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.