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IntroductiontoChem...
AuthoredbyTracy Poulsen
8.5" x 11.0" (21.59 x 27.94 cm)
Black & White onWhite paper
250 pages
ISBN-13: 9781478298601
ISBN-10: 147829860X
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IntroductiontoChemistry
Author: Tracy Poulsen
4. 7.4: BalancingChemical Equations.................................................................................148
7.5: Typesof Reactions.....................................................................................................153
7.6: Stoichiometry.............................................................................................................159
7.7: Reversible reaction&Equilibrium............................................................................165
7.8: EquilibriumConstant.................................................................................................168
7.9: The Effects of ApplyingStresstoReactionsatEquilibrium.....................................171
Chapter8: DescribingAcids&Bases..................................................................................177
8.1: ClassifyingAcidsandBases......................................................................................177
8.2: pH...............................................................................................................................180
8.3: Neutralization.............................................................................................................184
8.4: Titration.....................................................................................................................186
Chapter9: Energy of Chemical Changes..............................................................................190
9.1: Energy........................................................................................................................190
9.2: EndothermicandExothermicChanges......................................................................191
9.3: Oxidation –Reduction...............................................................................................194
Chapter10: NuclearChanges...............................................................................................201
10.1: Discoveryof Radioactivity......................................................................................201
10.2: Typesof Radiation...................................................................................................203
10.3: Half-life &Rate of Radioactive Decay....................................................................209
10.4: Applicationsof NuclearChanges............................................................................213
10.5: Big Bang Theory...................................................................................................... 219
Unit 3: Gases .........................................................................................................................222
11.1: Gases andKineticTheory........................................................................................222
11.2: Gas Laws..................................................................................................................226
11.3: Ideal Gas Law ..........................................................................................................231
AnswerstoSelectedProblems..............................................................................................234
Glossary................................................................................................................................246 4
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Course ObjectivesbyChapter
Unit 1: IntroductiontoChemistryandthe Nature of Science
Nature of Science Goal—Scienceisbasedonobservations,data,analysisandconclusions.
1. I can distinguishbetweenobservable (qualitative) andnumeric(quantitative) data.
2. I can construct andanalyze data tablesandgraphs.
3. I can identifyindependent,dependant,andcontrolledvariablesinanexperiment
description,datatable orgraph.
4. I can write a laboratorysummaryina Claim-Evidence Format
Unit 2: The Structure of the Atom
Nature of Science Goal—Scientificunderstandingchangesasnew dataiscollected.
1. I can use atomicmodelstoexplainwhytheoriesmaychange overtime.
2. I can identifythe relativesize,charge andpositionof protons,neutrons,andelectrons
inthe atom.
3. I can findthe numberof protons,neutronsandelectronsinagivenisotope of an
elementif Iamgivena nuclearsymbol orname of elementandmassnumber.
4. I can describe the differencebetweenatomicmassandmass number.
5. I can describe the relationshipbetweenwavelength,frequency,energyandcolorof
light(photons).
6. I can describe the processthroughwhichthe electronsgiveoff photons(energy) and
describe the evidence thatelectronshave specificamountsof energy.
7. I can identifyanunknownelementusingaflame testor bycomparisonto an emission
spectra.
8. I can write electronconfigurationsforelementsinthe groundstate.
Unit 3: The Organizationof the Elements
Nature of Science Goal—Classificationsystemsleadtobetterscientificunderstanding.
1. I can describe the advantagesof Mendeleev’sPeriodicTable overother
organizations.
6. 2. I can compare the propertiesof metals,nonmetals,andmetalloids.
3. I can determine the numberof valence electronsforelementsinthe mainblock.
4. I can explainthe similaritiesbetweenelementswithinagroupor family.
5. I can identifypatternsfoundonthe periodictable suchasreactivity,atomicradius,
ionizationenergyandelectronegativity.
Unit 4: DescribingCompounds
Nature of Science Goal—Vocabularyinscience hasspecificmeanings.
1. I can indicate the type of bondformedbetweentwoatomsandgive propertiesof
ionic,covalent,metallicbondsanddescribe the propertiesof materialsthatare
bondedineachof those ways.
2. I can compare the physical andchemical propertiesof acompoundtothe elements
that formit.
3. I can predictthe charge an atomwill acquire whenitformsanionby gainingor
losingelectronsusingthe octetrule.
4. I can write the namesandformulasof ioniccompounds.6
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5. I can indicate the shape andpolarityof simple covalentcompoundsfromamodel or
drawing.
6. I can describe howhydrogenbondinginwateraffectsphysical,chemical,and
biological phenomena.
Unit 5: ProblemSolvingandthe Mole
Nature of Science Goal— Mathematicsisa tool to increase scientificunderstanding.
1. I can describe the commonmeasurementsof the SIsystemof measurements
2. I can convertbetweenstandardnotationandscientificnotation.
3. I can convertbetweenmass,moles,andatomor moleculesusingfactor-label
methods.
Unit 6: Mixturesand TheirProperties
Nature of Science Goal-- Science providespredictable results.
7. 1. I can use the termssolute andsolventindescribingasolution.
2. I can sketcha solution,colloid,andsuspensionatthe particle level.
3. I can describe the relativeamountasolute particlesinconcentratedanddilute
solutions.
4. I can calculate concentrationintermsof molarityandmolality.
5. I can describe the colligative propertiesof solutions.(Boilingpointelevation,
Freezingpointdepression,Vaporpressure lowering) intermsof everyday
applications.
6. I can identifywhichsolutionof asetwouldhave the lowestfreezingpointorhighest
boilingpoint.
Unit 7: DescribingChemical Reactions
Nature of Science Goal—Conservationslawsare investigatedtoexplore science
relationships.
1. I can classifyachange as chemical orphysical andgive evidence of chemical
changesreactions.
2. I can describe the principlesof collisiontheoryandrelate frequency,energyof
collisions,andadditionof acatalystto reactionrate.
3. I can write a chemical equationtodescribe asimple chemical reaction.
4. I can balance chemical reactionsandrecognize thatthe numberof atomsina
chemical reactiondoesnotchange.
5. I can classifyreactionsassynthesis,decomposition,single replacement,double
replacementorcombustion.
6. I can use molarrelationshipsinabalancedchemical reactiontopredictthe massof
productproducedina simple chemicalreactionthatgoestocompletion.
7. I can explainthe conceptof dynamicequilibriumasitrelatestochemical reactions.
8. I can describe whetherreactantsorproductsare favoredinequilibriumwhengiven
the equilibriumconstant.
9. I can predictthe effectof addingorremovingeitheraproductor a reactantor the
effectof changingtemperature toshiftequilibrium.
8. Unit 8: DescribingAcidsandBases
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Nature of Science Goal--Nature ismovingtowardequilibrium
1. I can describe propertiesof acidsandbasesandidentifyif asolutionisacidicor
basic.
2. I can calculate the pH of a solution.
3. I can write a neutralizationreactionbetweenanacidandbase.
4. I can calculate the concentrationof anacid or base from data collectedinatitration.
Unit 9: Energy of Chemical Changes
Nature of Science Goal—Scienceprovidestechnologytoimprovelives.
1. I can classifyevidence of energytransformation(temperature change) asendothermic
or exothermic.
2. I can describe howelectrical energycanbe producedina chemical reactionand
identifywhichelementgainedandwhichelementlostelectrons.
3. I can identifythe partsof a battery,includinganode,cathode,andsaltbridge.
Unit 10: NuclearChanges
Nature of Science Goal—Correctinterpretationof datareplacesfearandsuperstition.
1. I can compare the charge, mass,energy,andpenetratingpowerof alpha,beta,and
gamma radiationandrecognize thatof the productsof the decayof an unstable
nucleusinclude radioactiveparticlesandwavelike radiation.
2. I can interpretgraphical dataof decayprocessestodetermine half-life andthe age of
a radioactive substance.
3. I can compare and contrast the amountof energyreleasedinanuclearreactiontothe
amountof energyreleasedinachemical reaction.
4. I can describe the differencesbetweenfissionandfusion.
5. I can describe scientificevidence thatall matterinthe universe hasacommonorigin.
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Chapter1: IntroductiontoChemistry&the Nature of Science
1.1: The Processof Science
Objectives
Explainthe necessityforexperimentation
Inan experiment,identifythe independent,dependent,andcontrolledvariables.
Introduction
Socrates(469 B.C. - 399 B.C.),Plato(427
B.C. - 347 B.C.),andAristotle (384 B.C. - 322 B.C.)
are amongthe most famousof the Greek
philosophers.Platowasastudentof Socrates,and
Aristotle wasastudentof Plato.These three were
probablythe greatestthinkersof theirtime.Aristotle's
viewsonphysical science profoundlyshaped
medieval scholarship,andhisinfluence extendedinto
the Renaissance (14thcentury - 16th century).
Aristotle'sopinionswere the authorityonnature until
well intothe 1300s. Unfortunately,manyof
Aristotle'sopinionswere wrong.Itisnotintended
here to denigrate Aristotle'sintelligence;he was
withoutdoubta brilliantman.Itwassimplythathe
was usinga methodfordeterminingthe nature of the
physical worldthatisinadequate forthattask.The
philosopher'smethodwaslogical thinking,not
makingobservationsonthe natural world.This ledto
manyerrors inAristotle'sthinkingonnature.Let's
considertwoof Aristotle'sopinionsasexamples.
In Aristotle'sopinion,menwere biggerand
strongerthan women;therefore,itwaslogical tohimthatmenwouldhave more teeththan
10. women.Thus,Aristotle concludeditwasa true fact that menhad more teeththanwomen.
Apparently,itneverenteredhismindtoactuallylookintothe mouthsof bothgendersand
count theirteeth.Hadhe done so,he wouldhave foundthatmenandwomenhave exactlythe
same numberof teeth.
In termsof physical science,Aristotle thoughtaboutdroppingtwoballsof exactlythe
same size andshape but of differentmassestosee whichone wouldstrike the groundfirst.In
hismind,itwas clearthat the heavierball wouldfall fasterthanthe lighterone andhe
concludedthatthiswasa lawof nature.Once again,he didnot considerdoinganexperiment
to see whichball fell faster.Itwaslogical tohim, andinfact, it still seemslogical.If
someone toldyouthatthe heavierball wouldfall faster,youwouldhave noreasonto
disbelieve it.Infact,itis nottrue and the bestwayto prove thisis totry it.
Eighteencenturieslater,Galileodecidedtoactuallygettwoballsof differentmasses,
but withthe same size andshape,anddrop themoff a building(Legendsaysthe Leaning
Towerof Pisa),andactuallysee whichone hitthe groundfirst.WhenGalileoactuallydidthe
experiment,he discovered,byobservation,thatthe twoballshitthe groundat exactlythe
same time . . . Aristotle'sopinionwas,once again,wrong.
Image obtainedfrom:
http://upload.wikimedia.org/wikipedia/c
ommons/a/ae/Aristotle_Altemps_Inv857
5.jpg
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ScientificMethodsof ProblemSolving
In the 16th and 17th centuries,innovativethinkerswere developinganew wayto
discoverthe nature of the worldaroundthem.Theywere developingamethodthatreliedupon
makingobservationsof phenomenaandinsistingthattheirexplanationsof the nature of the
phenomenacorrespondedtothe observationstheymade.
The scientificmethodisamethodof investigationinvolvingexperimentationand
observationtoacquire newknowledge,solve problems,andanswerquestions.Scientists
11. frequentlylistthe scientificmethodasaseriesof steps.Otherscientistsoppose thislistingof
stepsbecause notall stepsoccur ineverycase,and sometimesthe stepsare outof order.The
scientificmethodislistedinaseriesof stepshere because itmakesiteasier tostudy.Youshould
rememberthatnotall stepsoccur in everycase,nordo theyalwaysoccur inorder.
The Stepsin the ScientificMethod
Step1: Identifythe problemor
phenomenonthatneedsexplaining.This
issometimesreferredtoas"definingthe
problem."
Step2: Gather and organize dataon the
problem.Thisstepisalsoknownas
"makingobservations."
Step3: Suggesta possible solutionor
explanation.A suggestedsolutionis
calleda hypothesis.
Step4: Test the hypothesisbymaking
newobservations.
Step5: If the newobservationssupport
the hypothesis,youacceptthe hypothesis
for furthertesting.If the new
observationsdonotagree withyour
hypothesis,addthe newobservationsto
your observationlistandreturntoStep3.
Experimentation
Experimentationisthe primarywaythroughwhichscience gathersevidence for
ideas. Itis more successful forusto cause somethingtohappenata time andplace of our
choosing.When we arrange for the phenomenontooccur at our convenience,we canhave all
our measuringinstrumentspresentandhandytohelpusmake observations,andwe can
control othervariables.Experimentationinvolvescausingaphenomenontooccurwhenand
12. where we wantitand underthe conditionswe want.Anexperimentisacontrolledmethod
of testinganideaor to findpatterns.Whenscientistsconductexperiments,theyare usually
seekingnewinformationortryingtoverifysomeone else'sdata.
Experimentationinvolveschangingandlookingatmanyvariables.The independent
variable isthe part of the experimentthatisbeingchangedormanipulated.There canonly
be one independentvariable inanyexperiment. Consider,forexample,thatyouwere trying
to determine the bestfertilizerforyourplants. Itwouldbe importantforyou to grow your
plantswitheverythingelse abouthowtheyare grownbeingthe same exceptforthe fertilizer
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youwere using. You wouldbe changingthe type of fertilizeryougave the plantsandthis
wouldbe the independentvariable. If youalsochangedhow muchwater the plantsreceived,
the type of plantsyouwere growing,andsome of the plantswere grown inside andothers
outside,youcouldnotdeterminewhetherornotit wasactuallythe fertilizerthatcausedthe
plantsto growbetteror if it wassomethingelse youhadchanged. Thisiswhyitis important
that there isonlyone independentvariable.
The dependentvariable iswhatisobservedormeasuredasaresultof what
happenedwhenthe independentvariable waschanged. Inthe plantexperimentdescribed
above,youmightmeasure the heightof the plantandrecord theirappearance andcolor.
These wouldbe the dependentvariables. The dependentvariable isalsosometimescalled
the resultantvariable.
Controlledvariablesare conditionsof the experimentthatare keptthe same for
varioustrialsof the experiment. Once again,if we were testinghow fertilizeraffectedhow
well ourplantsgrew,we wouldwanteverythingelseabouthow the plantsare grownto be
keptthe same. We wouldneedtouse the same type of plant(maybe greenbeans),give them
the same amountof water,planttheminthe same location(all outsideinthe garden),give
themall the same pesticide treatment,etc. These wouldbe controlledvariables.
Suppose ascientist,while walkingalongthe beachona verycoldday followinga
rainstorm,observedtwopoolsof waterinbowl shapedrocksneareachother.One of the
poolswaspartiallycoveredwithice,while the otherpool hadnoice on it. The unfrozenpool
seemedtobe formedfromseawatersplashinguponthe rock fromthe surf,butthe otherpool
13. was toohighfor seawatertosplashin,so itwas more likelytohave beenformedfrom
rainwater.
The scientistwonderedwhyone pool waspartiallyfrozenandnotthe other,since
bothpoolswere at the same temperature.Bytastingthe water(nota goodidea),the scientist
determinedthatthe unfrozenpool tastedsaltierthanthe partiallyfrozenone.The scientist
thoughtperhapssaltwaterhad a lowerfreezingpointthanfreshwater,andshe decidedtogo
home and try an experimenttosee if thiswere true.Sofar, the scientisthasidentifieda
question,gatheredasmall amountof data, andsuggestedanexplanation.Inordertotestthis
hypothesis,the scientistwillconductanexperimentduringwhichshe canmake accurate
observations.
For the experiment,the scientistpreparedtwoidentical
containersof freshwaterandaddedsome saltto one of them.
A thermometerwasplacedineachliquidandthese wereputin
a freezer.The scientistthenobservedthe conditionsand
temperaturesof the twoliquidsatregularintervals.
The Temperature andConditionof Fresh
Water ina Freezer
Time (min) Temp(°C) Condition
0 25 Liquid
5 20 Liquid
14. 10 15 Liquid
15 10 Liquid
20 5 Liquid
25 0 Frozen
30 -5 Frozen
The Temperature andConditionof Salt
Water ina Freezer
Time (min) Temp(°C) Condition
0 25 Liquid
5 20 Liquid
10 15 Liquid
15 10 Liquid
20 5 Liquid
25 0 Liquid
30 -5 Frozen
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The scientistfoundsupportforthe hypothesisfromthisexperiment;freshwater
freezesata highertemperature thansaltwater.Muchmore supportwouldbe neededbefore
the scientistwouldbe confidentof thishypothesis.Perhapsshe wouldaskotherscientiststo
verifythe work.
In the scientist'sexperiment,itwasnecessarythatshe freeze the saltwaterandfresh
waterunderexactlythe same conditions.Why?The scientistwastestingwhetherornot the
presence of saltinwaterwouldalteritsfreezingpoint.Itisknownthatchangingairpressure
will alterthe freezingpointof water,sothisandothervariablesmustbe keptthe same,or
theymustbe controlledvariables.
Example: Inthe experimentdescribedabove, identifythe:
15. a) independentvariable(s)
b) dependentvariable(s)
c) controlledvariable(s)
Solution:
a) Remember,the independentvariable iswhatthe scientistchangedinhis/herexperiment.
In thiscase,the scientistaddedsalttoone containerandnot to anothercontainer. The
independentvariable iswhetherornot saltwas added.
b) Dependentvariablesare whatwe lookforas a resultof the change we made. The scientist
recordedthe temperature andphysical state (liquidorsolid) overtime. These are the
dependentvariables.
c) Controlledvariablesare keptthe same throughoutall of the trials. The scientistselected
identical containers,putthe same amountof waterinthe containers,andfroze theminthe
same conditionsinthe same freezer. These are all controlledvariables.
Suppose youwishtodetermine whichbrandof microwave popcorn(independent
variable) leavesthe fewestunpoppedkernels(dependentvariable).You will needasupplyof
variousbrandsof microwave popcorntotestand youwill needamicrowave oven.If you
useddifferentbrandsof microwave ovenswithdifferentbrandsof popcorn,the percentage of
unpoppedkernelscouldbe causedbythe different brandsof popcorn,butitcouldalsobe
causedby the differentbrandsof ovens.Undersuchcircumstances,the experimenterwould
not be able to conclude confidentlywhetherthe popcornorthe ovencausedthe difference.
To eliminate thisproblem,youmustuse the same microwave ovenforeverytest.Byusing
the same microwave oven,youcontrol manyof the variablesinthe experiment.Whatif you
allowedthe differentsamplesof popcorntobe cookedat differenttemperatures?Whatif you
allowedlongerheatingperiods?Inordertoreasonablyconclude thatthe change inone
variable wascausedbythe change inanotherspecificvariable,there mustbe noother
variablesinthe experiment.All othervariablesmustbe keptconstantorcontrolled.
Whenstatingthe purpose of anexperiment,itisimportanttoclarifythe independent
and dependentvariables. The purpose isfrequentlystatedinasentence suchas:
“To see howchanging_____________ affects____________.”
inwhichthe independentvariable islistedinthe firstblank,andthe dependentvariable is
16. listedinthe secondblank.
In the popcorn experiment,we wouldstate the purpose as: “Tosee how changingthe
brand of popcornaffectsthe percentage of unpoppedkernels”. The independentvariable is12
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the brand of popcorn andthe dependentvariableiswhatpercentage of the popcorndidn’t
pop. In the salt waterexperimentdescribedearlier,we wouldstate the purposeas“To see
howaddingsaltto water affectsthe temperature the waterfreezes.”
LessonSummary
Scientistsuse experimentationtotesttheirideas.
Inan experiment,itisimportanttoinclude onlyone independentvariable (tochange
onlyone thinginthe experiment)
The dependentvariable iswhatismeasuredorobservedasaresultof how the
independentvariable changed.
Controlledvariablesare those whichare keptthe same throughoutvarioustrialsinthe
experiment.
Vocabulary
Experiment:A controlledmethodof testingahypothesis.
Controlledexperiment:Anexperimentthatcomparesthe resultsof anexperimental
sample toa control sample.
FurtherReading/Supplemental Links
http://learner.org/resources/series61.html: The learner.org website allowsusersto
viewstreamingvideosof the Annenbergseriesof chemistryvideos.Youare required
to registerbefore youcanwatchthe videosbutthere isnocharge.The website has
twovideosthatapplyto thislesson.One isavideocalled The Worldof Chemistry
that relateschemistrytoothersciencesanddailylife.AnothervideocalledThinking
Like Scientistsrelatestothe scientificmethod.The audience onthe videoisyoung
childrenbutthe ideasare full grown.
Website of the JamesRandi Foundation.JamesRandi isastaunchopponentof fake
17. science.http://www.randi.org/site/
Websitesdealingwiththe historyof the scientificmethod.
http://www.historyguide.org/earlymod/lecture10c.html
http://www.history.boisestate.edu/WESTCIV/science/
1.1: ReviewQuestions
Use the followingparagraphtoanswerquestions1-4:
Gary noticedthat twoplantswhichhismotherplantedonthe same daythat were the same
size whenplantedwere differentinsize afterthree weeks.Since the largerplantwasinthe
full sunall dayand the smallerplantwasinthe shade of a tree most of the day,Gary
believedthe sunshine wasresponsibleforthe differenceinthe plantsizes.Inordertotest
this,Gary boughttensmall plantsof the same size andtype.He made sure theyhad the same
size andtype of pot.He alsomade sure theyhave the same amountandtype of soil.Then
Gary builta frame to holda canvas roof overfive of the plantswhile the otherfive were
nearby butout in the sun.Gary was careful tomake sure that eachplantreceivedexactlythe
same amountof waterand plantfoodeveryday.
1) What scientificreasonmightGaryhave forinsistingthatthe containersize forthe all
plantsbe the same?
a) Gary wantedto determine if the size of the containerwouldaffectthe plantgrowth.
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b) Gary wantedto make sure the size of the containerdidnot affectplantgrowthinhis
experiment.
c) Gary wantedtocontrol howmuch plantfoodhisplantsreceived.
d) Gary wantedhisgardento lookorganized.
e) There is nopossible scientificreasonforhavingthe same size containers.
2) What scientificreasonmightGaryhave forinsistingthatall plantsreceive the same
amountof watereveryday?
a) Gary wantedto testthe effectof shade onplantgrowth andtherefore,he wantedto
have no variablesotherthanthe amountof sunshine onthe plants.
b) Gary wantedto testthe effectof the amountof water onplantgrowth.
18. c) Gary's hypothesiswasthatwaterqualitywasaffectingplantgrowth.
d) Gary was conservingwater.
e) There is nopossible scientificreasonforhavingthe same amountof waterfor each
planteveryday.
3) What wasthe variable being testedinGary'sexperiment(whatisthe independent
variable)?
a) The amountof water
b) The amountof plantfood
c) The amountof soil
d) The amountof sunshine
e) The type of soil
4) Whichof the followingfactorsmaybe varyinginGary's experimental setupthathe did
not control?
a) Individual plantvariation
b) Soil temperature due todifferentcolorsof containers
c) Water lossdue to evaporationfromthe soil
d) The effectof insectswhichmayattack one setof plants butnot the other
5) A studentdecidestosetupan experimenttodeterminethe relationshipbetweenthe
growthrate of plantsand the presence of detergentinthe soil.He setsup10 seedpots.In
five of the seedpots,he mixesaprecise amountof detergentwiththe soil.The otherfive
seedpotshave no detergentinthe soil.The fiveseedpotswithdetergentare placedinthe
sunand the five seedpotswithnodetergentare placedinthe shade.All 10 seedpots
receive the same amountof waterandthe same numberandtype of seeds.He growsthe
plantsfortwo monthsand charts the growtheverytwodays.What iswrong withhis
experiment?
a) The studenthastoofewpots.
b) The studenthastwo independentvariables.
c) The studenthastwodependent(resultant) variables.
d) The studenthasno experimental control onthe soil.
A scientistplantstworowsof corn forexperimentation.She putsfertilizeronrow 1 but does
19. not putfertilizeronrow2. Bothrows receive the same amountof sunand water.She checks
the growthof the corn overthe course of five months.
6) What isthe independentvariable inthisexperiment?
7) What isthe dependentvariable inthisexperiment?
8) What variablesare controlled inthisexperiment?14
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1.2: Hypothesis,Law,&Theory
Objectives
Describe the difference betweenhypothesisandtheoryasscientificterms.
Describe the difference betweenatheoryandscientificlaw.
Explainthe conceptof a model.
Explainwhyscientistsuse models.
Explainthe limitationsof modelsasscientificrepresentationsof reality.
Introduction
Althoughall of us have takenscience classesthroughoutthe course of ourstudy,
manypeople have incorrectormisleadingideasaboutsome of the mostimportantandbasic
principlesinscience. We have all heardof hypotheses,theories,andlaws,butwhatdothey
reallymean? Before youreadthissection,thinkaboutwhatyouhave learnedaboutthese
termsbefore. Whatdo these termsmeantoyou? What do youread contradictswhatyou
thought? What do youread supportswhatyouthought?
Hypotheses
One of the most commontermsusedinscience classesisa“hypothesis”. The word
can have many differentdefinitions,dependingonthe contextinwhichitisbeingused:
“Aneducatedguess”– because itprovidesasuggestedsolutionbasedonthe
evidence. Note thatitisn’tjusta random guess. Ithas to be basedon evidence tobe
a scientifichypothesis.
Prediction –if youhave evercarriedout a science experiment,youprobablymade
thistype of hypothesis,inwhichyoupredictedthe outcome of yourexperiment.
Tentative orProposedexplanation –hypothesescanbe suggestionsaboutwhy
20. somethingisobserved,butinorderforit to be scientific,we mustbe able totestthe
explanationtosee if itworks,if itisable to correctlypredictwhatwill happenina
situation,suchas: if myhypothesisiscorrect,we shouldsee ___ resultwhenwe
perform___ test. A hypothesisisverytentative;itcanbe easilychanged.
Theories
The UnitedStatesNational Academyof Sciencesdescribeswhatatheoryisas
follows:
“Some scientificexplanationsare sowell establishedthatnonew
evidence islikelytoalterthem.The explanationbecomesascientifictheory.
In everydaylanguage atheorymeansahunchor speculation.Notsoin
science.Inscience,the wordtheoryreferstoacomprehensiveexplanationof
an importantfeature of nature supportedbyfactsgatheredovertime.Theories
alsoallowscientiststomake predictionsaboutasyetunobserved
phenomena.”
“A scientifictheoryisawell-substantiatedexplanationof some aspect
of the natural world,basedon a bodyof facts that have beenrepeatedly
confirmedthroughobservationandexperimentation.Suchfact-supported
theoriesare not"guesses"butreliable accountsof the real world.The theory
of biological evolutionismore than"justa theory."Itis as factual an
explanationof the universe asthe atomictheoryof matter(statingthat
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everythingismade of atoms) orthe germ theoryof disease (whichstatesthat
manydiseasesare causedbygerms).Ourunderstandingof gravityisstill a
workin progress.Butthe phenomenonof gravity,like evolution,isan
acceptedfact.“
Note some keyfeaturesof theoriesthatare importantto understandfromthis
description:
Theoriesare explanationsof natural phenomenon. Theyaren’tpredictions(although
21. we may use theoriestomake predictions). Theyare explanationswhywe observe
something.
Theoriesaren’tlikelytochange. Theyhave so muchsupportand are able toexplain
satisfactorilysomanyobservations,thattheyare notlikelytochange. Theoriescan,
indeed,be facts. Theoriescanchange,butitis a longand difficultprocess. Inorder
for a theoryto change,there mustbe manyobservationsor evidencethatthe theory
cannot explain.
Theoriesare notguesses. The phrase “justa theory”hasno room inscience. To be a
scientifictheorycarriesalot of weight;itisnot justone person’sideaabout
something.
Laws
Scientificlawsare similartoscientifictheoriesinthattheyare principlesthatcanbe
usedto predictthe behaviorof the natural world.Bothscientificlawsandscientifictheories
are typicallywell-supportedbyobservationsand/orexperimental evidence.Usuallyscientific
lawsreferto rulesforhownature will behave undercertainconditions,frequentlywrittenas
an equation.Scientifictheoriesare more overarchingexplanationsof how nature worksand
whyit exhibitscertaincharacteristics. Asa comparison, theoriesexplainwhywe observe
whatwe doand lawsdescribe whathappens.
For example,aroundthe year1800, JacquesCharlesandotherscientistswere
workingwithgasesto,amongotherreasons,improve the designof the hotairballoon.These
scientistsfound,aftermany,manytests,thatcertainpatternsexistedinthe observationson
gas behavior.If the temperature of the gasincreased,the volume of the gasincreased.Thisis
knownas a natural law.A lawis a relationshipthatexistsbetweenvariablesinagroupof
data. Lawsdescribe the patternswe see inlarge amountsof data,but do describe whythe
patternsexist.
A commonmisconceptionisthatscientifictheoriesare rudimentaryideasthatwill
eventuallygraduate intoscientificlawswhenenoughdataandevidence hasbeen
accumulated.A theorydoesnotchange intoa scientificlaw withthe accumulationof new or
betterevidence.Remember,theoriesare explanationsandlawsare patternswe see inlarge
amountsof data, frequentlywrittenasanequation.A theorywill alwaysremainatheory;a
22. lawwill alwaysremainalaw.
A model isa description,graphic,or3-D representationof theoryusedtohelp
enhance understanding.Scientistsoftenuse modelswhentheyneedawayto communicate
theirunderstandingof whatmightbe verysmall (suchas an atomor molecule) orverylarge
(suchas the universe). A model isanysimulation,substitute,orstand-inforwhatyouare
actuallystudying.A goodmodel containsthe essential variablesthatyouare concernedwith
inthe real system,explainsall the observationsonthe real system, andisas simple as16
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possible.A model maybe asuncomplicatedasasphere representingthe earthorbilliard
ballsrepresenting gaseousmolecules,orascomplex asmathematical equationsrepresenting
light.
Chemistsrelyonbothcareful observationandwell-knownphysical laws.Byputting
observationsandlawstogether,chemistsdevelopmodels.Modelsare reallyjustwaysof
predictingwhatwill happengivenacertainsetof circumstances.Sometimesthese models
are mathematical,butothertimes,theyare purelydescriptive.
If you were askedtodetermine the contentsof abox that cannotbe opened,you
woulddoa varietyof experimentsinordertodevelopanidea(ora model) of whatthe box
contains.Youwouldprobablyshake the box,perhapsputmagnetsnearitand/ordetermine
itsmass. Whenyoucompletedyourexperiments,youwoulddevelopanideaof whatis
inside;thatis,youwouldmake a model of whatisinside abox that cannot be opened.
A goodexample of howamodel isuseful toscientistsishow modelswere usedto
explainthe developmentof the atomictheory.Asyouwill learninalater chapter,the ideaof
the conceptof an atom changedovermanyyears.In orderto understandeachof the different
theoriesof the atomaccordingto the variousscientists,modelswere drawn,andthe concepts
were more easilyunderstood.
Chemistsmake upmodelsaboutwhathappenswhendifferentchemicalsare mixed
together,orheatedup,or cooleddown,orcompressed.Chemistsinventthesemodelsusing
manyobservationsfromexperimentsinthe past,andtheyuse these modelstopredictwhat
mighthappenduringexperimentsinthe future.Once chemistshave modelsthatpredictthe
outcome of experimentsreasonablywell,those workingmodelscanhelptotell themwhat
theyneedtodo to achieve acertaindesiredresult.Thatresultmightbe the productionof an
23. especiallystrongplastic,oritmightbe the detectionof a toxinwhenit’spresentinyour
food.
LessonSummary
A hypothesisisatentative explanationthatcanbe testedbyfurtherinvestigation.
A theoryis a well-supportedexplanationof observations.
A scientificlawisastatementthatsummarizesthe relationshipbetweenvariables.
Anexperimentisacontrolledmethodof testingahypothesis.
A model isa description,graphic,or3-D representationof theoryused tohelp
enhance understanding.
Scientistsoftenuse modelswhentheyneedawayto communicate their
understandingof whatmightbe verysmall (suchasan atom or molecule)orvery
large (suchas the universe).
Vocabulary
Hypothesis:A tentative explanationthatcanbe testedbyfurtherinvestigation.
Theory:A well-establishedexplanation
Scientificlaw:A statementthatsummarizesthe relationshipbetweenvariables.
Model:A description,graphic,or3-D representationof theoryusedtohelpenhance
understanding.
FurtherReading/Supplemental Links
http://en.wikipedia.org/wiki/Scientific_theory
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http://en.wikipedia.org/wiki/Hypothesis
VideoonDemand – Modelingthe Unseen
(http://www.learner.org/resources/series61.html?pop=yes&pid=793#)
1.2: ReviewQuestions
Multiple Choice
24. 1) A numberof people became ill aftereatingoystersinarestaurant.Whichof the
followingstatementsisahypothesisaboutthisoccurrence?
a) Everyone whoate oystersgotsick.
b) People gotsickwhetherthe oysterstheyate were raw orcooked.
c) Symptomsincludednauseaanddizziness.
d) Bacteriainthe oystersmayhave causedthe illness.
2) If the hypothesisisrejected(proved wrong) bythe experiment,then:
a) The experimentmayhave beenasuccess.
b) The experimentwasafailure.
c) The experimentwaspoorlydesigned.
d) The experimentdidn'tfollowthe scientificmethod.
3) A hypothesisis:
a) A descriptionof aconsistentpatterninobservations.
b) Anobservationthatremainsconstant.
c) A theorythat has beenproven.
d) A tentative explanationforaphenomenon.
4) A scientificlawis:
a) A descriptionof aconsistentpatterninobservations.
b) Anobservationthatremainsconstant.
c) A theorythat has beenproven.
d) A tentative explanationforaphenomenon.
5) A well-substantiatedexplanationof anaspectof the natural worldisa:
a) Theory.
b) Law.
c) Hypothesis.
d) None of these.
6) Whichof the followingwordsisclosesttothe same meaningashypothesis?
a) Fact
b) Law
c) Formula
d) Suggestion
25. e) Conclusion
7) Whydo scientistssometimesdiscardtheories?
a) The stepsin the scientificmethodwere notfollowedinorder.
b) Publicopiniondisagreeswiththe theory.
c) The theoryisopposedbythe church.
d) Contradictoryobservationsare found.
8) True/False: Whenatheoryhasbeenknownfora longtime,itbecomesalaw.
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1.3: Graphing
Objectives
Correctlygraphdata utilizingdependentvariable,independentvariable,scale and
unitsof a graph,and bestfitcurve.
Recognize patternsindatafroma graph.
Solve forthe slope of givenline graphs.
Introduction
Scientistssearchforregularitiesandtrendsin
data. Twocommon methodsof presentingdatathataid
inthe searchfor regularitiesandtrendsare tablesand
graphs.The table belowpresentsdataaboutthe pressure
and volume of asample of gas. You shouldnote thatall
tableshave a title andinclude the unitsof the
measurements.
You may note a regularitythatappearsinthis
table;as the volume of the gasdecreases(gets
smaller), itspressureincreases(getsbigger).This
regularityortrendbecomesevenmore apparentina
graph of thisdata. A graph isa pictorial
representationof patternsusingacoordinate system.
26. Whenthe data from the table isplottedasa graph, the
trendinthe relationshipbetweenthe pressureand
volume of a gas sample becomesmore apparent.The
graph givesthe scientistinformationtoaidinthe
searchfor the exactregularitythatexistsinthese data.
Whenscientistsrecordtheirresultsin adata table,the independentvariable isputin
the firstcolumn(s),the dependentvariable isrecordedinthe lastcolumn(s)andthe
controlledvariablesare typicallynotincludedatall. Note inthe data table that the first
columnislabeled“Volume (inliters)”andthatthe secondcolumnislabeled“Pressure(in
atm). That indicatesthatthe volume wasbeingchanged(the independentvariable) tosee
howit affectedthe pressure (dependentvariable).
In a graph, the independentvariableisrecordedalongthe x-axis(horizontal axis) or
as part of a keyfor the graph,the dependentvariable isrecordedalongthe y-axis(vertical
axis),andthe controlledvariablesare notincludedatall. Note inthe data table thatthe X-
axisislabeled“Volume (liters)”andthatthe Y-axisislabeled“Pressure (atm). That
indicatesthatthe volume wasbeingchanged(the independentvariable) tosee how itaffected
the pressure (dependentvariable).
DrawingLine Graphs
Readinginformationfromaline graphis easierandmore accurate as the size of the
graph increases.Inthe twographsshownbelow,the firstgraphusesonlyasmall fractionof
the space available onthe graphpaper.The secondgraphusesall the space available forthe
same graph. If you were attemptingtodetermine the pressure atatemperature of 260 K,
usingthe graph on the leftwouldgive alessaccurate resultthanusingthe graph onthe right.
Volume
(liters)
Pressure
(atm)
10.0 0.50
5.0 1.00
3.33 1.50
27. 2.50 2.00
2.00 2.50
1.67 3.00
CC – Tracy Poulsen
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Whenyoudraw a line graph,you
shouldarrange the numbersonthe axisto
use as much of the graphpaper as youcan.
If the lowesttemperature inyourdatais
100 K andthe highesttemperature inyour
data is160 K, youshouldarrange for 100
K to be on the extreme leftof yourgraph
and 160 K to be onthe extreme rightof
your graph.The creator of the graph on the
leftdidnottake thisadvice and didnot
produce a verygood graph.You should
alsomake sure that the axisonyour graph
are labeledandthatyourgraph has a title.
Whenconstructinga graph,there are some
general principlestokeepinmind:
Take up as much of the graph paper
as possible. The lowestx-value
shouldbe onthe far leftof the paper
and the highestx-valueshouldbe
on the far rightside of the paper.
Your lowesty-valueshouldbe near
the bottomof the graph and the
28. highesty-valuenearthe top.
Choose yourscale to allowyouto
do this. You do notneedto start
countingat zero.
Countyourx- andy-scalesbyconsistentamounts. If youstartcountingyourx-axis
where everybox countsas2-units,youmustcount that waythe course of the entire
axis. Your y-axismaycountby a differentscale (maybe everybox countsas5
instead),butyoumustcountthe entire y-axisbythatscale.
Bothof youraxisshouldbe labeled,includingunits. Whatwasmeasuredalongthat
axisand whatunitwas itmeasuredin?
For X-Yscatter plots,drawa best-fit-line orcurve thatfitsyourdata, insteadof
connectingthe dots. You wanta line thatshowsthe overall trendin the data, but
mightnot hitexactlyall of yourdata points. What isthe overall patterninthe data?
ReadingInformationfromaGraph
Whenwe draw a line graphfroma setof data points,we are creatingdatapoints
betweenknowndatapoints.This processiscalledinterpolation.Eventhoughwe mayhave
fouractual data pointsthatwere measured,we assume the relationshipthatexistsbetween
the quantitiesatthe actual data pointsalsoexistsatall the pointson the line graphbetween
the actual data points.Considerthe followingsetof datafor the solubilityof KClO3inwater.
The table showsthat there are exactlysix knowndatapoints.Whenthe datais
graphed,however,the graphmakerassumesthatthe relationshipbetweenthe temperature
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and the solubilityremainsthe same.The line isdrawnbyinterpolatingthe datapoints
betweenthe actual datapoints.
29. We can nowreasonablycertainlyreaddatafromthe graph forpointsthat were not
actuallymeasured.If we wishtodeterminethe solubilityof KClO3at70°C, we follow the
vertical gridline for70°C up to where ittouchesthe graphedline andthenfollow the
horizontal gridline tothe axistoread the solubility.Inthiscase,we wouldreadthe solubility
to be 30. g/100 mL of H2O at 70°C.
There are alsooccasionswhenscientists
wishto determinedatapointsfroma graphthat are
not betweenactual datapointsbutare beyondthe
endsof the actual data points.Creatingdatapoints
beyondthe endof the graph line,usingthe basic
shape of the curve as a guide iscalled
extrapolation.
Suppose the graphfor the solubilityof
potassiumchlorate hasbeenmade fromjustthree
actual data points.If the actual data pointsforthe
curve were the solubilityat60°C, 80°C, and
100°C, the graph wouldbe the solidline shownon
the graph above.If the solubilityat30°C wasdesired,we couldextrapolate (thedottedline)
fromthe graphand suggestthe solubilitytobe 5.0 g/100 mL of H2O. If we check onthe
more complete graphabove,youcan see thatthe solubilityat30°C isclose to 10 g/100 mL
of H2O. The reasonthe secondgraphproducessuch a pooransweristhat the relationshipthat
appearsinthe lesscomplete graphdoesnotholdbeyondthe endsof the graph.Forthis
reason,extrapolationisonlyacceptable forgraphswhere there isevidence thatthe
30. relationshipshowninthe graphwill be true beyondthe endsof the graph.Extrapolationis
more dangerousthatinterpolationintermsof possiblyproducingincorrectdata.
In situationsinwhichboththe independentanddependentvariablesare measured or
countedquantities,anX-Yscatter plotisthe most useful andappropriate type of graph. A
line graphcannotbe usedforindependentvariablesthatare groupsof data, or nonmeasured
data. In these situationsinwhichgroupsof data,ratherthan exact measurements,were
recordedas the independentvariable,abar graphcan typicallybe used. Considerthe datain
the followingtable.
For thisdata, a bar graph ismore appropriate because independentvariable isagroup,
not a measurement(forexample,everythingthathappenedin1980). The concept of the
average yearlyrainfall halfwaybetweenthe years1980 and 1981 doesnot make sense,soa
Temperature (°C)
Solubility
(g/100 mL H2O)
0 3.3
20 7.3
40 13.9
60 23.8
80 37.5
100 56.3
CC – Tracy Poulsen
CC – Tracy Poulsen
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Year
Rainfall
(inches)
1980 24.7
1981 21.2
31. 1982 14.5
1983 13.2
1984 21.1
1985 16.8
1986 19.9
1987 29.2
1988 31.6
1989 21.0
line graphdoesn'twork.Additionally,eachyearrepresentsagroupthat we are lookingat,
and nota measuredquantity.A bargraph isbettersuitedforthistype of data. Fromthisbar
graph,you couldveryquicklyanswerquestionslike,“Whichyearwasmostlikelyadrought
yearfor Trout Creek?”,and“Whichyear wasTrout Creekmostlikelytohave sufferedfrom
a flood?”
Findingthe Slope of aGraph
As youmay recall fromalgebra,the
slope of the line maybe determinedfromthe
graph.The slope representsthe rate at which
one variable ischangingwithrespecttothe
othervariable.Fora straight-linegraph,the
slope isconstantfor the entire line butfora
non-lineargraph,the slope isdifferentat
differentpointsalongthe line. Fora straight-
line graph,the slope forall pointsalongthe
line canbe determinedfromanysectionof the
graph.For a non-lineargraph,the mustbe
determinedforeachpointfromdata at that
point.Considerthe givendatatable andthe
lineargraphthat follows.
The relationshipinthissetof datais
linear,thatis,itproducesa straight-line graph.
32. The slope of thisline isconstantat all points
on the line.The slope of aline isdefinedasthe
rise (change invertical position) dividedbythe
run (change inhorizontal position).
Frequentlyinscience,all of ourdata pointsdo
not fall exactlyonaline.Inthissituation,we
draw a bestfitline,ora line thatgoesas close to all of ourpointsas possible.Whenfinding
the slope,itisimportantto use twopointsthat are on the bestfitline itself,insteadof our
measureddatapointswhichmaynot be on our bestfitline.Fora pairof pointsonthe line,
the coordinatesof the pointsare identifiedas(x1,y1) and (x2,y2).In thiscase,the points
selectedare (260, 1.3) and (180, 0.9). The slope canthenbe calculatedinthe manner:
CC – Tracy Poulsen
Temperature vs.Volume foraGas
Temperature (°C)
Volume of Gas
(mL)
20 60
40 65
60 70
80 75
100 80
120 85
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Therefore,the slope of the lineis0.005 atm/K.The fact that the slope ispositive indicates
that the line isrisingasit movesfromlefttorightand that the pressure increasesby0.005
33. atm for each1 Kelvinincreaseintemperature. A negativeslope wouldindicate thatthe line
was fallingasitmovesfromlefttoright.
LessonSummary
Twocommonmethodsof presentingdatathataid inthe search forregularitiesand
trendsare tablesandgraphs.
Whenwe drawa line graphfroma set of data points,we are creatingdata points
betweenknowndatapoints.Thisprocessiscalledinterpolation.
Creatingdatapointsbeyondthe endof the graph line,usingthe basicshape of the
curve as a guide iscalledextrapolation.
The slope of a graph representsthe rate atwhichone variable ischangingwith
respectto the othervariable.
Vocabulary
Graph: a pictorial representationof patternsusingacoordinate system
Interpolation:the processof estimatingvaluesbetweenmeasuredvalues
Extrapolation:the processof creatingdatapointsbeyondthe endof the graphline,
usingthe basicshape of the curve as a guide
Slope:the ratioof the change in one variable withrespecttothe othervariable.
FurtherReading/Supplemental Links
Use the followinglinktocreate bothx-yandbar graphs:
http://nces.ed.gov/nceskids/createagraph/default.aspx
These websitesoffermore tipsongraphingandinterpretingdata:
http://staff.tuhsd.k12.az.us/gfoster/standard/bgraph2.htmand
http://www.sciencebuddies.org/science-fair-projects/project_data_analysis.shtml
1.3: ReviewQuestions
1) Ona data table,where isthe independentvariabletypicallylisted? Whataboutthe
dependentvariable?
2) Ona graph,howdo youidentifythe
34. independentvariable anddependentvariable?
3) Andrewwascompletinghisdensitylabforhis
chemistrylabexam. He collectedthe given
data for volume andmass.
a) Identifythe independentanddependent
variablesinthisexperiment.
b) Draw a graph to representthe data,
includingabest-fit-line.
#3 data
Volume of
Solution(mL)
Mass of
Solution(g)
0.3 3.4
0.6 6.8
0.9 10.2
1.9 21.55
2.9 32.89
3.9 44.23
4.9 55.57
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c) If the graph isa straightline,calculate the slope,includingunits.
d) What wouldyouexpectthe massof 2.5 mL of solutiontohave?
e) What volume wouldyouexpect60g of the solutiontooccupy?
4) Donnaiscompletinganexperimenttofindthe effect
of the concentrationof ammoniaonrate (or speed) of
35. the reaction.She has collectedthe givendatafromher
time trialsand is readyforthe analysis.
a) Identifythe independentanddependentvariablesin
thisexperiment.
b) Draw a graph to representthe data,includinga
best-fit-line
c) If the concentrationof ammoniawas0.30 mol/L,
howmuch time haspassed?
d) After8 seconds,whatwill be the approximate
concentrationof ammonia?
5) Considerthe datatable foran experimentonthe
behaviorof gases.
a) Identifythe independentanddependentvariables
inthisexperiment.
b) Draw a graph to representthe data.
c) Calculate the slope,includingunits.
d) What wouldbe the pressure at55°C?
e) What wouldbe the pressure at 120°C?
#5 data
Temperature
(°C)
Pressure
(mmHg)
10 726
20 750
40 800
36. 70 880
100 960
#4 data
Time (s)
Concentration of
ammonia(mol/L)
0.71 2.40
1.07 2.21
1.95 2.00
5.86 1.53
10.84 1.30
14.39 1.08
20.43 0.81
29.67 0.60
39.80 0.40
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Chapter2: The Structure of the Atom
2.1: EarlyIdeasof Atoms
Objectives
Give a shorthistoryof the conceptof the atom.
Describe the contributionsof DemocritusandDaltontoatomictheory.
Summarize Dalton'satomictheoryandexplainitshistorical development.
Introduction
You learnedearlierhowall matterinthe universe ismade outof tinybuildingblocks
calledatoms.All modernscientistsacceptthe conceptof the atom, but whenthe conceptof
the atom was firstproposedabout2,500 yearsago,ancientphilosophers laughedatthe idea.
It has alwaysbeendifficulttoconvince peopleof the existenceof thingsthatare too small to
see.We will spendsome time consideringthe evidence (observations) thatconvince
37. scientistsof the existence of atoms.
Democritus andthe GreekPhilosophers
Before we discussthe experimentsandevidence
that have,overthe years,convincedscientiststhatmatteris
made up of atoms,it’sonlyfair to give credittothe man
whoproposed“atoms”inthe firstplace.About2,500 years
ago, earlyGreekphilosophersbelievedthe entire universe
was a single,huge,entity.Inotherwords,“everythingwas
one.”Theybelievedthatall objects,all matter,andall
substanceswere connectedasasingle,big,unchangeable
“thing.”
One of the firstpeople topropose “atoms”wasa
man knownasDemocritus.Asan alternative tothe beliefs
of the Greekphilosophers,he suggestedthatatomos,or
atomon– tiny,indivisible,solidobjects - make upall
matterin the universe.
Democritusthenreasonedthatchangesoccurwhen
the many atomosinan objectwere reconnectedor
recombinedindifferentways.Democritusevenextended
histheory,suggestingthatthere were differentvarietiesof
atomoswithdifferentshapes,sizes, andmasses.He
thought,however,thatshape,size andmasswere the onlypropertiesdifferentiatingthe
differenttypesof atomos.AccordingtoDemocritus,othercharacteristics,likecolorand
taste,didnot reflectpropertiesof the atomosthemselves, butrather,resultedfromthe
differentwaysinwhichthe atomoswere combinedandconnectedtoone another.
Greekphilosopherstrulybelievedthat,above all else,ourunderstandingof the world
shouldrelyon“logic.”Infact, theyarguedthat the worldcouldn’tbe understoodusingour
sensesatall,because oursensescoulddeceive us.Therefore,insteadof relyingon
observation,Greekphilosopherstriedtounderstandthe worldusingtheirmindsand,more
specifically,the powerof reason.
38. Democrituswasknownas “The
LaughingPhilosopher.”It’sagood
thinghe likedtolaugh,because most
otherphilosopherswere laughingat
histheories.
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So howcouldthe Greekphilosophershave knownthat
Democritushada good ideawithhistheoryof “atomos?" It
wouldhave takensome careful observationandafew simple
experiments.Now youmightwonderwhyGreekphilosophers
didn’tperformanyexperimentstoactuallytestDemocritus’
theory.The problem,of course,wasthatGreek philosophers
didn’tbelieve inexperimentsatall.Remember,Greek
philosophersdidn’ttrusttheirsenses,theyonlytrustedthe
reasoningpowerof the mind.
The earlyGreekphilosopherstriedtounderstandthe
nature of the worldthroughreasonandlogic,but notthrough
experimentandobservation.Asaresult,theyhadsome very
interestingideas,buttheyfeltnoneedtojustifytheirideas
basedon life experiences.Inalotof ways,youcan thinkof the
Greekphilosophersasbeing“all thoughtandnoaction.”It’s
trulyamazinghowmuch theyachievedusingtheirminds,but
because theyneverperformedanyexperiments,theymissedor
rejectedalotof discoveriesthattheycouldhave made otherwise. Greekphilosophers
dismissedDemocritus’theoryentirely.Sadly,ittookovertwomillenniabeforethe theoryof
atomos(or “atoms,”as they’re knowntoday) wasfullyappreciated.
Dalton'sAtomicTheory
Althoughthe conceptof atomsis nowwidelyaccepted,thiswasn’talwaysthe case.
Scientistsdidn’talwaysbelieve thateverythingwascomposedof small particlescalled
39. atoms. The workof several scientistsandtheirexperimental datagave evidence forwhatis
nowcalledthe atomictheory.
In the late 1700’s, Antoine Lavoisier,aFrenchscientist,experimentedwiththe
reactionsof manymetals. He carefullymeasuredthe massof a substance before reactingand
againmeasuredthe massaftera reactionhadoccurredin a closedsystem(meaningthat
nothingcouldenterorleave the container). He foundthatnomatter whatreactionhe looked
at, the mass of the starting materialswasalwaysequal tothe massof the endingmaterials.
Thisis nowcalledthe lawof conservationof mass. Thiswent contrary to whatmany
scientistsatthe time thought. Forexample,whenapiece of ironrusts,itappearsto gain
mass. Whena logis burned,itappearsto lose mass. Inthese examples,though,the reaction
doesnottake place ina closedcontainerandsubstances,suchasthe gasesinthe air, are able
to enteror leave. Whenironrusts,itiscombiningwithoxygeninthe air,whichiswhyit
seemstogainmass. What Lavoisierfoundwasthatno masswas actuallybeinggainedor
lost. It was comingfromthe air. Thiswas a veryimportantfirststepingivingevidence for
the ideathat everythingismade of atoms. The atoms(andmass) are not beingcreatedor
destroyed. The atomsare simplyreactingwithotheratomsthatare alreadypresent.
In the late 1700s andearly1800s, scientistsbegannoticingthatwhencertain
substances,like hydrogenandoxygen,were combinedtoproduce anew substance,like
water,the reactants(hydrogenandoxygen) alwaysreactedinthe same proportionsbymass.
In otherwords,if 1 gram of hydrogenreactedwith8 grams of oxygen,then2grams of
hydrogenwouldreactwith16 grams of oxygen,and3 grams of hydrogenwouldreactwith
24 grams of oxygen.Strangely,the observationthathydrogen andoxygenalwaysreactedin
Greekphilosopherstriedto
understandthe nature of the
worldthroughreasonand
logicbut notthrough
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the “same proportionsbymass” wasn’tspecial.Infact,itturnedout that the reactantsin
everychemical reactionreactedinthe same proportionsbymass. Thisobservationis
40. summarizedinthe lawof definiteproportions. Take,forexample,nitrogenandhydrogen,
whichreact to produce ammonia.Inchemical reactions, 1gram of hydrogenwill reactwith
4.7 grams of nitrogen,and2 gramsof hydrogenwill reactwith9.4 gramsof nitrogen.Can
youguesshowmuch nitrogenwouldreactwith3 grams of hydrogen? Scientistsstudied
reactionafterreaction,buteverytime the resultwasthe same.The reactantsalwaysreacted
inthe same proportions.
At the same time that scientistswerefindingthispattern
out,a mannamedJohnDalton wasexperimentingwithseveral
reactionsinwhichthe reactantelementsformedmore thanone
type of product,dependingonthe experimental conditionshe
used.One commonreactionthathe studiedwasthe reaction
betweencarbonandoxygen.Whencarbonandoxygenreact,
theyproduce twodifferentsubstances –we’ll call these
substances“A”and “B.” It turnedout that,giventhe same
amountof carbon,formingB alwaysrequiredexactlytwice as
much oxygenasformingA.Inotherwords,if you can make A
with3 gramsof carbon and 4 grams of oxygen,Bcan be made
withthe same 3 grams of carbon, butwith8 grams oxygen.
Daltonaskedhimself –whydoesB require 2timesas much
oxygenasA? Whynot 1.21 timesas much oxygen,or0.95
timesasmuch oxygen?Whya whole numberlike2?
The situationbecame evenstrangerwhenDaltontried
similarexperimentswithdifferentsubstances.Forexample,
whenhe reactednitrogenandoxygen,Daltondiscoveredthathe couldmake three different
substances – we’ll call them“C,”“D,” and “E.” As itturnedout,for the same amount of
nitrogen,Dalwaysrequiredtwice asmuchoxygenasC. Similarly,Ealwaysrequiredexactly
fourtimesas muchoxygenas C.Once again,Daltonnoticedthatsmall whole numbers(2
and 4) seemedtobe the rule.Thisobservationcame tobe knownas the law of multiple
proportions.
Daltonthoughtabouthis resultsandtriedtofindsome theorythatwouldexplainit,as
well asa theorythat wouldexplainthe Law of Conservationof Mass(massis neithercreated
41. nor destroyed,orthe massyouhave at the beginningisequal tothe massat the endof a
change).One wayto explainthe relationshipsthatDaltonandothershad observedwasto
suggestthatmaterialslike nitrogen,carbonandoxygenwere composedof small,indivisible
quantitieswhichDaltoncalled“atoms”(inreference toDemocritus’original idea).Dalton
usedthisideato generate whatisnowknownasDalton’sAtomicTheorywhichstatedthe
following:
1. Matter ismade of tinyparticlescalledatoms.
2. Atomsare indivisible(can’tbe brokenintosmallerparticles).Duringachemical
reaction,atomsare rearranged,buttheydonot breakapart, nor are theycreatedor
destroyed.
3. All atomsof a givenelementare identical inmassandotherproperties.
4. The atoms of differentelementsdifferinmassandotherproperties.
Unlike the Greek
philosophers,JohnDalton
believedinbothlogical
thinkingand
experimentation.
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5. Atomsof one elementcancombine withatomsof anotherelementtoform
“compounds”– new,complex particles.Inagivencompound,however,the
differenttypesof atomsare alwayspresentinthe same relative numbers.
LessonSummary
2,500 years ago,Democritussuggestedthatall matterinthe universe wasmade upof
tiny,indivisible,solidobjectshe called“atomos.”
OtherGreekphilosophersdislikedDemocritus’“atomos”theorybecause theyfeltit
was illogical.
Daltonusedobservationsaboutthe ratiosinwhichelementswillreact tocombine and
The Law of Conservationof Massto propose hisAtomicTheory.
42. Dalton’sAtomicTheorystates:
1. Matter ismade of tinyparticlescalledatoms.
2. Atomsare indivisible.Duringachemical reaction,atomsare rearranged,butthey
do notbreak apart,nor are theycreatedor destroyed.
3. All atomsof a givenelementare identical inmassandotherproperties.
4. The atomsof differentelementsdifferinmassandotherproperties.
5. Atomsof one elementcancombine withatomsof anotherelementtoform
“compounds”– newcomplex particles.Ina givencompound,however,the different
typesof atoms are alwayspresentinthe same relative numbers.
Vocabulary
Atom:Democritus’ wordforthe tiny,indivisible,solidobjectsthathe believedmade
up all matterin the universe
Dalton’sAtomicTheory:the firstscientifictheorytorelate chemical changestothe
structure,properties,andbehaviorof the atom
FurtherReading/Supplemental Links
To see a videodocumentingthe earlyhistoryof the conceptof the atom, go to
http://www.uen.org/dms/. Goto the k-12 library. Searchfor“historyof the atom”.
Watch part 01. (youcan getthe username andpasswordfromyour teacher)
VisionLearning:FromDemocritustoDalton:
http://visionlearning.com/library/module_viewer.php?c3=&mid=49&l=
2.1: ReviewQuestions
1) (Multiple choice) Whichof the followingisnotpart of Dalton’sAtomicTheory?
a) matteris made of tinyparticlescalledatoms.
b) duringa chemical reaction,atomsare rearranged.
c) duringa nuclearreaction,atomsare splitapart.
d) all atomsof a specificelementare the same.
2) DemocritusandDaltonbothsuggestedthatall matterwascomposedof small particles,
43. calledatoms. What isthe greatestadvantage Dalton’sAtomicTheoryhadover
Democritus’?
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3) Itturns out that a fewof the ideasinDalton’sAtomicTheoryaren’tentirelycorrect.Are
inaccurate theoriesanindicationthatscience isawaste of time?
2.2: FurtherUnderstandingof the Atom
Objectives
Explainthe observationsthatledtoThomson'sdiscoveryof the electron.
Describe Thomson's"plumpudding"mode of the atom andthe evidenceforit
Draw a diagramof Thomson's"plumpudding"modelof the atomand explainwhyit
has thisname.
Describe Rutherford'sgoldfoil experimentandexplainhow thisexperimentaltered
the "plumpudding"model.
Draw a diagramof the Rutherfordmodel of the atomandlabel the nucleusandthe
electroncloud.
Introduction
Dalton'sAtomicTheoryheldupwell toa lotof the
differentchemical experimentsthatscientistsperformedtotest
it.In fact, foralmost100 years,itseemedasif Dalton's
AtomicTheorywasthe whole truth.However,in1897, a
scientistnamedJ.J.Thomsonconductedsome researchthat
suggestedthatDalton’sAtomicTheorywasn’tthe entire story.
As itturns out,Daltonhad a lot right.He was rightin saying
matteris made up of atoms;he wasright insayingthere are
differentkindsof atomswithdifferentmassandother
properties;he was“almost”rightinsayingatomsof a given
elementare identical;he wasrightinsayingduringachemical
44. reaction,atomsare merelyrearranged;he wasrightinsayinga
givencompoundalwayshasatomspresentinthe same relative
numbers.Buthe was WRONG insayingatomswere
indivisible orindestructible.Asitturnsout,atomsare
divisible.Infact,atomsare composedof evensmaller,more fundamental particles.These
particles,calledsubatomicparticles,are particlesthatare smallerthanthe atom.We’ll talk
aboutthe discoveriesof these subatomicparticlesnext.
Thomson’sPlumPuddingModel
In the mid-1800s, scientistswere beginningtorealize thatthe studyof chemistryand
the studyof electricitywereactuallyrelated.First,amannamedMichael Faradayshowed
howpassingelectricitythroughmixturesof different chemicalscouldcause chemical
reactions.Shortlyafterthat,scientistsfoundthatbyforcingelectricitythroughatube filled
withgas,the electricitymade the gasglow!Scientistsdidn’t,however,understandthe
relationshipbetweenchemicalsand electricityuntil aBritishphysicistnamedJ.J.Thomson
beganexperimentingwithwhatisknownasa cathode ray tube.
The figure showsa basicdiagramof a cathode ray tube like the one J.J. Thomson
wouldhave used.A cathode ray tube isa small glasstube witha cathode (a negatively
chargedmetal plate) andan anode (apositivelychargedmetalplate) atoppositeends.By
J.J.Thomsonconducted
experimentsthatsuggested
that Dalton’satomictheory
wasn’ttellingthe entire
story.
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separatingthe cathode andanode by a shortdistance,the cathode ray tube can generate what
are knownascathode rays – rays of electricitythatflow fromthe cathode tothe anode.J. J.
Thomsonwantedtoknowwhat cathode rayswere,where cathode rayscame from, and
whethercathode rayshadany mass or charge.The techniquesthatJ.J.Thomsonusedto
answerthese questionswere verycleverandearnedhimaNobel Prize inphysics.First,by
45. cuttinga small hole inthe anode,J.J. Thomsonfoundthathe couldgetsome of the cathode
rays to flowthroughthe hole inthe anode andintothe otherendof the glass cathode ray
tube.Next,J.J.Thomsonfiguredoutthatif he paintedasubstance knownas“phosphor”
ontothe far endof the cathode ray tube,he couldsee exactlywhere the cathode rayshit
because the cathode raysmade the phosphorglow.
J. J.
Thomsonmusthave
suspectedthat
cathode rays were
charged,because
hisnextstepwasto
place a positively
chargedmetal plate
on one side of the
cathode ray tube
and a negatively
chargedmetal plate
on the otherside of
the cathode ray
tube,as shownin
Figure 3. The metal
platesdidn’tactuallytouchthe cathode raytube,buttheywere close enoughthata
remarkable thinghappened!The flow of the cathode rayspassingthroughthe hole inthe
anode wasbentupwardstowardsthe positive metal plate andawayfromthe negative metal
plate.Usingthe “opposite chargesattract, like chargesrepel”rule,J.J.Thomsonarguedthat
if the cathode rays were attractedto the positivelychargedmetal plate andrepelledfromthe
negativelychargedmetal plate,theythemselvesmusthave anegative charge!
J. J.Thomsonthendidsome rather complex experimentswithmagnets,andusedhis
resultstoprove that cathode rays were notonlynegativelycharged,butalsohadmass.
Rememberthatanythingwithmassispart of whatwe call matter.In otherwords,these
cathode rays mustbe the resultof negativelycharged“matter”flowingfromthe cathode to
46. the anode.But there wasa problem.AccordingtoJ.J. Thomson’smeasurements,eitherthese
cathode rays had a ridiculouslyhighcharge,orelse hadvery,verylittle mass–much less
mass thanthe smallestknownatom.How wasthispossible?How couldthe mattermaking
up cathode rays be smallerthanan atomif atoms were indivisible?J.J.Thomsonmade a
radical proposal:maybe atomsare divisible.J.J.Thomson suggestedthatthe small,
negativelychargedparticlesmakingupthe cathode ray were actuallypiecesof atoms.He
calledthese pieces“corpuscles,”althoughtodaywe know themaselectrons.Thankstohis
cleverexperimentsandcareful reasoning,J.J. Thomsoniscreditedwiththe discoveryof the
electron.
Thomson’sexperimentwithcathode raysfoundthatthe ray movedaway
fromnegativelychargedplatesandtowardpositivelychargesplates. What
doesthissay aboutthe charge of the ray?
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Nowimagine whatwouldhappenif atomswere made entirelyof electrons.Firstof
all,electronsare very,verysmall;infact,electronsare about2,000 timessmallerthanthe
smallestknownatom,soeveryatomwouldhave tocontaina whole lotof electrons.But
there’sanother,evenbiggerproblem:electronsare negativelycharged.Therefore,if atoms
were made entirelyoutof electrons,atomswouldbe negativelychargedthemselves…and
that wouldmeanall matterwasnegativelychargedaswell.Of course,matterisn’tnegatively
charged.In fact,most matteriswhat we call neutral – it has no charge at all.If matteris
composedof atoms,and atomsare composedof negative electrons,how canmatterbe
neutral?The onlypossible explanationisthatatomsconsistof more than justelectrons.
Atomsmustalso containsome type of positivelychargedmaterialthatbalancesthe negative
charge onthe electrons.Negativeandpositive chargesof equal size cancel eachother out,
justlike negative andpositive numbersof equal size.Whatdoyouget if youadd +1 and -1?
You get0, or nothing.That’strue of numbers,andthat’salso true of charges.If an atom
containsan electronwitha -1 charge,but alsosome formof material witha+1 charge,
overall the atommusthave a (+1) + (-1) = 0 charge – inotherwords,the atom must be
neutral,orhave no charge at all.
47. Basedon the fact that atomsare neutral, andbasedon J.J. Thomson’sdiscoverythat
atomscontainnegative subatomicparticlescalled“electrons,”scientistsassumedthatatoms
mustalso containa positive substance.Itturnedoutthatthispositive substance wasanother
kindof subatomicparticle,knownasthe proton.Althoughscientistsknewthatatomshadto
containpositive material,protonsweren’tactuallydiscovered,orunderstood,until quiteabit
later.
WhenThomsondiscoveredthe negative electron,he realizedthatatomshadto
containpositive materialaswell –otherwise theywouldn’tbe neutral overall.Asaresult,
Thomsonformulatedwhat’sknownasthe “plumpudding”modelforthe atom.Accordingto
the “plumpudding”model,the negative electronswere
like piecesof fruitandthe positivematerial waslikethe
batteror the pudding.Thismade alot of sense given
Thomson’sexperimentsandobservations.Thomsonhad
beenable toisolate electronsusingacathode ray tube;
howeverhe hadnevermanagedtoisolate positive
particles.Asa result,Thomsontheorizedthatthe positive
material inthe atom mustformsomethinglike the “batter”
ina plumpudding,while the negative electronsmustbe
scatteredthroughthis“batter.”(If you’ve neverseenor
tasteda plumpudding,youcan thinkof a chocolate chip
cookie instead.Inthatcase,the positive materialinthe
atom wouldbe the “batter”in the chocolate chipcookie,
while the negativeelectronswouldbe scatteredthrough
the batter like chocolate chips.)
Notice how easyit wouldbe topickthe piecesof fruitoutof a plumpudding.Onthe
otherhand,it wouldbe a lotharderto pick the batterout of the plumpudding,because the
batteris everywhere.If anatomwere similartoa plumpuddinginwhichthe electronsare
scatteredthroughoutthe “batter”of positive material,thenyou’dexpectitwouldbe easyto
pickout the electrons,buta lotharderto pickout the positive material.
Thomson’splumpuddingmodel
48. was muchlike a chocolate chip
cookie. Notice howthe chocolate
chipsare the negativelycharged
electrons,while the positivecharge
isspreadthroughoutthe entire
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J.J.Thomsonhad measuredthe charge tomass ratioof the electron,buthadbeen
unable toaccuratelymeasure the charge on the electron.Withhisoil dropexperiment,Robert
Millikanwasable toaccuratelymeasure the charge of the electron.Whencombinedwiththe
charge to massratio,he wasable to calculate the massof the electron.WhatMillikan didwas
to put a charge ontinydropletsof oil andmeasuredtheirrate of descent.Byvaryingthe
charge ondifferentdrops,he noticedthatthe electricchargesonthe drops were all multiples
of 1.6x10-19
C, the charge ona single electron.
Rutherford’sNuclearModel
EverythingaboutThomson’sexperimentssuggested
the “plumpudding”model wascorrect – butaccording to the
scientificmethod,anynewtheoryormodel shouldbe tested
by furtherexperimentationandobservation.Inthe case of the
“plumpudding”model,itwouldtake amannamedErnest
Rutherfordtoprove itinaccurate.Rutherfordandhis
experimentswill be the topicof the nextsection.
DisprovingThomson’s“plumpudding”modelbegan
withthe discoverythatan elementknownasuraniumemits
positivelychargedparticlescalledalphaparticlesasit
undergoesradioactivedecay.Radioactive decayoccurswhen
one elementdecomposesintoanotherelement.Itonlyhappens
witha fewvery unstable elements.Alphaparticlesthemselves
didn’tprove anythingaboutthe structure of the atom, they
49. were,however,usedtoconductsome veryinterestingexperiments.
ErnestRutherfordwasfascinatedbyall aspectsof alphaparticles.Forthe most part,
though,he seemedtoviewalphaparticlesastinybulletsthathe coulduse tofire at all kinds
of differentmaterials.One experimentinparticular,however,surprisedRutherford,and
everyone else.
Rutherfordfoundthat
whenhe firedalphaparticles
at a verythinpiece of gold
foil,aninterestingthing
happened.Almostall of the
alphaparticleswentstraight
throughthe foil as if they’d
hitnothingat all.Thiswas
whathe expectedtohappen.
If Thomson’smodel was
accurate,there was nothing
hard enoughforthese small
particlestohitthat would
cause any change in their
motion.
Everyso often,
though,one of the alpha
particleswouldbe deflected
ErnestRutherford
ErnestRutherford'sGoldFoil Experimentinwhichalphaparticleswere
shotat a piece of goldfoil. Mostof the particleswentstraightthrough,but
some bouncedstraightback,indicatingtheywere hittingaverysmall,very
dense particle inthe atom.
CC – Tracy Poulsen32
50. www.ck12.org
slightlyasif ithad bouncedoff of somethinghard.Evenlessoften,Rutherfordobserved
alphaparticlesbouncingstraightbackat the “gun” fromwhichtheyhad beenfired!Itwasas
if these alphaparticleshadhita wall “head-on”andhadricochetedrightback inthe direction
that theyhad come from.
Rutherfordthoughtthatthese experimental resultswereratherodd.Rutherford
describedfiringalphaparticlesatgoldfoil like shootingahigh-poweredrifle attissue paper.
Wouldyoueverexpectthe bulletstohitthe tissue paperandbounce backat you?Of course
not!The bulletswouldbreakthroughthe tissuepaperandkeepongoing,almostasif they’d
hitnothingat all.That’swhat Rutherfordhadexpectedwouldhappenwhenhe firedalpha
particlesatthe goldfoil.Therefore,the factthatmostalphaparticlespassedthroughdidn’t
shockhim.On the otherhand,how couldhe explainthe alphaparticlesthatgotdeflected?
Furthermore,howcouldhe explainthe alphaparticlesthatbouncedrightbackasif they’dhit
a wall?
Rutherforddecidedthatthe onlywaytoexplainhisresultswastoassume that the
positive matterformingthe goldatomswasnot,infact,distributedlikethe batterinplum
pudding,butrather,wasconcentratedinone spot,forminga small positivelychargedparticle
somewhere inthe centerof the goldatom.We now call thisclumpof positivelycharged
mass the nucleus.AccordingtoRutherford,the presence of anucleusexplainedhis
experiments,because itimpliedthatmostalphaparticlespassedthroughthe goldfoil without
hittinganythingatall.Once ina while,though,the alphaparticleswould actuallycollide
witha goldnucleus,causingthe alphaparticlestobe deflected,oreventobounce rightback
inthe directiontheycame from.
While Rutherford’sdiscoveryof the positively
chargedatomic nucleusofferedinsightintothe structure of
the atom, italsoledto some questions.Accordingtothe
“plumpudding”model,electronswerelikeplumsembedded
inthe positive “batter”of the atom.Rutherford’smodel,
though,suggestedthatthe positivecharge wasn’tdistributed
like batter,butrather,wasconcentratedintoa tinyparticle at
the centerof the atom,while mostof the restof the atom was
51. emptyspace.What didthat meanforthe electrons?If they
weren’tembeddedinthe positivematerial,exactlywhatwere
theydoing?Andhowwere theyheldinthe atom?Rutherford
suggestedthatthe electronsmightbe circlingor“orbiting”
the positivelychargednucleusassome type of negatively
chargedcloud,but at the time,there wasn’tmuchevidenceto
suggestexactlyhowthe electronswereheldinthe atom.
Despite the problemsandquestionsassociatedwith
Rutherford’sexperiments,hisworkwithalphaparticlesdefinitelyseemedtopointtothe
existence of anatomic“nucleus.”BetweenJ.J.Thomson,whodiscoveredthe electron,and
Rutherford,whosuggestedthatthe positive chargesinanatomwere concentratedatthe
atom’scenter,the 1890s andearly1900s saw huge stepsinunderstandingthe atomat the
“subatomic”(orsmallerthanthe size of an atom) level.Althoughthere wasstill some
uncertaintywithrespecttoexactlyhowsubatomicparticleswere organizedinthe atom, it
was becomingmore andmore obviousthatatomswere indeeddivisible.Moreover,itwas
clearthat an atom containsnegativelychargedelectronsandanucleuscontainingpositive
Rutherfordsuggestedthat
electronssurrounda
central nucleus.
(Obtainedfrom:
http://upload.wikimedia.org/wiki
pedia/commons/7/7d/Rutherford
sches_Atommodell.png)
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charges.In the nextsection,we’ll lookmore carefullyatthe structure of the nucleus,and
we’ll learnthatwhile the atomismade upof positive andnegative particles,italsocontains
neutral particlesthatneitherThomson,norRutherford,were able todetectwiththeir
experiments.
52. LessonSummary
Dalton’sAtomicTheorywasn’tentirelycorrect.Itturnsout thatatoms can be divided
intosmallersubatomicparticles.
AccordingtoThomson’s“plumpudding”model,the negativelychargedelectronsin
an atom are like the piecesof fruitinaplumpudding,while the positivelycharged
material islike the batter.
WhenErnestRutherfordfiredalphaparticlesatathingoldfoil,mostalphaparticles
wentstraightthrough;however,afewwere scatteredatdifferentangles,andsome
evenbouncedstraightback.
Inorder to explainthe resultsof hisGoldFoil experiment,Rutherfordsuggestedthat
the positive matterinthe goldatomswasconcentratedat the centerof the goldatom
inwhat we nowcall the nucleusof the atom.
Vocabulary
Subatomicparticles:particlesthatare smallerthanthe atom
Electron:a negativelychargedsubatomicparticle
Proton:a positivelychargedsubatomicparticle
Nucleus:the small,dense centerof the atom
FurtherReading/Supplemental Material
A short historyof the changesin ourmodel of the atom, an image of the plum
puddingmodel,andananimationof Rutherford'sexperimentcanbe viewedatPlum
PuddingandRutherfordPage (http://www.newcastle-
schools.org.uk/nsn/chemistry/Radioactivity/Plub%20Pudding%20and%20Rutherford
%20Page.htm).
To see a videodocumentingthe earlyhistoryof the conceptof the atom, go to
http://www.uen.org/dms/. Goto the k-12 library. Searchfor“historyof the atom”.
Watch part 02. (youcan getthe username andpasswordfromyour teacher)
VisionLearning:The EarlyDays(Thomson,etc)
http://visionlearning.com/library/module_viewer.php?mid=50&l=&c3=
Discoveryof Electron(YouTube):
54. charged
d. Nomatter whichelementThomsonputina cathode
ray tube,the same negative particleswiththe same
properties(suchascharge & mass) were ejected.
9) Atomshave a small,dense
nucleus
e.The particlesejectedinThomson’sexperimentbent
away fromnegativelychargedplates,buttoward
positivelychargedplates.
10) What isthe name givento the tinyclumpof positive material atthe centerof an atom?
11) Electronsare ______ negativelychargedmetalsplatesand______ positivelycharged
metal plates.
Considerthe followingtwoparagraphsfor#12-14
Scientist1:Althoughatomswere once regardedasthe smallestpartof nature,theyare
composedof evensmallerparticles. All atomscontainnegativelychargedparticles,
calledelectrons. However,the total charge of anyatom iszero. Therefore,thismeans
that there mustalsobe positive charge inthe atom. The electronssitina bedof
positivelychargedmass.
Scientist2:It is true that atomscontainsmallerparticles. However,the electronsare not
floatingina bedof positive charge. The positivecharge islocatedinthe central part of
the atom, ina verysmall,dense mass,called anucleus. The electronsare foundoutside
of the nucleus.
12) What isthe maindispute betweenthe twoscientists’theories?
13) Anotherscientistwasable tocalculate the exactcharge of an electrontobe -1.6x10-19
C.
What effectdoesthishave onthe claimsof Scientist1? (Pickone answer)
a) Goesagainsthis claim
b) Supportshisclaim
55. c) Has noeffectonhisclaim.
14) If a positivelychargedparticle wasshotata thinsheetof goldfoil,whatwouldthe
secondscientistpredictto happen?
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2.3: Protons,Neutrons,andElectronsinAtoms
Objectives
Describe the locations,charges,andmassesandthe three mainsubatomicparticles.
Define atomicnumber.
Describe the size of the nucleusinrelationtothe size of the atom.
Define massnumber.
Explainwhatisotopesare andhowisotopesaffectanelement’satomicmass.
Determine the numberof protons,neutrons,andelectronsinanatom.
Introduction
Dalton’sAtomicTheoryexplainedalotabout matter,chemicals,andchemical
reactions.Nevertheless,itwasn’tentirelyaccurate,becausecontrarytowhat Daltonbelieved,
atomscan, in fact,be brokenapartintosmallersubunitsorsubatomic particles.We have
beentalkingaboutthe electroningreatdetail,butthere are twootherparticlesof interestto
use:protonsand neutrons. Inthissection,we’lllookatthe atoma little more closely.
Protons,Electrons,andNeutrons
We alreadylearnedthatJ.J.Thomsondiscoveredanegativelychargedparticle,called
the electron. Rutherfordproposedthatthese electronsorbitapositive nucleus. In
subsequentexperiments,he foundthatthere isa smallerpositivelychargedparticlein the
nucleuswhichiscalledaproton. There is a thirdsubatomicparticle,knownasaneutron.
ErnestRutherfordproposedthe existence of aneutral particle,withthe approximate massof
a proton.Years later,JamesChadwickprovedthatthe nucleusof the atomcontainsthis
56. neutral particle thathadbeenproposedbyErnestRutherford. Chadwickobservedthatwhen
berylliumisbombardedwithalphaparticles,it
emitsanunknownradiationthathasapproximately
the same mass as a proton,but no electrical charge.
Chadwickwasable to prove thatthe beryllium
emissionscontainedaneutral particle - Rutherford’s
neutron.
As youmighthave alreadyguessedfromits
name,the neutronisneutral.Inotherwords,ithas
no charge whatsoever,andistherefore neither
attractedto nor repelledfromotherobjects.
Neutronsare ineveryatom(withone exception),
and they’re boundtogetherwithotherneutronsand
protonsinthe atomicnucleus.
Before we move on,we must discusshow the differenttypesof subatomicparticles
interactwitheachother.Whenit comesto neutrons,the answerisobvious.Since neutrons
are neitherattractedto,norrepelledfromobjects,theydon’treallyinteractwithprotonsor
electrons (beyondbeingboundintothe nucleuswiththe protons).
Eventhoughelectrons,protons,andneutronsare all typesof subatomicparticles,they
are notall the same size.Whenyoucompare the massesof electrons,protonsandneutrons,
whatyou findis that electronshave anextremelysmallmass,comparedtoeitherprotonsor
neutrons.Onthe otherhand,the massesof protonsandneutronsare fairlysimilar,although
technically,the massof aneutronisslightlylargerthanthe massof a proton.Because
Electronsare much smallerthan
protonsor neutrons. If an electronwas
the mass of a penny,aprotonor a
neutronwouldhave the massof a large
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protonsand neutronsare so much
57. more massive thanelectrons,almost
all of the massof any atomcomes
fromthe nucleus,whichcontainsall
of the neutronsandprotons.
The table showngivesthe
propertiesandlocationsof electrons,
protons,andneutrons.The third
columnshowsthe massesof the three
subatomicparticlesingrams.The
secondcolumn,however,showsthe massesof the three subatomicparticlesin“atomicmass
units”.Anatomicmass unit(amu) isdefinedasone-twelfththe massof acarbon-12 atom.
Atomicmassunits(amu) are useful,because,asyoucan see,the massof a proton and the
mass of a neutronare almostexactly1.0 inthisunitsystem.
In additiontomass,anotherimportantpropertyof subatomicparticlesistheircharge.
You alreadyknowthatneutronsare neutral,andthus have nocharge at all.Therefore,we say
that neutronshave acharge of zero.What aboutelectronsandprotons?Youknow that
electronsare negativelychargedandprotonsare positivelycharged,butwhat’samazingis
that the positive charge ona protonis exactlyequal inmagnitude(magnitudemeans
“absolute value”or“size whenyouignore positive andnegative signs”) tothe negative
charge onan electron. The thirdcolumninthe table showsthe chargesof the three
subatomicparticles.Notice thatthe charge onthe proton andthe charge on the electronhave
the same magnitude.
Negative andpositive chargesof equal magnitude cancel eachotherout.Thismeans
that the negative charge onan electronperfectlybalancesthe positive charge onthe proton.
In otherwords,a neutral atommust have exactlyone electronforeveryproton.If aneutral
atom has 1 proton,it musthave 1 electron.If aneutral atom has2 protons,itmust have 2
electrons.If aneutral atomhas 10 protons,it musthave 10 electrons.Yougetthe idea.In
orderto be neutral,anatommust have the same numberof electronsandprotons.
AtomicNumberandMass Number
Scientistscandistinguish
58. betweendifferentelementsbycounting
the numberof protons.If an atomhas
onlyone proton,we knowit’sa
hydrogenatom.Anatom withtwo
protonsisalwaysa heliumatom. If
scientistscountfourprotonsinan atom,
theyknowit’sa berylliumatom. An
atom withthree protonsisa lithium
atom,an atom withfive protonsisa
boronatom, an atomwithsix protonsis
a carbon atom… the listgoeson.
Since an atomof one element
can be distinguishedfromanatomof
anotherelementbythe numberof
It isdifficulttofindqualitiesthat are differentfromeach
elementanddistinguishonelementfromanother. Each
element,however,doeshave aunique numberof protons.
Sulfurhas16 protons,siliconhas14 protons,andgoldhas
79 protons.
Sub-AtomicParticles,PropertiesandLocation
Particle
Relative
Mass
(amu)
Electric
Charge
Location
electron -1
outside the
nucleus
59. proton 1 +1 nucleus
neutron 1 0 nucleus
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protonsinits nucleus,scientistsare alwaysinterestedinthisnumber,andhow thisnumber
differsbetweendifferentelements.Therefore,scientistsgive thisnumberaspecial name.An
element’satomicnumberisequal tothe numberof protonsinthe nuclei of anyof itsatoms.
The periodictable givesthe atomicnumberof eachelement.The atomicnumberisawhole
numberusuallywrittenabovethe chemical symbol of eachelement.The atomicnumberfor
hydrogenis1, because everyhydrogenatomhas1 proton.The atomic numberforheliumis2
because everyheliumatomhas2 protons.What is the atomicnumberof carbon?
Of course,since neutral atomshave tohave one electronforeveryproton,an
element’satomicnumberalsotellsyouhow manyelectronsare ina neutral atomof that
element.Forexample,hydrogenhasanatomic numberof 1. Thismeansthat an atom of
hydrogenhasone proton,and,if it’sneutral,one electronaswell.Gold,onthe otherhand,
has an atomicnumberof 79, whichmeansthat an atomof goldhas 79 protons,and,if it’s
neutral,and79 electronsaswell.
The mass numberof an atom isthe total numberof protonsand neutronsinits
nucleus.Whydoyou thinkthatthe “mass number”includesprotonsandneutrons,butnot
electrons?Youknowthatmost of the mass of an atomis concentratedinitsnucleus.The
mass of an atom dependsonthe numberof protonsandneutrons.Youhave alreadylearned
that the mass of an electronisvery,verysmall comparedtothe massof eitheraproton or a
neutron(like the massof a pennycomparedtothe mass of a bowlingball).Countingthe
numberof protonsand neutronstellsscientistsaboutthe total massof an atom.
mass numberA = (numberof protons) +(numberof neutrons)
An atom’smassnumberisa veryeasyto calculate providedyouknow the numberof protons
and neutronsinanatom.
Example:
60. What isthe massnumberof an atom of heliumthatcontains2 neutrons?
Solution:
(numberof protons) = 2 (Rememberthatanatom of heliumalwayshas2 protons.)
(numberof neutrons) =2
mass number= (numberof protons) +(numberof neutrons)
mass number= 2 + 2 = 4
There are twomainwaysin whichscientistsfrequentlyshow the massnumberof an
atom theyare interestedin. Itisimportanttonote that the mass numberisnot givenonthe
periodictable. These twowaysincludewritinganuclearsymbol orby givingthe name of
the elementwiththe massnumberwritten.
To write a nuclearsymbol,the massnumberisplacedatthe upperleft(superscript)
of the chemical symbol andthe atomicnumberisplacedatthe lowerleft(subscript) of the
symbol.The complete nuclearsymbol forhelium-4isdrawnbelow.
The followingnuclearsymbolsare foranickel nucleuswith31neutronsanda uranium
nucleuswith146 neutrons. 38
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In the nickel nucleusrepresentedabove,the atomicnumber28indicatesthe nucleuscontains
28 protons,and therefore,itmustcontain31neutronsinordertohave a massnumberof 59.
The uraniumnucleushas92 protonsas do all uraniumnuclei andthisparticularuranium
nucleushas146 neutrons.
The other wayof representingthesenucleiwouldbe Nickel-59andUranium-238,
where 59 and 238 are the massnumbersof the two atoms,respectively. Note thatthe mass
numbers(notthe numberof neutrons) isgiventothe side of the name.
Isotopes
Unlike the numberof protons,whichisalwaysthe same inatomsof the same
element,the numberof neutronscanbe different,eveninatomsof the same element.Atoms
61. of the same element,containingthe same numberof protons,butdifferentnumbersof
neutronsare knownas isotopes. Since the isotopesof anygivenelementall containthe same
numberof protons,theyhave the same atomicnumber(forexample,the atomicnumberof
heliumisalways2).However,since the isotopesof agivenelementcontaindifferent
numbersof neutrons,differentisotopeshave differentmassnumbers.The followingtwo
examplesshouldhelptoclarifythispoint.
Example:
a) What isthe atomic numberandthe mass numberof an isotope of lithiumcontaining3
neutrons.A lithiumatomcontains3protonsin itsnucleus.
b) What isthe atomicnumberand the mass numberof an isotope of lithiumcontaining4
neutrons.A lithiumatomcontains3protonsin itsnucleus.
Solution:
a) atomicnumber= (numberof protons) = 3
(numberof neutrons) =3
mass number= (numberof protons) +(numberof neutrons)
mass number= 3 + 3 = 6
b) atomic number= (numberof protons) = 3
(numberof neutrons) =4
mass number= (numberof protons) +(numberof neutrons)
mass number= 3 + 4 = 7
Notice thatbecause the lithiumatomalwayshas3 protons,the atomicnumberfor
lithiumisalways3.The mass number,however,is6 inthe isotope with3 neutrons,and7 in
the isotope with4 neutrons.Innature,onlycertainisotopesexist.Forinstance,lithiumexists
as an isotope with3 neutrons,andasan isotope with4 neutrons,butitdoesn’texistsasan
isotope with2neutrons,oras an isotope with5neutrons.
Thiswhole discussionof isotopesbringsusbackto Dalton’sAtomicTheory.
Accordingto Dalton,atomsof a givenelementare identical.Butif atomsof a givenelement
can have differentnumbersof neutrons,thentheycanhave differentmassesaswell!How
62. didDaltonmissthis?It turnsout that elementsfoundinnature existasconstantuniform
mixturesof theirnaturallyoccurringisotopes.Inotherwords,apiece of lithiumalways
containsbothtypesof naturallyoccurringlithium(the type with3neutronsandthe type with
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4 neutrons).Moreover,italwayscontainsthe twointhe same relativeamounts(or“relative
abundances”).Ina chunkof lithium,93%will alwaysbe lithiumwith4neutrons,while the
remaining7%will alwaysbe lithiumwith3neutrons.
Daltonalwaysexperimentedwithlarge chunksof anelement –chunksthat contained
all of the naturallyoccurringisotopesof thatelement.Asaresult,whenhe performedhis
measurements,he wasactuallyobservingthe averagedpropertiesof all the differentisotopes
inthe sample.Formostof our purposesinchemistry, we willdothe same thinganddeal
withthe average massof the atoms. Luckily,aside fromhavingdifferentmasses,mostother
propertiesof differentisotopesare similar.
We can use whatwe knowaboutatomic numberandmassnumberto findthe
numberof protons,neutrons,andelectronsinanygivenatomorisotope. Considerthe
followingexamples:
Example:Howmanyprotons,electrons,andneutronsare inanatom of ?
Solution:
Findingthe numberof protonsissimple. The atomicnumber,#of protons,islistedinthe
bottomrightcorner. # protons= 19.
For all atomswithno charge,the numberof electronsisequal tothe numberof protons. #
electrons=19.
The mass number,40, is the sumof the protonsand the neutrons. To findthe # of neutron,
subtract the numberof protonsfromthe massnumber. # neutrons= 40 – 19 = 21.
Example:Howmanyprotons,electrons,andneutronsinanatomof zinc-65?
Solution:
Findingthe numberof protonsissimple. The atomicnumber,#of protons,isfoundon the
periodictable. All zincatomshave # protons= 30.
63. For all atomswithno charge,the numberof electronsisequal tothe numberof protons. #
electrons =30.
The mass number,65, is the sumof the protonsand the neutrons. To findthe # of neutron,
subtract the numberof protonsfromthe massnumber. # neutrons= 65 – 30 = 35.
LessonSummary
Electronsare a type of subatomicparticle withanegative charge.
Protonsare a type of subatomicparticle withapositive charge.Protonsare bound
togetherinan atom’snucleusasa resultof the strongnuclearforce.
Neutronsare a type of subatomicparticle withnocharge (they’re neutral).Like
protons,neutronsare boundintothe atom’snucleusasa resultof the strongnuclear
force.
Protonsandneutronshave approximatelythe same mass,buttheyare bothmuch
more massive thanelectrons(approximately2,000 timesasmassive asan electron).
The positive charge ona protonis equal inmagnitude tothe negative charge onan
electron.Asa result,aneutral atommust have an equal numberof protonsand
electrons.
Each elementhasaunique numberof protons.Anelement’satomicnumberisequal
to the numberof protonsin the nuclei of anyof itsatoms.
The mass numberof an atom isthe sum of the protonsand neutronsinthe atom 40
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Isotopesare atomsof the same element(same numberof protons) thathave different
numbersof neutronsintheiratomicnuclei.
Vocabulary
Neutron:asubatomicparticle withnocharge
Atomicmassunit(amu):a unitof mass equal toone-twelfththe massof a carbon-
twelve atom
Atomicnumber:the numberof protonsinthe nucleusof an atom
Mass number:the total numberof protonsand neutronsinthe nucleusof anatom
Isotopes:atomsof the same elementthathave the same numberof protonsbut
64. differentnumbersof neutrons
FurtherReading/Supplemental Material
JeopardyGame: http://www.quia.com/cb/36842.html
For a Bill Nye videoonatoms,goto http://www.uen.org/dms/. Goto the k-12
library. Searchfor “Bill Nye atoms”. (youcan getthe username andpasswordfrom
your teacher)
2.3: ReviewQuestions
Label eachof the followingstatementsastrue or false.
1) The nucleusof an atomcontainsall of the protonsinthe atom.
2) The nucleusof an atomcontainsall of the electronsinthe atom.
3) Neutral atomsmustcontainthe same numberof neutronsasprotons.
4) Neutral atomsmustcontainthe same numberof electronsasprotons.
Match the subatomicpropertywithitsdescription.
Sub-AtomicParticle Characteristics
5) electron a. has a charge of +1
6) neutron b.has a massof approximately1/1840 amu
7) proton c. isneitherattractedto,nor repelledfromchargedobjects
Indicate whethereachstatementistrue orfalse.
8) Anelement’satomicnumberisequal tothe numberof protonsinthe nuclei of anyof its
atoms.
9) A neutral atom with4 protonsmusthave 4 electrons.
10) Anatom with7 protonsand 7 neutronswill have amassnumberof 14.
11) Anatom with7 protonsand 7 neutronswill have anatomicnumberof 14.
12) A neutral atom with7 electronsand7 neutronswill have anatomicnumberof 14.
Use the periodictable tofindthe symbol forthe elementwith:
65. 13) 44 electronsinaneutral atom
14) 30 protons
15) Anatomicnumberof 36
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In the table below,Column1containsdatafor 5 differentelements.Column2containsdata
for the same 5 elements,howeverdifferentisotopesof those elements.Matchthe atomin the
firstcolumnto itsisotope inthe secondcolumn.
Original element Isotopeof the same element
16) an atomwith2 protonsand 1 neutron
a. a C (carbon) atomwith6 neutrons
17) a Be (beryllium) atomwith5neutrons
b. an atom with2 protonsand 2
neutrons
18) an atomwithan atomic numberof 6 and mass
numberof 13
c. an atomwithan atomicnumberof 7
and a mass numberof 15
19) an atomwith1 protonand a mass numberof 1
d. an atom withan atomicnumberof 1
and 1 neutron
20) an atomwithan atomic numberof 7 and 7
neutrons
e. an atomwithan atomic numberof 4
and 6 neutrons
Write the nuclearsymbol foreach elementdescribed:
21) 32 neutronsinan atomwithmass numberof 58
