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