AP化学课件-bonding

APChemistryChapter8LectureNotes-BasicBonding8.1ChemicalBonds,LewisSymbols,andtheOctetRule•Thepropertiesofmanymaterialscanbeunderstoodintermsoftheirmicroscopicproperties.•Microscopicpropertiesofmoleculesinclude:•theconnectionsbetweenatomsand•the3-Dshapeofthemolecule.•Whenatomsorionsarestronglyattractedtooneanother,wesaythatthereisachemicalbondbetweenthem.•Inchemicalbonds,electronsaresharedortransferredbetweenatoms.•Typesofchemicalbondsinclude:•ionicbonds(electrostaticforcesthatholdionstogether,e.g.,NaCl);•covalentbonds(resultfromsharingelectronsbetweenatoms,e.g.,Cl2);•metallicbonds(referstometalnucleifloatinginaseaofelectrons,e.g.,Na).LewisSymbols•Theelectronsinvolvedinbondingarecalledvalenceelectrons.•Valenceelectronsarefoundintheincomplete,outermostshellofanatom.•Asapictorialunderstandingofwheretheelectronsareinanatom,werepresenttheelectronsasdotsaroundthesymbolfortheelement.•Thenumberofvalenceelectronsavailableforbondingareindicatedbyunpaireddots.•ThesesymbolsarecalledLewissymbols,orLewiselectron-dotsymbols.•Wegenerallyplacetheelectronsonfoursidesofasquarearoundtheelement’ssymbol.TheOctetRule•Atomstendtogain,lose,orshareelectronsuntiltheyaresurroundedbyeightvalenceelectrons;thisisknownastheoctetrule.•Anoctetconsistsoffullsandpsubshells.•Weknowthats2p6isanoblegasconfiguration.•Weassumethatanatomisstablewhensurroundedbyeightelectrons(fourelectronpairs).8.2IonicBonding•Considerthereactionbetweensodiumandchlorine:Na(s)+½Cl2(g)NaCl(s)∆Hºf=–410.9kJ/mol•Thereactionisviolentlyexothermic.•WeinferthattheNaClismorestablethanitsconstituentelements.•SodiumhaslostanelectrontobecomeNa+andchlorinehasgainedtheelectrontobecomeCl–.•NotethatNa+hasanNeelectronconfigurationandCl–hasanArconfiguration.•Thatis,bothNa+andCl–haveanoctetofelectrons.•NaClformsaveryregularstructureinwhicheachNa+ionissurroundedbysixCl–ions.•SimilarlyeachCl–ionissurroundedbysixNa+ions.•ThereisaregulararrangementofNa+andCl–.•Theionsarepackedascloselyaspossible.•Itisnoteasytofindamolecularformulatodescribetheioniclattice.EnergeticsofIonicBondFormation•TheheatofformationofNaCl(s)isexothermic:Na(s)+½Cl2(g)NaCl(s)∆Hºf=–410.9kJ/mol•SeparationoftheNaClintosodiumandchlorideionsisendothermic:NaCl(s)Na+(g)+Cl–(g)∆H=+788kJ/mol•Theenergyrequiredtoseparateonemoleofasolidioniccompoundintogaseousionsiscalledthelatticeenergy,∆Hlattice.•Latticeenergydependsonthechargeontheionsandthesizeoftheions.•Thestabilityoftheioniccompoundcomesfromtheattractionbetweenionsofunlikecharge.•ThespecificrelationshipisgivenbyCoulomb’sequation:dQQkE21=•whereEisthepotentialenergyofthetwointeractingchargedparticles,Q1andQ2arethechargesontheparticles,disthedistancebetweentheircenters,andkisaconstant(8.99x109J−m/C2).•AsQ1andQ2increase,Eincreases;andasdincreases,Edecreases.CalculationofLatticeEnergies:TheBorn-HaberCycle•TheBorn-Habercycleisathermodynamiccyclethatanalyzeslatticeenergyprecisely.•ConsideraBorn-HabercyclefortheformationofNaCl(s)fromNa(s)andCl2(g).•Thedirectrouteis:Na(s)+½Cl2(g)NaCl(s)∆Hºf=–411kJ•Alternatively,wecanform:•sodiumgas(108kJ;endothermic),then•chlorineatoms(122kJ;endothermic),then•sodiumions(ionizationenergyforNa,496kJ;endothermic),then•chlorideions(electronaffinityforCl,–349kJ;exothermic),then•formtheioniclattice(exothermic).•Thesumoftheaboveenthalpiesis–411kJ.•Thelatticeenergyis+788kJ.ElectronConfigurationofIonsoftheRepresentativeElements•Thesearederivedfromtheelectronconfigurationofelementswiththerequirednumberofelectronsaddedorremovedfromthemostaccessibleorbital.•Electronconfigurationofionscanpredictstableionformation:•Na:[Ne]3s1formsNa+:[Ne]writtenNa+:[He]2s22p6•Cl:[Ne]3s23p5formsCl–:[Ne]3s23p6=[Ar]Transition-MetalIons•Latticeenergiescompensateforthelossofuptothreeelectrons.•Weoftenencountercationswithchargesof1+,2+or3+inioniccompounds.•However,transitionmetalscannotattainanoblegasconformation(3electronsbeyondanoblegascore).•Transitionmetalstendtolosethevalenceshellelectronsfirstandthenasmanydelectronsasarerequiredtoreachthedesiredchargeontheion.•Thus,electronsareremovedfrom4sbeforethe3d,etc..PolyatomicIons•Polyatomicionsareformedwhenthereisanoverallchargeonacompoundcontainingcovalentbonds.•ExamplesareNH4+andCO22–.•Inpolyatomicions,twoormoreatomsareboundtogetherbypredominantlycovalentbonds.Thestablegroupingcarriesacharge.8.3CovalentBonding•Themajorityofchemicalsubstancesdonothavecharacteristicsofioniccompounds.•Achemicalbondformedbysharingapairofelectronsiscalledacovalentbond.•Bothatomsacquirenoble-gaselectronicconfigurations.•Thisisthe“glue”thatbindsatomstogether.LewisStructures•FormationofcovalentbondscanberepresentedusingLewissymbols.•ThestructuresarecalledLewisstructures.•Weshoweachelectronpairsharedbetweenatomsasalineandshowunsharedelectronpairsasdots.•Eachpairofsharedelectronsconstitutesonechemicalbond.•Example:H•+H•H:ΗhaselectronsonalineconnectingthetwoHnuclei(H−H).dQQkE21=MultipleBonds•Itispossibleformorethanonepairofelectronstobesharedbetweentwoatoms(e.g.,multiplebonding).•Onesharedpairofelectronsisasinglebond(e.g.,H2).•Twosharedpairsofelectronsformadou