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Chapter6Answers 1
6RelationshipsandPatternsinChemistry
6.1TheDevelopmentofthePeriodicTable
WarmUp(p.286)1.Studentswilloftensuggestthattheword“periodic”meansthattheelementswerediscoveredoverdifferentperiodsoftime.Thewordactuallymeansthatsimilarelementalpropertiesrecuronaregular
orperiodicbasis.2.Inthemodernperiodictable,theelementsarearrangedinorderofincreasingatomicnumber.
3.Elementsbelongingtothesameverticalcolumnorchemicalfamilydisplaysimilarproperties.
QuickCheck(p.288)1.EnglishchemistWilliamOldingwasthefirstpersontoarrangethechemicalelementsintogroups.
2.JohnNewlandsnoticedthatsimilarpropertiesseemedtorepeatevery8thelementinthesamewaythatnotesonamusicalscalerepeatevery8thtone.
3.AsMendeleevhadarrangedsimilarelementsverticallyonhistable,thelocationoftheblankspacespredictedthepropertiesthatelementswouldpossessuponbeingdiscovered.Whenthoseelements
wereeventuallydiscovered,Mendeleev’spredictionsturnedouttobeextremelyaccurate.Thishelpedhisarrangementoftheelementsinhisperiodictablegainwidespreadacceptance.
QuickCheck(p.291)
1.
ElementName
ElementSymbol
GroupNumber
PeriodNumber
Metal/Non‐metal/
Metalloid
silicon Si 14 3 metalloid
osmium Os 8 6 metal
chromium Cr 6 4 metal
meitnerium Mt 9 7 metal
antimony Sb 15 5 metalloid
iodine I 17 5 non‐metal
Chapter6Answers 2
2.(a)Theelementsrearrangedinorderofleastmetallictomostmetallicare:fluorine,oxygen,sulphur,aluminum,gallium,chromium,zirconium,cesium.
(b)Theelementwiththegreatesttendencytogainanelectronisfluorine.Theelementwiththegreatesttendencytoloseanelectroniscesium.
QuickCheck(p.293)1.
Property FamilyNumber FamilyName
(a)reactivenon‐metalspossessing7valenceelectrons 17 halogens
(b)reactivesolidsthatform2+cationsduringreactions 2 alkalineearthmetals
(c)invisiblegasesthatarealmosttotallyunreactive 18 noblegases
(d)soft,veryreactivesilverysolidswith1valenceelectron 1 alkalimetals
2.Thechemicalformulasare:(a)NaI(b)BaS(c)GaCl3(d)Rb2S(e)Mg3P2(f)H2Se
3.Thetwofamiliesoftheperiodictablecontainingthemostreactiveelementsarethehalogensandthealkalimetals.Elementsineachofthesefamiliesarewithinoneelectronofbeingisoelectronicwiththe
nearestnoblegasandreadilyeithergainorloseoneelectronrespectivelytoachievethatelectronarrangement.
6.1ReviewQuestions(p.297)1.Themostimportantthingtoknowabouttheperiodictableisthatelementsinthesamechemicalfamilyhavesimilarchemicalproperties.
2.In1913,datagatheredbytheyoungBritishchemistHenryMoseley,combinedwiththediscoveryofisotopes,resultedintheelementsoftheperiodictablebeingre‐orderedaccordingtotheiratomic
numbersratherthantheiratomicmasses.3.Elementsinthesamechemicalfamilyhavesimilarchemicalpropertiesbecausetheyhavesimilar
outerelectronconfigurationsandthevalenceelectronsaretheelectronsinvolvedinchemicalreactions.4.(a)Themostmetallicelementsarelocatedinthelowerleftportionoftheperiodictable.
(b)Themostnon‐metallic(orleastmetallic)elementsarelocatedintheupperrightportionoftheperiodictable.
Chapter6Answers 3
5.
Element Properties Letterand
Symbol
(a)chlorine foundincarbohydratesandanelementalgasin21%oftheatmosphere (e)O
(b)silver softconductorthatreactsexplosivelywithwaterproducingH2gas (d)Cs
(c)neon lessthan1ounceofthissolidradioactivenonconductorexistsonEarth (h)At
(d)cesium waxyyellowsolidnon‐metalfoundinmatchheads,fertilizers,anddetergents
(f)P
(e)oxygen blue‐graymetalloidusedextensivelyinthecomputerindustry (g)Si
(f)phosphorus veryreactivegreengasusedinthetrenchesinWorldWarI (a)Cl
(g)silicon shinysolidthatisthebestconductorofheatandelectricity (b)Ag
(h)astatine invisibleunreactivegasusedinlasersandsomeelectricstreetsigns (c)Ne
6.
ElementProperties FamilyNumber
unreactivegasusedinelectricstreetsignscomprising0.93%oftheatmosphere 18
shinymultivalentsolid,goodconductor,formscolouredcompounds 6
softsilverysolid,goodconductor,reactsvigorouslywithwater 1
gray‐whitemetalloidpredictedbyMendeleevanddiscoveredin1886 14
reactivemetalpresentinbonesandteethpossessingtwovalenceelectrons 2
yellow‐greengaseousnon‐metalandthemostreactiveofalltheelements 17
7.Propertiesofmetalsinclude:
• solidsatroomtemperature,exceptformercury,whichisaliquid• generallyshinyorlustrouswhenfreshlycutorpolished• goodconductorsofheatandelectricity• generallymalleable,whichmeanstheycanberolledorhammeredintothinsheets• generallyductile,whichmeanstheycanberolledorstretchedintowires• generallyflexibleasthinsheetsorwires• duringchemicalchanges,tendtogiveupelectronsrelativelyeasilytoformcations
8.Propertiesofnon‐metalsinclude:
• usuallygasesorbrittlesolidsatroomtemperature,exceptforliquidbromine• solidnon‐metalscanrangeinappearancefromdullorlustrousandtranslucenttoopaque• poorconductorsofheatandelectricity• duringchemicalchanges,theytendtogainelectronsfrommetalstoformanionsorshareelectronswithothernon‐metals.
9.Exceptwherelargeionicchargesresult,atomsofthemaingroupelementswilltendtogiveuporgainasmanyelectronsasarenecessarytoacquirethevalenceelectronconfigurationofthenearestnoble
gas.
Chapter6Answers 4
10.Theformulasforthestableionsare:(a)Be2+(b)Te2–(c)Cs+(d)Ra2+(e)Ga3+(f)Se2‐(g)In3+
11.(a)Propertiesofthealkalimetalsinclude:
• allsoft,silverysolids
• themostreactiveofallmetals
• theoxidecompoundsofthealkalimetalsdissolveinwatertoproducestronglybasicsolutions
• allcorroderapidlyinairtoadullgrayappearance,reactvigorouslywithwatertoproducehydrogengas
• readilyformcompoundswithnon‐metals
• readilylosethatouterelectrontoform1+cationsandsoassumetheelectronconfigurationofpreviousnoblegas
(b)Propertiesofthealkalineearthmetalsinclude:
• silver‐colouredreactivemetals.
• notasreactiveasthealkalimetals,butalsoreadilyformcompoundswithnon‐metals
• theiroxidesarealsoalkalineinsolutionbutunlikealkalicompounds,anumberofgroup2
compoundshavealowsolubilityinwater
• readilyform2+cationsbylosingthosetwovalenceelectronsandsowillachievetheidentical
electronconfigurationofthenearestnoblegas
(c)Propertiesofthehalogensinclude:
• themostreactivefamilyofelementsintheperiodictable
• theonlychemicalfamilyinwhichallthreestatesofmatterarerepresented
• elementalhalogensexistasdiatomicmolecules
• readilyformcompoundswithmetals,hydrogen,carbon,andothernon‐metals
• whenreactingwithmetals,halogenstypicallygainasingleelectronforming1–anions
• whenreactingwithnon‐metals,halogenswilloftensharevalenceelectrons
(d)Propertiesofthenoblegasesinclude:
• colourlessgases
• generallyunreactive
• allofthenoblegases,excepthelium,havefilledsandpsublevelsandthushave“stableoctets”
Chapter6Answers 5
12.
Group2 CoreNotation
Group17 CoreNotation
Group18 CoreNotation
Be [He]2s2 F [He]2s22p5 He 1s2
(nocorenotation)Mg [Ne]3s2 Cl [Ne]3s23p5 Ne [He]2s22p6Ca [Ar]4s2 Br [Ar]4s24p5 Ar [Ne]3s23p6Sr [Kr]5s2 I [Kr]5s24d105p5 Kr [Ar]4s23d104p6
Ba [Xe]6s2 At [Xe]6s24f145d106p5 Xe [Kr]5s24d105p6Ra [Rn]7s2 Rn [Xe]6s24f145d106p6
6.2PeriodicTrends—RegularChangesinElementalPropertiesWarmUp(p.299)
1.Thenegativelychargedelectroncloudsinatomsareheldinplacebytheattractiveforceofpositivelychargedprotonsinthenucleiofthoseatoms.
2.Wemightexpectthatthesizesofatomswoulddecreaseifthisattractiveforceincreasedinstrength.3.Increasingthepositivechargeonthenucleusshouldincreasetheattractiveforceontheelectron
cloud.Thiswouldoccurasanelement’satomicnumberincreased.4.Fewerprotonsinthenucleuswouldexertlessofanattractiveforceonanatom’selectroncloud,but
wemightalsoexpectthatasthedistancebetweenthenucleusandthevalenceelectronsincreasedtheattractiveforceexertedonthatouterchargecloudbythenucleuswoulddecrease.
QuickCheck(p.302)1.Aswemovelefttorightacrossachemicalperiodintheperiodictable,theenergylevelremainsthesameandsotheattractiveforcethatthevalenceelectronsexperiencefromthenucleusisthe
predominantforceoperatingthatinfluencesthesizeoftheatoms.Thisforceincreasesaswemoveacrosstheperiodandsotheatomsgenerallybecomesmaller.
2.Aswemovedownachemicalfamily,thenumberofenergylevelspresentincreasesinatoms.Eventhoughthenuclearchargealsoincreases,theincreasingenergylevelsarethepredominatingfactordeterminingatomicsize.Asaresult,theattractiveforcefeltbythevalenceelectronsdecreasesandthe
atomicsizeincreases.3.Effectiveshieldingismostprevalentmovingdownachemicalfamilyratherthanacrossachemical
period.Theevidenceofthisisthatthesizesofatomsincreaseaswemovedownachemicalfamilyanddecreaseaswemovelefttorightacrossachemicalperiod.
Chapter6Answers 6
PracticeProblems—TrendsinIonizationEnergy(p.305)
1.Theelementsrankedinorderofdecreasingionizationenergyare:
argon>sulphur>silicon>aluminum>magnesium>sodium>rubidium>cesium
2.MembersofthischemicalfamilyhavethehighestIE1intheirperiod.__18__MembersofthischemicalfamilyhavethelowestIE1intheirperiod.__1___
MembersofthischemicalperiodhavethehighestIE1intheirfamily.TheuppermostperiodinthatfamilyMembersofthischemicalperiodhavethelowestIE1intheirfamily.__7___
3.If2selectronsspendmoreoftheirtimenearerthenucleusthan2pelectrons,then2selectronsare
capableofpartiallyshielding2pelectronsfromthenuclearcharge.Thisshouldmakeiteasiertoremoveboron’ssingle2pelectron,whichexplainsboron’slowerfirstionizationenergy.
6.2Activity(p.308)Procedure1.
6.2ReviewQuestions(p.309)1.Theregularandpredictablechangesinelementalpropertiesaswemoveacrossaperiodordownachemicalfamilyintheperiodictableareknownasperiodictrends.
2.Thequantummechanicalmodeltellsusthattheouterboundariesofanatomarenothardand
definite,butratheraretheedgesofchargecloudsenclosingtheregionsofhighestprobabilityoffindinganatom’souterelectrons.
3.Ionizationenergyandelectronegativitybothincreaseaswemovelefttorightacrossaperiodandmoveupachemicalfamilyintheperiodictable.
4.Aswemoveinanydirectionhorizontallyacrossaperiodorverticallyinachemicalfamily,thetrendin
atomicsizeisgenerallyoppositetothetrendsseeninionizationenergyandelectronegativity.
5.Thevalenceelectronsofbothlithiumandfluorineareinthesecondenergylevelandasaresult,thosevalenceelectronsexperienceasimilaramountofelectronshielding.Becausefluorine’snucleushassixmoreprotons,theeffectivenuclearcharge(Zeff)seenbyfluorine’svalenceelectronsismuchgreater
MovingAcrossaPeriod MovingUpaChemicalFamily
AtomicSize decreases decreases
IonizationEnergy increases increases
Electronegativity increases increases
Chapter6Answers 7
thanthatexperiencedbylithium’ssinglevalenceelectron.Asaresult,fluorineatomsaresmallerthanlithiumatoms.
6.Thelargestatomsarelocatedinthelowerleftregionoftheperiodictableandthesmallestatomsare
locatedintheupperrightregionoftheperiodictable.
7.(a)Acationwillalwaysbesmallerthanitsparentneutralatombecauseofincreasedattractionoftheouterelectronsforthenucleusanddecreasedrepulsionoftheelectronsforeachother.
(b)Ananionwillalwaysbelargerthanitsparentneutralatombecauseofdecreasedattractionoftheouterelectronsforthenucleusandincreasedrepulsionoftheelectronsforeachother.
8.Inneror“core”electronsareeffectiveatshieldingtheouterelectronsfromtheattractiveforceofthe
nucleusandexertingarepulsiveforceonthoseouterclouds.Astheextentofthatshieldingincreases,(whichoccurswhendescendingachemicalfamily),theatomicsizesincreaseandtheionizationenergiesdecrease.
9.
WhereonthePeriodicTableElementsShow:
LargestAtomicRadii lowerleft
SmallestAtomicRadii upperright
LowestIonizationEnergy lowerleft
HighestIonizationEnergy upperright
LowestElectronegativity lowerleft
HighestElectronegativity upperright
10.Lithium’selectronconfigurationis1s22s1.Lithium’ssinglevalenceelectronisshieldedfromthenuclearchargebytheinnerorcore1s2electrons.Itisthereforerelativelyeasytoremovethatouter
electronandsoweseethatthefirstionizationenergyislow.Afterthatelectronisremovedhowever,thenextelectronremovedisaninnercoreelectron,whichisnotshieldedatallfromthenuclearcharge.Asaresult,lithium’ssecondionizationenergyisapproximately14timesgreaterthanthefirstionization
energy!
11.Whensuchreactionsoccur,elementswithlowionizationenergiesandlowelectronegativitieswilltendtolosevalenceelectronstoelementswhoseionizationenergiesandelectronegativitiesarehigh.
12.Whensuchreactionsoccur,elementswithhighionizationenergiesandhighelectronegativitieswilltendtogainvalenceelectronsfromelementswhoseionizationenergiesandelectronegativitiesarelow.
(Theresultsofthereactionsdescribedinquestions11and12willbetheformationofcationsbyatomslosingvalenceelectronsandtheformationofanionsbyatomsgainingvalenceelectrons.)
Chapter6Answers 8
13.Whentwonon‐metalatoms,eachwithrelativelyhighionizationenergiesandelectronegativitiesreact,asbothattracttheirvalenceelectronsstronglyandneitherhaveatendencytolosethem,theyare
likelytoformbondsbysharingvalenceelectronsratherthanbytransferringthem.14.Electronconfigurationfornickel:[Ar]3d84s2
Electronconfigurationforzinc:[Ar]3d104s2Noticethatwhentheelectronconfigurationsarewritteninorderofincreasingsublevelsize,ratherthanincreasingenergy,weseethatzinchasafilled3dsublevelwith2moreelectronsintheinner3dcloud
thannickel.These2additionalelectronsincreasetheeffectiveshieldingofzinc’s2outerelectronsandthusreducetheattractiveforcetheyfeelfromthenucleus.Asaresult,zincatomsarelargerthannickelatoms.
6.3DescribingChemicalBondingWarmUp(p.311)
1.Anatom’soutermostelectronsthattakepartinchemicalbondingareknownasvalenceelectrons.
2.Electronegativityisdefinedastherelativeabilityofabondedatomtoattractsharedelectronstoitself.
3.(a)Elementsthattendtoloseouterelectronsmosteasilyduringchemicalchangesarelocatedinthelowerleftregionoftheperiodicwhere.
(b)Elementsthattendtogainouterelectronsmosteasilyduringchemicalchangesarelocatedinthe
upperrightregionoftheperiodictable.
QuickCheck(p.314)1.Whenmetalatomsreactwithnon‐metalatoms,metalatomstendtoloseoneormorevalence
electronstonon‐metalatomsformingmetalcationsandnon‐metalanions.
2.Threetypesofchemicalbondsbasedonthedifferentelementsinvolved:
AtomsInvolvedinChemicalBond TypeofChemicalBond
metalbondedtonon‐metal ionicbond
non‐metalbondedtonon‐metal covalentbond
metalbondedtometal metallicbond
3.Ionicbondformationistypicallyassociatedwithmetalsfromgroups1and2reactingwithnon‐metals
fromgroups16and17oftheperiodictable.
Chapter6Answers 9
PracticeProblems—IonicCompounds(p.315)1.(a)BaBr2(b)BeO(c)Sr3N2(d)MgCl2(e)FrF
2.
3.(a)Na3N∆EN=2.1(b)SrBr2∆EN=1.8(c)LiCl∆EN=2.0
(d)CsF∆EN=3.3(e)Rb2O∆EN=2.7Compoundsinorderofincreasingionicbondcharacter:SrBr2<LiCl<Na3N<Rb2O<CsF
PracticeProblems—ComparingTypesofChemicalBonds,p.3191.(a)Acompoundwithanionicbond:NaCl(∆EN=2.1)(b)Acompoundwithapolarcovalentbond:AlN(∆EN=1.5)
(c)Acompoundwithanon‐polarcovalentbondNCl3(∆EN=0)
2.(a)H2O∆EN=1.4(b)PCl3∆EN=0.9
(c)CI4∆EN=0(d)SiO2∆EN=1.7(e)AlN∆EN=1.5
Compoundsinorderofmostequaltomostunequalelectronsharing:CI4PCl3H2OAlNSiO2
3.
ElementsPresent Formula ∆ENValue NatureofBonds
AtomPossessingGreaterElectronDensity
CandS CS2 0 covalent neither
BandCl BCl3 1.0 polarcovalent chlorine
AlandO Al2O3 2.0 ionic oxygen
NandI NI3 0.5 polarcovalent nitrogen
CaandF CaF2 3.0 ionic fluorine
6.3Activity(p.321)
ResultsandDiscussion1.Somemetal–non‐metalcombinationsresultinpolarcovalentbonds.Examplesinclude:AlCl3(∆EN=1.5)GaBr3(∆EN=1.4)
(a)RbF As∆EN=3.2bondisionic(b)RaCl2 As∆EN=2.1bondisionic(c)KBr As∆EN=2.0bondisionic(d)Na2O As∆EN=2.6bondisionic
Chapter6Answers 10
Someotherexamplesare:BeBr2(∆EN=1.3)MgS(∆EN=1.3)Ca3P2(∆EN=1.1)6.3ReviewQuestions(p.322)1.(a)Theenergyassociatedwiththebondedatomsmustbelessthanwhentheatomsareapart.
(b)Thistellsusthattheattractiveforcesexistingbetweenthebondedatomsexceedtherepulsiveforces.
2.Anioniccrystallatticeisthethree‐dimensionalsymmetricalarrangementofcationsandanionsina
solidioniccrystal.Thevastnumberofinterionicforcespresentinsuchacrystalmustbeovercometomeltanioniccompoundandthisexplainsthehighmeltingpointsofsuchcompounds.
3.Acrystallatticeshowsusthatnoneutralindependentmoleculesexistinioniccompounds.Theformulasthatwewritesimplyrepresentthesmallestwholenumberratiosofcationstoanionsthatexist
inioniccompounds.
4.(a)TheattractiveforcesassociatedwithIonicbondsaretheelectrostaticforcesbetweenpositivelychargedcationsandnegativelychargedanions.
(b)Theattractiveforcesassociatedwithcovalentbondsaretheelectrostaticforcesbetween
negativelychargedelectronsandadjacentpositivelychargednuclei.
5.(a)Similaritiesbetweenionicandcovalentbondsinclude:
• Bothinvolvevalenceelectronclouds.• Bothinvolveelectrostaticattractionsbetweenoppositelychargedspecies.• Theformationofbothbondsbeginswiththevalenceelectronsoftwoatomsexperiencingtheattractiveforceofadjacentpositivenuclei.
• Ionicandcovalentbondscanbothbestrong(b)Differencesbetweenionicandcovalentbondsinclude:
• Ionicbondsinvolvethetransferofvalenceelectronsformingionswhereascovalentbondsinvolvethe
sharingofvalenceelectronswithnoionformation.
• Ionicbondsformonlybetweenmetalsandnon‐metalswhereascovalentbondsusually(butnot
always)formbetweentwonon‐metals.
• Ionicbondingdoesnotresultinmoleculeformationwhereascovalentbondingusuallydoes.
• Theattractiveforcesassociatedwithcovalentbondsaretheelectrostaticforcesbetweennegativelychargedelectronsandadjacentpositivelychargednuclei.
• TheattractiveforcesassociatedwithIonicbondsaretheelectrostaticforcesbetweenpositivelychargedcationsandnegativelychargedanions.
Chapter6Answers 11
6.
Elements CompoundFormula ∆ENValue NatureofBondsPresent
(a)rubidiumandoxygen Rb2O 2.7 ionic
(b)strontiumandbromine SrBr2 1.8 ionic
(c)carbonandsulphur CS2 0 covalent
(d)siliconandchlorine SiCl4 1.2 polarcovalent
7.∆ENforMgS=1.3
∆ENforH2O=1.4Weseethatthebondsinwateractuallypossessslightlymoreioniccharacterthanthoseinmagnesiumsulphideeventhoughtheformercompoundcontainstwonon‐metalsandthelattercompoundcontains
ametalandanon‐metal.
8.Theformulaforglucoseandothermolecularcompoundsdoesnotrepresentaratioasdoionicformulas.Rather,theyrepresenttheactualnumberofatomsofeachelementexistinginanindividual
moleculeofthecompound.
9.Themeltingofmolecularcovalentcompoundsdoesnotinvolvethebreakingofcovalentchemicalbondswithinthemolecules,butratherovercomingtherelativelyweakattractiveforcesbetweenthose
moleculesinthesolidphase.
10.Diamondisatypeofcovalentsubstanceisknownasanetworkcovalentsolid.Ratherthanconsisting
ofindividualmoleculesasmolecularcovalentcompoundsdo,thesesubstancesareheldtogetherbycovalentbondsthatextendthroughouttheentiresample.Thismeansthatthe“molecule”isliterallyasbigasthesampleitself.Tomeltsuchasubstance,allofthecovalentbondswithinthisgiantmolecule
mustbebrokenandthisaccountsfortheveryhighmeltingpoint.
11.ThebondinHClisapolarcovalentbond(∆EN=0.9)whereasthebondinN2mustbepurecovalent(∆EN=0).ThismeansthatelectrondensityintheHClmoleculeisconcentratedonthechlorinesideof
themoleculemakingthatendofthemoleculesomewhatnegativeandthehydrogenendofthemoleculesomewhatpositive.Wemightexpecttherefore,thatHClmoleculeswouldattracteachothermorestronglythanN2moleculeswheretheelectrondensityisevenlydistributedthroughoutthe
molecule.(ThemeltingpointofHClis–114.2°CandthemeltingpointofN2is–210°C.)
Chapter6Answers 15
6.4Activity(p.337)Procedure1.1.ChemicalFormula
2.NumberofAtomsBondedtoCentralAtom
3.NumberofLonePairsonCentralAtom
4.SumofColumns2and3
NCl3 3 1 3+1=4 H2O 2 2 4 CO2 2 0 2 NH4
+ 4 0 4 XeF4 4 2 6 CO3
2– 3 0 3
4. CO2NCl3H2O NH4
+ XeF4 CO3
2–
Chapter6Answers 18
6.5TheShapeandBehaviourofMolecules(Extension)
WarmUp(p.341)1.Anelement’selectronegativitytellsushowwellitattractsapairofbondedelectronstoitselfinachemicalbond.
2.Apolarcovalentbondisabondinwhichelectronpairsareunequallysharedbetweentwodifferentatoms.Theatomhavingthehigherelectronegativitypullsthebondedelectronsclosertoitselfandaway
fromtheadjacentatomresultinginapartialpositivechargeontheatomwiththelowerelectronegativityandapartialnegativechargeontheatomwiththehigherelectronegativity.
3.Substancescomposedofpolarmoleculeswouldbeexpectedtohavehighermeltingandboilingpointsbecausethenegativeendsofthepolarmoleculeswillbeattractedtothepositiveendsofneighbouringpolarmolecules.
QuickCheck(p.346)1.Anygroupofvalenceelectronsassociatedwithacentralatom(suchasalonepair,bondingpair,or
multiplepairsinvolvedinadoubleortriplebond)willtendtoorientthemselvesinthree‐dimensionalspacearoundthatatomsoastominimizetherepulsionbetweenthem.
2.BP‐BP<BP‐LP<LP‐LP
leastintensemostintense
3.Inmethane(CH4)alloftheelectronpairinteractionsareBP‐BPbecausemethaneisanAX4molecule.
Asthesearetheleastintense,theH‐C‐Hbondanglesarethoseofaregulartetrahedron(109.5°).Ammonia(NH3)isanAX3Emoleculeandthelonepaironthecentralnitrogenexertsmoreofarepulsive
forceontheN‐HbondingelectronpairsthanabondingpairofelectronswouldandsotheH‐N‐Hbond
anglesaresmallerat107°becauseofthatLP‐BPinteraction.WaterisanAX2E2moleculeandthetwo
lonepairsonthecentraloxygenexertmorerepulsiveforcesontheO‐Hbondingelectronsthanasingle
lonepairwould.Inwater,LP‐LPaswellasLP‐BPinteractionsexists.Asaresult,theH‐O‐Hbondangleis
evensmallerat104.5°.
Chapter6Answers 19
PracticeProblems(p.348)1.
LewisStructure AXmEnNotation MolecularShape(NameandDiagram)(a)
AX5
trigonalbipyramidal
(b)
AX4E2
squareplanar
2.
ChemicalFormula
LewisStructure AXmEnNotation
MolecularShape(NameandDiagram)
(a)
CCl4
AX4
tetrahedral
(b)
PF3
AX3E
trigonalpyramidal
Chapter6Answers 20
(c)
SCl2
AX2E2
bentorangular
PracticeProblems(p.353)
1.
SymmetricMolecules AsymmetricMoleculesAXmEnNotation ShapeofMolecule AXmEnNotation ShapeofMolecule
AX2 linear AX2E bentorangular
AX3 trigonalplanar AX2E2 bentorangular
AX4 tetrahedral AX3E trigonalpyramidal
AX5 trigonalbipyramidal AX4E seesaw
AX6 octahedral AX3E2 t‐shaped
AX2E3 linear AX5E squarepyramidal
AX4E2 squareplanar
2.
LewisStructure AXmEnNotation
MolecularShape(NameandDiagram)
PolarMolecule?(Yes/No)
(a)
AX3
trigonalplanar
Yes
(b)
AX3E
trigonalpyramidal
Yes
Chapter6Answers 21
(c)
AX5
trigonalbipyramidal
No
6.5Activity(p.359)Procedure3.
AXmEnNotation SampleMolecule NameofShape PolarMolecule?Yes/No
AX2 CO2 linear no
AX3 BF3 trigonalplanar no
AX2E SO2 bentorangular yes
AX4 CH4 tetrahedral no
AX3E NH3 trigonalpyramidal yes
AX2E2 H2O bentorangular yes
AX5 PCl5 trigonalbipyramidal no
AX4E SF4 see‐saw yes
AX3E2 BrF3 t‐shaped yes
AX2E3 XeF2 linear no
AX6 SF6 octahedral no
AX5E BrF5 squarepyramidal yes
AX4E2 XeF4 squareplanar no
4.AsXeF4isasymmetricsquareplanarmolecule,anybonddipolesthatmightexistwouldcancelout.
Thismeansthattheelectronegativitiesoftheelementsinthemoleculeandalsothecorresponding∆ENvaluesareirrelevantwhenassessingthemolecularpolarity.
Chapter6Answers 22
6.5ReviewQuestions(p.360)1.(a)Thelettersstandfor:ValenceShellElectronPairRepulsion.
(b)VSEPRtheoryallowsustousetwo‐dimensionalLewisdiagramsformoleculestoquiteaccuratelypredictthethree‐dimensionalshapesofthosemolecules.
2.Aslone‐pairelectronsareattractedtoonlyoneatomicnucleus,theyareheldlesstightlythanbondingelectrongroups.Theirelectroncloudsthereforeoccupymorespaceandexertmorerepulsiveforceonbondingelectrongroupsthanthosegroupsexertoneachother.
3.BothBF3andCH2OhavethesameshapebecausebothofthemareAX3molecules.Eventhoughtheperipheralatomsineachmoleculepossessdifferentnumbersofnon‐bondingelectronsandoneofthe
AXbondsonCH2Oisadoublebond,theshapesareidentical.4.
AXmEnNotation MolecularShape AXmEnNotation MolecularShape
AX3 angular AX4E T‐shaped
AX2E3 trigonalbipyramidal AX2E octahedral
AX4 trigonalpyramidal AX3E2 squarepyramidal
AX3E trigonalplanar AX6 squareplanar
AX2E2 tetrahedral AX5E angular
AX5 linear AX4E2 seesaw
5.(a)TheX–A–Xbondanglesinammoniawillbesmallerthaninmethane(107°vs.109.5°)becauseammoniaisanAX3EmoleculeandmethaneisanAX4molecule.Thelonepaironthecentralnitrogenin
ammoniaoccupiesmorespacethanthebondedpairsonthecentralcarboninmethaneandwillthereforeforcethebondedelectronpairsinammoniaclosertogether.
(b)MethaneisasymmetricalAX4moleculeandisnon‐polar.Theintermolecularforcesactingare
thereforeweakLondondispersionforces.AmmoniaisanasymmetricalAX3Epolarmoleculethatexhibitsmuchstrongerhydrogenbonding.
6.
AXmEnCategory AX2 AX3 AX4 AX5 AX2E3 AX6 AX4E2X–A–XBondAngle 180° 120° 109.5° 120° 180° 180° 90° 90°
Chapter6Answers 23
7.ThemoleculesofeachcompoundareasymmetricAX3Emolecules.Ifweconsiderthe∆ENvaluesaswemoveupthefamilyfrombottomtotop,weseethefollowing:
Compound SbH3 AsH3 PH3 NH3∆ENValue 0.2 0.1 0.0 0.9
Eventhoughasymmetryexistsineachofthemolecules,becausethe∆ENvaluesforthefirstthreecompoundsaresosmall,allofthebondsinthosemoleculesarenon‐polarandsothemolecules
themselvesarenon‐polar.Asaresult,theintermolecularforcesactingbetweenthemareweakLondondispersionforces.However,thebondsinammoniaarequitepolarandsothemoleculeitselfisalsopolar.Asnitrogenisbondedtohydrogeninapolarmolecule,hydrogenbondsexistbetweenthe
ammoniamolecules.Wewouldthereforeexpectammoniatohavethehighestboilingpoint.8.BecausehydrogenbondsexistbetweenammoniamoleculesbutonlyLondondispersionforcesexist
betweenmoleculesofnitrogenandhydrogen,theammoniahasahigherboilingpointandthusliquefies(condenses)atahighertemperaturethannitrogenandhydrogen,whichremaininthegaseousphaseuponcooling.
9.Asaresultofhydrogenbonding,waterremainsliquiduntil100°C.Thisisveryimportantgiventhefact
thatmostofourEarthandmostofourbodiesarecomposedofandrequireliquidwatertosurvive.Thethree‐dimensionalstructureofproteinsandthebase‐pairinginthedoublehelixofDNAmolecules
dependonhydrogenbonds.Hydrogenbondformationinicemakesitlessdensethanliquidwater,whichiscrucialtoaquaticlifewhenbodiesofwaterfreeze.
10.I2isarelativelylargeandmassivediatomicmoleculeandthestrengthofLondondispersionforcesincreaseasthesizeofthemoleculesinvolvedincreases.Thisisbecauselargeelectroncloudsaremorelooselyheldthansmallercloudsandthusmoreeasilydeformedorpolarizedbyanearbydipolethan
compacttightlyheldclouds.Inaddition,largemoleculeswithmoresurfaceareahaveelectroncloudsthatarespreadoutandsoaremoreeasilydistortedbyneighboringdipoles.Asaresult,thedispersion
forcesarestrongenoughtokeepthemoleculesofI2attachedtoeachotherevenatroomtemperature.11.Althoughtheionicbondsholdingacrystallatticetogetherarestrong,whenthesurfaceofthat
latticeisincontactwithwater,eachiononthatsurfacewillattracttheoppositelychargedendofpolarwatermoleculesnearthem.Thatattractionbetweenanionandapolarmoleculeiscalledanion‐dipoleforce.Theseattractiveforcessoonovercomethoseexistingbetweentheionsthemselvesandsothe
crystalstructurebeginstobreakdownandtheioniccompounddissolves.Astheionsmoveawayfromthelatticesurface,theyimmediatelybecomesurroundedorenclosedinwhatchemistscallahydrationshell.Ion‐dipoleforcesaretheprimaryforceresponsibleforthesolubilityofioniccompoundsinwater.
Chapter6Answers 24
12.
LewisStructure AXmEn
Notation
ShapeofMolecule
(NameandDiagram)
TypeofIntermolecularForceActingBetween
Molecules(a)
dichloromethane
AX4
tetrahedral
dipole‐dipole
(b)
:
phosgene(agasusedinthetrenchesinWWI)
AX3
trigonalplanar
dipole‐dipole
(c)
sulphurhexafluoride
AX6
octahedral
Londondispersion
(d)
iodinepentafluoride
AX5E
squarepyramidal
dipole‐dipole
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