A-level化学 原子结构

RevisingAtomsLearningObjectivesCandidatesshouldbeableto:Identifyanddescribeprotons,neutronsandelectronsintermsoftheirrelativechargesandrelativemasses.Deducethebehaviourofbeamsofprotons,neutronsandelectronsinelectricfields.Describethedistributionofmassandchargeswithinanatom.Deducethenumberofprotons,neutronsandelectronspresentinbothatomsandionsgivenprotonandnucleonnumbers(andcharge).Distinguishbetweenisotopesonthebasisofdifferentnumbersofneutronspresent.StarteractivityDemocritus:AncientGreekPhilosopher-Scientist,HistoryoftheAtom‘atomos’–cannotbecut.Theproblem:hewasunabletoprovidetheevidenceneededtoconvincepeoplethatatomsreallyexisted.In1808,anEnglishschoolteachernamedJohnDaltonproposedthatatomscouldnotbedividedandthatallatomsofagivenelementwereexactlyalike.Dalton’stheoryisconsideredthefoundationforthemodernatomictheory.Dalton’stheorywasdevelopedwithscientificbasisandwasacceptedbyothers.HistoryoftheAtomAttheendofthenineteenthcentury,ascientistcalledJ.J.Thomsondiscoveredtheelectron.Thomsonsuggestedthattheycouldonlyhavecomefrominsideatoms.SoDalton'sideaoftheindestructibleatomhadtoberevised.ThomsonimaginedtheelectronsasthebitsofpluminaplumpuddingThisimpliesadense,positivelychargedcentralregioncontainingmostoftheatomicmassandthattheatomismostlyspace.In1872-1937,Rutherfordetal.ranexperimentstodeterminethestructureofanatom.Whenpositivelychargedparticlesarefiredintogoldfoil,mostpassstraightthroughwhileafewareviolentlydeflected.Rutherfordexpectedthealphaparticlestogostraightthroughthegoldfoil.Instead,someofthealphaparticlesweredeflected,implyingacentralpositivelychargedregion(nucleus).In1913,theDanishscientistNielsBohrsuggestedthatelectronsinanatommoveinsetpaths(energylevels)aroundthenucleusmuchliketheplanetsorbitthesun.Electronscanonlybeincertainenergylevelsandmustgainenergytomovetoahigherenergylevelorloseenergytomovetoalowerenergylevel.Inthe1920’sdeBroglie&Shrodingershowedthatthe“solarsystem”modeloftheatomwasincorrect.Instead,electronsorbitthenucleusinorbitals.Thisiscalledquantummechanics.Wewilllookatthisinournextlesson.•Forsometimepeoplethoughtatomswerethesmallestparticlesandthattheycouldnotbebrokenintoanythingsmaller.•Wenowknowthatatomsarethemselvesmadefromevensmallerandsimplerparticles.•Theseparticlesare•Protons•Neutrons•ElectronsJ.J.Thompson–discoveredpresenceofelectronsandproposed‘PlumPudding’modeloftheatom.Rutherford’s‘Goldfoil’experimentconcludedthatanatom'smassmustbeconcentratedinasmallpositivelychargednucleusandthatmostoftheatommustbeemptyspace.Thisspacemustcontaintheelectrons.Therearetwopropertiesofsub-atomicparticlesthatareespeciallyimportant:◦Mass◦ElectricalchargeParticleChargeRelativeMassProtons+11Neutrons01Electrons-1negligibleElementatomscontainequalnumbersofprotonsandelectronsandsohavenooverallchargeABC–+PropertiesofSub-atomicParticlesprotonelectronneutronProtons,neutronsandelectronsareNOTevenlydistributedinatoms.Theprotonsandneutronsexistinadensecorecalledthenucleus.Aroundtheoutsideareverythinlyspreadelectrons.Theseelectronsexistinlayerscalledshells.TheNucleusadensecoreofprotonsandneutronscontainingnearlyallthemassoftheatom‘Shells’ofelectronselectronsarereallyveryverytinysotheatomismostlyemptyspace.Theatomofanyparticularelementalwayscontainsthesamenumberofprotons.E.g.◦Hydrogenatomsalwayscontain1proton◦Carbonatomsalwayscontain6protons◦Magnesiumatomsalwayscontain12protonsThenumberofprotonsinanatomisknownasitsatomicorprotonnumber.Itisthesmalleroftwonumbersshowninmostperiodictables12C6Notethatanyelementhasadefiniteandfixednumberofprotons.Ifwechangethenumberofprotonsinanatomthenthischangesthatatomintoadifferentelement.Changesinthenumberofparticlesinthenucleus(protonsorneutrons)isveryrare.Itonlytakesplaceinnuclearprocessessuchasradioactivedecay,nuclearbombsornuclearreactors.Themassofeachatomresultsalmostentirelyfromthenumberofprotonsandneutronsthatarepresent.(Rememberthatelectronshavearelativelytinymass).Thesumofthenumberofprotonsandneutronsinanatomisthemassnumber.AtomProtonsNeutronsMassNumberHydrogen101Lithium347Aluminium131427Electronsarenotevenlyspread.Theexistinenergylevelsknownasshells.Thearrangementofelectronsintheseshellsisoftencalledtheelectronconfiguration.2ndShell1stShell3rdShell4thShellEachshellhasamaximumnumberofelectronsthatitcanhold.1stShell:2electrons2ndShell:8electrons3rdShell:Initially8electronsThemaximumOppositesattract.Protonsare+andelectronsare–charged.Electronswilloccupytheshellsnearestthenucleusunlesstheseshellsarealreadyfull.1stShell:Fillsthisfirst2ndShell:Fillthisnext3rdShell:AndsoonHowmanyelectronsdotheelementatomshave?(Thiswillequaltheatomicnumber).Keepingtrackofthetotalused,feedthemintotheshellsworkingoutwardsuntilyouhaveusedthemallup.1stShell:Fillsthisfirst2ndShell:FillthisnextDrawingneatdiagramshelpsyoukeeptrack!Itisnotstrictlytruetosaythatelementsconsistofonetypeofatom.Whilstatomsofagivenelementalwayshavethesamenumberofprotons,theymayhavedifferentnumbersofneutrons.Atomsthatdifferinthiswayarecalledisotopes.Remember:Thenumberofprotonsde