搜索
1.1AtomicstructureTheStructureoftheAtomMassSpectrometryElectronicStructureIonisationEnergiesTHESTRUCTUREOFTHEATOMa)Protons,neutronsandelectronsAtomsaremadeupofthreefundamentalparticles:protons,neutronsandelectrons.Protonsandneutronsarefoundinthenucleusandarecollectivelycallednucleons.Electronsorbitthenucleusinasimilarwaytothatinwhichplanetsorbitasun.Inbetweentheelectronsandnucleusthereisnothing(emptyspace).Thenucleusisverysmall;ifanatomwerethesizeofafootballpitch,thenucleuswouldbethesizeofadrawingpin.Thebasicpropertiesofthesethreeparticlescanbesummarizedinthefollowingtable:ParticleChargeMassProton+1unitApprox1unitNeutronNochargeApprox1unitElectron-1unitApprox1/1840units(verysmall)1unitofchargeis1.602x10-19coulombs.Aprotonisgivenachargeof+1andanelectronachargeof-1.Allchargesaremeasuredintheseunits.1unitofmassis1.661x10-27kg.Thisisalsonotaconvenientnumber,soweuse“atomicmassunits”.Sincethemassofprotonsandneutronsvariesslightlydependingonthenucleus,theninordertodefinean“atomicmassunit”weneedtochooseonenucleusasastandard.Forthispurpose126C,or“carbon-12”,waschosenbecauseitsmasspernucleon(1.661x10–27kg)isaroundaverage,whichmeansalltheothernucleihavemassesclosetowholenumbers.Anatomicmassunitisthusdefinedas1/12thofthemassofoneatomofcarbon-12.Everythingelseismeasuredrelativetothisquantity.b)Atomicnumbers,massnumbersandisotopesAnatomisnamedafterthenumberofprotonsinitsnucleus.Ifthenucleusofanatomhas1proton,itishydrogen;ifithastwoprotons,itishelium;ifithas3,itislithiumetc.Thenumberofprotonsinthenucleusofanatomiscalledtheatomicnumber.IthasthesymbolZ.TheatomicnumberisthenumberofprotonsinthenucleusofanatomNotallatomsofthesameelementhaveequalnumbersofneutrons;thismayvaryslightly.Thesumofthenumberofprotonsandneutronsinthenucleusofanatomiscalleditsmassnumber.ItisrepresentedbythesymbolA.ThemassnumberisthesumofthenumberofprotonsandneutronsinthenucleusofanatomThenucleusofanatomcanthusbecompletelydescribedbyitsmassnumberanditsatomicnumber.Itisgenerallyrepresentedasfollows:AZEEg.94Be,126C,2412MgAtomswiththesameatomicnumberbutwithdifferentmassnumbers(iedifferentnumbersofneutrons)arecalledisotopes.IsotopesareatomswiththesameatomicnumberbutwithdifferentmassnumbersEgmagnesium(atomicnumber12)has3naturallyoccurringisotopes:2412Mg:12protons,12neutrons2512Mg:12protons,13neutrons2612Mg:12protons,14neutronsInaneutralatom,thenumberofprotonsandelectronsarethesame.However,manyelementsdonotexistasneutralatoms,butexistasions.Ionsarespeciesinwhichtheprotonandelectronnumbersarenotthesame,andhencehaveanoverallpositiveornegativecharge.Thenumberofelectronsinaspeciescanbededucedfromitscharge:Eg2412Mg2+:12p,12n,10e2412Mg+:12p,12n,11e2412Mg12p,12n,12e2412Mg-:12p,12n,13ec)RelativeatomicmassThemassofanatomismeasuredinatomicmassunits,whereoneunitis12thofthemassofoneatomofcarbon-12.Therelativeisotopicmassofanisotopeistheratioofthemassofoneatomofthatisotopeto1/12thofthemassofoneatomofcarbon-12.Itisusuallyveryclosetoawholenumberratio:IsotopeMassnumberRelativeisotopicmass11H11.00642He44.00394Be99.0122713Al2726.9195927Co5958.933Themassesofprotonsandneutronsvaryslightlyfromisotopetoisotope,sotherelativeisotopicmassisnotexactlyawholenumber.Therelativeatomicmassofanatomistheratiooftheaveragemassofoneatomofthatelementto1/12thofthemassofoneatomofcarbon-12.TheRAMistheaveragemassofalltheisotopes,andisoftennotclosetoawholenumber:ElementCommonmassnumbersRelativeatomicmassMg24,25,2624.32Cl35,3735.45Br79,8179.91Ba134,135,136,137,138137.33Someelementsandcompoundsexistasmolecules;thesealsohaveacharacteristicmass:Therelativemolecularmassofamoleculeistheratiooftheaveragemassofthatmoleculeto1/12thofthemassofanatomofcarbon-12.Therelativemolecularmassofamoleculeisthesumoftherelativeatomicmassesofitsconstituentatoms.EgTherelativemolecularmassofCO2is12.0+16.0+16.0=44.0MASSSPECTROMETRYThemassspectrometerisaninstrumentusedformeasuringthemassesofatomsandmolecules.Itcanalsobeusedtomeasuretherelativeabundanceofdifferentisotopesandtopredictthestructureofmorecomplexmolecules.1.HowthemassspectrometerworksTheworkingsofthemassspectrometercanbesummarizedinfivestages:1-Gaseousmaterialreleasedintoionizationchamber2-Particlesbombardedwithelectronsandionized,mostlyto+1ions(IONISATION)Ametalwireisheateduntilitstartsemittinghighenergyelectrons.Theseelectronshittheparticles,knockingmoreelectronsoff.Mostoftheparticlesareionizedto+1ions3-Ionsacceleratedtouniformspeedbyelectricfield(ACCELERATION)Thepositiveionsareattractedtothenegativeplateandacceleratetowardsit4-Ionsdeflectedbymagneticfield;deflectiondependsonm/eratio(DEFLECTION)Theheaviertheparticle,thelessthedeflection5-Electriccurrentmeasuredasionslandonplate(DETECTION)Thegreatertheabundanceoftheisotope,thelargerthecurrentThedegreeofdeflectiondependsonthemassandthecharge;thegreaterthemass,thelessthedeflection,andthegreaterthecharge,thegreaterthedeflection.Itcanbeshownthatthedeflectionisinverselyproportionaltothem/eratio.Inmostcases,however,thechargeis+1,sothedeflectiondependsessentiallyontherelativemassofthespeciesinthemassspectrometer.Ifthespectr