Chapter 2. Atoms and Elements
Student Objectives
2.1 BrownianMotion:AtomsConfirmed
• DescribeBrownianmotion
• Describescanningtunnelingmicroscopy(STM)andhowatomsareimagedonsurfaces.
• Define atom and element.
2.2 EarlyIdeasabouttheBuildingBlocksofMatter
• Describetheearliestdefinitionsofatomsandmatter(Greeks).
• Knowthatgreateremphasisonobservationandthedevelopmentofthescientificmethodled tothescientificrevolution.
2.3 ModernAtomicTheoryandtheLawsThatLedtoIt
• Stateandunderstandthelawofconservationofmass(alsofromSection1.2).
• Stateandunderstandthelawofdefiniteproportions.
• Stateandunderstandthelawofmultipleproportions.
• KnowthefourpostulatesofDalton’satomictheory.
2.4 TheDiscoveryoftheElectron
• DescribeJ.J.Thomson’sexperimentswiththecathoderaytubeandunderstandhowthey provideevidencefortheelectron.
• DescribeRobertMillikan’soil-dropexperimentandunderstandhowitenablesmeasurement ofthechargeofanelectron.
2.5 TheStructureoftheAtom
• Define radioactivity, nucleus, proton,and neutron.
• UnderstandThomson'splum-puddingmodelandhowErnestRutherford’sgold-foil experimentrefuteditbygivingevidenceforanuclearstructureoftheatom.
2.6 SubatomicParticles:Protons,Neutrons,andElectronsinAtoms
• Define atomic mass unit, atomic number,and chemical symbol.
• Recognizechemicalsymbolsandatomicnumbersontheperiodictable.
• Define isotope, mass number,and natural abundance.
• Determinethenumberofprotonsandneutronsinanisotopeusingthechemicalsymboland themassnumber.
• Define ion, anion,and cation
• Understandhowionsareformedfromelements.
2.7 FindingPatterns:ThePeriodicLawandthePeriodicTable
• Definethe periodic law
• Knowthatelementswithsimilarpropertiesareplacedintocolumns(calledgroups)inthe periodictable.
• Defineanddistinguishbetweenmetals,nonmetals,andmetalloids.
• Identifymain-groupandtransitionelementsontheperiodictable.
• Knowthegeneralpropertiesofelementsinsomespecificgroups:noblegases,alkalimetals, alkalineearthmetals,andhalogens.
• Knowandunderstandtherationaleforelementsthatformionswithpredictablecharges.
2.8 AtomicMass:TheAverageMassofanElement’sAtoms
• Calculateatomicmassfromisotopemassesandnaturalabundances.
• Define mass spectrometry andunderstandhowitcanbeusedtomeasuremassandrelative abundance.
2.9 MolarMass:CountingAtomsbyWeighingThem
• Understandtherelationshipbetweenmassandcountofobjectssuchasatoms.
• Define mole and Avogadro’s number.
• Calculateandinterconvertbetweennumberofmolesandatoms.
• Calculateandinterconvertbetweennumberofmolesandmass.
Section Summaries
LectureOutline
• Terms,Concepts,Relationships,Skills
• Figures,Tables,andSolvedExamples
TeachingTips
• SuggestionsandExamples
• MisconceptionsandPitfalls
Lecture Outline
Terms,Concepts,Relationships,Skills
2.1 Brownian Motion: Atoms Confirmed
• Descriptionofscanningtunnelingmicroscopy (STM)
• Introductiontomacroscopicandmicroscopic perspectives.
• Definitionsofatomandelement.
2.2 Early Ideas about the Building Blocks of Matter
• Historyofchemistryfromantiquity(~450bc)
• Scientificrevolution(1400s-1600s)
2.3 Modern Atomic Theory and the Laws That Led to It
• Lawofconservationofmass
o Matterisneithercreatednordestroyed.
o Atomsatthestartofareactionmay recombinetoformdifferentcompounds, butallatomsareaccountedforatthe end.
o Massofreactants=massofproducts.
• Lawofdefiniteproportions
o Differentsamplesofthesamecompound havethesameproportionsof constituentelementsindependentof samplesourceorsize.
• Lawofmultipleproportions
• JohnDalton’satomictheory
Figures,Tables,andSolvedExamples
• Figure2.1ImagingAtoms
• unnumberedfigure:modelsand photosofNaandCl2 forming NaCl
• Example2.1LawofDefinite Proportions
• unnumberedfigure:modelsof COandCO2 illustratingthelaw ofmultipleproportions
• Example2.2LawofMultiple Proportions
• ChemistryinYourDay:Atoms andHumans
SuggestionsandExamples
2.1 Brownian Motion: Atoms Confirmed
• OtherSTMimagescanbefoundreadilyonthe Internet.
• Itisusefultoreiteratetheanalogiesaboutsize;for example,comparinganatomtoagrainofsandand agrainofsandtoalargemountainrange.
2.2 Early Ideas about the Building Blocks of Matter
• Theviewofmatterasmadeupofsmall, indestructibleparticleswasignoredbecausemore popularphilosopherslikeAristotleandSocrates haddifferentviews.
• LeucippusandDemocritusmayhavebeenproven correct,buttheyhadnomoreevidencefortheir ideasthanAristotledid.
• Observationsanddataledscientiststoquestion models;thescientificmethodpromotestheuseofa cycleofsuchinquiry.
2.3 Modern Atomic Theory and the Laws That Led to It
• Thatmatteriscomposedofatomsgrewfrom experimentsandobservations.
• ConceptualConnection2.1TheLawofConservation ofMass
• Investigatingthelawofdefiniteproportions requirespreparingordecomposingasetofpure samplesofacompoundlikewater.
• Investigatingthelawofmultipleproportions requirespreparingordecomposingsetsofpure samplesfromrelatedcompoundslikeNO,NO2,and N2O5
• ConceptualConnection2.2TheLawsofDefiniteand MultipleProportions
MisconceptionsandPitfalls
• STMisnotactuallyshowing imagesofatomslikeonemight imagineseeingwithalight microscope.
• Atomsarenotcoloredspheres; theimagesusecolorto distinguishdifferentatoms.
• Theoriesarenotautomatically acceptedandmaybeunpopular forlongperiodsoftime.
• Philosophyandreligioncanbe supportedbyarguments; sciencerequiresthattheoriesbe testableandtherefore falsifiable.
• Measurementstoestablishearly atomictheorieswereperformed atthemacroscopiclevel.The scientistsobservedproperties forwhichtheycouldcollectdata (e.g.,massorvolume).
Lecture Outline
Terms,Concepts,Relationships,Skills
2.4 The Discovery of the Electron
• Thomson’scathode-raytubeexperiments
o Highvoltageproducedastreamof particlesthattraveledinstraight lines.
o Eachparticlepossessedanegative charge.
o Thomsonmeasuredthecharge-tomassratiooftheelectron.
• Millikan’soil-dropexperiments
o Oildropletsreceivedchargefrom ionizingradiation.
o Chargeddropletsweresuspended inanelectricfield.
o Themassandchargeofeachoil dropwasusedtocalculatethemass andchargeofasingleelectron.
2.5 The Structure of the Atom
• Thomson’splum-puddingmodel: negativelychargedelectronsinaseaof positivecharge
• Radioactivity
o Alphadecayprovidesthealpha particlesforRutherford’s experiment.
• Rutherford’sexperiment
o Alphaparticlesdirectedatathin goldfilmdeflectinalldirections, includingbackatthealphasource.
o Onlyaconcentratedpositivecharge couldcausethealphaparticlesto bounceback.
• Rutherford’snucleartheory
o mostmassandallpositivecharge containedinasmallnucleus
o mostofatombyvolumeisempty space
o protons:positivelycharged particles
o neutralparticleswithsubstantial massalsoinnucleus
Figures,Tables,andSolvedExamples
• Figure2.2CathodeRayTube
• unnumberedfigure:propertiesof electricalcharge
• Figure2.3Thomson’sMeasurementof theCharge-to-MassRatiooftheElectron
• Figure2.4Millikan’sMeasurementofthe Electron'sCharge
• unnumberedfigure:plum-pudding model
• Figure2.5Rutherford’sGoldFoil Experiment
• Figure2.6TheNuclearAtom
• unnumberedfigure:scaffoldingand emptyspace
SuggestionsandExamples
2.4 The Discovery of the Electron
• Reviewtheattraction,repulsion,andadditivityof charges.
• Discussthephysicsofelectricfieldsgeneratedby metalplates.
• Ademonstrationofacathoderaytubewillhelp studentsbetterunderstandThomson’sexperiments.
• DemonstratehowMillikan’scalculationworksand whyhecoulddeterminethechargeofasingle electron.
2.5 The Structure of the Atom
• Itmaybeusefultogiveabriefdescriptionof radioactivity.Rutherford’sexperimentmakesmore senseifoneknowssomepropertiesofthealpha particleandfromwhereitcomes.
• Thomsonidentifiedelectronsandsurmisedthe existenceofpositivechargenecessarytoforma neutralatom.Theplum-puddingmodelisthe simplestwaytoaccountfortheobservations.
• Thefigureaboutscaffoldingsupportsdiscussion aboutanatombeingmostlyemptyspacebutstill havingrigidityandstrengthinthemacroscopicview. Thisisanotherexampleofapparentdifferences betweenthemicroscopicandmacroscopic properties.
MisconceptionsandPitfalls
• Millikandidnotmeasurethe chargeofasingleelectron;he measuredthechargeofa numberofelectronsand deducedthechargeofasingle electron.
• Studentsoftendon’t understandthe source ofalpha particlesinRutherford’s experiments.
Lecture Outline
Terms,Concepts,Relationships,Skills
2.6 Subatomic Particles: Protons, Neutrons, and Electrons in Atoms
• Propertiesofsubatomicparticles
o atomicmassunits(amu)
proton,neutron:~1amu
electron:~0.006amu
o charge
relativevalue: 1for electron,+1forproton
absolutevalue:1.6 × 10 19 C
• Atomicnumber(numberofprotons): definingcharacteristicofanelement
• Isotope:sameelement,differentmass (differentnumberofneutrons)
• Ion:atomwithnonzerocharge
o anion:negativelycharged(more electrons)
o cation:positivelycharged(fewer electrons)
2.7 Finding Patterns: The Periodic Law and the Periodic Table
• Periodiclawandtheperiodictable
o generallyarrangedbyascending mass
o recurring,periodicproperties; elementswithsimilarproperties arrangedintocolumns:groups(or families)
• Majordivisionsoftheperiodictable
o metals,nonmetals,metalloids
o main-groupelements,transition elements
• Groups(families)
o noblegases(group8A)
o alkalimetals(group1A)
o alkalineearthmetals(group2A)
o halogens(group7A)
• Ionswithpredictablecharges:basedon stabilityofnoble-gaselectroncount
o group1A:1+
o group2A:2+
o group3A:3+
o group5A:3
o group6A:2
o group7A:1
Figures,Tables,andSolvedExamples
• unnumberedfigure:baseball
• Table2.1SubatomicParticles
• unnumberedfigure:lightningand chargeimbalance
• Figure2.7HowElementsDiffer
• Figure2.8ThePeriodicTable
• unnumberedfigure:portraitof MarieCurie
• Example2.3AtomicNumbers,Mass Numbers,andIsotopeSymbols
• ChemistryinYourDay:WhereDid ElementsComeFrom?
• unnumberedfigure:discoveryof theelements
• Figure2.9RecurringProperties
• Figure2.10MakingaPeriodicTable
• unnumberedfigure:stamp featuringDmitriMendeleev
• Figure2.11Metals,Nonmetals,and Metalloids
• Figure2.12ThePeriodicTable: Main-GroupandTransition Elements
• unnumberedfigure:thealkali metals
• unnumberedfigure:thehalogens
• Example2.4PredictingtheCharge ofIons
• Figure2.13ElementsThatForm IonswithPredictableCharges
• ChemistryandMedicine:The ElementsofLife
• Figure2.14ElementalComposition ofHumans(byMass)
Teaching Tips
SuggestionsandExamples
2.6 Subatomic Particles: Protons, Neutrons, and Electrons in Atoms
• Theanalogyofthebaseballandagrainofricetoa protonandanelectronismeanttoillustratethe differenceinmassbutnotsize.
• Electricalchargecanbedemonstratedwithstatic electricity.Twoballoonschargedwithwoolor humanhairwillrepeleachother.
• Namesofelementscomefromvarioussources.Tom Lehrer’s“ElementSong”canbefoundonthe Internet.
• Isotopicabundancesareinvariantintypicallabsizedsamplesbecauseofsuchlargenumbersof atoms.
• ConceptualConnection2.5TheNuclearAtom, Isotopes,andIons
• Thehistoryofchemistryinvolvesconsiderable culturalandgenderdiversity.Examplesinclude bothLavoisiers(French),Dalton(English), Thomson(English),MarieCurie(Polish/French), Mendeleev(Russian),Millikan(American),Robert Boyle(Irish),AmedeoAvogadro(Italian).
• TheChemistryinYourDayboxgivesabroad descriptionoftheoriginofatoms.
2.7 Finding Patterns: The Periodic Law and the Periodic Table
• Otherdisplaysoftheperiodictablecanbefoundin journals(Schwartz, J. Chem. Educ. 2006, 83,849; Moore, J. Chem. Educ. 2003, 80,847;Bouma, J. Chem. Educ. 1989, 66,741),books,andontheInternet.
• Periodictablesarearrangedaccordingtothe periodiclawbutcancomparemanyfeatures,e.g. phasesofmatter,sizesofatoms,andcommonions. Thesearepresentedasaseriesoffiguresinthetext.
• ChemistryandMedicine:TheElementsofLife providesanopportunitytorelatethetopicsto everydaylife.Someoftheotherelementsinthe figureandtablerepresenttracemineralsthatare partofgoodnutrition.Theperiodiclawaccounts forwhysomearenecessaryandothersaretoxic.
MisconceptionsandPitfalls
• Studentssometimesconfusethe massnumberasbeingequalto thenumberofneutrons,notthe numberofneutronsplusthe numberofprotons.
• Studentslogically(but mistakenly)presumethatthe massofanisotopeisequalto thesumofthemassesofthe protonsandneutronsinthat isotope.
• Theperiodictableisbetterat predictingmicroscopic properties,thoughmacroscopic propertiesarealsooften illustrated.
Lecture Outline
Terms,Concepts,Relationships,Skills
2.8 Atomic Mass: The Average Mass of an Element’s Atoms
• Averageatomicmassisbasedonnatural abundanceandisotopicmasses.
• Massspectrometry
o atomsconvertedtoionsand deflectedbymagneticfieldsto separatebymass
o outputdata:relativemass vs. relativeabundance
2.9 Molar Mass: Counting Atoms by Weighing Them
• MoleconceptandAvogadro’snumber
• Convertingbetweenmolesandnumberof atoms
• Convertingbetweenmassandnumberof moles
Figures,Tables,andSolvedExamples
• unnumberedfigure:periodictablebox forCl
• Example2.5AtomicMass
• Figure2.15TheMassSpectrometer
• Figure2.16TheMassSpectrumof Chlorine
• unnumberedfigure:pennies containing~1molofCu
• unnumberedfigure:1tbspofwater contains~1molofwater
• Example2.6 Convertingbetween NumberofMolesandNumberof Atoms
• unnumberedfigure:relativesizesof Al,C,He
• unnumberedfigure:balancewith marblesandpeas
• Example2.7ConvertingbetweenMass andAmount(NumberofMoles)
• Example2.8TheMoleConcept–ConvertingbetweenMassand NumberofAtoms
• Example2.9TheMoleConcept
Teaching Tips
SuggestionsandExamples
2.8 Atomic Mass: The Average Mass of an Element's Atoms
• Themassesofisotopesmustbereconciledwithan elementhavingonlywholenumberquantitiesof protonsandneutrons;thevaluesshouldbenearly integralsincethemassofelectronsissosmall.
• Massspectrometryisaneffectivewayto demonstratewherevaluesofnaturalabundance areobtained.
2.9 Molar Mass: Counting Atoms by Weighing Them
• Reviewthestrategyforsolvingnumerical problems:sort,strategize,solve,check.
• Estimatinganswersisanimportantskill;the numberofatomswillbeverylarge(i.e.somelarge poweroften)evenfromasmallmassorsmall numberofmoles.
• ConceptualConnection2.7Avogadro’sNumber
• ConceptualConnection2.8TheMole
MisconceptionsandPitfalls
• Studentsaretemptedto calculateaverageatomicmass byaddingtogetherisotopic massesanddividingbythe numberofisotopes.
• Atomicmassontheperiodic tableisusuallynotintegraleven thoughelementshaveonly wholenumbersofprotonsand neutrons.
• Manystudentsareintimidated byestimatinganswersin calculationsinvolvingpowers often.
Additional Problem for Converting between Number of Moles and Number of Atoms (Example 2.6)
Sort
You are given a number of iron atoms and asked to find the amount of iron in moles.
Strategize
Convert between number of atoms and number of moles using Avogadro’s number.
Calculate the number of moles of iron in a sample that has 3.83 × 1023 atoms of iron.
Given 3.83 × 1023 Fe atoms
Find mol Fe
Conceptual Plan atoms → mol × 23 1 mol Fe 6.022 10 Fe atoms
Relationships Used
6.022 × 1023 = 1 mol (Avogadro’s number)
Solve
Follow the conceptual plan. Begin with 3.83 × 1023 Fe atoms and multiply by the ratio that equates moles and Avogadro’s number.
Check
Solution × × × = 23 23 1 mol Fe 3.8310 Fe atoms 6.02210 Fe atoms
0.636 mol Fe
The sample was smaller than Avogadro’s number so the answer should be a fraction of a mole. The value of the sample has 3 significant figures, and the answer is provided in that form.
Additional Problem for Converting between Mass and Number of Moles (Example 2.7)
Sort
You are given the amount of silver in moles and asked to find the mass of silver.
Strategize
Convert amount (in moles) to mass using the molar mass of the element.
Calculate the number of grams of silver in an American Silver Eagle coin that contains 0.288 moles of silver.
Given 0.288 mol Ag
Find g Ag
Conceptual Plan
mol Ag → g Ag
107.87 g Ag 1 mol Ag
Relationships Used
107.87 g Ag = 1 mol Ag
Solve
Follow the conceptual plan to solve the problem. Start with 0.288 mol, the given number, and multiply by the molar mass of silver.
Solution
0.288molAg × 107.87gAg 1molAg = = 31.07gAg
31.07g31.1gAg
Check The magnitude of the answer makes sense since we started with an amount smaller than a mole. The molar amount and answer both have 3 significant figures.
Additional Problem for the Mole Concept—Converting between Mass and Number of Atoms (Example 2.8)
Sort
You are given a number of iron atoms and asked to find the mass of Fe.
Strategize
Convert the number of Fe atoms to moles using Avogadro’s number. Then convert moles Fe into grams of iron using the molar mass of Fe.
What mass of iron (in grams) contains 1.20 × 1022 atoms of Fe? A paperclip contains about that number of iron atoms.
Given 1.20 × 1022 Fe atoms
Find g Fe
Conceptual Plan
Solve
Follow the conceptual plan to solve the problem. Begin with 1.20 × 1022 atoms of Fe, multiply by the ratio derived from Avogadro’s number, and finally multiply by the atomic mass of Fe.
Check
Relationships Used 6.022 × 1023 = 1 mol (Avogadro’s number) 55.85 g Fe = 1 mol Fe
Solution × 221.2010 Fe atoms × 1 mol Fe × 236.02210 Fe atoms × 55.85 g Fe 1 mol Fe = 1.11 g Fe
The units and magnitude of the answer make sense. The sample is smaller than a mole. The number of atoms and mass both have 3 significant figures.
Additional Problem for the Mole Concept
(Example 2.9)
Sort
You are given the number of iron atoms in a sphere and the density of iron. You are asked to find the radius of the sphere.
Strategize
The critical parts of this problem are density, which relates mass to volume, and the mole, which relates number of atoms to mass:
(1) Convert from the number of atoms to the number of moles using Avogadro’s number;
(2) Convert from the number of moles to the number of grams using the molar mass of iron;
(3) Convert from mass to volume using the density of iron;
(4) Find the radius using the formula for the volume of a sphere.
An iron sphere contains 8.55 × 1022 iron atoms. What is the radius of the sphere in centimeters? The density of iron is 7.87 g/cm3 .
Given 8.55 × 1022 Fe atoms
d = 7.87 g/cm3
Find radius (r) of a sphere
Conceptual Plan
Relationships Used
g Fe = 1 mol Fe
Solve
number)
(density of Fe) = 7.87 g/cm3 V = 4/3 πr3 [volume of a sphere with a radius of r]
Follow the conceptual plan to solve the problem. Begin with 8.55 × 1022 Fe atoms and convert to moles, then to grams and finally to a volume in cm3. Solve for the radius using the rearranged equation. Solution
Check
The units (cm) are correct and the magnitude of the answer makes sense compared with previous problems.
