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Chemistry · Glossary

What is Radius ratio?

Definition 6.3 University Chemistry — Year 1 · Chapter 6 — Crystals II: Ionic, Covalent and Molecular Solids

In an ionic crystal of a cation of radius r+r_+ and an anion of radius r−r_-, the radius ratio is x=r+/r−x = r_+/r_- (usually x<1x < 1, cations being smaller than anions).

The radius-ratio rule: the coordination number of the cation allowed by x, and the measured ratios of four salts (Shannon radii). Rubidium chloride, at 0.84, should be of the CsCl type but adopts the NaCl type: the rule is a guide, not a law ().
The radius-ratio rule: the coordination number of the cation allowed by xx, and the measured ratios of four salts (Shannon radii). Rubidium chloride, at 0.84, should be of the CsCl\ce{CsCl} type but adopts the NaCl\ce{NaCl} type: the rule is a guide, not a law (Exercise 6.12).
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