CHEMICAL AND THEORETICAL BACKGROUND
Box 5.4 Radius ratio rules
The structures of many ionic crystals can be rationalized to a
first approximation by considering the relative sizes and
relative numbers of the ions present. For monatomic ions,
cations are usually smaller than anions (see Appendix 6),
although examples such as KF and CsF show that this is
not always true. The radius ratio
rþ
r
can be used to make a
first guess at the likely coordination number and geometry
around the cation using a set of simple rules: