Fajan’s Rule : Fajan Discussed the covalent character in ionic compounds and the rule can be summarised in following points.
(i) Smaller the cation, greater is its polarising power due to more effective nuclear charge, higher will be the covalent character. For example the polarising power of alkali metal cations follows the order Li+> Na+> K+> Rb+> Cs and the covalent character in their chlorides also follows the order LiCl > NaCl > KCl > RbCl > CsCl.
(ii) Larger the anion, more is the polarisability, hence more is the covalent character. For example covalent character in lithium halides follows the order LiF< LiCl< LiBr< LiI.
(iii) Higher is the oxidation state of the cation, again power of polarisation increases hence the covalent character also increases. For example FeCl3 has more covalent character than FeCl2 .