Valence electrons of atoms in covalent compounds may be shared between atoms to form covalent bonds or they may be unshared. Pairs of electrons that are not shared in covalent bonds are lone pairs. Lone pairs of electrons play a critical role in determining the three-dimensional shape of molecules. A molecule's shape, in turn, determines many of its physical properties and its chemical reactivity.

A very useful method for understanding and predicting molecular shapes that relies upon the role of the lone pair is called the Valence Shell Electron Pair Repulsion (VSEPR) theory. This method has been developed extensively by Professor R. J. Gillespie of MacMaster University. It correctly accounts for the structures of most covalent compounds of elements other than transition metal complexes.

The basic tenet of the VSEPR theory is that atoms in a covalent molecule are arranged to minimize the repulsions of valence...