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The thory (as the name suggests) has its foundation in three types of repulsions taking place in these molecules between the electrons of the molecule:
The molecule must avoid these repulsions to remain stable. The theory states that the repulsion becomes zero at ~115-120°. Sometimes, when repusion cannot be avoided, the less severe repulsion is preffered. The lone pair-lone pair (lp-lp) repulsion is said to exceed the lone pair-bonding pair (lp-bp) repulsion which is further said to exceed the bonding pair-bonding pair (bp-bp) repulsion. Hence, the lesser bp-bp repulsion is preffered over the lp-lp or lp-bp repulsion.
Larger molecules which fail to even maintain 90° between their electron pairs prefer to lie in more than one plane.
To predict the structure of a molecule, the total coordination number of the central atom is taken into account. The total coordination number is the sum of the total number of neighbours (of the central molecule) and the number of lone pairs on it. The structure is given taking into account not merely the coordination number but also how it is derived (from the sum of the numbers).
A small table to show a few of the geometries:
# neighbours #lp Shape Angle
2 0 Linear 180 3 0 Trigonal Planar 120(This table is incomplete)
Chemistry