Chapter 8 Basic Concepts of Chemical Bonding

Sec$on8.1TypesofChemicalBonds

Chapter 8

Basic Concepts of Chemical Bonding

Chapter8

Ques$onstoConsider

§ Whatismeantbytheterm“chemicalbond”?§ Whydoatomsbondwitheachothertoformcompounds?

§  Howdoatomsbondwitheachothertoformcompounds?

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AChemicalBond§  Nosimple,andyetcomplete,waytodefinethis.§  Forcesthatholdgroupsofatomstogetherandmakethemfunc$onasaunit.

§  Abondwillformiftheenergyoftheaggregateislowerthanthatoftheseparatedatoms.



TypesofBonds

§  IonicBonding–electronsaretransferred§  CovalentBonding–electronsaresharedequallybynuclei

§  Whataboutintermediatecases?



PolarCovalentBond

§  Unequalsharingofelectronsbetweenatomsinamolecule.

§  Resultsinachargesepara$oninthebond(par$alposi$veandpar$alnega$vecharge).

Sec$on8.2Electronega7vity

§  TheabilityofanatominamoleculetoaWractsharedelectronstoitself.

§  ForamoleculeHX,therela$veelectronega$vi$esoftheHandXatomsaredeterminedbycomparingthemeasuredH–Xbondenergywiththe“expected”H–Xbondenergy.


Electronega$vity


§  Ontheperiodictable,electronega$vitygenerallyincreasesacrossaperiodanddecreasesdownagroup.

§  Therangeofelectronega$vityvaluesisfrom4.0forfluorine(themostelectronega$ve)to0.7forcesium(theleastelectronega$ve).


Electronega$vity

Sec$on8.2Electronega7vityThePaulingElectronega$vityValues



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Electronega$vityandBondType

Sec$on8.3BondPolarityandDipoleMoments

§  Propertyofamoleculewhosechargedistribu$oncanberepresentedbyacenterofposi$vechargeandacenterofnega$vecharge.

§  Useanarrowtorepresentadipolemoment.§  Pointtothenega$vechargecenterwiththetailofthearrowindica$ngtheposi$vecenterofcharge.


Polarity-DipoleMoment

Sec$on8.3BondPolarityandDipoleMomentsPolarity-DipoleMoment

11

Sec$on8.3BondPolarityandDipoleMomentsNoNetDipoleMoment(DipolesCancel)


Sec$on8.3BondPolarityandDipoleMoments

§  A dipole moment ( µ )

produced by two equal but opposite charges separated by a distance, r, is calculated:

µ = Qr §  It is measured in debyes (D).

DipoleMoment-Calcula$on

Sec$on8.4Ions:ElectronConfigura7onsandSizes

ElectronConfigura$onsinStableCompounds

§  Atomsinstablecompoundsusuallyhaveanoblegaselectronconfigura$on.


Sec$on8.4Ions:ElectronConfigura7onsandSizesElectronConfigura$onsinStableCompounds

§  Whentwononmetalsreacttoformacovalentbond,theyshareelectronsinawaythatcompletesthevalenceelectronconfigura$onsofbothatoms.

§  Whenanonmetalandarepresenta7ve-groupmetalreacttoformabinaryioniccompound,theionsformsothatthevalenceelectronconfigura$onofthenonmetalachievestheelectronconfigura$onofthenextnoblegasatom.Thevalenceorbitalsofthemetalareemp$ed.


Sec$on8.4Ions:ElectronConfigura7onsandSizesIsoelectronicSeries

§  Aseriesofions/atomscontainingthesamenumberofelectrons. O2-,F-,Ne,Na+,Mg2+,andAl3+

§  Ca#onisalwayssmallerthanatomfromwhichitisformed.

§  Anionisalwayslargerthanatomfromwhichitisformed.


Sec$on8.4Ions:ElectronConfigura7onsandSizesIonicRadii(inppm)


Sec$on8.4Ions:ElectronConfigura7onsandSizesPeriodicTableAllowsUstoPredictManyProper$es

§  Trendsfor:§  Atomicsize,ionradius,ioniza$onenergy,electronega$vity

§  Electronconfigura$ons§  Formulapredic$onforioniccompounds§  Covalentbondpolarityranking


Sec$on8.5EnergyEffectsinBinaryIonicCompounds

§  Whatarethefactorsthatinfluencethestabilityandthestructuresofsolidbinaryioniccompounds?

§  HowstronglytheionsaWracteachotherinthesolidstateisindicatedbythelahceenergy.


EnergyEffectsinBinaryIonicCompounds

Sec$on8.5EnergyEffectsinBinaryIonicCompoundsLahceEnergy

§  Thechangeinenergythattakesplacewhenseparatedgaseousionsarepackedtogethertoformanionicsolid.

k=propor$onalityconstantQ1andQ2=chargesontheionsr= shortestdistancebetweenthecentersofthe

ca$onsandanions


1 2Lattice energy = ⎛ ⎞⎜ ⎟⎝ ⎠

QQkr

Sec$on8.5EnergyEffectsinBinaryIonicCompoundsBorn-HaberCyclefordeterminingLahceEnergyofLiF

21ΔHoverall = ΔH1 + ΔH2 + ΔH3 + ΔH4 + ΔH5

Sec$on8.5EnergyEffectsinBinaryIonicCompoundsForma$onofanIonicSolid

1.Sublima$onofthesolidmetal.• M(s)M(g)[endothermic]

2.Ioniza$onofthemetalatoms.•  M(g)M+(g)+e-[endothermic]

3.Dissocia$onofthenonmetal.•  1/2X2(g)X(g)[endothermic]


Sec$on8.5EnergyEffectsinBinaryIonicCompounds

Forma$onofanIonicSolid(con$nued)

4Forma$onofnonmetalionsinthegasphase.•  X(g)+e-X-(g)[exothermic]

5.Forma$onofthesolidioniccompound.• M+(g)+X-(g) MX(s) [quiteexothermic]


Sec$on8.5EnergyEffectsinBinaryIonicCompoundsComparingEnergyChanges

Sec$on8.6Par7alIonicCharacterofCovalentBonds

§  Nobondsreach100%ioniccharacterevenwithcompoundsthathavethemaximumpossibleelectronega$vitydifference.


+measured dipole moment of X Y% ionic character of a bond = 100%calculated dipole moment of X Y−⎛ ⎞−

×⎜ ⎟⎜ ⎟⎝ ⎠

Par$alIonicCharacterofCovalentBonds

Sec$on8.6Par7alIonicCharacterofCovalentBondsTherela$onshipbetweentheioniccharacterofacovalentbondandtheelectronega$vitydifferenceofthebondedatoms


Sec$on8.6Par7alIonicCharacterofCovalentBonds

Opera$onalDefini$onofIonicCompound

§  Anycompoundthatconductsanelectriccurrentwhenmeltedwillbeclassifiedasionic.


Sec$on8.7TheCovalentChemicalBond:AModel

TheCovalentChemicalBond:AModel

§  ModelsareaWemptstoexplainhownatureoperatesonthemicroscopiclevelbasedonexperiencesinthemacroscopicworld.


Sec$on8.7TheCovalentChemicalBond:AModelFundamentalProper$esofModels

1.  Amodeldoesnotequalreality.2.  Modelsareoversimplifica$ons,andarethereforeopen

wrong.3.  Modelsbecomemorecomplicatedandaremodifiedas

theyage.4.  Wemustunderstandtheunderlyingassump$onsina

modelsothatwedon’tmisuseit.5.  Whenamodeliswrong,weopenlearnmuchmorethan

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Sec$on8.8CovalentBondEnergiesandChemicalReac7ons

CovalentBondEnergies

§  Tobreakbonds,energymustbeaddedtothesystem(endothermic,energytermcarriesaposi$vesign).

§  Toformbonds,energyisreleased(exothermic,energytermcarriesanega$vesign).


Sec$on8.8CovalentBondEnergiesandChemicalReac7onsCovalentBondEnergies

ΔH=Σn×D(bondsbroken)–Σn×D(bondsformed)

Drepresentsthebondenergypermoleof bonds(alwayshasaposi$vesign).


Sec$on8.8CovalentBondEnergiesandChemicalReac7onsExample

FromthefigureonthelastslideCH4(g)+Cl2(g)⎯⎯→CH3Cl(g)+HCl(g)§  Inthisexample,oneC—HbondandoneCl—Clbondarebroken;oneC—ClandoneH—Clbondareformed.

Sec$on8.8CovalentBondEnergiesandChemicalReac7onsAnswer

ΔH=[D(C—H)+D(Cl—Cl)]−[D(C—Cl)+D(H—Cl)]

=[(413kJ)+(242kJ)]−[(328kJ)+(431kJ)]=(655kJ)−(759kJ)=−104kJ

Sec$on8.8CovalentBondEnergiesandChemicalReac7onsBondEnthalpyandBondLength

§  Wecanalsomeasureanaveragebondlengthfordifferentbondtypes.

§  Asthenumberofbondsbetweentwoatomsincreases,thebondlengthdecreases.

Sec$on8.9TheLocalizedElectronBondingModel

The“LocalizedElectron”BondingModel(LE)

§  Amoleculeiscomposedofatomsthatareboundtogetherbysharingpairsofelectronsusingtheatomicorbitalsoftheboundatoms.



§  Electronpairsareassumedtobelocalizedonapar$cularatomorinthespacebetweentwoatoms:§  Lonepairs–pairsofelectronslocalizedonanatom§  Bondingpairs–pairsofelectronsfoundinthespace

betweentheatoms




1.  Descrip$onofvalenceelectronarrangement(Lewisstructure).

2.  Predic$onofgeometry(VSEPRmodel).3.  Descrip$onofatomicorbitaltypesusedbyatomsto

shareelectronsorholdlonepairs.



Sec$on8.10LewisStructures

LewisStructure

§  Showshowvalenceelectronsarearrangedamongatomsinamolecule.

§  Reflectscentralideathatstabilityofacompoundrelatestonoblegaselectronconfigura$on.



§  Hydrogenformsstablemoleculeswhereitsharestwoelectrons.

DuetRule

Sec$on8.10LewisStructuresOctetRule

§  Elementsformstablemoleculeswhensurroundedbyeightelectrons.

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SingleCovalentBond

§  Acovalentbondinwhichtwoatomsshareonepairofelectrons.

H–H



DoubleCovalentBond

§  Acovalentbondinwhichtwoatomssharetwopairsofelectrons.

O=C=O


Sec$on8.10LewisStructuresTripleCovalentBond

§  Acovalentbondinwhichtwoatomssharethreepairsofelectrons.


N N≡

Sec$on8.10LewisStructuresStepsforWri$ngLewisStructures

1.  Sumthevalenceelectronsfromalltheatoms.2.  Useapairofelectronstoformabondbetweeneach

pairofboundatoms.3.  Atomsusuallyhavenoblegasconfigura$ons.Arrange

theremainingelectronstosa$sfytheoctetrule(orduetruleforhydrogen).



StepsforWri$ngLewisStructures

1.  Sumthevalenceelectronsfromalltheatoms.(Usetheperiodictable.) Example: H2O 2(1e–)+6e–=8e–total



StepsforWri$ngLewisStructures

2.  Useapairofelectronstoformabondbetweeneachpairofboundatoms.

Example: H2O


O HH

Sec$on8.10LewisStructuresStepsforWri$ngLewisStructures

3.  Atomsusuallyhavenoblegasconfigura$ons.Arrangetheremainingelectronstosa$sfytheoctetrule(orduetruleforhydrogen). Examples:H2O,PBr3,andHCN


O HHP

BrBr Br

H C N

Sec$on8.11Excep7onstotheOctetRule

§  Borontendstoformcompoundsinwhichtheboronatomhasfewerthaneightelectronsaroundit(itdoesnothaveacompleteoctet). BH3=6e–


B

H

H H


§  Whenitisnecessarytoexceedtheoctetruleforoneofseveralthird-row(orhigher)elements,placetheextraelectronsonthecentralatom. SF4=34e– AsBr5=40e–


S

F

F F

F

AsBr

Br BrBr

Br

Sec$on8.11Excep7onstotheOctetRuleLet’sReview

§  C,N,O,andFshouldalwaysbeassumedtoobeytheoctetrule.

§  BandBeopenhavefewerthan8electronsaroundthemintheircompounds.

§  Second-rowelementsneverexceedtheoctetrule.§  Third-rowandheavierelementsopensa$sfytheoctetrulebutcanexceedtheoctetrulebyusingtheiremptyvalencedorbitals.



Let’sReview

§  Whenwri$ngtheLewisstructureforamolecule,sa$sfytheoctetrulefortheatomsfirst.Ifelectronsremainapertheoctetrulehasbeensa$sfied,thenplacethemontheelementshavingavailabledorbitals(elementsinPeriod3orbeyond).


Sec$on8.12Resonance§  MorethanonevalidLewisstructurecanbewriWenforapar$cularmolecule.

NO3–=24e–


NO

O

ON

O

O

ON

O

O

O↔ ↔

Sec$on8.12Resonance§  Actualstructureisanaverageoftheresonancestructures.

§  Electronsarereallydelocalized–theycanmovearoundtheen$remolecule.


NO

O

ON

O

O

ON

O

O

O↔ ↔

Sec$on8.12Resonance

FormalCharge

§  UsedtoevaluatenonequivalentLewisstructures.§  Atomsinmoleculestrytoachieveformalchargesasclosetozeroaspossible.

§  Anynega$veformalchargesareexpectedtoresideonthemostelectronega$veatoms.


Sec$on8.12ResonanceFormalCharge

§  Formalcharge=(#valencee–onfreeneutralatom)–(#valencee–assignedtotheatominthemolecule)

§  Assume:§  Lonepairelectronsbelongen$relytotheatominques$on.

§  Sharedelectronsaredividedequallybetweenthetwosharingatoms.


Sec$on8.12Resonance

RulesGoverningFormalCharge

§  Tocalculatetheformalchargeonanatom:1.  Takethesumofthelonepairelectronsandone-half

thesharedelectrons.2.  Subtractthenumberofassignedelectronsfromthe

numberofvalenceelectronsonthefree,neutralatom.


Sec$on8.12Resonance

RulesGoverningFormalCharge

§  Thesumoftheformalchargesofallatomsinagivenmoleculeorionmustequaltheoverallchargeonthatspecies.


Sec$on8.12ResonanceRulesGoverningFormalCharge

§  IfnonequivalentLewisstructuresexistforaspecies,thosewithformalchargesclosesttozeroandwithanynega$veformalchargesonthemostelectronega$veatomsareconsideredtobestdescribethebondinginthemoleculeorion.


CO O CO O

Chapter 8 Basic Concepts of Chemical Bonding

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