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Chapter Ten

Chemical Bonding llMolecular Geometry and

Hybridization of Atomic Orbitals

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Molecular Geometry

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The Valence-Shell Electron-Pair Repulsion (VSEPR)Method based on the idea that pairs of valence

electrons in bonded atoms repel one another.Assumes electron pairs try to get as far apart as possibleEach electron pair or bond takes up ~ same amount of space# of bonds or pairs determines molecular geometry

Molecular Geometry: The shape of a molecule thatdescribes the location of nuclei & the connections between them.

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Molecules with No Lone PairsBond angles due to # of repulsions

Each bond takes up space of 1 electron pair

AB2 AB3 AB4Linear Trigonal planar Tetrahedral

AB5 AB6Trigonal bipyramid Octahedral

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Molecules with Lone Pairs: Know table 10.2Lone pair electrons not seen but take up space

Act as “invisible bond”Count electrons as E’sSingle, double or triple bonds count as 1 bond

To determine molecular geometryAdd up all the B’s and E’s on the molecule

AB2E2The sum equals number of spaces needed

2B + 2E = 4 # spaces = 4Match to table of geometries without lone pairs

Electron pair geometry: TetrahedralMolecular Geometry: Bent

HOH

BAB

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Molecules with More than 1 Central AtomVSEPR must be done separately for each atom

May result in a different molecular geometry around each one

Methanol CH3OHC: 4 spaces: tetrahedralO: 4 spaces: bent

Oxoacids: Hydrogen goes on oxygensHNO3, H2SO4, etc. will also use this method

C

H

H

H O H

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Dipole Moments

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Dipole Moments and Polar Molecules

H F

electron richregionelectron poor

region

= Q x rQ is the charge

r is the distance between charges1 D = 3.36 x 10-30 C m

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Predict molecular shapeVSEPR AB2Linear O=C=O

Predict bond dipolesC less electronegative than O C - O O=C=O

Bond dipoles cancel or combine?Cancel Nonpolar = 0

Predicting Polarity: CO2

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Predict molecular shape.VSEPR AB3ETetrahedral

Predict bond dipoles.H less electronegative than N N - H lone pair more electronegative than N

Bond dipoles cancel or combine?Combine: Polar molecule >0

Predicting Polarity: NH3

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Isomers: Same molecular formulaDifferent structure

CisLarge groups on same side of double bond plane

TransLarge groups across plane of double bond

Polarity of different MoleculesDichloroethylene: C2H2Cl2

2 possible isomers

Cis-dichloroethylene

Trans-dichloroethylene

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Valence Bond Theory and

Hybridization

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Valance Bond Theory: Formation of H2(g)Covalent bond formation

Electrons in 1s atomic orbitals pairOpposing spins occupy the overlap

region between 2 atomsShield nuclei from each other

DelocalizationArea of high electron density (red)Lowers energy, provides stabilityBonding electrons are found in the

overlap region (covalent bond)

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Carbon BondingElectronic configuration

C should have 2 bonds2 half-filled orbitals on C [He] 2s22px

12py12pz

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ExperimentallyC has 4 identical bonds: CH4Implies 4 half-filled orbitals [He] 2s12px

12py12pz

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Excite 2s1 electron to 2pz orbital

Problems with Theory4 bonds, but orbitals of differing energies, bond lengths 3 bonds: H 1s C 2p Higher energy1 bond: H 1s C 2s Lower energy

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Hybrid OrbitalsModel that predicts shape based on atomic orbitals

Allows use of electrons in s, p and d orbitals in bondCreates several identical bonds“Averages” orbital energies to equalize bonds

Used for central atoms in covalent bonds# hybrid orbitals = # combining atomic orbitals

Use VSEPR and Lewis theory to predict geometryDetermine Lewis structure and VSEPR notationOrientation of determines electron geometryDetermine hybridization based on VSEPR modelElectron pairs may occupy 1 or more of the hybrid orbitals

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sp3 Hybridization4 equivalent orbitals

1part s to 3 parts p: sp3 CH4 : 4 valence electronsNH3 : 5 valence electrons

Orbitals point toward corners of tetrahedron

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sp2 Hybridization3 sp2 hybrid orbitals in a plane

1 2s orbital & 2 2p orbitals Forms 3 sp2 hybrids with 1 empty 2p orbitalTrigonal planar geometry: 120o angles.Often involves double bonds

19sp HybridizationTwo sp orbitals in a plane

1 2s orbital & 1 2p orbitals Forms 2 sp hybrids and 1 empty 2p orbital Linear geometry: 180o anglesTriple bonds may be present

BE: 2 2s valence electrons2 sp hybrid orbitals

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Hybrid Orbitals Involving d SubshellsAllows for expanded valence shell compounds

3s electron promoted to 3d subshellFive sp3d hybrid orbitals. Trigonal bipyramidal molecular geometry

3s & 3p electrons promoted to 3d subshellSix sp3d2 hybrid orbitalsOctahedral molecular geometry.

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Hybrid Orbitals: d and f SubshellsAllows for expanded valence shell compounds

A 3s and a 3p electron are promoted to 3d subshell Makes 6 sp3d2 hybrid orbitals

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Bonding Scheme for Iodine Pentafluoride (IF5)

VSEPRAX5E

Electron GeometryOctahedron

Molecular GeometryTetragonal Pyramid

Bonding5 sp3d2 I - F bonds

1 electron pair in sp3d2 orbital

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Hybrid Orbitals and Geometric Orientations

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Hybridization of

Double and Triple Bonds

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10.5

Carbon Bonding: sp2 Hybridization of CH2O

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Sigma and Pi Bonding in Ethylene

Sigma Bonding () End to ends,p (or d) orbitalsSingle bonds

Ethylene

C: 3 sp2 orbitalsH: 1s orbital

Pi Bonding () Parallel side to side

C: 1 p orbitalDouble bond1e- from each C

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Bonding in Acetylene, C2H2

HCCH

Sigma BondingEnd to End

C: 2 sp orbitalsH: 1s orbital

Pi BondingSide to Side

2 p orbitals per C

spHybridization

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Molecular Orbital Theory

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Molecular Orbitals and Bonding In H2 (1s orbitals)Molecular orbitals (MOs)

Orbitals that result from the interaction of atomic orbitalsInteraction can stabilize or destabilize molecule

Bonding OrbitalLower energy than atomic orbitals: High charge density in centerAntibonding OrbitalHigher in energy, designated with a *

Higher energy than atomic orbitals No electron density in center.

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Molecular Orbitals From 2p Atomic Orbitals6 atomic orbitals 6 molecular orbitals

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Bond Order: Can the Molecule Exist?BO = ½ (# bonding electrons - # antibonding electrons)