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1、Chapter1TheoreticalandExperimentalPrinciples1.1GeneralCharacteristicsofIsotopesIsotopesareatomswhosenucleicontainthesamenumberofprotonsbutadiffer-entnumberofneutrons.ThetermisotopesisderivedfromGreek(meaningequalplaces)andindicatesthatisotopesoccupythesamepositionintheperiodictable.Itisconvenientto
2、denoteisotopesintheformmnE,wherethesuperscriptmde-notesthemassnumber(i.e.,sumofthenumberofprotonsandneutronsinthenucleus)andthesubscriptndenotestheatomicnumberofanelement,E.Forex-ample,12Cistheisotopeofcarbonwhichhassixprotonsandsixneutronsinits6nucleus.Theatomicweightofeachnaturallyoccurringelemen
3、tistheaverageoftheweightscontributedbyitsvariousisotopes.Isotopescanbedividedintotwofundamentalkinds,stableandunstable(radioac-tive)species.Thenumberofstableisotopesisabout300;whilstover1,200unstableoneshavebeendiscoveredsofar.Thetermstableisrelative,dependingonthede-tectionlimitsofradioactivedecay
4、times.Intherangeofatomicnumbersfrom1(H)to83(Bi),stablenuclidesofallmassesexcept5and8areknown.Only21elementsarepureelements,inthesensethattheyhaveonlyonestableisotope.Allotherelementsaremixturesofatleasttwoisotopes.Therelativeabundanceofdifferentisotopesofanelementmayvarysubstantially.Incopper,forex
5、ample,63Cuaccountsfor69%and65Cufor31%ofallcoppernuclei.Forthelightelements,however,oneisotopeispredominant,theothersbeingpresentonlyintraceamounts.Thestabilityofnuclidesischaracterizedbyseveralimportantrules,twoofwhicharebrie?ydiscussedhere.The?rstistheso-calledsymmetryrule,whichstatesthatinastable
6、nuclidewithlowatomicnumber,thenumberofprotonsisapproximatelyequaltothenumberofneutrons,ortheneutron-to-protonratio,N/Z,isapproxi-matelyequaltounity.Instablenucleiwithmorethan20protonsorneutrons,theN/Zratioisalwaysgreaterthanunity,withamaximumvalueofabout1.5fortheheavieststablenuclei.Theelectrostati
7、cCoulombrepulsionofthepositivelychargedprotonsgrowsrapidlywithincreasingZ.Tomaintainthestabilityinthenuclei,moreJ.Hoefs,StableIsotopeGeochemistry,1?Springer-VerlagBerlinHeidelberg200921TheoreticalandExperim
