i. The stoichiometry and the size of the cation and anion determines the structure of ionic compounds.
ii. Smaller cations occupy tetrahedral voids and bigger cations occupy octahedral voids or cubic voids.
iii. The coordination number of the ion also determines the geometry of the unit cell.
iv. The ratio of the radius of the cation (r+ ) to the radius of the anion (r- ) is the radius ratio \(\left(\cfrac{r^+}{r^-}\right)\). For a given coordination number, the radius ratio has a limiting value.
v. The ionic structure is unstable when the radius ratio is less than the expected value.
vi. By using radius ratio rule, the structures of ionic solids can be predicted. This rule is not applicable to covalent compounds.
