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Fajan Rules

(i)    Small, highly charged cations

Small, highly charged cations have higher polarizing effects an anions than large, lowly charged cation. In other words, induced covalent character in ionic compounds increases with decrease in size of the ctionor increase in charge on the cation.

(ii)    Large, highly charged anions

Covalent character induced in ionic compounds due to polarization of the anion increases with an increase in size and charge of the anion. The electron cloud on the large-sized anion is less strongly held by its nucleus and hence it is easily polarized by the approaching cation. In other words, the large the anion charge, the higher is its polarizability.

(iii)    Cation configuration

Cations with pseudo noble gas configuration (electron outer shell) have higher polarizing power than cations with noble gas configuration. In other words, cations with ns2 np6 configuration cause less distortion than those with ns2 np6 nd10 configuration. This is because d-electrons shield the nucleus poorly with the consequence that such a cation will exert a higher nuclear charge on the interacting anion than a cation with 8 electron configuration.

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