Ch 10. Chemical Bonding (Representing Bonded Structures)

Chem 30A

Types of Compounds

MolecularCompounds•  Covalentbond•  Oneunit:Molecule

O HH

IonicCompounds•  Ionicbond•  Oneunit:Formulaunit

+ –

Molecular (Covalent) Compounds

•  HeldtogetherthroughCovalentBonds:Bondsinwhichelectronsaresharedbetweenthebondedatoms.

H H H2

Ionic Compounds

•  HeldtogetherthroughIonicBonds:Bondsinwhichoppositely-chargedionsareheldtogetherbyelectrosta@caArac@onbetweenthem.

+ –

Lewis Structures

Lewis Symbols for Atoms and Ions

•  LewisSymbol:Awayofshowingthevalenceelectronsofanatomorion.Consistsofthechemicalsymbolforelementandadotforeachvalenceelectron.

•  eg.SulfurElectronconfigura@on[Ne]3s23p4

Lewissymbol

•  Eachsidecanaccommodateupto2electrons;allfoursidesareequivalent

Lewis Symbols for Some Main Group Elements

ExProbs

Lewis Structures

•  Lewisstructure:arepresenta@onofacompoundthatshowshowthevalenceelectronsarearrangedamongtheatomsofthecompound

•  Lewisstructureshelpusunderstandbondingincompounds.

Octet Rule

•  OctetRule:Inreac@ons,atomstendtogain,lose,orshareelectronsun@ltheyaresurroundedbyeightvalenceelectrons(aAainnoblegasconfigura@on)

– Noblegaselectronconfigura@on:Highestenergylevelorbitals(sandp)arecompletelyfilled.Verystable.

•  DuetRule:HandLitendtogain/lose/shareelectronsun@ltheyaresurroundedbytwoelectrons.

Lewis Structures of Ionic Compounds

Inionicbonding,atomstransferelectronsun@leachatomissurroundedbyeightvalenceelectrons.[Ne]3s1 [Ne]3s23p5 [Ne] [Ar]

ExProbs

Lewis Structures for Molecular Compounds

Incovalentbonding,atomsshareelectronsun@leachatomissurroundedbyeightvalenceelectrons:

Cl[Ne]3s23p5(orduetruleforH,Li):

Lewis Structure Terminology

•  Apairofsharedelectrons=bond•  Apairofunsharedelectrons=lonepairs

•  Abondisrepresentedbyaline.

F F

F F

Multiple Covalent Bonds

Some@mes,twoatomsinamoleculeneedtosharemorethan2electronstoachievetheoctet.àDoubleandtriplebonds

Single, Double, Triple Bonds

•  Singlebond–covalentbondinwhich1pairofelectrons(2electrons)issharedbytwoatoms.

H–H•  Doublebond–covalentbondinwhich2pairsofelectronsaresharedbytwoatoms.

•  Triplebond–covalentbondinwhich3pairsofelectronsaresharedbytwoatoms.

O C O

N N

Steps to Drawing Lewis Structures

Goalistogetoctetaroundeachatom!1.  Addupvalenceelectronsfromallatoms.2.  Draw“skeletonstructure”:atomsconnectedby

singlebonds.3.  Arrangetheremainingelectronsaslonepairson

terminalatomstosa@sfytheoctetrule(orduetruleforhydrogen).

4.  Placeanylec-overelectronsaslonepairsonthecentralatom.

5.  Iftherearenotenoughelectronstogivethecentralatomanoctet,useelectronsfromsurroundingatomstomakemul@plebonds.

ExProbs

Exceptions to Octet Rule

Moleculescontaining:1.  anatomwithfewerthananoctetofvalence

electrons2.  anatomwithmorethananoctetofvalence

electrons(“expandedvalenceshell”)

3.  anoddnumberofelectrons

Multiple Lewis Structures

Some@mes,morethanoneLewisstructurethatsa@sfiestheoctetrulecanbedrawnforamolecule:Resonancestructure

Resonance Structures

•  Resonancestructures:thestructuresofamoleculewhoseaveragegivesanaccuratedescrip@onoftherealmolecule

•  Representselectrondelocaliza@on.

C

O

O O

C

O

C

O

O

O O

O2– 2– 2–

Molecular Shape

Molecular Shape

•  Lewisstructuresshowcomposi@onandatom-to-atomconnec@vityofmolecules.

•  Buttheydon’tshowthe3Dshapesofmolecules(bondangles)àUseVSEPRModel

Predicting Molecular Structure: VSEPR Model

VSEPRModel(Valence-ShellElectron-PairRepulsionModel):ThestructurearoundagivenatomisdeterminedbyminimizingrepulsionsbetweenelectrondomainsàElectrondomainsareasfarapartaspossible.

Anelectrondomaincanbe:§  abond(singleormul@ple)ORalonepair

Electron Domain Geometries that Minimize Repulsion

Electron Domain Geometry vs. Molecular Geometry

•  Electrondomaingeometry:Geometryoftheelectrondomainsaboutacentralatom

•  Moleculargeometry:Geometryofthebondedatomsaboutacentralatom

Electrondomain MolecularGeometry GeometryTetrahedral TrigonalPyramidal

Electron Domain and Molecular Geometry #ElectronDomains

ElectronDomainGeometry

No.ofBonds

No.ofLonePairs

MolecularGeometry

Example

Electron Domain and Molecular Geometry #ElectronDomains

ElectronDomainGeometry

No.ofBonds

No.ofLonePairs

MolecularGeometry

Example

Electron Domain and Molecular Geometry

Structures for Molecules with Double Bond

•  Remember,anelectrondomaincanbeasingle,double,ortriplebond(oralonepair).§  Eg.CarboninCO2hastwoelectrondomains.

Steps to Predict Molecular Structure Using VSEPR

1.  DrawtheLewisstructureforthemolecule.2.  Counttheelectrondomainsandarrange

theminthewaythatminimizesrepulsionàelectrondomaingeometry

3.  Lookattheposi@onsoftheatomsàmoleculargeometry

ExProbs

Polar Covalent Bonds

Types of Bonds

1.  Ionicbond2.  Covalentbond

a)  Nonpolarcovalentbondb)  Polarcovalentbond

Polar Covalent Bonding

•  Nonpolarcovalentbonding:Electronsaresharedequallybetweentwoatoms.

•  Polarcovalentbonding:Electronsaresharedunequallybetweentwoatoms(OneatomaAractstheelectronsmorethantheotheratom).àResultsinapar@alchargesepara@oninthebond:polarity

H H

NonpolarCovalent PolarCovalent

Bond Polarity Represented by a Dipole Moment

•  Dipolemoment:Ameasureofpolarity(qxd)•  Thedipolemomentisavector(hasbothmagnitudeanddirec@on),representedbyanarrow.

Electronegativity

•  Thepolarityofabonddependsontheelectronega@vityoftheatomsinthebond.

•  Electronega@vity:Therela@veabilityofanatominamoleculetoaAractsharedelectronstoitself.

Electronegativity Trend

Range0.7–4.0 ExProbs

Electronegativity and Bond Type

H-H H-F K+---Cl-noorv.small largerv.largeelectroneg electroneg electronegdifferencedifference differencenonpolarpolarcovalentionicbond

covalentbondcovalentbond

Thebondtypeisdeterminedbythedifferenceintheelectronega@vityoftheatomsinthebond.

General Guidelines: Electronegativity and Bond Type

Nonpolar

Observations: Electronegativity Diffs and Bond Polarity

•  Lowerleccornerofperiodictablehaslowestelectronega@vityvalues;upperrighthashighestelectronega@vityvalues.

•  N,O,F,Clhavethehighestelectronega@vityvalues.

•  HandChaveelectronega@vityvaluesthataresimilartoeachother,andthataresignificantlylowerthanthoseofN,O,F,orCl.

•  C-Hbondsarenonpolar.CorHbondedtoN,O,F,Clarepolar.

ExProbs

Polar Molecules

•  Dipolemomentsofbondsinamoleculecanaddorsubtract,togiveanetmoleculardipolemoment.

•  Polarmolecule:Amoleculewithanetmoleculardipolemoment.Moleculeasawholehasapar@al+endandpar@al–end.

Polar Molecules •  Molecularpolaritydependsonbothbonddipolemomentsandmoleculargeometry!

C

O

H3C CH3

N

HH H OH H

Nonpolar Molecules

Nonpolarmolecule:HasNOnetdipolemoment,b/c:(1)haspolarbondswhosedipolemomentscancel,or

Anyhydrocarbonmolecules

CCl

Cl

Cl

ClC OO

Br BrCHH

HH

CHH

CH H

HCH

HH

(2)hasnopolarbonds(think“oily”or“greasy”).

Determining Whether a Molecule is Polar or Nonpolar

1.  Determinethemoleculargeometry.2.  Determinewhethereachbondinthe

moleculeispolarornonpolar.•  Iftherearenopolarbonds,moleculeisnonpolar.

3.  Drawdipolemomentsforthepolarbonds.4.  Adddipolemoments:

•  Ifdipolemomentscancel,moleculeisnonpolar.•  Iftherethereisanetdipolemoment,moleculeispolar.

Common Cases of Adding Dipole Moments

ExProbs

“Like Dissolves Like”

•  Polarsolventsdissolvepolarsolutesandionicsolutes.

•  Nonpolarsolventsdissolvenonpolarsolutes.

Ionic Solute is Soluble in Polar Solvent

•  PolarsolventmoleculesareaAractedtotheca@onsandanionsofanionicsolid,andhelpbreakuptheionicsolidintoindividualca@onsandanions.

Polar Solute is Soluble in Polar Solvent

•  PolarsoluteaAractspolarsolvent.

δ-δ-δ+

δ+

Eg.Water+Ethanol(C2H5OH)

Polar Solute is Soluble in Polar Solvent

•  Whyissolidsugarsolubleinwater?

•  Whyisacetonesolubleinwater?

Nonpolar Solute is Soluble in Nonpolar Solvent

•  NonpolarsoluteaAractsnonpolarsolvent.

Eg.Fat Hexane

Polar Solvent and Nonpolar Solute Do Not Mix

•  NonpolarsubstancesdonothavestrongaArac@ontopolarsubstances.

•  Eg.Oilandwatermixture:TherearestrongeraArac@onsbetweenthewatermoleculesandbetweentheoilmoleculesthanbetweenwaterandoilmolecules.