Section 8.3 Bonding Theories. VSEPR Theory Electron dot structures fail to reflect the three dimensional shapes of the molecules. VSEPR Valence Shell.

Section 8.3

Bonding Theories

VSEPR TheoryElectron dot structures fail to reflect the three dimensional shapes of the molecules.

Valence Shell Electron Pair Repulsion Theory (VSEPR)

According to VSEPR theory, the repulsion between electron pairs causes molecular shapes to adjust so that the valence electron pairs stay as far apart as possible.

Unshared pairs of electrons – no bonding atom is vying for these unshared electrons so they are held closer to the central atom than are the bonding pairs.

The unshared pair strongly repels the bonding pairs pushing them together.

In predicting molecular shapes, it may be useful to start with an electron dot structure.

The electron dot structure shows both the bonding and nonbonding pairs of electrons around the central atom.

When using VSEPR theory to predict molecular shape, double and triple bonds are viewed as single bonds.

When 4 pairs of electrons must be accommodated around the central atom – tetrahedral (109.5º)

When 3 pairs – a trigonal planar maximizes space (120º)

When 2 pairs – a linear arrangement (180º)

VSEPR Theory

VSEPR for methane (a gas):

• Single bonds fill all atoms.

• There are 4 pairs of electrons pushing away.

• The furthest they can get away is 109.5°

C HHH

H

This 2-dimensional drawing does not show a true representation of the chemical arrangement.

4 atoms bonded• Basic shape is

tetrahedral.• A pyramid with a

triangular base.• Same shape for

everything with 4 pairs.

109.5°

VSEPR Structure

3 bonded - 1 lone pair• Still basic tetrahedral but you

can’t see the electron pair.• Shape is called

trigonal pyramidal.N HHH

107°

VSEPR Structure

3 atoms no lone pair• The farthest you can the

electron pair apart is 120º.

• Shape is flat and called trigonal planar.

• Will require 1 double bond

CH

HO

120°

2 bonded - 2 lone pair• Still basic tetrahedral but you can’t

see the 2 lone pair.• Shape is called bent.

OHH

105°

VSEPR Structure

2 atoms no lone pair• With three atoms the farthest they

can get apart is 180°.• Shape called linear.• Will require 2 double bonds or one

triple bond

C OO180°

180°

pages 232 - 233

• Ammonia (NH3) = 107o

• Water (H2O) = 105o

• Carbon dioxide (CO2) = 180o

• Methane (CH4) = 109.5o

• Boron fluoride (BF3) = 120o