VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) Theory

Lewis concept is unable to explain the shapes of molecules but VSEPR theory provides a simple method to predict the shapes of covalent molecules. This theory, was developed earlier by Sidwick and Powell in 1940, and it was further improved by Gillespie and Nyholm in (1957).

The theory is primarily based upon the fact that in a polyatomic molecule the direction of bonds around the central atom depends upon the total number of electron pairs (bonding as well as non bonding) in its valence shell. These electron pairs place themselves as far apart as possible in space so as to have minimum repulsive interaction between them. The minimum repulsions correspond to the state of minimum energy and maximum stability of the molecule.

Postulates of VSEPR theory :

1. The geometry and shape of the molecule depends upon the number of electron pairs (bonded as well as non-bonded) in the valence shell of the central atom.

2. Pairs of the electrons in the valence shell repel one another since their electron clouds are negatively charged.

3. The position of electron pairs in space around central atom are such that minimum repulsion & thus maximum distance between them.

4. A multiple bond is considered as if it is a single electron pair and the two or three electron pairs of a multiple bond are treated as a single super pair.

5. The repulsive interaction between two Ione pairs (lp) are different from those between two bond pairs (bp) or those between a lone pair and a bond pair. The magnitude of repulsive interaction of electron pairs decreases in the order.

lp  – lp  > lp – bp > bp – bp

Nyholm and Gillespie in 1957, refined the VSEPR model by explaining the important difference between the lone pairs and bonding pairs of electrons. They postulated that bond pair of electrons is shared by the two atoms whereas lone pair is under the influence of only central atom. As a result, in a molecule , the electron cloud containing lone pair is more spread out and occupy more space as compared to the electron cloud containing bond pair.That’s why relatively greater repulsive interactions between the lone pairs in comparison to lone pair – bond pair and bond pair-bond pair repulsive interactions.

These repulsive effects result in deviations from idealised shapes and alterations in bond angles in molecules.

For the prediction of geometrical shapes of molecules with the help of VSEPR theory, it is convenient to divide molecules into two categories are

(i) molecules in which the central atom has no availability of lone pair

(ii) molecules in which the central atom has one or more lone pairs.

First category molecules possess regular geometry and the second category molecules have irregular or distorted geometry.

Likewise, VSEPR theory gives the geometry of simple molecules but theoretically it could not explain them and has limited applications.