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.