4. THE VALENCE SHELL ELECTRON PAIR REPULSION (VSEPR) THEORY

THE VALENCE SHELL ELECTRON PAIR REPULSION (VSEPR) THEORY

Sidgwick and Powell in 1940, proposed a simple theory based on repulsive character of electron pairs in the valence shell of the atoms. It was further developed by Nyholm and Gillespie (1957).
Main Postulates are the following:
(i) The exact shape of molecule depends upon the number of electron pairs (bonded or non bonded) around the central atoms.
(ii) The electron pairs have a tendency to repel each other since they exist around the central atom and the electron clouds are negatively charged.
(iii) Electron pairs try to take such position which can minimize the rupulsion between them.
(iv) The valence shell is taken as a sphere with the electron pairs placed at maximum distance.

(v) A multiple bond is treated as if it is a single electron pair and the electron pairs which constitute the bond as single pairs.