Computing Formal Charges - Indiana University...

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ComputingFormalCharges

•  formalcharge(fc)=(#valencee‐)–(#ofbonds+#ofunsharede‐)

•  Inaneutralmolecule,thesumoftheformalchargesiszero.Foranion,thesumoftheformalchargesisequaltothechargeoftheion.

•  AnatominaLewisstructurethathasthenumberofbondsexpectedonthebasisofitsgroupnumberhasnoformalcharge.


ComputingFormalCharges•  “Normal”covalentbondingpatterns:

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ComputingFormalCharges•  Commonstructureswithformalcharges:

Theelementsarearrangedintheperiodictablesothatthosewithsimilarchemicalpropertiesarearrangedinthesameverticalcolumn,calledagrouporafamily.

Eachhorizontalrowiscalledaperiod.

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Lewisstructuresindicatehowtheatomswithinamoleculeareconnected,whetherthebondsaresingle,double,ortriple,andthelocationsoftheextra,ornon‐bondingelectrons.

However,bythemselves,Lewisstructuresdonotprovideanyinformationaboutthethree‐dimensionalshapeofthemolecule.

Thevalence‐shellelectron‐pairrepulsiontheory,orVSEPR,enablesustopredictthethreedimensionalshapeofamoleculefromthecompletedLewisstructureofthemolecule.

Thistheoryisbasedupontheideathatallofthepairofelectronsonacentralatominamoleculewillrepeleachotherandtrytogetasfarapartaspossible.

Thisincludesboththebondingpairsofelectronsandthenon‐bondingpairsontheatom.

VSEPRTheory

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Wewillfirstconsideraseriesofmoleculeswhichpossessnonon‐bondingelectrons:

BeCl2(berylliumchloride)isanelectron‐deficientcompoundpossessingtwobondingpairsofelectronsandnonon‐bondingpairs.

Thetwopairsofbondingelectronswillbeasfarapartaspossibleifthebondspointinexactlyoppositedirections(180oapart).

Thismoleculeislinear.

AlCl3(aluminumchloride)isanelectron‐deficientcompoundpossessingthreebondingpairsofelectronsandnonon‐bondingpairs.

Thethreepairsofbondingelectronswillbeasfarapartaspossibleifthebondspointatanglesof120otoeachother.

Thismoleculeistrigonalplanarortriangularplanar.

CH4(methane)possessesfourbondingpairsofelectronsandnonon‐bondingpairs.

Fourpairsofelectronscangetasfarapartinspaceaspossibleifthefourbondspointananglesinspaceof109.5otoeachother.

Theshapeofthemethanemoleculeistetrahedral.

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Inmoleculessuchaswater,H2O,andammonia,NH3,non‐bondingaswellasbondingelectronssurroundthecentralatom.

Thenon‐bondingelectronsandbondingelectronsasagroupwillrepeleachotherandwillpointindirectionsinspacetotrytogetasfarapartaspossible.

Thenon‐bondingelectronsrequireaslightlylargervolumeinspacethandothebondingelectronsand,asaresult,causetheanglesbetweenthebondingelectronstobeslightlysmallerthanonemightotherwisepredict.

WhenapplyingtheVSEPRtheorytomoleculescontainingnon‐bondingelectrons,theshapeofthemoleculeisdeterminedbythepositionsoftheouteratomsonly.Thepositionsoftheseouteratomsandthebondanglesaredeterminedbyconsideringalloftheelectrons,bothbondingandnon‐bonding.

Non‐bondingElectrons

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NH3(ammonia)containsthreebondingpairsofelectronsandonenon‐bondingpair.

Thefourpairsofelectronswillpointinspaceinthedirectionsofaregulartetrahedron,however,theshapeofthemoleculeisnamedfromthepositionsofthehydrogenatomsonly,ignoringthelonepairofelectrons.Theshapeofammoniaiscalledatrigonalpyramid.

H2O(water)containstwobondingpairsofelectronsandtwonon‐bondingpairs.

Thefourpairsofelectronswillpointinspaceinthedirectionsofaregulartetrahedron,however,theshapeofthemoleculeisnamedfromthepositionsofthehydrogenatomsonly,ignoringthelonepairofelectrons.Theshapeofwateriscalledbentorangular.

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AtomicOrbitals

•  sorbital

•  porbitals

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BondinginOrganicCompounds

•  Generatehybridorbitalsby“mixing”atomicorbitalsinordertoachievetheVSEPRgeometries.


Methane•  VSEPRtheorysuggestsatetrahedralstructure:

•  Theatomicorbitalsoncarbondonothavethecorrectgeometrytogiveatetrahedron.

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Methane•  VSEPRtheorysuggestsatetrahedralstructure:

•  But,theatomicorbitalsoncarbondonothavethecorrectgeometrytogiveatetrahedron.


Methane•  However,mixingthecarbon2sorbitalandthe3carbon2porbitalsgives4sp3hybridorbitals:

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Methane•  However,mixingthecarbon2sorbitalandthe3carbon2porbitalsgives4sp3hybridorbitals:


Methane•  These4sp3hybridorbitalspointtowardthecornersofatetrahedron:

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Methane•  Orbitaloverlapofthecarbonsp3withthehydrogen1sorbitalsgivesthefourσbonds:

Atetrahedralatomhassp3hybridization.


Ethene

•  Themolecularorbitalsofethene:– 1sorbitalsofthefourhydrogenatoms

– 2s,2px,2py,and2pzorbitalsofthetwocarbon atoms

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Ethene•  The2s,2px,and2pyorbitalsoncarbonformthreesp2hybridorbitalsoneachcarbonatom.

•  Thethreesp2hybridorbitalspointtowardthecornersofatriangle.

Atrigonalplanaratomhassp2hybridization.


Ethene•  Thesp2hybridsandthe1sorbitalsonhydrogenmakeuptheσ‐framework—σbondsthatarerelativelystrongbonds.

Atrigonalplanaratomhassp2hybridization.

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Ethene•  Aweakerbondisformedthatismadeupentirelyfromtheremaining2pzorbitalsofthetwocarbonatomsoverlappinginaπbond.


CarbonDioxide•  VSEPRtheorysuggestswhattypeofstructure?

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CarbonDioxide•  VSEPRtheorysuggestsalinearstructure:

•  MixaC2sorbitalandaC2porbitaltogivetwosphybridorbitalsthatpointinoppositedirections:

•  p‐bondscompletethebondingpicture:


CarbonDioxide•  VSEPRtheorysuggestsalinearstructure:

•  MixaC2sorbitalandaC2porbitaltogivetwosphybridorbitalsthatpointinoppositedirections:

•  πbondscompletethebondingpicture:

Alinearatomhassphybridization.

Computing Formal Charges - Indiana University...

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