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Molecular StructuresMolecular StructuresChapter 9Chapter 9

Linus Pauling 1627-1691.** Defined electronegativity. Wrote “The Nature of the Chemical Bond” 1939.Pioneered crystal and protein structures.

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Molecular ShapesMolecular Shapes

•Lewis structures give atomic connectivity: they tell us which atoms are physically connected to which.

•Lewis structures do not give us information about geometry or shape of molecules.

For example, methane, CH4, is drawn as:

Where all the angles appear to be 90o.

Actually, the bond angles are 109.5o, (tetrahedral angle)

H CH

H

H

HH C H H

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Molecular ShapesMolecular Shapes

Methane: Tetrahedral geometry

Tetrahedron(sp3-hybridization)

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Some Molecular GeometriesSome Molecular Geometries

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Molecular ShapesMolecular Shapes

In order to predict molecular shape, we assume the valence electrons repel each other.

Therefore, the molecule adopts whichever 3D geometry minimized this repulsion.

This is the idea behind Valence Shell Electron Pair Repulsion (VSEPR) theory.

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Consider water, H2O

Dot structure is:

BD: 2

. .H : O : H. .

NBD: 2TED: 4

TED=4 tells us thatelectron domain geometry (EDG) is tetrahedral and,

BD=2 tells us that: molecular geometry (MG) is bent

H

O

H

7

8

9

PCl5

SF6

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Some examples:

PBr3 SO3 CO32-

SO3

2- H2O CH2=CH2

BeF2 BCl3 HCN

Work out the molecular geometries.

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Polarity of MoleculesPolarity of MoleculesPolar molecules interact with electric fields.If the centers of negative and positive charge do not coincide, then the molecule is polar.

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Polarity of MoleculesPolarity of MoleculesDipole Moments of Polyatomic MoleculesDipole Moments of Polyatomic MoleculesExample: in CO2, each C-O dipole is canceled because the molecule is linear. In H2O, the H-O dipoles do not cancel because the molecule is bent.

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Polarity of MoleculesPolarity of MoleculesDipole Moments of Polyatomic MoleculesDipole Moments of Polyatomic Molecules

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Covalent Bonding and Orbital OverlapCovalent Bonding and Orbital Overlap

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Hybrid OrbitalsHybrid Orbitals

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Hybrid OrbitalsHybrid Orbitals

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Multiple BondsMultiple Bonds -Bonds: electron density lies on the axis between the nuclei.All single bonds are -bonds.

-Bonds: electron density lies above and below the plane of the nuclei.

A double bond consists of one -bond and one -bond.A triple bond has one -bond and two -bonds.

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Multiple BondsMultiple Bonds

Two p orbitalsoverlap to forma π-bond

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Multiple BondsMultiple BondsEthylene, C2H4, H2C=CH2, has:

•one - and one -bond between the carbon atoms;•both C atoms sp2 hybridized;•both C atoms with trigonal planar molecular geometries.

2 p-orbitals1 π-bond5 σ-bonds

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Multiple BondsMultiple BondsConsider acetylene, C2H2, H-CC-H, which has

•one - and two -bonds between the carbon atoms;•both C atoms sp-hybridized;•both C atoms with linear molecular geometries.

•When triple bonds form (e.g. HCCH, N2) one -bond is always above and below and the other is in front and behind the axis of the nuclei.

C CH H NN ::4 p-orbitals2 π-bonds3 σ-bonds

4 p-orbitals2 π-bonds1 σ-bond

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Delocalized Delocalized Bonding BondingIn the case of benzene (C6H6) there are

6 C-C bonds, all equal; 6 C-H bonds, all equal. Each C atom is sp2 hybridized.• There are 6 p orbitals on each C atom.

σ-bonds p orbitalsmolecular

orbital