James Richard Fromm
Once the substituents on the central atom of the molecule are arranged in one of the basic shapes, a qualitative picture of the actual molecular geometry can be obtained by considering the distortions of the basic structure which will be introduced by the repulsion which exists between the unshared electron pairs and the electron pairs used in bonding to the substituents. The effect of this repulsion will be to move the substituents closer to each other, and farther away from any unshared electron pairs, than they would be found in the basic structure.
In a perfectly tetrahedral molecule, such as methane, the bond angle between any two adjacent atoms must be, and is, 109.5o. In ammonia, NH3, there is one orbital occupied by a lone pair of nonbonding electrons; this repels the three bonding electron pairs so that the H-N-H angle is
107.3o. In water, H2O, there are two unshared electron pairs or lone pairs; the repulsion is greater and the H-O-H angle is 104o.
It is possible for a single atom, particularly an atom of oxygen or sulfur, to share two of the electron pairs on a neutral atom rather than one. The compounds SOCl2 and POCl3 are examples of compounds in which this happens. The two electron pairs shared must occupy roughly the same space, the space between the two atoms to which they are bound, and so the two bonding pairs act as if they were a single electron pair in establishing the basic molecular geometry. For this reason SOCl2 is a planar triangular molecule while POCl3 is tetrahedral.