Question

Explain why for the ionic solids all of the ions do not have the same coordination number.

Coordination Numbers (Cation:Anion): NaTl 5:5, Cu₃Au 2:2, ZnS 4:4, CdI₂ 6:3, CsCl 8:8

Answer 1

In ideal ionic crystals, coordination numbers are determined largely by electrostatic considerations.

Cations surround themselves with as many anions as possible and vice-versa.

This can be related to the relative sizes of the ions. ⇒ radius ratio rule

Radius ratio rule states:

As the size (ionic radius r) of a cation increases, more anions of a particular size can pack around it.

Ratio of the ionic radii= r⁺ / r^-

Since different ions have different ionic radii,the radius ratios will be different in different crystals

This accounts for the fact that in ionic solids all of the ions do not have the same coordination number.