Valence Shell Electron Pair Repulsion Theory

 Valence Shell Electron Pair Repulsion Theory

The basic concept of this theory as suggested by Sidgwick and Powell (1940) has provided a highly useful idea for predicting geometries of molecules. According to this model, the arrangement of bonds around the central atom depends upon the repulsions operating between pairs of electrons (bonded or non-bonded) around the central atom.


The main postulates of VSEPR theory are

(i) For polyatomic molecules containing 3 or more atoms, one of the atoms is called the central atom to which other atoms are linked.
(ii) 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.
(iii) If the central atom is linked to similar atoms the repulsion between them are also similar. It gives the symmetrical shape to the molecule.
(iv) If the central atom is linked to atoms of different sizes or is surrounded by lone pairs as well as bond pairs of electrons then the molecule has a distorted geometry.

The relative order of repulsion between electron pairs is as follows

l.p. – l.p. > l.p. – b.p. > b.p. – b.p.

Shape of molecules in which the central atom has no lone pair of electrons.

Shapes of some simple molecules/ions with central ions having one or more lone pairs of electrons.


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