how to predict a geometry for a atom in a molecule

Dear student,

The VSEPR theory, proposed by R.J.Gillespie and R.S. Nyholmm in 1957, is based on the repulsions between the electron-pairs in the valence-shell of the atoms in the molecule. It was developed to predict the shapes of the molecules in which the atoms are bonded together with single bonds only.

The main postulates of VSEPR theory are:

Postulates

• The shape of the molecule is determined by both the total number of electron pairs (bonding and non-bonding) around the molecules central atom and the orientation of these electron pairs in the space around the central atom.

• In order to minimize the repulsion forces between them, electron pairs around the molecules central atom, tend to stay as far away from each other as possible.

• Electron pairs around the molecule's central atom can be shared or can be lone pairs. The 'shared pairs' of electrons are also called bond pairs of electrons. The presence of lone pair(s) of electrons on the central atom causes some distortions in the expected regular shape of the molecule.

• The strength of repulsions between different electron pairs follows the order:

Lone pair - Lone pair > Lone pair - Shared pair > Shared pair - Shared pair.

Predicting the shape of molecules on the basis of VSEPR theory

To use this theory for predicting the shapes of molecules, the number of electron pairs (both, shared and lone pairs) is simply counted. This is illustrated by taking a typical molecule of the type AB_n. 'A' is the central atom, 'B' atoms are bonded to 'A' by single covalent bonds (single electron pair bonds), and 'n' is the number of 'B' atoms bonded to one atom of 'A'.