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©BoardworksLtd20071of50P51-chapter4bondingandstructure1.Chemicalbond化学键©BoardworksLtd20072of501.Chemicalbond(1)DefinitionItisaninteractionbetweenatomsorionsthatresultsinareductioninthepotentialenergyofthesystemwhichhencebecomesmorestable.©BoardworksLtd20073of50Whydoatomsformbonds?Theatomsofnoblegaseshavecompletelyfulloutershellsandsoarestable.Theatomsofotherelementshaveincompleteouterelectronshellsandsoareunstable.Thismakesthenoblegasesveryunreactiveandsotheydonotusuallyformbonds.Byformingbonds,theatomsoftheseelementsareabletohavefilledoutershellsandbecomestable.©BoardworksLtd20074of50ChemicalbondCompoundsareformedwhendifferentelementschemicallyreactandformbondswitheachother.Somecompounds,likewater(H20),havesmall,simplestructureswithjustafewatomsbondedtogether.Otherscompounds,likeDNA,havelarge,complexstructurescontainingthousandsorevenmillionsofbondedatoms.Eachelementismadeupofjustonetypeofatom,usuallyjoinedtootheratomsofthesameelementbybonds.Thisformsmoleculessuchaschlorine(Cl2).©BoardworksLtd20075of50(2)Division•Thebondtypedependsontheirthecharacteroftheelement.•non-metalandmetalIonicbonding•non-metalandnon-metalCovalentbonding•metalsMetallicbonding©BoardworksLtd20076of50Howdoatomsformpositiveions?Thenumberofchargeisusuallythesameasthenumberofelectronsintheatom’soutershell.Formetal,itloseselectronstoformapositivelychargedion.(cation).Formaingroup,metalslosealltheirelectronsintheirvalenceshell..lithiumion[2]lithiumatom2.1aluminiumatom2.8.3aluminiumion[2.8]magnesiumatom2.8.2magnesiumion[2.8]=Mg2+=Li+=Al3+©BoardworksLtd20077of50Fortransitionmetals,someofthemcanformstableionswithdifferentcharges.e.g.Fe2+,Fe3+However,weusuallyusebracketedRomannumbertorepresentthechemicalvalence.iron(II)hydroxide→Fe(OH)2iron(III)hydroxide→Fe(OH)3©BoardworksLtd20078of50Howdoatomsformnegativeions?Fornon-metal,itgainselectronstoformanegativelychargedion.(anion)Formaingroup,non-metalsgainelectronstofillthevalenceleveloftheatom.chlorineatomchlorideion[2.8.8]=Cl-oxygenatomoxideion[2.8]nitrogenatomnitrideion[2.8]2.8.72.62.5=N3-=O2-©BoardworksLtd20079of502.Ionicbonding•(1)Definition:Thepositiveandnegativeionsarestronglyattractedtoeachother.Thiselectrostaticattraction(静电吸引力)iscalledionicbonding.•Noelectronsharesbetweentheionsinionicbonding.©BoardworksLtd200710of502.Ionicbonding•Whydoesnon-metalandmetalalwaysformionicbond?•Ionicbondingoccursbetweenelementsthathavealargedifferenceinelectronegativity.©BoardworksLtd200711of50Review:electronegativity•Electronegativityisachemicalpropertythatdescribesthetendencyofanatomtoattractelectronstowardsitselfandthusthetendencytoformanions.©BoardworksLtd200712of50Theelectronegativitiesofnon-metalsarealwayshigherthanthoseofmetals.©BoardworksLtd200713of50Formationofanionicbond©BoardworksLtd200714of50(3)TherelationshipofcationandanionintheionicbondingThetotalnumberofelectronslostbymetalsareequaltothenumberofelectronsgainedbynon-metals.KBr:potassiumtransfersanelectrontothebromineatom.CaCl2:calciumtransferoneelectrontoonechlorineatom,andoneelectrontotheotheratom.©BoardworksLtd200715of50HowtowritetheformulaofanioniccompoundToworkouttheformulaofanioniccompound:1.Writedownthesymbolforeachelement–themetalisalwayswrittenfirst.2.Calculatethechargeforeachtypeofion.3.Balancethenumberofionssothatthepositiveandnegativechargesarebalancedandequalzero.Thisgivestheratioofions.4.Usetheratiotowritedowntheformulaoftheioniccompound.Theformulaofacompounduseschemicalsymbolsandnumberstoshowtheratioofatomsofeachelementpresent.©BoardworksLtd200716of50FormulaofaluminiumoxideWhatistheformulaofaluminiumoxide?AlO+3-2Al2O32aluminiumionsareneededfor3oxideions2:3SymbolBalancethenumberofionsIonchargeRatioofionsFormulaAlAl3+AlAl3+OO2-OO2-OO2-2electronsforeachoxygenatom©BoardworksLtd200717of503.Ioniccrystal•(1)what’sacrystal?•Asolidsubstanceinwhichtheatoms,molecules,orionsarearrangedinanorderlyrepeatingpatternextendinginallthreespatialdimensions(3D).•(2)Definitionofioniccrystal:•Acrystalconsistingofionsboundtogetherbytheirelectrostaticattraction.•Thismeansthattherearenomoleculespresentinioniccrystal.©BoardworksLtd200718of50©BoardworksLtd200719of50•Eachatomhassixnearestneighbors,withoctahedral(正八面体)geometry.Thisarrangementisknownascubicclosepacked(ccp立方密堆积).•EachNa+issurroundedby6Cl-ions.•EachCl-issurroundedby6Na+ions.©BoardworksLtd200720of50(2)Characterofioniccrystal1.Hard,brittle(易碎)crystallinesolids.2.Relativelyhighmeltingandboilingpoints.3.Donotconductelectricitywhensolid,butconductelectricitywhenmoltenorinaqueoussolution.4.Aremoresolubleinwaterthanothersolvents.©BoardworksLtd200721of50Whyareioniccompoundsbrittle?Ioniccompoundsarebrittle–theyshatterwhentheyarehit.Whenthelatticeishit,alayerofionsisshiftedsothationswiththesamechargesarelineduptogether.Theselikechargesrepeleachotherandsosplittheioniclatticecausingittoshatter.repulsionforceIonicbondsarestrong,sowhydoesthishappen?©BoardworksLtd200722of50Whyareioniccompoundshavehighmeltingpointsandboilingpoints?Largerionicchargesproducestrongerionicbondsandsomuchmoreheatisrequiredtobreaktheion
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