Chapter4ChemicalBonds

Mrs.Valen6nePhysicalScience4thand6thPeriod

Sec6on1-IonicBonds

•  Elementswithfewerthan8valenceelectronsbehaveasthesetwofriends.

•  Elementswithmorethan4valenceelectronsarelikethefriendwithnotenoughmoney,sotheyborrowenoughelectronstohave8.

•  Elementswithfewerthan4valenceelectronstendtodonateenoughtotheirfriendsothattheyhave0,andtheirfriendhas8.

5+valencee-

3orfewervalencee-

Apples

WhatareIons?•  Thisway,bothatomsareinalowerenergystate,andaremorestable.

•  Defini&on:ion–anatomorgroupofatomsthathasbecomeelectricallycharged.

•  Whenanatomlosesanelectron,itlosesanega6vechargeandbecomesaposi6veion.

-

11protons+11electrons=neutral11protons+10electrons=+1charge

Posi6veorNega6ve?•  Whenanatomgainsanelectron,itgainsanega6vechargeandbecomesanega6veion.

•  Sodiumandfluorineformionsandbondtocreatesodiumfluoride.

Na+F→NaFNaFismadeofNa+andF-

-

9protons+9electrons=neutral

9protons+10electrons=-1charge

DeterminingIonicCharge•  Atomstransferelectronstobecomemorestable.

•  Recall:Atomsaremoststablewith0or8valenceelectrons.

•  Therefore,theywilleithergiveupalloftheirvalenceelectronsorgainenoughtohave8.

•  Theresul6ngionicchargehasthemagnitudeofthenumberofelectronstransferred.Itwillbeposi6veifelectronswerelostornega6veifelectronsweregained.

FormingIonsPrac6ceProblems(p.1inpacket)

•  Usingyourperiodictableonly,determinetheionsthateachofthefollowingelementswillform.Na OMg NAl ClSr SLi F

Na+Mg2+Al3+Sr2+Li+

O2-N3-Cl-S2-F-

ForminganIonicBond• Defini&on:IonicBond–thea`rac6onbetweentwooppositelychargedions.

•  Inthecaseofsodiumfluoride,theposi6velychargedsodiumion(Na+)isa`ractedtothenega6velychargedfluorideion(F-).

•  Everysodiumion(posi6vecharge)isbalancedoutwithafluorideion(nega6vecharge).

•  Theformula(NaF)showsthatthesodiumandfluorineareina1:1ra6o.

IonicCompounds•  Compoundsareelectricallyneutral.

•  Therefore,theionsmustcometogetherinsuchawayastocanceloutanycharge.

•  Sowhatifwewantedtocombinemagnesiumandchlorine?

•  Magnesiumformsanionwitha2+charge.Chlorineformsanionwitha1-charge.

•  Thecross-overmethodcanbeusedtodeterminetheformulaforthiscompound.

Cross-OverMethod•  Thecross-overmethodinvolvescrossingoverthechargesofthe

ionstodeterminehowmanyofeachisneeded.

•  Thesetup:

•  Let’ssetitupforourexample:

•  Next,taketheabsolutevalueofthecharges.

•  Then,crossthechargesintothecountboxes.

•  Aswecanseeabove,therewouldneedtobetwochlorideionsforeachmagnesiuminordertogetaneutralcompound.

Ioncharge

count

Posi6veIon Nega6veIon

Mg2+

Cl1-

Ioncharge

count

|| ||

21

IonicCompounds

•  Therefore,thecompoundformedwithmagnesiumandchlorineisMgCl2.

•  Thesubscript2indicatesthattherearetwooftheseions.Subscriptsof1areassumed,andthereforearenotincludedintheformula.

•  Subscriptsarereducedtothesimplestwhole-numberra6oforioniccompounds.

•  Typically,ionsareformedfromthecombina6onofametalwithanonmetal.

•  Trycomingupwiththeformulasforthecompoundsformedfromthefollowing(p.2inpacket):CaandO BeandCl GaandOKandBr AlandF LiandS

CaO BeCl2 Ga2O3KBr AlF3 Li2S

PolyatomicIons

•  Defini&on:PolyatomicIons–ionsmadeofmorethanoneatom.

•  Theprefixpolymeans“many,”sopolyatomicmeans“manyatoms.”

•  Thinkoftheseasgroupsofatomsac6ngasone.

•  Let’slookattheexampletotheright.

•  OtherexamplescanbefoundinFigure2.

PolyatomicIonCompounds•  Whenapolyatomicionreactswithanotherionofoppositecharge,anioniccompoundforms.

•  Forexample,calciumcarbonate(CaCO3)ismadeofcalciumions(Ca2+)andcarbonateions(CO3

2-).Calciumcarbonateisamaincomponentoflimestone.

• Anotherexampleincludessodiumsulfate(Na2SO4).Thisismadeofsodiumions(Na+)andsulfateions(SO4

2-).

Cross-OverMethodwithPolyatomicIons

•  Aspolyatomicionsaresingleunits,thecross-overmethodcanbeusedtodetermineformulascontainingthem.

•  Let’sdeterminetheformulaforthecompoundmadefromcalciumion(Ca2+)andphosphateion(PO4

3-).Sincephosphateisasingleunit,alloftheion’satomsgointhelargebox:

•  This6me,thelargeboxforthepolyatomiciongoesinparentheses.Thisistoavoidconfusionwiththesubscripts.

•  Then,proceednormally.

•  Inthiscase,theformulawillbeCa3(PO4)2.

Ca2+

PO4

3-|| ||

23 ()

IonTypesandNamingIonicCompounds

•  Defini&on:Ca6ons–ionswithaposi6vecharge.Thesecanbeeithermonatomicorpolyatomicions.

• Defini&on:Anions–ionswithanega6vecharge.Thesecanbeeithermonatomicorpolyatomicions.

•  Whennaminganioniccompound,theca6onalwayscomesfirst,anionsecond.

•  Usually,thenameoftheposi6veionisthenameofametal.

NamingIonicCompounds•  Theca6onmayalsobeapolyatomicion,suchasammonium.

•  Iftheanionisanelement,theendofthenamechangesto–ide.

•  Forexample,MgOiscalled“magnesiumoxide.”

•  Trynamingthefollowingcompounds(p.2inpacket):NaCl LiI CaO MgS BeF2 Na2O

sodiumchloride

magnesiumsulfide

lithiumiodide

berylliumfluoride

calciumoxide

sodiumoxide

NamingIonicCompounds

•  Iftheanionispolyatomic,itsnameisunchanged.

•  Forexample,Na2CO3iscalled“sodiumcarbonate.”

•  Trynamingtheseothercompounds(p.3inpacket):

Al2(SO4)3 NH4Cl MgCO3

K3PO4 CaHCO3 LiNO3

aluminumsulfate

potassiumphosphate

ammoniumchloride

calciumbicarbonate

magnesiumcarbonate

lithiumnitrate

Proper6esofIonicCompounds•  Eventhoughsomeioniccompoundsmaynotappeartobeanythingalike,theymayhavesomesimilari6es.

•  Forexample,limestone,rust,andtablesalt.

•  Youmightnotbeabletousethemforthesamethings,andtheymaynotlookthesame,buttheys6llsharesomeproper6es.

•  Thecharacteris&cproper&esofioniccompoundsincludecrystalshape,highmel&ngpoints,andelectricalconduc&vity.

CrystalShape

•  Allhalite(tablesalt)sampleshavesharpedges,corners,andflatsurfaces,whetherthehaliteisinbricksorsmallcrystals.

•  Thesodiumchloridepackstogetherinanalterna6ng pa`ern.

• Defini&on:Crystal–anorderly,three-dimensionalarrangementofatomsorionsinasolid.

•  Crystalsofasinglecompoundtendtohaveasimilarshape,whilecrystalsofdifferentcompoundsdifferinshape.

CrystalShape•  Inanioniccompound,everyionisa`ractedtoionsnearitthat

haveanoppositecharge.Likechargesrepeleachother.

•  Onesodiumiondoesnotjustbondtoonechlorideion.Theybondtoallthosearoundthem(inalldirec6ons)toformthecrystal.

•  Thenumberofsodiumionsandchlorideionsexistina1:1ra6o.Theformula,NaCl,representsthisra6o.

HighMel6ngPoints•  Inordertomeltthecrystalofsalt,thecrystalstructuremustbebrokenapart.

•  Sincethestructureisheldtogetherbyionicbonds,thesebondsmustbebroken.

•  Fortablesalt,thistakesatemperatureof801°C.

•  Ionicbondsareverystrong,strongenoughtocauseallsortsofcompoundstobesolidatroomtemperature.

ElectricalConduc6vity•  Whenioniccompoundsdissolveinwater,thesolu6onconductselectricity.

•  Defini&on:Electricity–theflowofelectriccharge.

•  Ionshaveelectriccharge.However,asolidioniccompoundwillnotconductelectricity.

•  Thisisbecausetheionsareso6ghtlyboundtogetherwhentheyaretogetherasasolid.

ElectricalConduc6vity

•  Whenioniccompoundsaredissolvedinwater,theybreakapartandseparate.

•  Thesefree-movingionsinthesolu6onarewhatenablesittoconductelectricity.

•  Mel6ngalsoallowsthemtoconductelectricity.

•  Backtotheappleanalogy.Let’ssaythatyoucangetoneapplefor40cents,buttwofor70cents.

•  Theycouldpooltheirmoneytogethersothattheycouldeachgetanapple.Thisislikecovalentbonds.

Sec6on2-CovalentBonds

1/40¢or2/70¢

ApplesX

ElectronSharing•  Defini&on:CovalentBond–achemicalbondformedwhentwo

atomsshareelectrons.

•  Unlikeionicbonds,whichformbetweenmetalsandnonmetals,covalentbondsformbetweentwononmetals.

•  Oxygen,carbon,nitrogen,andthehalogensareallexamplesofnonmetalsthatfrequentlybondwithothernonmetals.

Forexample,whenthesefluorineatomsapproacheachother,theatomsshareelectronsandstaytogether.

ElectronSharing•  Recallthatfluorineformsadiatomicmolecule.Eachofthefluorineatomshas7valenceelectrons.

•  Eachonesharesoneofthesewiththeother.

•  Whenyoucountthenumberofelectronsononeatominthemolecule,youcountbothofthesharedelectronseach6me.

•  Inacovalentbond,bothatomsa`ractthetwosharedelectronsatthesame6me.

HowManyBonds?•  Let’slookattheelectrondotdiagramsforhydrogen,oxygen,nitrogen,andcarbonatoms.

•  Thenumberofbondseachofthesecanformequalsthenumberofelectronsneededtomakeatotalofeight.

•  Forexample,oxygenhassixvalenceelectrons,soitcanformtwocovalentbonds.

•  Sincenitrogenhas5valenceelectrons,itcanformthreebonds.

Mul6pleBonds•  Someofthemoleculesformedbytheseelementscanbeseeninthefigurebelow.

•  Defini&on:DoubleBond–twopairsofelectronsaresharedbetweentwoatoms.

•  Some6mes,twoatomscanevensharethreepairsofelectrons.Thisiscalledatriplebond.

•  Rememberthatsharedpairsofelectronscounttowardsbothatoms.

Proper6esofMolecularCompounds

•  Defini&on:MolecularCompounds–moleculeshavingcovalentlybondedatoms.

•  Theproper6esofmolecularcompoundsareverydifferentthanioniccompounds.

•  Theseareverydifferentthantheveryhightemperaturesthationiccompoundsrequire.

Proper6esofMolecularCompounds

•  Theforcesbetweenanytwomoleculesaremuchweakerthanthoseholdingionstogether.

•  Somemolecularcompoundsdoformcrystals,suchaswaterandsugar,buttheymeltandboilatmuchlowertemperaturesthanioniccrystals.

•  Mostmolecularcompoundsarepoorconductors.Thisiswhysomemolecularcompounds,suchasrubber,areusedasinsulatorsforelectricalwires.

SharpenYourSkills•  Hypothe6cally,youhavetwosamplesofcolorless,odorlessgas.

•  Oneismethane(CH4)andtheotheriscarbondioxide(CO2).

•  UseFigure8onpage122tofindawaytodeterminewhichgasiswhich.

•  Whatcondi6onswouldyouchange?Whatcondi6onswouldyoucontrol?

•  Whatresultwouldyoulookfortogetananswer?

UnequalSharingofElectrons

•  Thesharingofelectronscanbelikeagameoftug-of-war.

•  Ifthepullisnotthesameonbothsides,theelectronswillmoveclosertotheatomwiththestrongerpull.

•  Thiscausestheatomstohaveslightelectricalcharges.

•  Thesechargesarenotasstrongasthoseinions.

δ- δ+Note:δ=par6al

UnequalSharingofElectrons

•  Defini&on:Polar–describesacovalentbondinwhichelectronsaresharedunequally.

•  Iftwoatomspullequallyontheelectrons,thenneitheratombecomescharged.

•  Thisisthecasewhenthetwoatomsareiden6cal,asinfluorinegas(F2).

•  Defini&on:Nonpolar–describesacovalentbondinwhichelectronsaresharedequally.

NonpolarMolecules

•  Let’slookatcarbondioxide.

•  Sincethetwooxygenatomsarepullinginequalandoppositedirec6ons,they,inasense,canceleachotherout.

•  NOTE:Amoleculecanbenonpolar,eventhoughithaspolarbonds.

PolarBonds•  Let’slookatthewatermolecule.

•  Theoxygenatomispullingtheelectronsfromthehydrogenatomsclosertoit.

•  Overall,themoleculeispolar,sincethetwopullsarenotopposite,thoughtheyareequalinmagnitude.

A`rac6onBetweenMolecules•  Theslightlyposi6veandslightlynega6veendsofawatermoleculeactlikethepolesofabarmagnet.

•  Theya`racttheoppositeendsofotherwatermolecules.

•  DifferencesintheaDrac&onsbetweenmoleculesleadtodifferentproper&esinpolarandnonpolarcompounds.

•  Forexample,waterandvegetableoildon’tmix.Oilisnonpolar,andnonpolarcompoundsdonotdissolveinwater.

HowSoapWorks•  Soaphasapolarendandanonpolarend.

•  Thenonpolardirtoroilisa`ractedtothenonpolarendofthesoap.

•  Thepolarendofthesoapisthena`ractedtothepolarwater.

•  Asthewaterflows,ittakesthesoapwithit,and,therefore,thedirt.

(+)

(-)

WaterSoap–polarendSoap–nonpolarendDirt

Sec6on3CrystalChemistry

•  Defini&on:Mineral–anaturallyoccurringsolidthathasacrystalstructureandadefinitechemicalcomposi6on.

•  Thereareafewmineralsthatareelements,suchassulfurandgold.

•  Mineralscanbeiden6fiedbytheirproper6es,suchascolor,shininess,density,crystalshape,hardness,andmagne6sm.

MineralProper6es•  Someoftheseproper6esareeasilyobserved,suchascolor.

•  Othersrequiredmeasurementsortes6ng,suchasdensity.

•  Talc,thesosestmineral,caneasilybescratchedwithyourfingernail.

•  Diamondisthehardestmineral.

•  Othersareinbetween.

MineralProper6es

•  Anotherkeypropertyisthewayamineralbreaksapart.

•  Somebreakapartintoregularshapes.

•  Mica,forexample,breaksapartalongflatsurfacesatsharpedges.

• Crystalsalsogrowincharacteris6cshapes.

BondinginMineralCrystals•  Thestructureofacrystalisacharacteris6cpropertyofamineral.

•  Mineralscanbemadeofeitherioniccompoundsormolecularcompounds.

•  Thearrangementofpar&clesinamineralandthekindofbondsholdingthemtogetherdetermineproper&es,suchascrystalshape,hardness,andthewaythecrystalbreaksapart.

IonicCrystals•  Halite,forexample,isanioniccrystal.

•  Theoppositechargeofthesodiumandchlorideionsarelaidoutlikea3Dcheckerboard.

•  Let’slookathowioniccrystalsbreak.

+-+--+-++-+--+-+

+-+--+-++-+--+-+

AMolecularCrystal

•  Quartz(SiO2)isanexampleofamolecularmineral.

•  Thecovalentbondsinquartzaremuchstrongerthantheionicbondsinhalite.

•  Itcannotbecrushedintopredictableshapeswithahammer.

•  Instead,itbreaksintosmaller,irregularlyshapedpieceswithshell-likeridgessimilartobrokenglass.

ComparingCrystals•  Notallioniccrystalsaresimilarinproper6estohalite,andnotallmolecularcrystalsaresimilarinproper6estoquartz.

•  Somemolecularcrystalsarestrongerthanquartz.Thestrongerthebondsare,thestrongerthecrystalsare.

•  Manycrystalsareiden6fiablesimplybysight.However,ifthereisdoubt,furthertestsmaybeperformed.

Chapter4Review

Answerques6ons1-10,12,14,16,20-23onpages132-133.

Frank, D. V., Little, J. G., Miller, S., Pasachoff, J. M., & Wainwright, C. L. (2001). Physical science. Needham, Mass.: Prentice Hall.