Write about VSEPR theory.

Explanation

VSEPR theory was proposed by Gillespy and Nyholm. It predicts the shapes of molecules without reference to the hybridisation.
The important points in the theory are :
1) The shape of a molecule depends upon the number of electron pairs (bond pairs) present on the central atom. If there are only 'bond pairs' and no 'lone pairs' on the central atom, the molecules assumes a regular geometrical shape.
2) If there are 'lone pairs' also on the central atom, then the structure gets distorted and the bond angle changes.
3) A 'lone pair of electrons' occupy more space around the central atom than a bond pair, because the 'lone pair' is attracted by only one nucleus, while the 'bond pair' is attracted by two nuclei.
4) The electron pairs orient in space so as to have minimum repulsions among them, which determines the 'bond angles'.
5) The order of repulsion between various electron pairs is
`l.p-l.p gt l.p-b.p gt b.p-b.p`
6) The magnitude of repulsion between bond pairs of electrons depends on the electronegativity differences between the central atom and the other atoms.
7) The order of repulsion between different bonds is :
Tiple bond `gt` Double bond `gt` Single bond

Applications :
1. Structure of `NH_(3)` molecule : In `NH_(3)` molecule, the central 'N' atom contains 3 bond pairs one lone pair. In total, there are four pair of electrons. Hence, according to VSEPR theory, the expected structure of the molecule in tetrahedral with a bond angle of `109^(@)28^(1)`. But, because of the presence of lone pair, the shape of the molecule gets distorted. Therefore, the real structure of the molecule is pyramidal with a bond angle of `107^(@)`.

2. Structure of `H_(2)O` molecule : In `H_(2)O` molecule, the central 'O' atom contains two bond pair and two lone pairs. In total there are four pair of electrons. Hence, according to VSEPR theory, the expected structure of the molecule is tetrahedral with a bond angle of `109^(@)28^(1)`. But, because of the presence of lone pair, the shape of the molecule gets disorted. Therefore, the real structure of the molecule is V - shape with a bond angle of `104.5^(@)`.