The ratio of the radius of a cation (r+) and the radius of an anion (r–) is known as the radius ratio (r+/r–).

Using the radius ratio, we can predict the coordination number and structure of ionic crystals.

The corresponding limiting radius ratio values for coordination numbers 2, 3, 4, 6, and 8 are

The radius ratio is rarerly 1; in such a case the coordination number is 12

Calculation Of Some Limiting Radius Ratio Values

1. Coordination number 3 (planar triangle)

The smaller positive ions with radius r+ interacted with 3 larger negative ions with radius r-. Plainly AB=BC=AC=2r–, BE=r–, BD=r+ + r–, the angle D-B-E is 30o.

Hence, in the coordination number 3 the radius ratio is 0.155.