Covalent Character of Ionic Bond

Even ionic bonds have some degree of covalent character. The polarising power of the cation distorts the electron cloud of the anion, inducing covalency. This is explained by Fajan's Rules:

Covalent character increases when:

• The cation is small and highly charged (high charge density → high polarising power)
• The anion is large (easily polarised / high polarisability)
• Both ions have high charge

\[\text{Polarising power}\propto\frac{1}{\text{Size of cation}}\propto\text{Size of anion}\propto\text{Charge on ions}\]

Greater the polarising power → More covalent character in the ionic bond.