Vsepr Theory (Valence Shell Electron Pair Repulsion Theory)

Chemical Bonding of Class 11

VSEPR THEORY (VALENCE SHELL ELECTRON PAIR REPULSION THEORY)

This theory starts from the general principle that valence shell electrons occupy essentially localised orbitals. Mutual interaction among the electrons orient the orbitals in space to an equilibrium position where repulsion becomes minimum. The extent of repulsive interaction then follows the order.

Lone pair – lone pair > lone pair – bond pair> bond pair – bond pair: 

A lone pair is concentrated around the central atom while a bond pair is pulled out between two bonded atoms. As such repulsion becomes greater when a lone pair is involved. Let’s take an example to illustrate this theory. CH4 contains no lone pairs. The bond pair – bond pair interactions brings about the most stable equilibrium bond angle of 109°28′, the angle predicted from sp3 hybridisation.

“If the electronegativity of the peripheral atoms is more, then the bond angle will be less”. For example - if we consider NH3 and NF3, F – N – F bond angle will be lower than H – N – H bond angle. This is because in NF3 the bond pair is displaced more towards F and in NH3 it is displaced more towards N. So accordingly the b.p. – b.p. interaction is less in NF3 and more in NH3.

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