VSEPR theory

VSEPR which stands for Valence shell electron pair repulsion, is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. VSEPR theory proposes that the geometric arrangement of terminal atoms, or groups of atoms about a central atom in a covalent compound, or charged ion, is determined solely by the repulsions between electron pairs present in the valence shell of the central atom.

History- The idea of a correlation between molecular geometry and number of valence electrons (both shared and unshared) was first presented in a Bakerian lecture in 1940 by Nevil Sidgwick and Herbert Powell at Oxford University.

To start, we need to know the Lewis structure of a molecule. Then we count how many pairs of electrons are around the central atom. 

If there are two pairs of electrons, they must be positioned 180° apart from each other and the shape is therefore linear. 

Three pairs are best positioned 120° apart and the shape is thus trigonal planar. Here the shape is referred to include the non-bonding electron pairs. For the shape of a molecule without counting non-bonding electron pairs, make a normal prediction then look at the molecule without non-bonding electrons showing. 

Four pairs of electrons are best positioned as tetrahedral shape. Depending upon the number of non-bonding electron pairs, the shape of the molecule not counting non-bonding electron pairs can be: 

a) tetrahedral (no non-bonding pairs); 

b) trigonal pyramidal (one non-bonding pair); or 

c) "bent" or "V" (two non-bonding pairs). For five pairs of electrons, the shape is predicted to be trigonal bipyramidal. 

Lastly, the octahedral is the shape predicted for six pairs of electrons.