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what is VSEPR theory

 VSEPR theory
This theory was given by Gillespie and Nyholmn in 1957 to predict the observed shape of the molecules in which the atoms are linked together by covalent bonds only. In a covalent molecule, the central atom is surrounded by shared pairs and lone pairs of electrons. These electron pairs repel one another and as a result, the molecule tends to become unstable. In order to minimize the force of repulsion, these electron pairs are directed in such a way so that they are as apart as possible. This means that a covalent molecule has a geometry of its own. Since the different molecules differ in the nature as well as the number of electron pairs around the central atom, they have therefore, different geometries as well as shapes. Thus, it is quite clear that the geometry and the shape of a covalent molecule to a large extent depends upon the force of repulsion in the electron pairs around the central atom.
The main features of the VSEPR theory are listed :
1. The shape of a molecule depends upon the number of the electron pairs (bonded or non-bonded) around the central atom.
2. The electron pairs around the central atom tend to repel one another since the electron clouds are negatively charged.
3. The electron pairs in space tend to occupy such positions that they are at maximum distance apart and the repulsive interactions are minimum.
 4. A multibond is treated as if it is a single bond and the electron pairs which constitute the bond may be regarded as pairs.
The order of repulsive interactions of the electron pairs. We have learnt that the electron pairs involved in the bond formation are known as bond pairs or shared pairs (bp) while those not involved in any bonding are called lone pairs (lp). n and Gillespie stated that the electron pairs existing as lone pairs cause greater repulsive interactions as compared to electron pairs. In the light of this, the repulsive interactions follow the order :
Lone pair-Lone pair > Lone pair-Bond pair > Bond pair-Bond pair
The repulsive interactions also influence the geometries of the covalent molecules whether regular or irregular.