Give an account of VSEPR theory and its applications.

VSEPR theory (Valency shell electron pair repulsion theory) :
Postulates :
1) Shape of the molecule is determined by repulsions between all the electron pairs that mean both lone pairs and bond pairs present in the valance cell of central atom.
2) Llone pair of electrons occupies more space than bond pair of electrons.
3) The order of repulsions between them is LP– LP > LP – BP > BP – BP.
4) Order of repulsive forces exerted by various bonds is $\equiv >=>-$
5) The magnitude of repulsions between bond pair of electrons depends on electronegativity difference between central atom and bonded atom.
6) The electron pairs are oriented around the central atom is such a way, repulsions among them are minimum.
Applications :
i) NH₃ molecule : In NH₃ molecule N-atom undergoes sp³ hybridisation and it has three Bond pairs and one lone pair. According to VSEPR theory there exist repulsions between Bond pair and lone pair, so its bond angle is deviated from 109° 28' to 107° and shape is also deviated for tetrahedral to pyramidal.

ii) H₂O molecule : In H2O molecule O-atom undergoes sp³ hybridisation and O-atom has two bond pairs and two lone pairs according to VSEPR theory there exist repulsion between lone pair and lone pair, lone pair and bond pair. So, the bond angle is deviated from 109° 28' to 104° 30' and shape is also deviated from tetrahedral to angular.