Atomic Model Project 1st six weeks PROJECT 5 – ATOMIC MODELS BACKGROUND: We have learned about atomic structure, what atoms are made of and how they look Now it is time to build a model of an atom ASSIGNMENT You will be assigned an element You will build a 3-D representation of one atom of that element, no “flat” models will be allowed
atom This is an individual project - each student will build their own atom model outside of class using scrap material available at home Materials must be non-perishable (if it is edible, DO NOT use it) and APPROPRIATE for classroom display Project Deadline: Monday, October 27, 2014 Scoring Rubric: Your project will be scored as follows:
HISTORY OF THE ATOM 1856 -1940 Joseph John Thompson In 1904, Thompson develops the idea that an atom was made up of electrons scattered unevenly within an elastic sphere surrounded by a soup of positive charge to balance the electron's charge It is called the plums pudding model He was awarded a Nobel Prize in 1906 for discovering the Electron
atom: the smallest particle of an element that has all the properties of that element The atom has nocharge The 3 main subatomic particles that make up the atom are the proton, neutron and electron nucleus: small, dense positively charged center of an atom protons and neutrons are found in the nucleus
As technology continues to improve, it is the hope of this project group that stu-dents can now benefit from such programs as well Research was done to determine con-tent areas high school chemistry students typically struggle with that fall under the State of Massachusetts chemistry educational standards A teaching module consisting of a se-
A project the children enjoy is making marshmallow molecules This activity will help your students understand the difference between an atom and a molecule Tell the children that each marshmallow represents an atom Toothpicks are used to join the atoms When atoms are joined they will form a molecule To begin, each child should receive
The atomic theory of matter is an excellent illustration of the process of science You must hand in a bibliography with your project teach high school
Directors of Harvard Project Physics F James Rutherford, Capuchino High School, Dalton's atomic theory and the laws of chemical combination 11
This project is modified from Susan Schmidt's “A Webquest for High School Chemistry ” history, scientists have accepted five major different atomic models
grade band High School Chemistry Project: Which model of the atom is best supported by evidence? Estimated Time ~225 minutes Grade Level Standard(s )
but occasionally they have a division for junior high school entrants High School, Staten Island, New York, If you are interested in a project related to atomic
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TheProjectPhysicsCourseTextandHandbook
ModelsoftheAtom
/•.••
TheProjectPhysicsCourse
TextandHandbook
UNIT5
ModelsoftheAtom
AComponentofthe
ProjectPhysicsCourse
Publishedby
HOLT,RINEHARTandWINSTON,Inc.NewYork,Toronto
DirectorsofHarvardProjectPhysics
GeraldHolton,DepartmentofPhysics,Harvard
University
F.JamesRutherford,CapuchinoHighSchool,
SanBruno,Cahfornia,andHarvardUniversity
FletcherG.Watson,HarvardGraduateSchool
ofEducation
Acknowledgments,TextSection
Theauthorsandpublisherhavemadeeveryeffort
totracetheownershipofallselectionsfoundinthis bookandtomakefullacknowledgmentfortheiruse.
Manyoftheselectionsareinthepublicdomain.
Gratefulacknowledgmentisherebymadetothe
followingauthors,pubUshers,agents,andindivid- ualsforuseoftheircopyrightedmaterial.
SpecialConsultanttoProjectPhysics
AndrewAhlgren,HarvardGraduateSchoolof
Education
ApartialUstofstaffandconsultantstoHarvard
ProjectPhysicsappearsonpageiv.
ThisText-Handbook,Unit5isoneofthemanyin-
structionalmaterialsdevelopedfortheProject
PhysicsCourse.ThesematerialsincludeTexts,
Handbooks,TeacherResourceBooks,Readers,
ProgrammedInstructionbooklets,FilmLoops,
Transparencies,16mmfilms,andlaboratory
equipment.
Copyright©1970ProjectPhysics
AllRightsReservedSBN03-084501-7
123403998765432
ProjectPhysicsisaregisteredtrademark
P.3ExcerptsfromTheWayThingsAre:TheDe
RerumNaturaofTitusLucretiusCaius,atransla-
tionbyRolfeHumphries,copyright©1969by
IndianaUniversityPress.
P.5From'TheFirstChapterofAristotle's
'FoundationsofScientij&cThought'(Metaphysica,
LiberA),"translatedbyDanielE.Gershensonand
DanielA.Greenburg,inTheNaturalPhilosopher,
Vol.II,copyright©1963bytheBlaisdellPub-
lishingCompany,pp.14 - 15.
P.7FromTheLifeoftheHonorableHenry
Cavendish,byGeorgeWilson,printedforthe
CavendishSociety,1851,pp.186-187.
Pp.7-8From"ElementsofChemistry"byAntoine
LaurentLavoisier,translatedbyRobertKerrinGreat
BooksoftheWesternWorld,Vol.45,copyright1952
byEncyclopaediaBritannica,Inc.,pp.3-4.
P.11From"TheAtomicMolecularTheory"by
LeonardK.NashinHarvardCaseHistoriesin
ExperimentalScience,Case4,Vol.1,copyright1950
byHarvardUniversity,p.228.
P.21FromThePrinciplesofChemistrybyDmitri
Mendeleev,translatedbyGeorgeKamensky,copy-
right1905byLongmans,GreenandCompany,
London,p.27.
P.22Mendeleev,Dmitri,1872.
P.29From"ExperimentalResearchesinElec-
tricity"byMichaelFaradayfromGreatBooksofthe
WesternWorld,Vol.45,copyright1952by
EncyclopaediaBritannica,Inc.,pp.389-390.
Pp.43-44Einstein,Albert,trans,byProfessor
IrvingKaplan,MassachusettsInstituteofTech-
nology.
P.48Roentgen,W.K.
P.57From"Opticks"byIsaacNewtonfromGreat
BooksoftheWesternWorld,Vol.34,copyright1952
byEncyclopaediaBritannica,Inc.,pp.525-531.
P.67FromBackgroundtoModeimScience,
Needham,JosephandPagel,Walter,eds.,copyright
1938byTheMacmillanCompany,pp.68-69.
P.88LetterfromRutherfordtoBohr,March1913.
P.91From"Opticks"byIsaacNewtonfromGreat
BooksoftheWesternWorld,Vol.34,copyright1952
byEncyclopaediaBritannica,Inc.,p.541.
P.113FromAtom,icPhysicsbyMaxBom,copy-
right1952byBlackie&Son,Ltd.,p.95. p.114LetterfromAlbertEinsteintoMaxBom, 1926.
P.119FromAPhilosophicalEssayonPossibilities
byPierreSimonLaplace,translatedbyFrederickW.
TruscottandFrederickL.Emory,copyright1951
byDoverPublications,Inc.,p.4.
PictureCredits,TextSection
Coverphoto:CourtesyofProfessorErwinW.
Mueller,ThePennsylvaniaStateUniversity.
P.1(top)MerckSharp&DohmeResearch
Laboratories;(center)LoomisDean,LIFE
MAGAZINE,©TimeInc.
P.2(charioteer)HirmerFotoarchiv,Munich;
(architecturalruins)GreekNationalTourist
Office,N.Y.C.
P.4Electrumpendant(enlarged).Archaic.
Greek.Gold.Courtesy,MuseumofFineArts,
Boston.HenryLilliePierceResiduaryFund.
P.7FisherScientificCompany,Medford,Mass.
P.10fromDalton,John,ANewSystemof
ChemicalPhilosophy,R.BickerstafF,London,
1808-1827,asreproducedinAHistoryof
ChemistrybyCharles-AlbertReichen,c1963,
HawthornBooksInc.,70FifthAve.,N.Y.C.
P.13EngravedportraitbyWorthingtonfroma
paintingbyAllen.TheScienceMuseum,London.
P.15(drawing)Reprintedbypermissionfrom
CHEMICALSYSTEMSbyChemicalBondApproach
Project.Copyright1964byEarlhamCollegePress,
Inc.PublishedbyWebsterDivision,McGraw-Hill
BookCompany.
P.20MoscowTechnologicalInstitute.
P.26(portrait)TheRoyalSocietyofLondon.
P.27CourtesyofAluminumCompanyofAmerica.
P.32ScienceMuseum,London.LentbyJ.J.
Thomson,M.A.,TrinityCollege,Cambridge.
P.35CourtesyofSirGeorgeThomson.
P.39(top)CaliforniaInstituteofTechnology.
P.45(left,top)CourtesyofTheNewYorkTimes;
(left,middle)AmericanInstituteofPhysics; (middle,right)CourtesyofCaliforniaInstituteof
TechnologyArchives;(left,bottom)Courtesyof
EuropaVerlag,Zurich.
P.47(left,top)Dr.MaxF.Millikan;(right,top)
HarperLibrary,UniversityofChicago;(right
margin)R.Diihrkoopphoto.
P.48TheSmithsonianInstitution.
P.49BurndyLibrary,Norwalk,Conn.
P.51EastmanKodakCompany,Rochester,N.Y.
P.52HighVoltageEngineeringCorp.
P.53(rose)EastmanKodakCompany;(fish)
AmericanInstituteofRadiology;(reactorvessel)
NuclearDivision,CombustionEngineering,Inc.
P.58ScienceMuseum,London.LentbySir
LawrenceBragg,F.R.S.
P.64CourtesyofDr.OwenJ.Gingerich,
SmithsonianAstrophysicalObservatory.
P.67CourtesyofProfessorLawrenceBadash,
Dept.ofHistory,UniversityofCalifornia,
SantaBarbara.
P.76(top)AmericanInstituteofPhysics;
(bottom,right)CourtesyofNielsBohrLibrary,
AmericanInstituteofPhysics.
P.80(ceremony)CourtesyofProfessorEdward
M.Purcell,HarvardUniversity;(medal)Swedish
InformationService,N.Y.C.
P.93ScienceMuseum,London.LentbySir
LawrenceBragg,F.R.S.
P.94fromtheP.S.S.C.filmMatterWaves.
P.100AmericanInstituteofPhysics.
P.102ProfessorHarryMeiners,Rensselaer
PolytechnicInstitute.
P.106AmericanInstituteofPhysics.
P.107(deBroglie)AcademicdesSciences,Paris;
(Heisenberg)ProfessorWernerK.Heisenberg; (Schrodinger)Ameriq^nInstituteofPhysics.
P.109(top,left)Perkin-ElmerCorp.
P.112Orear,Jay,FundamentalPhysics,©1961
byJohnWiley&Sons,Inc.,NewYork.
P.115TheGraphicWorkofM.C.Escher,
HawthornBooksInc.,N.Y."LuchtenWater2."
PictureCredits,HandbookSection
Cover:DrawingbySaulSteinberg,from
TheSketchbookfor1967,HallmarkCards,Inc.
P.130Thesetablesappearonpp.122,157and
158ofTypesofGraphicRepresentationofthe
PeriodicSystemofChemicalElementsby
EdmundG.Mazurs,publishedin1957bythe
author.Theyalsoappearonp.8ofChemistry magazine,July1966.
P.136CourtesyL.J.Lippie,DowChemical
Company,Midland.Michigan.
P.149FromthecoverofTheScienceTeacher,
Vol.31,No.8,December1964,illustrationfor
thearticle,"ScientistsonStamps;Reflectionsof
Scientists'PublicImage,"byVictorShowalter,
TheScienceTeacher,December1964,pp.40 - 42.
Allphotographsusedwithfilmloopscourtesy
ofNationalFilmBoardofCanada.
Photographsoflaboratoryequipmentandof
studentsusinglaboratoryequipmentweresupplied withthecooperationoftheProjectPhysicsstaff andDamonCorporation.
PartialListofStaffandConsultants
Theindividualslistedbelow(andonthefollowingpages)haveeachcontributedinsomewaytothe developmentofthecoursematerials.Theirperiodsofparticipationrangedfrombriefconsultationsto full-timeinvolvementintheteamforseveralyears.Theaffiliationsindicatedarethosejustpriorto orduringtheperiodofparticipation.
AdvisoryCommittee
E.G.Begle,StanfordUniversity,Calif.
PaulF.Brandwein,Harcourt,Brace&World,
Inc.,SanFrancisco,Calif.
RobertBrode,UniversityofCalifornia,Berkeley
ErwinHiebert,UniversityofWisconsin,Madison
HarryKelly,NorthCarolinaStateCollege,Raleigh
WilliamC.Kelly,NationalResearchCouncil,
Washington,D.C.
PhilippeLeCorbeiller,NewSchoolforSocial
Research,NewYork,N.Y.
ThomasMiner,GardenCityHighSchool,New
York.
PhilipMorrison,MassachusettsInstituteof
Technology,Cambridge
ErnestNagel,ColumbiaUniversity,NewYork,
N.Y.
LeonardK.Nash,HarvardUniversity
I.I.Rabi,ColumbiaUniversity,NewYork.N.Y.
StaffandConsultants
L.K.Akers,OakRidgeAssociatedUniversities,
Tenn.
RogerA.Albrecht,OsageCommunitySchools,
Iowa
DavidAnderson,OberlinCollege,Ohio
GaryAnderson,HarvardUniversity
DonaldArmstrong,AmericanScienceFilm
Association,Washington,D.C.
ArnoldArons,UniversityofWashington
SamAscher,HenryFordHighSchool,Detroit,
Mich.
RalphAtherton,TalawandaHighSchool,Oxford,
Ohio
AlbertV.Baez,UNESCO,Paris
WilliamG.Banick,FultonHighSchool.Atlanta,
Ga.
ArthurBardige,NovaHighSchool,Fort
Lauderdale,Fla.
RollandB.Bartholomew,HenryM.GunnHigh
School,PaloAlto,Calif.
O.TheodorBenfey,EarlhamCollege,Richmond,
Ind.
RichardBerendzen,HarvardCollegeObservatory
AlfredM.Bork,ReedCollege,Portland,Ore.
F.DavidBoulanger,MercerIslandHighSchool,
Washington
AlfredBrenner,HarvardUniversity
RobertBridgham,HarvardUniversity
RichardBrinckerhoff,PhillipsExeterAcademy,
Exeter.N.H.
DonaldBrittain,NationalFilmBoardofCanada.
Montreal
JoanBromberg,HarvardUniversity
VinsonBronson,NewtonSouthHighSchool,
NewtonCentre,Mass.
StephenG.Brush,LawrenceRadiationLaboratory,
UniversityofCalifornia.Livermore
MichaelButler.CIASAFilmsMundiales.S.A..
Mexico
LeonCallihan,St.Mark'sSchoolofTexas.Dallas
DouglasCampbell,HarvardUniversity
J.ArthurCampbell,HarveyMuddCollege,
Claremont,California
DeanR.Casperson.HarvardUniversity
BobbyChambers.OakRidgeAssociated
Universities.Tenn.
RobertChesley.ThacherSchool,Ojai,Calif.
JohnChristensen.OakRidgeAssociated
Universities,Tenn.
DavidClarke.BrowneandNicholsSchool.
Cambridge.Mass.
RobertS.Cohen.BostonUniversity.Mass.
BrotherColumbanFrancis.F.S.C..MaterChristi
DiocesanHighSchool.LongIslandCity.N.Y.
ArthurCompton.PhillipsExeterAcademy,
Exeter.N.H.
DavidL.Cone,LosAltosHighSchool,CaUf.
WilliamCooley.UniversityofPittsburgh.Pa.
AnnCouch.HarvardUniversity
PaulCowan,Hardin-SimmonsUniversity.
Abilene,Tex.
CharlesDavis.FairfaxCountySchoolBoard.
Fairfax.Va.
MichaelDentamaro.SennHighSchool.Chicago,
111.
RaymondDittman.NewtonHighSchool.Mass.
ElsaDorfman.EducationalServicesInc..
Watertown.Mass.
VadimDrozin.BucknellUniversity.Lewisburg,
Pa.
NeilF.Dunn.BurlingtonHighSchool.Mass.
R.T.Ellickson.UniversityofOregon.Eugene
ThomasEmbry.NovaHighSchool.Fort
Lauderdale.Fla.
WalterEppenstein.RensselaerPolytechnic
Institute,Troy,N.Y.
HermanEpstein.BrandeisUniversity.Waltham.
Mass.
ThomasF.B.Ferguson.NationalFilmBoardof
Canada.Montreal
ThomasvonFoerster.HarvardUniversity
(continuedonp.122) Scienceisanadventureofthewholehumanracetolearntoliveinand perhapstolovetheuniverseinwhichtheyare.Tobeapartofitisto understand,tounderstandoneself,tobegintofeelthatthereisacapacity withinmanfarbeyondwhathefelthehad,ofaninfiniteextensionof humanpossibilities... Iproposethatsciencebetaughtatwhateverlevel,fromthelowesttothe highest,inthehumanisticway.Itshouldbetaughtwithacertainhistorical understanding,withacertainphilosophicalunderstanding,withasocial understandingandahumanunderstandinginthesenseofthebiography,the natureofthepeoplewhomadethisconstruction,thetriumphs,thetrials,the tribulations.
I.I.RABI
NobelLaureateinPhysics
Preface
BackgroundTheProjectPhysicsCourseisbasedontheideasand researchofanationalcurriculumdevelopmentprojectthatworkedin threephases.First,theauthors - ahighschoolphysicsteacher,a universityphysicist,andaprofessorofscienceeducation - collaborated tolayoutthemaingoalsandtopicsofanewintroductoryphysics course.Theyworkedtogetherfrom1962to1964withfinancialsupport fromtheCarnegieCorporationofNewYork,andthefirstversionof thetextwastriedoutintwoschoolswithencouragingresults. ThesepreliminaryresultsledtothesecondphaseoftheProject whenaseriesofmajorgrantswereobtainedfromtheU.S.Officeof EducationandtheNationalScienceFoundation,startingin1964. Invaluableadditionalfinancialsupportwasalsoprovidedbythe FordFoundation,theAlfredP.SloanFoundation,theCarnegie Corporation,andHarvardUniversity.Alargenumberofcollaborators werebroughttogetherfromallpartsofthenation,andthegroup workedtogetherforoverfouryearsunderthetitleHarvardProject Physics.AttheProject'scenter,locatedatHarvardUniversity, Cambridge,Massachusetts,thestaffandconsultantsincludedcollege andhighschoolphysicsteachers,astronomers,chemists,historians andphilosophersofscience,scienceeducators,psychologists, evaluationspecialists,engineers,filmmakers,artistsandgraphic designers.Theteachersservingasfieldconsultantsandthestudents inthetrialclasseswerealsoofvitalimportancetothesuccessof HarvardProjectPhysics.Aseachsuccessiveexperimentalversionof thecoursewasdeveloped,itwastriedoutinschoolsthroughoutthe UnitedStatesandCanada.Theteachersandstudentsinthoseschools reportedtheircriticismsandsuggestionstothestaffinCambridge, andthesereportsbecamethebasisforthesubsequentrevisionsof thecoursematerials.InthePrefacetoUnit1Textyouwillfindalistofthe majoraimsofthecourse. Wewishitwerepossibletolistindetailthecontributionsofeach personwhoparticipatedinsomepartofHarvardProjectPhysics. Unhappilyitisnotfeasible,sincemoststaffmembersworkedona varietyofmaterialsandhadmultipleresponsibilities.Furthermore, everytextchapter,experiment,pieceofapparatus,filmorotheritem intheexperimentalprogrambenefittedfromthecontributionsofa greatmanypeople.Ontheprecedingpagesisapartiallistof contributorstoHarvardProjectPhysics.Therewere,infact,many othercontributorstoonumeroustomention.Theseincludeschool administratorsinparticipatingschools,directorsandstaffmembers oftraininginstitutesforteachers,teacherswhotriedthecourseafter theevaluationyear,andmostofallthethousandsofstudentswho notonlyagreedtotaketheexperimentalversionofthecourse,but whowerealsowillingtoappraiseitcriticallyandcontributetheir opinionsandsuggestions. TheProjectPhysicsCourseToday.Usingthelastoftheexperimental versionsofthecoursedevelopedbyHarvardProjectPhysicsin
1964-68asastartingpoint,andtakingintoaccounttheevaluation
resultsfromthetryouts,thethreeoriginalcollaboratorssetoutto developtheversionsuitableforlarge-scalepublication.Wetake particularpleasureinacknowledgingtheassistanceofDr.Andrew AhlgrenofHarvardUniversity.Dr.Ahlgrenwasinvaluablebecause ofhisskillasaphysicsteacher,hiseditorialtalent,hisversatility andenergy,andaboveall,hiscommitmenttothegoalsofHarvard
ProjectPhysics.
WewouldalsoespeciallyliketothankMissJoanLawswhose
administrativeskills,dependability,andthoughtfulnesscontributedso muchtoourwork.Thepublisher.Holt,RinehartandWinston,Inc. ofNewYork,providedthecoordination,editorialsupport,andgeneral backingnecessarytothelargeundertakingofpreparingthefinal versionofallcomponentsoftheProjectPhysicsCourse,including texts,laboratoryapparatus,films,etc.Damon,acompanylocatedin Needham,Massachusetts,workedcloselywithustoimprovethe engineeringdesignofthelaboratoryapparatusandtoseethatitwas properlyintegratedintotheprogram. Intheyearsahead,thelearningmaterialsoftheProjectPhysics Coursewillberevisedasoftenasisnecessarytoremoveremaining ambiguities,clarifyinstructions,andtocontinuetomakethematerials moreinterestingandrelevanttostudents.Wethereforeurgeall studentsandteacherswhousethiscoursetosendtous(incareof Holt,RinehartandWinston,Inc.,383MadisonAvenue,NewYork, NewYork10017)anycriticismorsuggestionstheymayhave.
F.JamesRutherford
GeraldHolton
FletcherG.Watson
ContentsTEXTSECTION,Unit5
Prologue1
Chapter17TheChemicalBasisofAtomicTheory
Dalton'satomictheoryandthelawsofchemicalcombination11
Theatomicmassesoftheelements14
Otherpropertiesoftheelements:combiningcapacity16
Thesearchfororderandregularityamongtheelements18
Mendeleev'speriodictableoftheelements19
Themodernperiodictable23
ElectricityandMatter:qualitativestudies25
Electricityandmatter:quantitativestudies28
Chapter18ElectronsandQuanta
Theideaofatomicstructure33
Cathoderays34
Themeasurementofthechargeoftheelectron:Millikan'sexperiment37
Thephotoelectriceffect40
Einstein'stheoryofthephotoelectriceffect43
Xrays48
Electrons,quantaandtheatom54
Chapter19TheRutherford-BohrModeloftheAtom
Spectraofgases59
Regularitiesinthehydrogenspectrum63
Rutherford'snuclearmodeloftheatom66
Nuclearchargeandsize69
TheBohrtheory:thepostulates71
Thesizeofthehydrogenatom72
OtherconsequencesoftheBohrmodel74
TheBohrtheory:thespectralseriesofhydrogen75
Stationarystatesofatoms:theFranck-Hertzexperiment79
Theperiodictableoftheelements82
TheinadequacyoftheBohrtheoryandthestateofatomictheoryintheearly1920's86
Chapter20SomeIdeasfromModernPhysicalTheories
Someresultsofrelativitytheory95
Particle-hkebehaviorofradiation99
Wave-likebehaviorofparticles101
Mathematicalvsvisuahzableatoms104
Theuncertaintyprinciple108
Probabihtyinterpretation111
Epilogue116
Contents - HandbookSection125
Index/TextSection159
Index/HandbookSection163
AnswerstoEnd-of-SectionQuestions165
BriefAnswerstoStudyGuideQuestions168
.*^*". UNIT5
ModelsoftheAtom
CHAPTERS
17TheChemicalBasisoftheAtomicTheory
18ElectronsandQuanta
19TheRutherford-BohrModeloftheAtom
20SomeIdeasfromModernPhysicalTheories
PROLOGUEIntheearlierunitsofthiscoursewestudiedthemotion ofbodies:bodiesofordinarysize,suchaswedealwithineverydaylife, andverylargebodies,suchasplanets.Wehaveseenhowthelawsof motionandgravitationweredevelopedovermanycenturiesandhow theyareused.Wehavelearnedaboutconservationlaws,aboutwaves, aboutlight,andaboutelectricandmagneticfields.Allthatwehave learnedsofarcanbeusedtostudyaproblemwhichhasintrigued peopleformanycenturies:theproblemofthenatureofmatter.The phrase,"thenatureofmatter,"mayseemsimpletousnow,butits meaninghasbeenchangingandgrowingoverthecenturies.Thekind ofquestionsandthemethodsusedtofindanswerstothesequestions arecontinuallychanging.Forexample,duringthenineteenthcentury thestudyofthenatureofmatterconsistedmainlyofchemistry:inthe twentiethcenturythestudyofmatterhasalsomovedintoatomicand nuclearphysics. Since1800progresshasbeensorapidthatitiseasytoforgetthat peoplehavetheorizedaboutmatterformorethan2,500years.Infact someofthequestionsforwhichanswershavebeenfoundonlyduring thelasthundredyearsbegantobeaskedmorethantwothousand yearsago.Someoftheideasweconsidernewandexciting,suchas theatomicconstitutionofmatter,weredebatedinGreeceinthefifth andfourthcenturiesB.C.Inthisprologueweshallthereforereview brieflythedevelopmentofideasconcerningthenatureofmatterupto about1800.ThisreviewwillsetthestageforthefourchaptersofUnit5, whichwillbedevoted,ingreaterdetail,totheprogressmadesince
1800onunderstandingtheconstitutionofmatter.Itwillbeshownin
thesechaptersthatmatterismadeupofdiscreteparticlesthatwecall atoms,andthattheatomsthemselveshavestructure. Opposite:Monolith - TheFaceofHalfDome(PhotobyAnselAdams)
Thephotographsonthesetwo
pagesillustratesomeofthevariety offormsofmatter:largeandsmall, stableandshifting. microscopiccrystals condensedwatervapor
GreekIdeasofOrder
TheGreekmindlovedclarityandorder,expressedin
awaythatstilltouchesusdeeply.Inphilosophy,litera- ture,artandarchitectureitsoughttointerpretthingsin termsofhumaneandlastingqualities.Ittriedtodiscover theformsandpatternsthoughttobeessentialtoan understandingofthings.TheGreeksdelightedinshow- ingtheseformsandpatternswhentheyfoundthem.Their artandarchitectureexpressbeautyandintelligibility bymeansofbalanceofformandsimpledignity.
TheseaspectsofGreekthoughtarebeautifullyex-
pressedintheshrineofDelphi.Thetheater,whichcould seat5,000spectators,impressesusbecauseofthesize anddepthofthetieredseatingstructure.Butevenmore strikingisthenaturalandorderlywayinwhichthetheater isshapedintothelandscapesothattheentirelandscape takesontheaspectofagianttheater.TheTreasurybuild- ingatDelphihasanorderlysystemofproportions,with formandfunctionintegratedintoalogical,pleasing whole.ThestatueofthecharioteerfoundatDelphi,with itsbalanceandfirmness,representsagenuineidealof malebeautyatthattime.Aftermorethan2,000yearswe arestillstruckbytheeleganceofGreekexpression. v^'K^ r^.^5><^>
Prologue3
TheRomanpoetLucretiusbasedhisideasofphysicsonthe
traditionofatomismdatingbacktotheGreekphilosophersDemocritus andLeucippus.ThefollowingpassagesarefromhispoemDeRerum Natura(OntheNatureofThings),aneloquentstatementofatomism: ...Ifyouthink
Atomscanstoptheircourse,refrainfrommovement,
Andbycessationcausenewkindsofmotion,
Youarefarastrayindeed.Sincethereisvoid
Throughwhichtheymove,allfundamentalmotes
Mustbeimpelled,eitherbytheirownweight
Orbysomeforceoutsidethem.Whentheystrike
Eachother,theybounceoff;nowonder,either.
Sincetheyareabsolutesolid,allcompact.
Withnothingbackofthemtoblocktheirpath.
...noatomeverrests
Comingthroughvoid,butalwaysdrives,isdriven
Invariousways,andtheircollisionscause.
Asthecasemaybe,greaterorlessrebound.
Whentheyareheldinthickestcombination,
Atcloserintervals,withthespacebetween
Morehinderedbytheirinterlockoffigure.
Thesegiveusrock,oradamant,oriron.
Thingsofthatnature.(Notverymanykinds
Gowanderinglittleandlonelythroughthevoid.)
Therearesomewhosealternatemeetings,partings,are
Atgreaterintervals;fromthesewearegiven
Thinair,theshiningsunlight...
*** ...It'snowonder
Thatwhiletheatomsareinconstantmotion,
Theirtotalseemstobeattotalrest,
Savehereandtheresomeindividualstir.
Theirnatureliesbeyondourrangeofsense.
Far,farbeyond.Sinceyoucan'tgettosee
Thethingsthemselves,they'reboundtohidetheirmoves,
Especiallysincethingswecansee,often
Concealtheirmovements,too,whenatadistance.
Takegrazingsheeponahill,youknowtheymove,
Thewoollycreatures,tocropthelovelygrass
Whereveritmaycalleachone,withdew
Stillsparklingitwithjewels,andthelambs.
Fedfull,playlittlegames,flashinthesunlight.
Yetallthis,faraway,isjustablue,
Awhitenessrestingonahillofgreen.
Orwhengreatarmiessweepacrossgreatplains
Inmimicwarfare,andtheirshininggoes
Uptothesky,andalltheworldaround
Isbrilliantwiththeirbronze,andtrampledearth
Tremblesunderthecadenceoftheirtread,
Whitemountainsechotheuproartothestars,
Thehorsemengallopandshaketheveryground,
Andyethighinthehillsthereisaplace
Fromwhichthewatcherseesahostatrest.
Andonlyabrightnessrestingontheplain.
[translatedfromtheLatinbyRolfeHumphries]
ModelsoftheAtom
Thisgoldearring,madeinGreece
about600B.C.,showsthegreatskill withwhichancientartisansworked metals.[MuseumofFineArts,Boston] Earlysciencehadtodevelopoutoftheideasavailablebefore sciencestarted - ideasthatcamefromexperiencewithsnow,wind, rain,mistandclouds;withheatandcold;withsaltandfreshwater; wine,milk,blood,andhoney;ripeandunripefruit;fertileandinfertile seeds.Themostobviousandmostpuzzlingfactswerethatplants, animals,andmenwereborn,thattheygrewandmatured,andthatthey agedanddied.Mennoticedthattheworldaboutthemwascontinually changingandyet,onthewhole,itseemedtoremainmuchthesame. Theunknowncausesofthesechangesandoftheapparentcontinuity ofnaturewereassignedtotheactionsofgodsanddemonswhowere thoughttocontrolnature.Mythsconcerningthecreationoftheworld andthechangesoftheseasonswereamongtheearliestcreative productionsofprimitivepeopleseverywhere,andhelpedthemtocome totermswitheventsmancouldseehappeningbutcouldnotrationally understand. Overalongperiodoftimemendevelopedsomecontrolovernature andmaterials:theylearnedhowtokeepwarmanddry,tosmeltores,to makeweaponsandtools,toproducegoldornaments,glass,perfumes, andmedicines.Eventually,inGreece,bytheyear600B.C.,philosophers - literally"loversofwisdom" - hadstartedtolookforrationalexplana- tionsofnaturalevents,thatis,explanationsthatdidnotdependonthe actionsorthewhimsofgodsordemons.Theysoughttodiscoverthe enduring,unchangingthingsoutofwhichtheworldismade,andhow theseenduringthingscangiverisetothechangesthatweperceive, aswellasthegreatvarietyofmaterialthingsthatexists.Thiswasthe beginningofman'sattemptstounderstandthematerialworldrationally, anditledtoatheoryofthenatureofmatter. TheearliestGreekphilosophersthoughtthatallthedifferentthings intheworldweremadeoutofasinglebasicsubstance.Somethought thatwaterwasthefundamentalsubstanceandthatallothersubstances werederivedfromit.Othersthoughtthatairwasthebasicsubstance; stillothersfavoredfire.Butneitherwater,norair,norfirewassatis- factory;noonesubstanceseemedtohaveenoughdifferentproperties togiverisetotheenormousvarietyofsubstancesintheworld.According toanotherview,introducedbyEmpedoclesaround450B.C.,thereare fourbasictypesofmatter - earth,air,fire,andwater - andallmaterial thingsaremadeoutofthem.Thesefourbasicmaterialscanmingle andseparateandreuniteindifferentproportions,andsoproduce thevarietyoffamiliarobjectsaroundusaswellasthechangesin suchobjects.Butthebasicfourmaterials,calledelements,were supposedtopersistthroughallthesechanges.Thistheorywasthe firstappearanceinourscientifictraditionofamodelofmatter, accordingtowhichallmaterialthingsarejustdifferentarrangements ofafewexternalelements. ThefirstatomictheoryofmatterwasintroducedbytheGreek philosopherLeucippus,bornabout500B.C.,andhispupilDemocritus, wholivedfromabout460B.C.to370B.C.Onlyscatteredfragmentsof thewritingsofthesephilosophersremain,buttheirideasweredis- cussedinconsiderabledetailbytheGreekphilosophersAristotle (389-321B.C.)andEpicurus(341-270B.C.),andbytheLatinpoet
Prologue
Lucretius(100-55B.C.).Itistothesementhatweowemostofour knowledgeofancientatomism. Thetheoryoftheatomistswasbasedonanumberofassumptions: (1)matteriseternal - nomaterialthingcancomefromnothing, norcananymaterialthingpassintonothing; (2)materialthingsconsistofverysmallindivisibleparticles - the word"atom"meant"uncuttable"inGreekand,indiscussingtheideas oftheearlyatomists,wecouldusetheword"indivisibles"insteadof theword"atoms"; (3)atomsdifferchieflyintheirsizesandshapes; (4)theatomsexistinotherwiseemptyspace(thevoid)whichsepa- ratesthem,andbecauseofthisspacetheyarecapableofmovement fromoneplacetoanother; (5)theatomsareinceaselessmotion,althoughthenatureand causeofthemotionarenotclear; (6)inthecourseoftheirmotionsatomscometogetherandform combinationswhicharethematerialsubstancesweknow;whenthe atomsformingthesecombinationsseparate,thesubstancesdecayor breakup.Thus,thecombinationsandseparationsofatomsgiveriseto thechangeswhichtakeplaceintheworld; (7)thecombinationsandseparationstakeplaceinaccordwith naturallawswhicharenotyetclear,butdonotrequiretheactionof godsordemonsorothersupernaturalpowers. Withtheaboveassumptions,theancientatomistswereableto workoutaconsistentstoryofchange,ofwhattheysometimescalled "coming-to-be"and"passingaway."Theycouldnotdemonstrate experimentallythattheirtheorywascorrect,andtheyhadtobesatis- fiedwithanexplanationderivedfromassumptionsthatseemed reasonabletothem.Thetheorywasa"likelystory."Itwasnot usefulforthepredictionofnewphenomena;butthatbecamean importantvalueforatheoryonlylater.Totheseatomists,itwasmore significantthatthetheoryalsohelpedtoallaytheunreasonablefear ofcapriciousgods. TheatomictheorywascriticizedseverelybyAristotle,whoargued logically - fromhisownassumptions - thatnovacuumorvoidcould existandthattheideasofatomswiththeircontinualmotionmustbe rejected.(Aristotlewasalsoprobablysensitivetothefactthatinhis timeatomismwasidentifiedwithatheism.)ForalongtimeAristotle's argumentagainstthevoidwaswidelyheldtobeconvincing.Onemust hererecallthatnotuntiltheseventeenthcenturydidTorricelli's experiments(describedinChapter11)showthatavacuumcouldindeed exist.Furthermore,Aristotlearguedthatmatteriscontinuousand infinitelydivisiblesothattherecanbenoatoms. Aristotledevelopedatheoryofmatteraspartofhisgrandscheme oftheuniverse,andthistheory,withsomemodifications,wasthought tobesatisfactorybymostphilosophersofnaturefornearlytwo thousandyears.Histheoryofmatterwasbasedonthefourbasic elements.Earth,Air,Fire,andWater,andfour"qualities,"Cold,Hot, Moist,andDry.Eachelementwascharacterizedbytwoqualities(the
AccordingtoAristotleinhisMeta-
physics,"Thereisnoconsensus concerningthenumberornatureof thesefundamentalsubstances.
Thales,thefirsttothinkaboutsuch
matters,heldthattheelementary substanceisclearliquid....He mayhavegottenthisideafromthe observationthatonlymoistmatter canbewhollyintegratedintoan object - sothatallgrowthdepends onmoisture.... "AnaximenesandDiogenesheld thatcolorlessgasismoreelemen- tarythanclearliquid,andthatin- deed,itisthemostelementaryof allsimplesubstances.Ontheother handHippasusofMetpontumand
HeraclitusofEphesussaidthatthe
mostelementarysubstanceisheat.
Empedoclesspokeoffourelemen-
tarysubstances,addingdrydustto thethreealreadymentioned...
AnaxagorasofClazomenaesays
thatthereareaninfinitenumberof elementaryconstituentsofmat- ter...."[Fromatranslationby
D.E.GershensonandD.A.Green-
berg.]
6ModelsoftheAtom
FIRE WATER
Laboratory
chemist. ofa16th-centuryal- nearertwotoeachside,asshowninthediagramattheleft).Thus theelement
EarthisDryandCold,
WaterisColdandMoist,
AirisMoistandHot,
FireisHotandDry.
AccordingtoAristotle,itisalwaysthefirstofthetwoqualitieswhich predominates.Inhisversiontheelementsarenotunchangeable;any oneofthemmaybetransformedintoanyotherbecauseofoneorboth ofitsqualitieschangingintoopposites.Thetransformationtakesplace mosteasilybetweentwoelementshavingonequalityincommon;thus EarthistransformedintoWaterwhendrynesschangesintomoistness. EarthcanbetransformedintoAironlyifbothofthequalitiesofearth (dryandcold)arechangedintotheiropposites(moistandhot). AswehavealreadymentionedintheTextChapter2,Aristotlewas abletoexplainmanynaturalphenomenabymeansofhisideas.Like theatomictheory,Aristotle'stheoryofcoming-to-beandpassing-away wasconsistent,andconstitutedamodelofthenatureofmatter.Ithad certainadvantagesovertheatomictheory:itwasbasedonelements andqualitiesthatwerefamiliartopeople;itdidnotinvolveatoms, whichcouldn'tbeseenorotherwiseperceived,oravoid,whichwas mostdifficulttoimagine.Inaddition,Aristotle'stheoryprovidedsome basisforfurtherexperimentation:itsuppliedwhatseemedlikea rationalbasisforthetantalizingpossibilityofchanginganymaterial intoanyother. Althoughtheatomisticviewwasnotaltogetherabandoned,itfound fewsupportersduringtheperiod300A.D.toabout1600A.D.Theatoms ofLeucippusandDemocritusmovedthroughemptyspace,devoidof spirit,andwithnodefiniteplanorpurpose.Suchanidearemained contrarytothebeliefsofthemajorreligions.JustastheAtheniansdid inthetimeofPlatoandAristotle,thelaterChristian,Hebrew,and Moslemtheologiansconsideredatomiststobeatheisticand"mate- rialistic"becausetheyclaimedthateverythingintheuniversecanbe explainedintermsofmatterandmotion. About300or400yearsafterAristotle,akindofresearchcalled alchemyappearedintheNearandFarEast.AlchemyintheNearEast wasacombinationofAristotle'sideasaboutmatterwithmethodsof treatingoresandmetals.Oneoftheaimsofthealchemistswasto change,or"transmute"ordinarymetalsintopreciousmetals.Although theyfailedtodothis,thealchemistsfoundandstudiedmanyofthe propertiesofsubstancesthatarenowclassifiedaschemicalproperties. Theyinventedsomepiecesofchemicalapparatus,suchasreaction vesselsanddistillationflasks,that(inmodernform)arestillcommon inchemicallaboratories.Theystudiedsuchprocessesascalcination, distillation,fermentation,andsublimation.Inthissensealchemymay beregardedasthechemistryoftheMiddleAges.Butalchemyleft unsolvedthefundamentalquestions.Attheopeningoftheeighteenth centurythemostimportantofthesequestionswere:(1)whatisa chemicalelement;(2)whatisthenatureofchemicalcompositionand chemicalchange,especiallyburning;and(3)whatisthechemical
Prologue
natureoftheso-calledelements,Earth,Air,FireandWater.Untilthese questionswereanswered,itwasimpossibletomakerealprogressin findingoutthestructureofmatter.Oneresultwasthatthe"scientific revolution"oftheseventeenthcentury,whichclarifiedtheproblemsof astronomyandmechanics,didnotincludechemistry. Duringtheseventeenthcentury,however,someforwardstepswere madewhichsuppliedabasisforfutureprogressontheproblemof matter.TheCopernicanandNewtonianrevolutionsunderminedthe authorityofAristotletosuchanextentthathisideasaboutmatter werealsomoreeasilyquestioned.Atomicconceptswererevived,and offeredawayoflookingatthingsthatwasverydifferentfromAristotle's ideas.Asaresult,theoriesinvolvingatoms(or"particles"or"corpus- cles")wereagainconsideredseriously.Boyle'smodelswerebasedon theideaof"gasparticles."Newtonalsodiscussedthebehaviorofa gas(andevenoflight)bysupposingittoconsistofparticles.In addition,therewasnowasuccessfulscienceofmechanics,through whichonemighthopetodescribehowtheatomsinteractedwitheach other.Thusthestagewassetforageneralrevivalofatomictheory. Intheeighteenthcentury,chemistrybecamemorequantitative; weighinginparticularwasdonemorefrequentlyandmorecarefully. Newsubstanceswereisolatedandtheirpropertiesexamined.The attitudethatgrewupinthelatterhalfofthecenturywasexemplifiedby thatofHenryCavendish(1731-1810),who,accordingtoabiographer, regardedtheuniverseasconsisting
Oneofthosewhocontributed
greatlytotherevivalofatomism wasPierreGassendi(1592 - 1655),a
Frenchpriestandphilosopher.He
avoidedthecriticismofatomism asatheisticbysayingthatGodalso createdtheatomsandbestowed motionuponthem.Gassendiac- ceptedthephysicalexplanationsof theatomists,butrejectedtheirdis- beliefintheimmortalityofthesoul andinDivineProvidence.Hewas thusabletoprovideaphilosophical justificationofatomismwhichmet someoftheseriousreligious objections. ...solelyofamultitudeofobjectswhichcouldbeweighed, numbered,andmeasured;andthevocationtowhichhecon- sideredhimselfcalledwastoweigh,number,andmeasure asmanyofthoseobjectsashisallotedthreescoreyearsand tenwouldpermit....HeweighedtheEarth;heanalysedthe
Air;hediscoveredthecompoundnatureofWater;henoted
withnumericalprecisiontheobscureactionsoftheancient elementFire.
ItwasCavendish,remember,who
designedthesensitivetorsional balancethatmadeitpossibleto findavalueforthegravitational constantG.(TextSec.8.8.) Eighteenth-centurychemistryreacheditspeakintheworkof AntoineLavoisier(1743-1794),whoworkedoutthemodernviewsof combustion,establishedthelawofconservationofmass,explainedthe elementarynatureofhydrogenandoxygen,andthecompositionof water,andaboveallemphasizedthequantitativeaspectsofchemistry. Hisfamousbook,TraiteElementairedeChimie(orElementsof Chemistry),publishedin1789,establishedchemistryasamodern science.Init,heanalyzedtheideaofanelementinawaywhichisvery closetoourmodernviews: ...if,bythetermelementswemeantoexpressthosesimple andindivisibleatomsofwhichmatteriscomposed,itisex- tremelyprobablethatweknownothingatallaboutthem;but ifweapplythetermelements,orprinciplesofbodies,to expressourideaofthelastpointwhichanalysisiscapable ofreaching,wemustadmitaselementsallthesubstances intowhichwearecapable,byanymeans,toreducebodies bydecomposition.Notthatweareentitledtoaffirmthat
Lavoisier'sworkontheconserva-
tionofmasswasdescribedinText
Chapter9.
ModelsoftheAtom
TRAITEELEMENTAIRE
DECHIMIE,
PRfeSENTtDANSUNORDRENOUVEAU
ETd'aPR^SLESD^COUVERTESUODERNES}
AvecFigures:
TarM.LavoistEA,deCAcaJimUdit
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laSocieteRoyaUdeLondres,deI'lnftiiutde
Bologiie,delaSocieteHelvitiquedeBajle,dt
celtesdePhUadelphle,Harlem,Manchefler,Padoue,&c. |.TOMEPREMIER.
APARIS,
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M.DCC.LXXXIX.
SmaItPriviUgtdeTAcaidrnitdttScieru-ei6dtUSociMRoyaUdtMidteint
TitlepageofLavoisier'sIratteEle-
mentairedeChimie(1789) thesesubstancesweconsiderassimplemaynotbecom- poundedoftwo,orevenofagreaternumberofprinciples; butsincetheseprinciplescannotbeseparated,orrather sincewehavenothithertodiscoveredthemeansofsepa- ratingthem,theyactwithregardtousassimplesubstances, andweoughtnevertosupposethemcompoundeduntilex- perimentandobservationhaveprovedthemtobeso. Duringthelatterhalfoftheeighteenthcenturyandtheearlyyearsof thenineteenthcenturygreatprogresswasmadeinchemistrybecause oftheincreasinguseofquantitativemethods.Chemistsfoundoutmore andmoreaboutthecompositionofsubstances.Theyseparatedmany elementsandshowedthatnearlyallsubstancesarecompounds - combinationsofafairlysmallnumberofchemicalelements.They learnedagreatdealabouthowelementscombinetoformcompounds andhowcompoundscanbebrokendownintotheelementsofwhich theyarecomposed.Thisinformationmadeitpossibleforchemiststo establishmanyempiricallawsofchemicalcombination.Thenchemists soughtanexplanationfortheselaws. Duringthefirsttenyearsofthenineteenthcentury,theEnglish chemistJohnDaltonintroducedamodifiedformoftheoldGreek atomictheorytoaccountforthelawsofchemicalcombination.Itis herethatthemodernstoryoftheatombegins.Dalton'satomictheory wasanimprovementoverthatoftheGreeksbecauseitopenedthe wayforthequantitativestudyoftheatominthenineteenthcentury. Todaytheexistenceoftheatomisnolongeratopicofspeculation. Therearemanykindsofexperimentalevidence,notonlyforthe existenceofatomsbutalsofortheirinnerstructure.Inthisunitwe shalltracethediscoveriesandideasthatprovidedthisevidence. Thefirstconvincingmodernideaoftheatomcamefromchemistry. Weshall,therefore,startwithchemistryintheearlyyearsofthenine- teenthcentury;thisisthesubjectofChapter17.Thenweshallseethat chemistryraisedcertainquestionsaboutatomswhichcouldonlybe answeredbyphysics.Physicalevidence,accumulatedinthenineteenth centuryandtheearlyyearsofthetwentiethcentury,madeitpossible toproposemodelsforthestructureofatoms.Thisevidencewillbe discussedinChapters18and19.Someofthelatestideasaboutatomic theorywillthenbediscussedinChapter20.
Chemicallaboratoryofthe18thcentury
17.1Dalton'satomictheoryandthelawsofchemicalcombination11
17.2Theatomicmassesoftheelements14
17.3Otherpropertiesoftheelements:combiningcapacity16
17.4Thesearchfororderandregularityamongtheelements18
17.5Mendeleev'speriodictableoftheelements19
17.6Themodernperiodictable23
17.7Electricityandmatter:qualitativestudies25
17.8Electricityandmatter:quantitativestudies28
OCD^O®
oo®®©©®©®o
Dalton'ssymbolsfor'elements
"(1808)
CHAPTERSEVENTEEN
TheChemicalBasisof
AtomicTheory
17.1Dalton'satomictheoryandthelawsofchemicalcombination
TheatomictheoryofJohnDaltonappearedinhistreatise,A NewSystemofChemicalPhilosophy,publishedintwoparts,in
1808and1810.Themainpostulatesofhistheorywere:
(1)Matterconsistsofindivisibleatoms. ...matter,thoughdivisibleinanextremedegree,is neverthelessnotinfinitelydivisible.Thatis,theremust besomepointbeyondwhichwecannotgointhedivision ofmatter.Theexistenceoftheseultimateparticlesof mattercanscarcelybedoubted,thoughtheyareprobably muchtoosmallevertobeexhibitedbymicroscopicim- provements.Ihavechosenthewordatomtosignifythese ultimateparticles.... (2)Eachelementconsistsofacharacteristickindofidentical atoms.Thereareconsequentlyasmanydifferentkindsofatomsas thereareelements.Theatomsofanelement"areperfectlyalikein weightandfigure,etc." (3)Atomsareunchangeable. (4)Whendifferentelementscombinetoformacompound,the smallestportionofthecompoundconsistsofagroupingofadefinite numberofatomsofeachelement. (5)Inchemicalreactions,atomsareneithercreatednor destroyed,butonlyrearranged. Dalton'stheoryreallygrewoutofhisinterestinmeteorology andhisresearchonthecompositionoftheatmosphere.Hetriedto explainmanyofthephysicalpropertiesofgasesintermsofatoms (forexample,thefactthatgasesreadilymix,andthefactthatthe pressuresoftwogasesaddsimplywhenbotharecombinedina fixedenclosure).Hethoughtoftheatomsofdifferentelementsas beingdifferentinsizeandinmass.Inkeepingwiththequantitative spiritofthetime,hetriedtodeterminethenumericalvaluesfortheir relativemasses.Thiswasacrucialstepforward.Butbeforeconsider- inghowtodeterminetherelativemassesofatomsofthedifferent elements,letusseehowDalton'spostulatesmakeitpossibletoac- countfortheexperimentallyknownlawsofchemicalcombination. 11
SG17.1
Meteorologyisasciencethatdeals
withtheatmosphereandits phenomena - weatherforecasting isonebranchofmeteorology.
12TheChemicalBasisoftheAtomicTheory
Recallthatempiricallaws(suchas
these,orKepler'slawsofplanetary motion)arejustsummariesof experimentallyobservedfacts.They cryoutforsometheoreticalbase fromwhichtheycanbeshownto followasnecessaryconsequences.
Physicalsciencelooksforthese
deepernecessitiesthatdescribe nature,andisnotsatisfiedwith meresummariesofobservation, usefulthoughthesemaybeinitially. Dalton'satomictheoryaccountsinasimpleanddirectwayfor thelawofconservationofmass.AccordingtoDalton'stheory (postulates4and5),chemicalchangesareonlytherearrangements ofunionsofatoms.Sinceatomsareunchangeable(accordingto postulate3)rearrangingthemcannotchangetheirmasses.Hence, thetotalmassofalltheatomsbeforethereactionmustequalthe totalmassofalltheatomsafterthereaction.
Anotherwellknownempiricallawwhichcouldbeexplained
easilywithDalton'stheoryisthelawofdefiniteproportions.This lawstatesthatanyparticularchemicalcompoundalwayscontains thesameelements,andtheyareunitedinthesameproportionsof weight.Forexample,theratioofthemassesofoxygenandhy- drogenwhichcombinetoformwaterisalways7.94to1 : massofoxygen_7.94 massofhydrogen1 Ifthereismoreofoneelementpresentthanisneededforfull combinationinachemicalreaction,say10gramsofoxygenand onegramofhydrogen,only7.94gramsofoxygenwillcombine withthehydrogen.Therestoftheoxygen,2.06grams,remains uncombined. Thefactthatelementscombineinfixedproportionsimpliesthat eachchemicalcompoundwillalsodecomposeintodefinitepropor- tionsofelements.Forexample,thedecompositionofsodium chloride(commonsalt)alwaysgivestheresults:39percent sodiumand61percentchlorinebyweight. NowletusseehowDalton'smodelcanbeappliedtoachemical reaction,say,totheformationofwaterfromoxygenandhydrogen. AccordingtoDalton'ssecondpostulate,alltheatomsofoxygen havethesamemass;andalltheatomsofhydrogenhavethesame mass,whichisdifferentfromthemassoftheoxygenatoms.To expressthetotalmassofoxygenenteringintothereaction,we multiplythemassofasingleoxygenatombythenumberofoxygen atoms: SG17.2,17.3"/massof\massofoxygen=(oxygenatom)numberof^ oxygenatoms, Similarly,forthetotalmassofhydrogenenteringintothereaction: ",,/massof\/numberof\massofhydrogen-(^hydrogenatom]^\hydrogenatoms) Wecanfindtheratioofthemassofoxygentothemassofhydrogen bydividingthefirstequationbythesecondequationasshownat thetopofthenextpage:
Section17.113
massofmassofoxygen_oxygenatom massofhydrogenmassof hydrogenatom numberof oxygenatoms numberof hydrogenatoms Ifthemassesoftheatomsdonotchange(postulate3),thefirst ratioontherightsideoftheequationhasacertainunchangeable value.Accordingtopostulate4,ifthesmallestportionofthecom- poundwaterconsistsofadefinitenumberofatomsofeachelement (postulate4),thesecondratioontherightsideoftheequationhas acertainunchangeablevaluealso.Theproductofthetworatioson therightsidewillalwayshavethesamevalue.Thisequation, basedonanatomictheory,thustellsusthattheratioofthe massesofoxygenandhydrogenthatcombinetoformwaterwill alwayshavethesamedefinitevalue.Butthisisjustwhatthe experimentallawofdefiniteproportionssays.Dalton'stheory accountsforthislawofchemicalcombination - andthissuccess tendstoconfirmDalton'sconception.Dalton'stheorywasalso consistentwithanotherempiricallawofchemicalcombination,the lawofmultipleproportion.Forsomecombinationsofelements thereareasetofpossiblevaluesfortheirproportionsinforminga
SG17.4
^J^3r^
ApagefromDalton'snotebook,
showinghisrepresentationoftwo adjacentatoms(top)andofamole- culeorcompoundatom'(bottom)
JohnDalton(1766-1844).Hisfirst
lovewasmeteorology,andhekept carefuldailyweatherrecordsfor
46years - atotalof200,000observa-
tions.Hewasthefirsttodescribe colorblindnessinapublicationand wascolor-blindhimself,notexactly anadvantageforachemistwhohad toseecolorchangesinchemicals. (Hiscolorblindnessmayhelpto explainwhyDaltonissaidtohave beenaratherclumsyexperimenter.)
However,hisaccomplishmentsrest
notonsuccessfulexperiments,but onhisingeniousinterpretationof theknownresultsofothers.Dalton's notionthatallelementswerecom- posedofextremelytiny,indivisible andindestructibleatoms,andthat allsubstancesarecomposedof combinationsoftheseatomswas acceptedsoonbymostchemists withsurprisinglylittleopposition.
Thereweremanyattemptstohonor
him,butbeingaQuakerheshunned anyformofglory.Whenhereceived adoctor'sdegreefromOxford,his colleagueswantedtopresenthimto
KingWilliamIV.Hehadalways
resistedsuchapresentationbe- causehewouldnotwearcourt dress.However,hisOxfordrobes satisfiedtheprotocol.
14TheChemicalBasisoftheAtomicTheory
Dalton'svisualizationofthecom-
positionofvariouscompounds. setofcompounds.Daltonshowedthatthesecasescouldallbe accountedforbydifferentcombinationsofwholenumbersofatoms.
Thereareotherlawsofchemicalcombinationwhichare
explainedbyDalton'stheory.Becausetheargumentwouldbe lengthyandrelativelylittlethatisnewwouldbeadded,weshallnot elaborateonthemhere. Dalton'sinterpretationoftheexperimentalfactsofchemical combinationmadepossibleseveralimportantconclusions:(1)that thedifferencebetweenonechemicalelementandanotherwould havetobedescribedintermsofthedifferencesbetweentheatoms ofwhichtheseelementsweremadeup;(2)thattherewere,there- fore,asmanydifferenttypesofatomsastherewerechemical elements;(3)thatchemicalcombinationwastheunionofatomsof differentelementsintomoleculesofcompounds.Dalton'stheory alsoimpliedthattheanalysisofalargenumberofchemicalcom- poundscouldmakeitpossibletoassignrelativemassvaluesto theatomsofdifferentelements.Thispossibilitywillbediscussed inthenextsection.
Q1WhatdidDaltonassumeabouttheatomsofanelement?
Q2WhattwoexperimentallawsdidDalton'stheoryexplain?
Whatfollowsfromthesesuccesses?
17.2Theatomicmassesoftheelements
Thefirstgoodestimatesof
molecularsizecamefromthekinetic theoryofgasesandindicatedthat atoms(ormolecules)haddiameters oftheorderof10'"meter.Atoms arethusmuchtoosmallforordinary massmeasurementstobemadeon singleatoms.
SG17.5
SG17.6
OneofthemostimportantconceptstocomefromDalton'swork isthatofatomicmassandthepossibilityofdeterminingnumerical valuesforthemassesoftheatomsofdifferentelements.Dalton hadnoideaoftheactualabsolutemassofindividualatoms. Reasonablygoodestimatesofthesizeofatomsdidnotappearuntil about50yearsafterDaltonpublishedhistheory.Nevertheless,as Daltonwasabletoshow,relativevaluesofatomicmassescanbe foundbyusingthelawofdefiniteproportionsandexperimental dataonchemicalreactions.
Toseehowthiscouldbedonewereturntothecaseofwater,
forwhich,theratioofthemassofoxygentothemassofhydrogen isfoundbyexperimenttobe7.94:1.Ifoneknewhowmanyatoms ofoxygenandhydrogenarecontainedinamoleculeofwaterone couldcalculatetheratioofthemassoftheoxygenatomtothemass ofthehydrogenatom.ButDaltondidn'tknowthenumbersof oxygenandhydrogenatomsinamoleculeofwatersohemadean assumption.Asisdoneoften,Daltonmadethesimplestpossible assumption:thatamoleculeofwaterconsistsofoneatomof oxygencombinedwithoneatomofhydrogen.Bythisreasoning Daltonconcludedthattheoxygenatomis7.94timesmoremassive thanthehydrogenatom.Actually,thesimplestassumptionproved inthiscasetobeincorrect:twoatomsofhydrogencombinewith oneatomofoxygentomakeamoleculeofwater.Theoxygenatom has7.94timesthemassofthetwohydrogenatoms,andtherefore has15.88timesthemassofasinglehydrogenatom. Moregenerally,Daltonassumedthatwhenonlyonecompound
Section17.215
ofanytwoelementsisknowntoexist,moleculesofthecompound alwaysconsistofoneatomofeach.WiththisassumptionDalton couldfindvaluesfortherelativemassesofdifferentatoms - but laterworkshowedthatDalton'sassumptionofone-to-oneratioswas oftenasincorrectasitwasforwater.Bystudyingthecomposition ofwateraswellasmanyotherchemicalcompounds,Daltonfound thatthehydrogenatomappearedtohaveasmallermassthanthe atomsofanyotherelement.Therefore,heproposedtoexpressthe massesofatomsofallotherelementsrelativetothemassofthe hydrogenatom.Daltondefinedtheatomicmassofanelementas themassofanatomofthatelementcomparedtothemassofa hydrogenatom.Forexample,themassesofchlorineandhydrogen gasthatreacttoformhydrogenchloride(theonlyhydrogenand chlorinecompound)areintheratioofabout35V2to1;therefore thechlorineatomwouldbesupposedtohaveanatomicmassof
35V2atomicmassunits.Thisdefinitioncouldbeusedbychemists
inthenineteenthcenturyevenbeforetheactualvaluesofthe massesofindividualatoms(sayinkilograms)couldbemeasured directly. Duringthenineteenthcenturychemistsextendedandimproved Dalton'sideas.Theystudiedmanychemicalreactionsquantita- tively,anddevelopedhighlyaccuratemethodsfordetermining relativeatomicandmolecularmasses.Becauseoxygencombined readilywithmanyotherelementschemistsdecidedtouseoxygen ratherthanhydrogenasthestandardforatomicmasses.Oxygen wasassignedanatomicmassof16sothathydrogenwouldhave anatomicmassclosetoone.Theatomicmassesofotherelements couldbeobtainedbyapplyingthelawsofchemicalcombinationto thecompoundsoftheelementswithoxygen.Throughoutthenine- teenthcenturymoreandmoreelementswereidentifiedandtheir atomicmassesdetermined.Forexample,thetableonthenextpage lists63elementsfoundby1872,togetherwiththemodernvalues fortheatomicmasses.Thistablecontainsmuchvaluableinforma- tion,whichweshallconsideratgreaterlengthinSec.17.4.(The specialmarksonthetable - circlesandrectangles - willbeuseful then.) Q3Wasthesimplestchemicalformulaforthecompositionof amoleculenecessarilythecorrectone? Q4WhydidDaltonchoosehydrogenastheunitofatomicmass?
SG17.7
SG17.8
Thesystemofatomicmassesused
inmodernphysicalscienceisbased onthisprinciple,althoughitdiffers indetails(andthestandardfor comparisonbyinternationalagree- mentisnowcarboninsteadof hydrogenoroxygen.)
Theprogressmadeinidentifying
elementsinthe19thcenturymay beseeninthefollowingtable.
Totalnumberof
Year
16TheChemicalBasisoftheAtomicTheory
Elementsknownby1872,inorderof
increasingrelativeatomicmass.
Section17.317
cleaning,hastheformulaCCI4whereCstandsforacarbonatom thatcombineswithfourchlorineatoms.Anothercommonsub- stance,ammonia,hastheformulaNH3;inthiscaseoneatomof nitrogen(N)combineswiththreeatomsofhydrogen. Thereareespeciallysignificantexamplesofcombiningcapacity amongthegaseouselements.Forexample,thegashydrogenoccurs innatureintheformofmolecules,eachofwhichcontainstwo hydrogenatoms.Themoleculeofhydrogenconsistsoftwoatoms andhastheformulaHg.Similarly,chlorinehasthemolecular formulaCI2.Chemicalanalysisalwaysgivestheseresults.Itwould beinconsistentwithexperimenttoassigntheformulaH3orH4toa moleculeofhydrogen,orCI,CI3,orCI4toamoleculeofchlorine. Moreover,eachelementshowsgreatconsistencyinitscombining proportionswithotherelements.Forexample,calciumandoxygen seemtohavetwicethecombiningcapacityofhydrogenand chlorine - oneatomofhydrogenisenoughforoneatomofchlorine, buttwohydrogensareneededtocombinewithoxygenandtwo chlorinesarerequiredtocombinewithcalcium.
Theaboveexamplesindicatethatdifferentelementshave
differentcapacitiesforchemicalcombination.Itappearedthat eachspeciesofatomischaracterizedbysomedefinitecombining capacity(whichissometimescalledvalence).Atonetimecombin- ingcapacitywasconsideredasthoughitmightrepresentthe numberof"hooks"possessedbyagivenatom,andthusthenumber oflinksthatanatomcouldformwithothersofthesameordifferent species.Ifhydrogenandchlorineatomseachhadjustonehook (thatis,acombiningcapacityof1)wewouldreadilyunderstand howitisthatmoleculeslikeH2,CI2,andHClarestable,while certainotherspecieslikeH3,H2CI,HCI2,andCI3don'texistatall. Andifthehydrogenatomisthusassignedacombiningcapacity of1,theformulaofwater(H2O)requiresthattheoxygenatomhas twohooksoracombiningcapacityof2.TheformulaNH3for ammonialeadsustoassignacombiningcapacityofthreetonitro- gen;theformulaCH4formethaneleadsustoassignacapacityof
4tocarbon;andsoon.Proceedinginthisfashion,wecanassign
acombiningcapacitynumbertoeachoftheknownelements. Sometimescomplicationsariseas,forexample,inthecaseof sulfur.InH2Sthesulfuratomseemstohaveacombiningcapacity of2,butinsuchacompoundassulfurtrioxide(SO3),sulfurseems tohaveacombiningcapacityof6.Inthiscaseandothers,then, wemayhavetoassigntwo(orevenmore)differentpossiblecapaci- tiestoanelement.Attheotherextremeofpossibilitiesarethose elementslikeheliumandneonwhichhavenotbeenfoundasparts ofcompounds - andtotheseelementswemayappropriatelyassign acombiningcapacityofzero.
Theatomicmassandcombiningcapacitiesarenumbersthat
canbeassignedtoanelement;theyare"numericalcharacteriza- tions"oftheatomsoftheelement.Thereareothernumberswhich representpropertiesoftheatomsoftheelements,butatomicmass andcombiningcapacitywerethetwomostimportanttonineteenth-
Inthethirteenthcenturythe
theologianandphilosopherAlbert
Magnus(AlberttheGreat)intro-
ducedtheideaofaffinitytodenote anattractiveforcebetweensub- stancesthatcausesthemtoenter intochemicalcombination.Itwas notuntil600yearslaterthatit becamepossibletoreplacethis qualitativenotionbyquantitative concepts.Combiningcapacityisone oftheseconcepts.
Representationsofmoleculesformed
from"atomswithhooks. "Ofcourse thisconceptionisjustaguidetothe imagination.Therearenosuchme- chanicallinkagesamongatoms.
SG17.9
Sinceoxygencombineswitha
greatervarietyofelements, combiningcapacityofanelement wascommonlydeterminedbyits combinationwithoxygen.For example,anelementXthatisfound tohavean"oxideformula"XO wouldhaveacombiningcapacity equaltooxygen's:2.
18TheChemicalBasisoftheAtomicTheory
centurychemists.Thesenumberswereusedinattemptstofind orderandregularityamongtheelements - aproblemwhichwillbe discussedinthenextsection.
Q5Atthispointwehavetwonumberswhicharecharacter-
isticoftheatomsofanelement.Whatarethey?
Q6Assumethecombiningcapacityofoxygenis2.Ineachof
thefollowingmolecules,givethecombiningcapacityoftheatoms otherthanoxygen:CO,CO2,N2O5,Na^OandMnO.
Therewerealsomanyfalsetrails.
Thusin1829theGermanchemist
JohannWolfgangDbbereiner
noticedthatelementsoftenformed groupsofthreememberswith similarchemicalproperties.He identifiedthe"triads":chlorine, bromineandiodine:calcium, strontiumandbarium:sulfur, seleniumandtellurium:iron,cobalt andmanganese.Ineach"triad,"the atomicmassofthemiddlemember wasapproximatelythearithmetical averageofthemassesoftheother twoelements.Butallthisturned outtobeoflittlesignificance.
17.4Thesearchfororderandregularityamongtheelements
By1872sixty-threeelementswereknown;theyarelistedin thetableonp.16withtheiratomicmassesandchemicalsymbols. Sixty-threeelementsaremanymorethanAristotle'sfour:and chemiststriedtomakethingssimplerbylookingforwaysof organizingwhattheyhadlearnedabouttheelements.Theytriedto findrelationshipsamongtheelements - aquestsomewhatlike Kepler'searliersearchforrulesthatwouldrelatethemotionsof theplanetsofthesolarsystem. Inadditiontorelativeatomicmasses,manyotherpropertiesof theelementsandtheircompoundsweredetermined.Amongthese propertieswere:meltingpoint,boilingpoint,density,electrical conductivity,heatconductivity,heatcapacity(theamountofheat neededtochangethetemperatureofasampleofasubstanceby1 C)hardness,andrefractiveindex.Theresultwasthatby1870an enormousamountofinformationwasavailableaboutalarge numberofelementsandtheircompounds.
ItwastheEnglishchemistJ.A.R.Newlandswhopointedout
in1865thattheelementscouldusefullybelistedsimplyinthe orderofincreasingatomicmass.Whenthiswasdone,acuriousfact becameevident;similarchemicalandphysicalpropertiesappeared overandoveragaininthelist.Newlandsbelievedthattherewas inthewholelistaperiodicrecurrenceofelementswithsimilar properties:"...theeighthelement,startingfromagivenone,isa kindofrepetitionofthefirst,liketheeighthnoteinanoctaveof music."Newlands'proposalwasmetwithskepticism.Onechemist evensuggestedthatNewlandsmightlookforasimilarpatternin analphabeticallistofelements. Yet,existentrelationshipsdidindeedappear.Thereseemedto befamiliesofelementswithsimilarproperties.Onesuchfamily consistsoftheso-calledalkalimetals - hihium.sodium,potassium, rubidiumandcesium.WehaveidentifiedtheseelementsbyaDin thetableonp.16.Allthesemetalsaresimilarphysically.Theyare softandhavelowmeltingpoints.Thedensitiesofthesemetalsare verylow;infact,lithium,sodiumandpotassiumarelessdense thanwater.Thealkalimetalsarealsosimilarchemically.Theyall havecombiningcapacity1.Theyallcombinewiththesameother elementstoformsimilarcompounds.Theyformcompoundsreadily withotherelements,andsoaresaidtobehighly"reactive";conse-
Section17.519
quently,theydonotoccurfreeinnature,butarealwaysfoundin combinationwithotherelements. Anotherfamilyofelements,calledthehalogens,includes fluorine,chlorine,bromineandiodine.Thehalogensmaybefound inthetableonp.16identifiedbysmallcircles.
Althoughthesefourhalogenelementsexhibitsomemarked
dissimilarities(forexample,at25°Cthefirsttwoaregases,the thirdaliquid,thelastavolatilesolid),theyalsohavemuchincom- mon.Theyallcombineviolentlywithmanymetalstoformwhite, crystallinesalts(halogenmeans"salt-former");thosesaltshave similarformulas,suchasNaF,NaCl,NaBrandNal,orMgFz, MgCla,MgBraandMgla.Frommuchsimilarevidencechemists noticedthatallfourmembersofthefamilyseemtohavethesame valencewithrespecttoanyotherparticularelement.Allfourele- mentsfromsimplecompoundswithhydrogen(HF,HCI,HBr,HI) whichdissolveinwaterandformacids.Allfour,underordinary conditions,existasdiatomicmolecules;thatis,eachmolecule containstwoatoms.Butnotice:eachhalogenprecedesanalkali metalinthelist,althoughthelistingwasorderedsimplyby increasingatomicmass.Itisasifsomenewpatterniscomingout ofajig-sawpuzzle. Theelementswhichfollowthealkalimetalsinthelistalso formafamily,theonecalledthealkalineearthfamily;thisfamily includesberyllium,magnesium,calcium,strontiumandbarium. Theirmeltingpointsanddensitiesarehigherthanthoseofthe alkalimetals.Thealkalineearthsallhaveavalenceoftwo.They reacteasilywithmanyelements,butnotaseasilyasdothealkali metals.
Recognitionoftheexistenceofthesefamihesofelements
encouragedchemiststolookforasystematicwayofarrangingthe elementssothatthemembersofafamilywouldgrouptogether. Manyschemesweresuggested;themostsuccessfulandfarreach- ingwasthatoftheRussianchemistD.I.Mendeleev. Q7Whatarethosepropertiesofelementswhichrecursystem- aticallywithincreasingatomicmass?
17.5Mendeleev'speriodictableoftheelements
Mendeleev,examiningthepropertiesoftheelements,reached theconclusionthattheatomicmasswasthefundamental"numeri- calcharacterization"ofeachelement.Hediscoveredthatifthe elementswerearrangedinatableintheorderoftheiratomic masses - butinaspecialway,abitlikecardslaidoutinthegame ofsolitaire - thedifferentchemicalfamiliesturnedouttofallinto thedifferentverticalcolumnsofthetable.Therewasnoevident physicalreasonwhythisshouldbeso,butitwasahinttoward someremarkableconnectionamongallelements.
Modernchemistsusetheword
'valence"lessandlessinthesense weuseithere.Theyaremorelikely todiscuss"combiningnumber"or "oxidationnumber."Eventhe ideaofadefinitevalencenumber foranelementhaschanged,since combiningpropertiescanbedif- ferentunderdifferentconditions. Li7
20TheChemicalBasisoftheAtomicTheory
DmitriIvanovichMendeleev(men-
deh-lay>'-ef)(1834-1907)receivedhis firstsciencelessonsfromapolitical prisonerwhohadbeenpreviously banishedtoSiberiabytheCzar.Un- abletogetintocollegeinMoscow,he wasacceptedinSt.Petersburg,where afriendofhisfatherhadsomein- fluence.In1866hebecameaprofes- sorofchemistrythere:in1869hepub- lishedhisfirsttableofthesixty-three thenknownelementsarrangedac- cordingtoincreasingatomicmass.
HispaperwastranslatedintoGerman
atonceandsobecameknowntosci- entistseverywhere.Mendeleevcame totheUnitedStates,wherehestudied theoilfieldsofPennsylvaniainorder toadvisehiscountryonthedevelop- mentoftheCaucasianresources.His liberalpoliticalviewscausedhim oftentobeintroublewiththeoppres- siveregimeoftheCzars.
Asinthetableontheprecedingpage,Mendeleevsetdown
sevenelements,fromlithiumtofluorine,inorderofincreasing atomicmasses,andthenputthenextseven,fromsodiumto chlorine,inthesecondrow.Theperiodicityofchemicalbehavioris alreadyevidentbeforewegoontowritethethirdrow.Inthefirst columnontheleftarethefirsttwoalkalimetals.Intheseventh columnarethefirsttwomembersofthefamilyofhalogens.Indeed, withineachofthecolumnstheelementsarechemicallysimilar, having,forexample,thesamecharacteristiccombiningcapacity. WhenMendeleevaddedathirdrowofelements,potassium(K) camebelowelementsLiandNa,whicharemembersofthesame familyandhavethesameoxideformula,X2O,andthesame combiningcapacity1.NextintherowisCa,oxideformulaXOas withMgandBeaboveit.Inthenextspacetotheright,theelement ofnexthigheratomicmassshouldappear.Oftheelementsknown atthetime,thenextheavierwastitanium(Ti),anditwasplacedin thisspace,rightbelowaluminum(Al)andboron(B)byvarious workerswhohadtriedtodevelopsuchschemes.Mendeleev,how- ever,recognizedthattitanium(Ti)haschemicalpropertiessimilar tothoseofcarbon(C)andsilicon(Si).Forexample,apigment, titaniumwhite,Ti02,hasaformulacomparabletoCO2andSi02. Thereforeheconcludedthattitaniumshouldbeputinthefourth column.Then,ifallthisisnotjustagamebuthasdeepermeaning. Mendeleevthought,thereshouldexistahithertounsuspectedele- mentwithatomicmassbetweenthatofcalcium(40)andtitanium (50),andwithanoxideX2O3.Herewasadefiniteprediction. Mendeleevfoundalsoothercasesofthissortamongtheremaining elementswhentheywereaddedtothistableofelementswithdue regardtothefamilypropertiesofelementsineachcolumn. ThetablebelowisMendeleev'speriodicsystem,or"periodic table"oftheelements,asproposedin1872.Hedistributedthe63 elementsthenknown(with5indoubt)in12horizontalrowsor series,startingwithhydrogeninauniqueseparatedpositionatthe topleft,andendingwithuraniumatthebottomright.Allelements
Periodicclassificationoftheele-
ments;Mendeleev,1872.
GROUP - *
Section17.521
werelistedinorderofincreasingatomicmass(Mendeleev'svalues giveninparentheses),butweresoplacedthatelementswithsimilar chemicalpropertiesareinthesameverticalcolumnorgroup. ThusinGroupVIIareallthehalogens;inGroupVIII,onlymetals thatcaneasilybedrawnintowires;inGroupsIandII,metalsof lowdensitiesandmeltingpoints;andinI,thefamilyofalkali metals. Thetableatthebottomofthepreviouspageshowsmanygaps. Also,notallhorizontalrows(series)haveequallymanyelements. Nonetheless,thetablerevealedanimportantgeneralization; accordingtoMendeleev,
Foratruecomprehensionofthematteritisveryimpor-
tanttoseethatallaspectsofthedistributionofthe elementsaccordingtotheorderoftheiratomicweights expressessentiallyoneandthesamefundamentaldepen- dence - periodicproperties. Thereisgradualchangeinphysicalandchemicalpropertieswithin eachverticalgroup,butthereisamorestrikingperiodicchangeof propertiesinthehorizontalsequence. Thisperiodiclawistheheartofthematterandarealnovelty. PerhapswecanbestillustrateitasLotharMeyerdid,bydrawing agraphthatshowsthevalueofsomemeasureablephysicalquantity asafunctionofatomicmass.Belowisaplotoftherelative atomicvolumesoftheelements,thespacetakenupbyanatomin theliquidorsolidstate.Eachcircledpointonthisgraphrepresents anelement;afewofthepointshavebeenlabeledwiththe identifyingchemicalsymbols.Viewedasawhole,thegraph demonstratesastrikingperiodicity:asthemassincreasesstarting withLi,theatomicvolumefirstdrops,thenincreasestoasharp maximum,dropsoffagainandincre
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