Ch 10. Chemical Bonding (Representing Bonded Structures) Chem 30A
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.