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Molecular Geometry

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Molecular Structure

Molecular geometry is the general shape of a molecule or the arrangement of atoms in three dimensional space.

Physical and chemical properties depend on the geometry of a molecule.

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3-D Molecular Structures

3-D Model

3-D Drawing

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Does it matter?The Thalidomide Story

The chemical structure of thalidomide.

“The Same and Not the Same”, by Roald Hoffmann1995, Columbia University Press

HC

HC

CH

C

C

HC

C

N

C

CH

C NH

C

CH2H2C

O

O

O

O

models – enantiomers (mirror image)

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H3C

H2C

CH2

H2C

CH2

H2C

CH2

HC

CH

H2C

CH2

H2C

CH2

H2C

CH2

O

OH

H3C

H2C

CH2

H2C

CH2

H2C

CH2

HC

CH

H2CCH2

H2CCH2

H2CC

O

OH

Does it matter?Fatty Acids

‘trans’ fatty acid

‘cis’ fatty acid

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VSEPR Model

The Valence Shell Electron Pair Repulsion model predicts the shapes of molecules and ions by assuming that the valence shell electron pairs are arranged as far from one another as possible to minimize the repulsion between them.

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AXE shorthand notation:

• A - central atom• X - substituent atoms• E - lone pair electrons

AX3

VSEPR Notation

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Only five basic shapes. When a lone pair replaces an atom, the

molecular geometry changes as well as the angles.

Double and triple bonds are treated as single bonds.

Predicting Molecular Geometry: VSEPR

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Linear

bond lone

Molecularpairs pairs

Geometry

2 0 linear....1 1-3 linear

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Geometry is Linear. Bond angle is 180o.

AX2

Example 1: BeCl2

Cl Be Cl

1. Draw the Lewis structure

2. Two electron pairs around the central atom.

Two substituentsZero lone pairs.

Predicting Molecular Geometry

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Predicting Molecular Geometry

Example 2: HF

1. Draw the Lewis structure

2. One electron pair between the two atoms.Three lone pairs on fluorine.

Geometry is Linear.

AX

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bond loneMolecular

pairs pairsGeometry

3 0 trigonal planar

Trigonal Planar

Three e- pairs about central atom

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Predicting Molecular Geometry

trigonal planar

AX3

..

F: :FB

:F:

.. ......

Example 3: BF3

Three electron pairs around the central atom. Three substituents and Zero lone pairs.

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Tetrahedralbond lone

Molecular pairs pairs

Geometry 4 0

tetrahedral

Four e- pairs about central atom

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Predicting Molecular Geometry

CH4

AX4

Example 4:

H

H

H C H

Four electron pairs around the central atom. Four substituents and Zero lone pairs.

tetrahedral

Trigonal Pyramidal

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3 1 trigonal

pyramidal....

bond loneMolecular

pairs pairsGeometry

Four e- pairs about central atom

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Predicting Molecular Geometry

NH3

AX4 :AX3

molecular geometry = triangular pyramidal

Example 5:

HH N

H

H N H

H

Four electron pairs around the central atom. Three substituents and One lone pair.

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....

....

2 2 bent

Bentbond lone

Molecular pairs pairs

Geometry

Four e- pairs about central atom

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Predicting Molecular Geometry

H2O

AX4 :AX2

molecular geometry = angular or bent

Example 6:

H O H

Four electron pairs around the central atom. Two substituents and Two lone pairs.

O H H

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Predicting Molecular Geometry

Tetrahedral - bond angles

Order of increasing repulsion:

bonding pair-bonding pair < bonding pair-lone pair < lone pair-lone pair