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Fajan's rules were developed based on the concept, "the greater the polarization of anion, the greater is the covalent nature".

The polarizing power increases with increase in the size of cation i.e. smaller cations are very effective in the polarization of anion. Polarizability of an anion is the tendency of an anion to undergo polarization. It indicates the easiness with which an anion undergoes distortion in presence of a cation. Due to polarization of anion and the increase in covalent nature. Lithium ion is smaller cation, hence, lithium fluoride has more covalent character than the potassium fluoride. Iodide ion is larger than fluoride ion, hence, potassium iodide has more covalent character than potassium fluoride. ${\mathrm{Sn}}^{4+}$ is smaller cation than ${\mathrm{Sn}}^{2+}$. ${\mathrm{Zn}}^{2+}$ ion is having pseudo inert gas configuration and ${\mathrm{Na}}^{+}$ has inert gas configuartion. The ions with pseudo inert gas configuration readily forms the covalent compounds.