Although radius is a useful concept of determining coordination number and in understanding the structure of ionic compounds yet there are a few limitations to this rule.

(i)    Each ion is considered as a hard sphere for determining the optimum arrangement of ions in the crystal lattice. This is far form reality and serious errors can be made if anions get polarized and the bonding the intermediate (partially covalent) in character.

(ii)    Some compounds may crystallize in more than one modification with different coordination numbers. In such case, anion-anion repulsions and hence intern clear distance would be different.

(iii)    Effective radius of a cation is greatly influenced by the anions with the consequence that the radius ratio changes. For example, A_gF and N_aF crystallize out in N_aCI type of structure with coordination No. 6 and if we assume that the size of F^- remains constant than Ag⁺ is bigger than Na^+. On the other hand, for a given size for a chloride ion in A_gCI and N_aCI, the sizes of cations are reversed, that is, Na⁺ is bigger than Ag^+. It is also true for their bromides. This is understandable because Ag⁺ is softer than N⁺ and introduces relatively more covalent character with Polarizable anions like CI^- and  Br^- 

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