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