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1、4.2CONDUCTIONELECTRONSWhatareconductionelectrons?Toanswer,consideranNagasconsistingofatoms,eachhaving11electronsorbitingthenucleus:•10coreelectronscomprisingstructureofthefilledfirstandsecondshells;•avalenceelectronlooselyboundtotherestofthesystem.Valenceelectron,occupying3rdat
2、omicshellisresponsibleformostoftheordinarychemicalpropertiesofNa.Theradiusofthe3rdshellis0.19nm.Inchemicalreactions:•Naatomlosesitsvalenceelectron=>anNa+ionisformed;•Forexample,inNaCltheelectronistransferredfromtheNatotheClatom.Inthemetallicstate,Nahasbccstructure=>distancebe
3、tweennearestneigh-boorsis0.37nm.Inthesolidstatetwoatomsslightlyoverlap(seeFig.4.1):•valenceelectronisnolongerattachedtooneparticularatom=>itbelongstobothneighboringatomsatthesametime;•Infact,thevalenceelectronbelongstothewholecrystal,becauseitcanmovereadilyfromoneiontothenext
4、.•Thevalenceelectroninafreeatom,becomesaconductionelectroninasolid.Eachatomcontributesitsownconductionelectron,andeachofthesebelongtothewholecrystal.Theyarecalledconductionelectronsbecausetheycancarryanelectriccurrentundertheactionofanelctricfield.Insummary:•Whenfreeatomsforma
5、metal,allthevalenceelectronsbecomeconduc-tionelectronsandtheirstatesareprofoundlymodified;conductionelec-tronsareresponsibleformostofthepropertiesofmetals.•Thecoreelectronsremainlocalized,andtheirpropertiesremainessentiallyun-changed.Theno.ofconductionelectronsfollowsfromtheno
6、.ofvalenceelectronsandthedensityofthemetal.Thus:•inNa(andothernoblemetals)theno.ofconductionelectronsisthesameastheno.ofatoms;•indivalentmetals(Be,Mg,Zn,andCd)theno.ofconductionelectronsistwicetheno.ofatoms.Ifthedensityofthesubstanceisρm,thentheatomconcentrationis:!(ρm/M)NAwh
7、ereM!=>atomicweight;NA=>Avogadro’snumber.IftheatomicvalenceisZv,thentheelectronconcentrationis:ρmNAN=Zv.M!4.3THEFREEELECTRONGASInthefreeelectronmodel,theconductionelectronsareassumedtobecompletelyfree,exceptforthepotentialatthesurface(seeFig.4.2).Inthismodel:•electronsmoveabo
8、utinsidethespecimenwithoutanycollisions:theyformafreeelectrongas.But
