To explain the bond angle in the molecules with three or more than three atoms with all atoms attached to a central atom through covalent bond Valence - Shell - Electron- Pair - Repulsion Theory was proposed by Sidgwick and Powell and further improved by Gillespie and Nyholm.

Important Postulates : 

(i) VSEPR Theory considers electrons in the valence shell which are in covalent bonds and in lone pair as charge clouds that repel one another and stay as far apart as possible. This is the reason why molecule gets specific shapes. 

(ii) By knowing the total number of electron pairs in the valence shell as covalent bonds and lone pairs in the central atom, it will help us to predict the arrangement of those pairs around the nucleus of the central atom and from that the shape of the molecule. 

(iii) Lone pairs occupy more space around the central atom than bond pairs. Lone pair means unshared electron pairs or non-bond pairs and are shared by two nuclei. 

(iv) The presence of lone pairs on the central atom causes slight distortion of the bond angles from the expected regular shape. If the angle between lone pair and bond pair increases at the central atom due to more repulsion, the actual bond angles between atoms must be decreased.