L 5Bonding+Crystalline 01.docx
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L 5Bonding+Crystalline 01.docx
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L5Bonding+Crystalline01
MetallicBonding
Elementscanbedividedintometalsandnon-metals.Mostelementsaremetals.Metalshavelowelectro-negativities–theylosetheiroutershellelectronseasilytoformpositivelychargedionscalledcations.
Structureofmetals
▪Metalsoccurascrystallatticesmadeupofcloselypackedcations.
▪Theelectronslostfromeachcationaredelocalised–theyarefreetomovebetweenallcations.
▪Thusametalcanbedescribedas‘closelypackedarrayofcationssurroundedbyaseaofmobileelectrons’.
▪Thenegativeelectronseaandthepositivecationsattracteachother.Thisistheforcewhichholdsametaltogether.Itiscalledmetallicbonding.
Theelectronscanmovefreelywithinthesemolecularorbitals,andsoeachelectronbecomesdetachedfromitsparentatom.Theelectronsaresaidtobedelocalised.Themetalisheldtogetherbythestrongforcesofattractionbetweenthepositivenucleiandthedelocalisedelectrons.
Theelectronscanmovefreelywithinthesemolecularorbitals,andsoeachelectronbecomesdetachedfromitsparentatom.Theelectronsaresaidtobedelocalised.Themetalisheldtogetherbythestrongforcesofattractionbetweenthepositivenucleiandthedelocalisedelectrons.
Sincethemetalatomshavealllosttheirvalenceelectrons,theyarenowallidenticaltocations.However,becausetheyarenotassociatedwithionicbonding,thesecationshaveaspecialname.ThecationswithinametallicsolidareknownasKernels.
Themechanismthatholdsametallicbondtogetheristheattractionbetweenthepositivekernelsandthenegativeelectronsea.Thestrengthofthemetallicbondisderivedprimarilyfromthechargesinthesystem.
∙Thelargerthemagnitudeofthepositivechargeonthemetallicnuclei,thegreaterthestrengthofthemetallicbond.
∙Thegreaterthenumberofvalenceelectronscontributedtotheelectronsea,thegreaterthestrengthofthemetallicbond.
Metallicbondsareomnidirectional.Theydonothaveanygeometricrequirementswhichneedtobefulfilled.Thinkofthemarblessurroundedbywater,inabox.Themarblescanbepushedanywherewithintheboxandthewaterwillfollowthem,alwayssurroundingthemarbles.Becauseofthisuniqueproperty,metallicbondscanmaintaintheirexistencewhenpushedandpulledinallsortsofways.Asaresult,metalsareknownfortheirflexibility--thustheyaremalleableandductile.
∙Ifametalissubjectedtoaforce,thekernelscanslidearoundonthelayerofelectrons.
∙Asthekernelsmovetonewpositions,thebondswillnotbreak,becauseoftheiromnidirectionalnature.
Inadditiontobeingmalleableandductile,theyarealsoverygoodconductorsofelectricity.Electricitydependsupontheflowof
electrons.Wheneverelectronscanfloweasilythroughastructure,thenthatstructureissaidtobeagoodelectricalconductor.Clearly,theveryfluidnatureoftheelectronseaallowsittobeaverygoodelectricalconductor.Becauseofthisquality,metalsareusuallyusedintheelectronicsindustry.
Metalsarealsoknownasbeinggoodconductorsofheat,orthermalconductors.Heatiskineticenergy.Inorderforasubstancetoconductheat,itmustbeabletotransmitkineticenergy.
∙Ifheatisappliedtoonesideofapieceofmetal,thenthekernelswillstarttovibrate.Becausetheyaresolooselyheldintothecrystalstructure,theywillbeabletovibratefreely.
∙Withtheincreaseintheamountoftheirvibration,theywillrunintothekernelslocatednexttothem.Thatwillstartmorekernelstovibrate.
∙Inthisway,theprocesscontinuesuntilallofthekernelsinthesystemarevibrating.
Anymaterialthathashighlyrigidstructuresbecauseofstrong,rigidbondingwillnothavethefreedomofmotionthatisneededforthetransmissionofthekineticenergy.
Thepropertiesofmetalsusingthemetallicbondingmodel
Generallyhavehighmeltingandboilingpoints
Theforcebetweencationsandtheelectronsea(metallicbonding)isstrong.
Generallyhard
Metallicbondingisstrong.
Lustrous
Themobileelectronsreflectlightwhichcausesthemetaltolookshiny.
Goodconductorsofheat
Themobileelectronscangainheat(kineticenergy)inhotterareasofthemetalandmovetootherareastransferringheat.
Goodconductorofelectricity
Themobileelectronsarefreetomoveandsocancarryanelectriccurrent.
Malleable(canbehammeredintosheets)andductile(canbedrawnintowires).
Thebondsarenon-directional-theforceofattractionbetweenelectronseaandthecationlatticeextendsinalldirections.Whenthelayersofcationsaredisrupted(e.gwhenthemetalishammeredordrawnintowires)thebondingisnotdisrupted.Theelectronscanmovetomaintainthebonding.
Dense
Thecationsinthemetallatticearecloselypacked.
The‘BallBearingModel’-ithelpstoexplainmetalstrength
Themoreeasilylayersofmetalatomsslideovereachotherthemoreeasilyametalwillbend.
Themoreperfectthepackingofmetalatomsthemoreeasilythelayersofatomscanslideovereachotherandthemoreeasilyametalwillbend.
Thepackingofmetalcationsisnotperfectthroughoutthemetal–metalshaveareasofperfectpackingcalledgrains.
Thereareareasofdisorderbetweenthegrainscalleddislocations.
Metalswithlargegrainshavefewerdislocations–theybendeasily.
Metalswithsmallgrainshavemoredislocations–theyarehardbuttheyaremorebrittle.
Modifyingmetals:
1.WorkHardening
▪Bendingorhammeringacoldmetalmakesthegrainssmaller
▪Themetalbecomesharderbutmorebrittle
▪e.gbendingapaperclipbackandforwardsmakesiteventuallysnap.
2.UsingHeat
Thesizeofthegrainsinametalcanbechangedbyheating
(i)Annealing
Themetalisheatedtillredhotandcooledslowly
Largegrainsform–themetalissofterandmoremalleable.
(ii)Quenching
Themetalisheatedandcooledquickly
Smallcrystalsform–themetalisharderbutmorebrittle.
(iii)Tempering
Quenchedmetalsarewarmedtoalowertemperaturethancooledslowly.
Thegrainsare‘mediumsized’-themetalislessbrittlebutretainsitshardness.
3.Alloying
▪Alloysaremadebymixingametalwithanothermetal(orsometimesanonmetal)
▪Themetalsareheatedtogetherandthemixtureiscooled.(Alloysareamixture)
▪Mostmetalsinusetodayarealloys.Alloyingaltersthepropertiesofmetalsandcanmakethemmoreusefule.gpureironissoftbuttheadditionof<1%carbonmakesitstrongandhard(steel).
▪Therearetwotypesofalloys
Substitutionalalloys–themetalatomsmakingthealloyareaboutthesamesizeandsoreplaceeachotherinthemetallattice.
Interstitialalloys–smalleratomsfitintospacesbetweenthelargeratoms.
▪Alloystendtobeharderandhavelowermeltingpointsthanthemetalsfromwhichtheyaremadebecausetheregularbondingisdisruptedbythepresenceofforeignatoms.
▪Thelayersdonotslideaseasilyovereachother,whichleadstolowermalleability.
▪Theforcesbetweentheparticlesarereducedbecauseofthemoreirregularanddisruptedlayers.Thisleadstoalowermeltingpoint.
CovalentBonding
Covalentbonds
▪Nonmetalatomsoftenbondtogether.Thisbondingcannotbeexplainedbyelectrontransferbecausebothnonmetalatomsneedtogainelectronstoattainafulloutershellofelectrons.
▪Nonmetalscanbondtogether,achievingastableelectronicconfigurationbysharingelectrons.
▪Thissharingofelectronsbetweentwononmetalatomiscalledacovalentbond.
▪Therearetwotypesofcovalentcompounds:
1.Molecularcompounds
2.Threedimensionallattices
1.Covalentmolecularsubstances:
▪Amoleculeisasmallgroupofatomsheldtogetherbycovalentbonds.Theatomsarecombinedinfixedratios.Moleculesareelectricallyneutral.
▪Molecularsubstancescanbeelements(e.gO2,Cl2)orcompounds(e.gH2O,CO2,CH4)
Propertiesofmolecularsubstances
Property
Explanation
Theyhavelowmeltingandboilingtemperatures
Forcesbetweenmoleculesareveryweak.
Theyarenon-conductorsinthesolidandliquidstate
Theyarenofreemovingcharges
Forcesincovalentsubstances
Theyaretwosortsofforcestobeconsidered
▪Forceswithinmolecules(covalentbonds)theseareverystrongforces
▪Forcesbetweenmolecules–thesearerelativelyweakforces
Forceswithinmolecules–covalentbonding
Non-metalshavehighelectronegativities–theyattractelectronseasily.
Whentwononmetalsformacovalentbondtheysharepairsofelectronstoachieveastable‘octet’–8electronsintheoutershell.
Hydrogen,H2
▪Ahydrogenatomhasoneoutershellelectron.Forhydrogenafulloutershellistwoelectrons.
▪Whentwohydrogenatomsbondtwoelectrons(onefromeachatom)aresharedbetweenthetwoatoms.
Bysharingthesingle,unpairedelectrons,bothatomscancompletetheiroutershells,asshowninthefigurebelow.Acovalentbondresultsfromthesimultaneousattractionofthetwopositivechlorinenucleitothesamesharedpairofelectrons.Thisbondholdsthetwoatomstogether.AmoleculeofchlorineformsandisrepresentedbythesymbolCl2.
▪Thesharedelectronsnowbelongtobothatoms.Theyspendmuchoftheirtimeinthespacebetweenthetwoatoms.
▪Althoughtherearenoionspresent,covalentbondingiselectrostaticinnature.Itarisesbecauseoftheattractionbetweenthepositiveatomicnucleiandthenegativesharedelectroncloud.
▪Moleculescanberepresentedby:
∙Electrondotformulae(Lewisdiagram)whereeachoftheoutershellelectronsisrepresentedbyadot.
∙Structuralformulaewhereasinglelinerepresentsonepairofelectrons.
Thereareseveralusefulwaysofrepr
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