Chemistry Ch 3 Atomic StructureCH 3原子结构化学.docx

Chemistry Ch 3 Atomic StructureCH 3原子结构化学.docx

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ChemistryCh3AtomicStructureCH3原子结构化学

ChemistryCh3:

AtomicStructure

Objectives

3-1

∙Definethetermatom

∙ListthepostulatesofDalton’satomictheory

3-2

∙Discusshowatomicstructureisrelatedtoelectricity

∙Explainwhatstudiesofcathoderaysandradioactivityrevealedabouttheatom

∙DiscussRutherford’salpha-scatteringexperimentandhowitshowedtheexistenceofthenucleus

3-3

∙Nameanddescribethethreesubatomicparticlesinanatom

∙Determinethenumberofprotons,neutrons,andelectronsinanatomorion

∙Defineisotopeandatomicmass

3-4

∙Describethechangesthataccompanynuclearreactions

∙Defineradioactivity

3-1Earlymodelsoftheatom

Watchvideo

∙Whatisthesmallestpossibleparticleofasubstance?

∙Canyoukeepdividingapieceofaluminumfoilinhalfindefinitely?

∙In450BC,theGreekphilosopherDemocritusproposedthatallmatteriscomposedoftiny,indivisibleparticlescalledatomos.Aristotleandotherphilosophersdisagreed,askingwhatheldtheparticlestogether.Democrituscouldn’tanswerthissohistheorywasrejected.

∙Todayscientistsdefineanatomasthesmallestparticleofanelementthatretainsthechemicalidentityofthatelement

∙Acoupleofimportantprincipleswerediscoveredinthelate1700’s:

1.Lavoisier’sConservationofMatter（whichstateswhat?

）

2.Proust’sLawofConstantComposition-agivencompoundalwayscontainsthesameelementsinthesameproportionsbymass.I.E.Waterisalways88.9%Oand11.1%H.

∙In1803anEnglishmath&physicsschoolteacher,JohnDalton,wrotedowntheatomictheoryofmatterbasedonthefollowingpostulates:

1.Eachelementiscomposedofextremelysmallparticlescalledatoms

2.Allatomsofagivenelementareidentical,buttheydifferfromthoseofanyotherelement.

3.Atomsareneithercreatednordestroyedinanychemicalreaction.

4.Agivencompoundalwayshasthesamerelativenumbersandkindsofatoms.

∙Thereareabout100differentelements-whichmeansabout100differentkindsofatoms

∙Theseatomscombinetoformeverything,allmatter.Kindoflikethelettersinthealphabetformwords.

3-2DiscoveringAtomicStructure

StaticElectricity

∙Daltonthoughtatomsmustbehardandround

∙MichaelFaraday,anEnglishchemist,（1791-1867）suggestedstructureofatomsrelatedtoelectricity.Atomsdocontainparticlesthathaveelectricalcharge.ThewordelectricitycomesfromtheGreekword

elektronforamber,ifamberwererubbedw/aclothitwouldattractdustorotherparticles

∙BenjaminFranklin（1706-1790）didearlyexperimentsw/electricity.（KITE）

oHefoundtwokindsofchargesandcalledthempositiveandnegative.

oParticleswithlikechargesrepelandoppositechargesattract

CathodeRays&Electrons

∙Amovingstreamofelectricalchargesiscalledanelectricalcurrent

∙Inmid1800’sscientistsbegantoinvestigatemovementofelectriccurrentthroughevacuatedglasstubes（verylittleairinthem）seefigure3.9p.97

∙CathodeRayTube-Negativeelectrode（cathode）andpositiveelectrode（anode）connectedtoabattery.Lineglassw/fluorescentmaterial（glowsinpresenceofelectricity）.Energyflowedfromthecathodetotheanode,calleditacathoderay.Byendof1800’smanynewdiscoveriesaboutcathoderays:

oTheycouldspinapaddlewheelantheirway（suggestedithadparticles）

oDeflectedbyamagnetinadirectionconsistentw/annegativeelectricalcharge（suggestedithadaneg.charge）

∙Decidedtotryandfindthemassof1particle

∙EnglishphysicistJ.J.Thomson（1856-1940）startedaseriesofexperiments.seetransparency

oAnodehadholesothatcathoderaywentthrough++-chargedplatesandthroughmagneticfield

oHefoundhecouldmathematicallypredicttheamountofdeflectioncreated.Heconcludedthatacathoderayiscomposedofnegativeparticlesthatcomefromthecathode.Thismeantthatatomswerenotsolid,indivisibleballsbuthadasubstructure.

oHenamedthesenegativeparticleselectrons.Hecouldnotcomputethemassbutdidfiguretheratiooftheelectricchargetoitsmass.1.76x10（8）coulombs/gram

∙1909AmericanphysicistRobertMilliken（1868-1953）measuredchargeofelectron.Usedoildropletstofindcharge.Foundchargeofeveryoildropletwasamultipleof1.6x10（-19）coulomb-sothechargeof1electron（e-）mustbethat.Usingthechargetomassratiohefoundthemassof1e-=9.11x10（-28）,very,verylight!

∙1896-HenriBequerelaccidentallydiscoveredradioactivitywhenheplacedasampleofuraniumonphotographicfilm.Radioactivityisthespontaneousemissionofradiationfromanelement.

∙Early1900’sNewZealandScientistErnestRutherfordshowedtwokindsofradiation,alpha&beta.Gammawasdiscoveredlater.

∙JJThomsonconcludedthatsinceatomshaveelectronswithanegativechargebuthaveaneutraloverallchargetheymustcontainparticleswithapositivecharge.Plumpuddingmodel.

∙Rutherford’sgoldfoilexperimentshowedthatabeamwasscatteredwentitwentthroughthingoldfoilthusprovingthattheparticlesweredeflectingoffthenucleus,asmallconcentratedcoreatthecenter.Thiscontradictedtheplumpuddingmodel.Finallyhecameupwiththenuclearmodeloftheatom（seetransparency）

∙Readlastparagraphpage102

3-3ModernAtomicTheory

∙Partsofanatom

oNucleus-protons（positive）andneutrons（neutral）,mostofmass.Neutronshaveslightlymoremassthanprotons.

oElectrons-（negativecharge）moveinspacearoundthenucleus,smallmasstakesabout2000to=1proton.Negativechargeattractedtopositivechargeofnucleus（protons）

∙Rutherford’s（actuallyBohr’s）atom-sawanatomasaminiaturesolarsystem.Notcorrecttheydonotorbitinawelldefinedpath,donotknowexactlywheretheyare.Insteadelectronsaremoreaccuratelydepictedasindistinctcloudsaroundthenucleus.Seetransparency

∙Forconveniencesake（althoughnotcompletelyaccurate,seechartpage104）:

oChargeofproton=+1,actually+1.062x10（-19）.Mass=1amu（atomicmassunit）,actually1.673x10（-24）g.

oChargeofneutron=0.Mass=1amu,actually1.675x10（-24）

oChargeofelectron=-1,actually-1.062x10（-19）.Mass=0,actually9.109x10（-28）g.

∙Atomicnumber-discoveredbyHenryMoseley（1887-1915）astudentofRutherford,itisthe#ofprotonsinanatom.Inperiodictabletheatomicnumberiswrittenabovetheelement,sotheidentityoftheatomcomesfromthe#ofprotonsinthenucleus.Sinceanatomundernormalconditionsisneutral,its#ofelectrons=#ofprotons.Sotheatomic#alsoreflectsthe#ofelectrons.

（Talkthroughsampleproblem1,practiceprob.1,2,alt.pract.1,2）

∙Ions-whenanatomgainsorlosesandelectronitthenhaseitherapositiveornegativenetelectricalcharge.Thechargeoftheionisthedifferencebetweenthe#ofprotonsandthe#ofelectrons.Ioniccharge=#protons-#electrons.Example:

1.Nalosesoneelectron（e-）,so11-10=+1

2.Clgainsonee-,so17-18=-1

3.Caloses2,so20-18=+2

∙Ionsarewritten-chemicalsymbolwiththechargeinsuperscript.I.E.the+/-canbebeforeorafterthenumber.Ifthechargeis+/-1the1isomitted.I.E.

∙（Dosampleprob.2,Practice3&4,Altsample2,Altsample3&4）

∙Isotopes-everyelementhasaset#ofprotons,howevertheymayhavedifferent#’sofneutrons.I.E.AllClhave17protonsbutsome18neutrons,othershave20.Atomsw/same#protonsbutdifferent#ofneutronsarecalledisotopes.

∙Generallyoneformisthemostcommon.

∙I.E.themostcommonformofHhas0neutrons.

∙Hw/oneneutroncalleddeuterium（rare）

∙Hw/twoneutronscalledtritium（veryrare）

∙Innatureelementsarefoundasamixtureofisotopes.Thesemixturesusuallystayinthesameratios-nomatterwhereyoufindthem.

∙Howdifferentareisotopes?

Theyhavedifferentmassesbecausewheredoesthemassofanatomcomefrom?

Protons+neutrons.Ifdifferent#ofneutronsthendifferentmasses.Cl-Mass#37atomic#17.Sincemass=proton+neutronthen37-17=20neutrons.

∙Talkthroughsampleproblem3

∙Dopractice5&6.Alt.sampleprob3&alt.practice5&6

∙Massofanatom

∙Sincethemassofanatomissoincrediblysmallitisimpracticaltousegrams

∙ChemistsuseanotherunitcalledAtomicMassUnit（amu）

∙1amu=1/12（massofaC-12atom）=1.66x10（-24）g

∙atomicmass（averageatomicmassoratomicweight）=theaveragemassofanelementsatoms

∙Homework3-3Practice,3-3Review

3-4Changesinthenucleus

∙ChemicalReactions-atomsinteractthroughouterelectrons

∙Changesinthenucleusarecallednuclearreactions.Thesechangethecompositionofanatom’snucleus

NuclearStability

∙Almostallatomsinnaturearestable-notradioactive.Averyfewarenaturallyradioactive.

∙Whyaresomestableandsomenot?

Ithastodowiththe#ofprotonsandneutrons-notallcombinationsarestable

∙Nucleusisprotonsandneutronspackedverycloselytogether.Butsinceprotonshavea+chargetheyrepeleachother.

∙Whydon’ttheyflyapart?

∙StrongNuclearForce-aforcethatovercomestheelectricrepulsionbetweenprotons.Itisaforcethatisonlysignificantbetweensubatomicparticles.

∙Neutronshavenocharge,soexperiencenoelectricalrepulsion.HowevertheydohaveStrongNuclearForce,whichhelpsholdthemtogetherwiththeprotons.Thepresenceoftheneutronsaddsanetattractiveforcetotheinsideofthenucleus.Thinkofneutronsasthegluethatholdstogetherthenucleus.

oNeutrons-don’trepeltheyattract

oProtons-bothrepel/attractotherprotonsandattractneutrons.Why?

∙Allstablenucleibetweenatomicnumbers1-20haveaboutequal#’sofprotonsandneutrons

∙Beyond20needincreasinglymoreneutronstoholdnucleustogether

∙Whenatomic#excee