Chemical Bonding and Molecular Structure
9.0 Valence Shell Electron Pair Repulsion (VSEPR) Theory
9.0 Valence Shell Electron Pair Repulsion (VSEPR) Theory
In $1957$ Gillespie and Nyhom gave this theory to predict and explain molecular shapes and bond angles more exactly. The theory was developed extensively by Gillespie as the Valence Shell Electron Pair Repulsion (VSEPR) theory. This may be summarized as:
1. The shape of the molecule is determined by repulsions between all of the electron pairs present in the valence shell.
2. A lone pair of electrons takes up more space round the central atom than a bond pair, since the lone pair is attracted to one nucleus whilst the bond pair is shared by two nuclei.
It follows that repulsion between two lone pairs is greater than repulsion between a lone pair and a bond pair, which in turn is greater than the repulsion between two bond pairs.
Thus the presence of lone pairs on the central atom causes slight distortion of the bond angles from the ideal shape. If the angle between a lone pair, the central atom and a bond pair is increased, it follows that the actual bond angles between the atoms must be decreased.
The order of repulsion between lone pairs and bond pairs of electrons follows the order as: $$Lone{\text{ }}pair{\text{ }} - {\text{ }}lone{\text{ }}pair{\text{ }}repulsion{\text{ }} > {\text{ }}lone{\text{ }}pair{\text{ }}-{\text{ }}bond{\text{ }}pair{\text{ }}repulsion{\text{ }} > {\text{ }}bond{\text{ }}pair{\text{ }}-{\text{ }}bond{\text{ }}pair{\text{ }}repulsion$$
3. The magnitude of repulsions between bonding pairs of electrons depends on the electronegativity difference between the central atom and the other atoms.
4. Double bonds cause more repulsion than single bonds, and triple bonds cause more repulsion than a double bond.
