molecular-orbital-theory

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MolecularOrbitalTheoryLuisBonillaAbelPerezUniversityofTexasatElPasoMolecularElectronics,Chem5369AtomicOrbitalsHeisenbergUncertaintyPrinciplestatesthatitisimpossibletodefinewhattimeandwhereanelectronisandwhereisitgoingnext.Thismakesitimpossibletoknowexactlywhereanelectronistravelinginanatom.Sinceitisimpossibletoknowwhereanelectronisatacertaintime,aseriesofcalculationsareusedtoapproximatethevolumeandtimeinwhichtheelectroncanbelocated.TheseregionsarecalledAtomicOrbitals.Thesearealsoknownasthequantumstatesoftheelectrons.Onlytwoelectronscanoccupyoneorbitalandtheymusthavedifferentspinstates,½spinand–½spin(easilyvisualizedasoppositespinstates).Orbitalsaregroupedintosubshells.Thisfieldofstudyiscalledquantummechanics.AtomicSubshellsThesearesomeexamplesofatomicorbitals:Ssubshell:(Sphericalshape)ThereisoneSorbitalinanssubshell.Theelectronscanbelocatedanywherewithinthespherecenteredattheatom’snucleus.POrbitals:(Shapedliketwoballoonstiedtogether)Thereare3orbitalsinapsubshellthataredenotedaspx,py,andpzorbitals.Thesearehigherinenergythanthecorrespondingsorbitals.://(cont’d)DOrbitals:Thedsubshellisdividedinto5orbitals(dxy,dxz,dyz,dz2anddx2-y2).Theseorbitalshaveaverycomplexshapeandarehigherinenergythanthesandporbitals.ElectronicConfigurationEveryelementisdifferent.Thenumberofprotonsdeterminestheidentityoftheelement.Thenumberofelectronsdeterminesthecharge.Thenumberofneutronsdeterminestheisotope.Allchemistryisdoneattheelectroniclevel(thatiswhyelectronsareveryimportant).Electronicconfigurationisthearrangementofelectronsinanatom.TheseelectronsfilltheatomicorbitalsAtomicorbitalsarearrangebyenergylevel(n),subshells(l),orbital(ml)andspin(ms)-inorder:LithiumElectronicConfigurationThearrowsindicatethevalueofthemagneticspin(ms)quantumnumber(upfor+1/2anddownfor-1/2)Theoccupationoftheorbitalswouldbewritteninthefollowingway:1s22s1or,1stwo,2sone.–BoxDiagramThetwoelectronsinHeliumrepresentthecompletefillingofthefirstelectronicshell.Thus,theelectronsinHeareinaverystableconfigurationForBoron(5electrons)the5thelectronmustbeplacedina2porbitalbecausethe2sorbitalisfilled.Becausethe2porbitalsareequalenergy,itdoesn'tmatterwhich2porbitalisfilled.Electronicconfigurationscanalsobewritteninashorthandwhichreferencesthelastcompletedorbitalshell(i.e.allorbitalswiththesameprinciplequantumnumber'n'havebeenfilled)TheelectronicconfigurationofNacanbewrittenas[Ne]3s1TheelectronicconfigurationofLicanbewrittenas[He]2s1Theelectronsinthestable(Noblegas)configurationaretermedthecoreelectronsTheelectronsintheoutershell(beyondthestablecore)arecalledthevalenceelectronsElectronConfiguration:Electron_orbitals.svgTwowaystoremembertheorderofelectronsValenceElectronsThevalenceelectronsaretheelectronsinthelastshellorenergylevelofanatom.(K),cancontain2electrons.Thenextlevel(L)cancontain8electrons.Thenextlevel(M)cancontain8electrons.Ne1s22s22p6(10electrons)F1s22s22p5(9electrons)F-1s22s22p6(10electrons)Mg1s22s22p63s2(12electrons)Mg2+1s22s22p6(10electrons)Notice–differentelementscanhavethesamenumberofelectronsMolecularOrbitalTheoryThegoalofmolecularorbitaltheoryistodescribemoleculesinasimilarwaytohowwedescribeatoms,thatis,intermsoforbitals,orbitaldiagrams,andelectronconfigurations.FormingaCovalentBondMoleculescanformbondsbysharingelectronTwosharedelectronsformasinglebondAtomscanshareone,twoorthreepairsofelectronsformingsingle,doubleandtriplebondsOthertypesofbondsareformedbychargedatoms(ionic)andmetalatoms(metallic).AtomicandMolecularOrbitals(cont’d)OrbitalMixingWhenatomsshareelectronstoformabond,theiratomicorbitalsmixtoformmolecularbonds.Inorderfortheseorbitalstomixtheymust:Havesimilarenergylevels.Overlapwell.Beclosetogether.Thisisandexampleoforbitalmixing.Thetwoatomsshareoneelectroneachfromthereoutershell.Inthiscaseboth1sorbitalsoverlapandsharetheirvalenceelectrons.Inatoms,electronsoccupyatomicorbitals,butinmoleculestheyoccupysimilarmolecularorbitalswhichsurroundthemolecule.Thetwo1satomicorbitalscombinetoformtwomolecularorbitals,onebonding(s)andoneantibonding(s*).•ThisisanillustrationofmolecularorbitaldiagramofH2.•Noticethatoneelectronfromeachatomisbeing“shared”toformacovalentbond.Thisisanexampleoforbitalmixing.MolecularOrbitalTheoryEachlineinthediagramrepresentsanorbital.Themolecularorbitalvolumeencompassesthewholemolecule.Theelectronsfillthemolecularorbitalsofmoleculeslikeelectronsfillatomicorb

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