MolecularGeometriesand BondingMolecular Shapes• The shape of amolecule plays animportant role in itsreactivity.• By noting the numberof bonding andnonbonding electronpairs we can easilypredict the shape ofthe molecule.
MolecularGeometriesand BondingWhat Determines the Shape of aMolecule?• Simply put, electronpairs, whether they bebonding or nonbonding,repel each other.• By assuming the electronpairs are placed as far aspossible from each other,we can predict the shapeof the molecule.
MolecularGeometriesand BondingElectron Domains• We can refer to theelectron pairs as electrondomains.• In a double or triple bond,all electrons sharedbetween those two atomsare on the same side ofthe central atom;therefore, they count asone electron domain.• This molecule hasfour electrondomains.
MolecularGeometriesand BondingValence Shell Electron PairRepulsion Theory (VSEPR)“The bestarrangement of agiven number ofelectron domains isthe one thatminimizes therepulsions amongthem.”
MolecularGeometriesand BondingElectron-DomainGeometriesThese are theelectron-domaingeometries for twothrough six electrondomains around acentral atom.
MolecularGeometriesand BondingElectron-Domain Geometries• All one must do iscount the number ofelectron domains inthe Lewis structure.• The geometry will bethat whichcorresponds to thatnumber of electrondomains.
MolecularGeometriesand BondingMolecular Geometries• The electron-domain geometry is often notthe shape of the molecule, however.• The molecular geometry is that defined by thepositions of only the atoms in the molecules,not the nonbonding pairs.
MolecularGeometriesand BondingMolecular GeometriesWithin each electrondomain, then, theremight be more thanone moleculargeometry.
MolecularGeometriesand BondingLinear Electron Domain• In this domain, there is only one moleculargeometry: linear.• NOTE: If there are only two atoms in themolecule, the molecule will be linear nomatter what the electron domain is.
MolecularGeometriesand BondingTrigonal Planar Electron Domain• There are two molecular geometries:Trigonal planar, if all the electron domains arebondingBent, if one of the domains is a nonbonding pair.
MolecularGeometriesand BondingNonbonding Pairs and Bond Angle• Nonbonding pairs are physicallylarger than bonding pairs.• Therefore, their repulsions aregreater; this tends to decreasebond angles in a molecule.
MolecularGeometriesand BondingMultiple Bonds and Bond Angles• Double and triplebonds place greaterelectron density onone side of thecentral atom than dosingle bonds.• Therefore, they alsoaffect bond angles.
MolecularGeometriesand BondingTetrahedral Electron Domain• There are three molecular geometries:Tetrahedral, if all are bonding pairsTrigonal pyramidal if one is a nonbonding pairBent if there are two nonbonding pairs
MolecularGeometriesand BondingTrigonal Bipyramidal ElectronDomain• There are twodistinct positions inthis geometry:AxialEquatorial
MolecularGeometriesand BondingTrigonal Bipyramidal ElectronDomainLower-energy conformations result fromhaving nonbonding electron pairs inequatorial, rather than axial, positions in thisgeometry.
MolecularGeometriesand BondingTrigonal Bipyramidal ElectronDomain• There are fourdistinct moleculargeometries in thisdomain:Trigonal bipyramidalSeesawT-shapedLinear
MolecularGeometriesand BondingOctahedral Electron Domain• All positions areequivalent in theoctahedral domain.• There are threemoleculargeometries:OctahedralSquare pyramidalSquare planar
MolecularGeometriesand BondingLarger MoleculesIn larger molecules,it makes moresense to talk aboutthe geometry abouta particular atomrather than thegeometry of themolecule as awhole.
MolecularGeometriesand BondingLarger MoleculesThis approachmakes sense,especially becauselarger moleculestend to react at aparticular site in themolecule.
MolecularGeometriesand BondingPolarity• In Chapter 9 wediscussed bond dipoles.• But just because amolecule possessespolar bonds does notmean the molecule as awhole will be polar.
MolecularGeometriesand BondingPolarityBy adding theindividual bonddipoles, one candetermine theoverall dipolemoment for themolecule.
MolecularGeometriesand BondingOverlap and Bonding• We think of covalent bonds forming throughthe sharing of electrons by adjacent atoms.• In such an approach this can only occur whenorbitals on the two atoms overlap.
MolecularGeometriesand BondingOverlap and Bonding• Increased overlap bringsthe electrons and nucleicloser together whilesimultaneouslydecreasing electron-electron repulsion.• However, if atoms get tooclose, the internuclearrepulsion greatly raisesthe energy.
MolecularGeometriesand BondingHybrid OrbitalsBut it’s hard to imagine tetrahedral, trigonalbipyramidal, and other geometries arisingfrom the atomic orbitals we recognize.
MolecularGeometriesand BondingHybrid Orbitals• Consider beryllium:In its ground electronicstate, it would not beable to form bondsbecause it has nosingly-occupied orbitals.
MolecularGeometriesand BondingHybrid Orbitals• But if it absorbs thesmall amount of energyneeded to promote anelectron from the 2s to the2p orbital, it can form twobonds.• However the observedgeometry of berylliumcompounds like BeF2 islinear• does not explain thegeometry
MolecularGeometriesand BondingHybrid Orbitals• Mixing the s and p orbitals yields two degenerateorbitals that are hybrids of the two orbitals.These sp hybrid orbitals have two lobes like a p orbital.One of the lobes is larger and more rounded as is the sorbital.
MolecularGeometriesand BondingHybrid Orbitals• These two degenerate orbitals would alignthemselves 180° from each other.• This is consistent with the observed geometry ofberyllium compounds: linear.
MolecularGeometriesand BondingHybrid Orbitals• With hybrid orbitals the orbital diagram forberyllium would look like this.• The sp orbitals are higher in energy than the1s orbital but lower than the 2p.
MolecularGeometriesand BondingHybrid OrbitalsUsing a similar model for boron leads to…
MolecularGeometriesand BondingHybrid OrbitalsFor geometries involving expanded octets onthe central atom, we must use d orbitals inour hybrids.
MolecularGeometriesand BondingHybrid OrbitalsThis leads to five degeneratesp3d orbitals……or six degenerate sp3d2orbitals.
MolecularGeometriesand BondingHybrid OrbitalsOnce you know theelectron-domaingeometry, you knowthe hybridizationstate of the atom.
MolecularGeometriesand BondingValence Bond Theory• Hybridization is a major player in thisapproach to bonding.• There are two ways orbitals can overlapto form bonds between atoms.
MolecularGeometriesand BondingSigma (σ) Bonds• Sigma bonds are characterized byHead-to-head overlap.Cylindrical symmetry of electron density about theinternuclear axis.
MolecularGeometriesand BondingPi (π) Bonds• Pi bonds arecharacterized bySide-to-side overlap.Electron densityabove and below theinternuclear axis.
MolecularGeometriesand BondingSingle BondsSingle bonds are always σ bonds, because σoverlap is greater, resulting in a stronger bondand more energy lowering.
MolecularGeometriesand BondingMultiple BondsIn a multiple bond one of the bonds is a σ bondand the rest are π bonds.
MolecularGeometriesand BondingMultiple Bonds• In a molecule likeformaldehyde (shownat left) an sp2orbitalon carbon overlaps inσ fashion with thecorresponding orbitalon the oxygen.• The unhybridized porbitals overlap in πfashion.
MolecularGeometriesand BondingMultiple BondsIn triple bonds, as inacetylene, two sporbitals form a σbond between thecarbons, and twopairs of p orbitalsoverlap in π fashionto form the two πbonds.
MolecularGeometriesand BondingDelocalized Electrons: ResonanceWhen writing Lewis structures for species likethe nitrate ion, we draw resonance structures tomore accurately reflect the structure of themolecule or ion.
MolecularGeometriesand BondingDelocalized Electrons: Resonance• In reality, each of the fouratoms in the nitrate ion has ap orbital.• The p orbitals on all threeoxygens overlap with the porbital on the central nitrogen.
MolecularGeometriesand BondingDelocalized Electrons: ResonanceThis means the π electrons arenot localized between thenitrogen and one of theoxygens, but rather aredelocalized throughout the ion.
MolecularGeometriesand BondingResonanceThe organic moleculebenzene has six σbonds and a p orbitalon each carbon atom.
MolecularGeometriesand BondingResonance• In reality the π electrons in benzene are notlocalized, but delocalized.• The even distribution of the π electrons in benzenemakes the molecule unusually stable.
MolecularGeometriesand BondingMolecular Orbital (MO) TheoryThough valence bondtheory effectively conveysmost observed propertiesof ions and molecules,there are some conceptsbetter represented bymolecular orbitals.
MolecularGeometriesand BondingMolecular Orbital (MO) Theory• In MO theory, weinvoke the wave natureof electrons.• If waves interactconstructively, theresulting orbital is lowerin energy: a bondingmolecular orbital.
MolecularGeometriesand BondingMolecular Orbital (MO) TheoryIf waves interactdestructively, theresulting orbital ishigher in energy: anantibonding molecularorbital.
MolecularGeometriesand BondingMO Theory• In H2 the two electrons gointo the bonding molecularorbital.• The bond order is one halfthe difference between thenumber of bonding andantibonding electrons.• Bond order indicates theapproximate strength of abond.• Bond orders can be fractionsbut a bond order of zeromeans the bond has nostability (the molecule cannotexist)
MolecularGeometriesand BondingMO TheoryFor hydrogen, with twoelectrons in the bondingMO and none in theantibonding MO, thebond order is12(2 - 0) = 1
MolecularGeometriesand BondingMO Theory• In the case of He2,the bond orderwould be12(2 - 2) = 0• Therefore, He2does not exist.
MolecularGeometriesand BondingMO Theory• For atoms with both sand p orbitals, there aretwo types ofinteractions:The s and the p orbitalsthat face each otheroverlap in σ fashion.The other two sets of porbitals overlap in πfashion.
MolecularGeometriesand BondingMO Theory• The resulting MOdiagram looks like this.• There are both σ and πbonding molecularorbitals and σ* and π*antibonding molecularorbitals.
MolecularGeometriesand BondingMO Theory• The smaller p-block elements inthe second period have asizeable interaction between thes and p orbitals.• This flips the order of the s and pmolecular orbitals in theseelements.
MolecularGeometriesand BondingSecond-Row MO Diagrams
MolecularGeometriesand BondingEnd of Chapter 10