In Valence Shell Electron Pair Repulsion (VSEPR) theory, pairs of electrons that surround the central atom of a molecule or ion are arranged as far apart as possible to minimise electron-electron repulsion. This simple idea can be used to predict the shapes of molecules by following a simple procedure:
Decide which is the central atom in a molecule.
- In cases of ambiguity, pick the least electronegative atom as this atom will be better able to share its electrons with the other atoms in the molecule.
- Count up the valence (outer shell) electrons on the central atom.
- Count up the electrons used by the outer atoms to make bonds with the cental atom.
- The sum of (2) + (3) divided by two gives the Valence Shell Electron Pair (VSEP) count.
- The predicted geometry of the molecule is based on the number of VSEP.