Valence Shell Electron Pair Repulsion Theory (VSEPR):

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The basic concept of the theory was suggested by Sidgwick and Powell (1940). It provides useful idea for predicting shapes and geometries of molecules. The concept tells that, the arrangement of bonds around the central atom depends upon the repulsion’s operating between electron pairs(bonded or non bonded) around the central atom. Gillespie and Nyholm developed this concept as VSEPR theory.


The Main Postulates of VSEPR Theory are:

1. For polyatomic molecules containing 3 or more atoms, one of the atoms is called the central atom to which other atoms are linked.
2. The geometry of a molecule depends upon the total number of valence shell electron pairs (bonded or not bonded) present around the central atom and their repulsion due to relative sizes and shapes.
3. If the central atom is surrounded by bond pairs only. It gives the symmetrical shape to the molecule.
4. If the central atom is surrounded by lone pairs (lp) as well as bond pairs (bp) of then the molecule has a distorted geometry.
5. The relative order of repulsion between electron pairs is as follows : lp – lp > lp – bp > bp – bp.
6. 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.

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