VSEPR Theory

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:

  1. 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.
  2. Count up the valence (outer shell) electrons on the central atom.
  3. Count up the electrons used by the outer atoms to make bonds with the cental atom.
  4. The sum of (2) + (3) divided by two gives the Valence Shell Electron Pair (VSEP) count.
  5. The predicted geometry of the molecule is based on the number of VSEP.
3Trigonal Planar
5Trigonal Bipyramidal