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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.