Fajan's Rules state that the covalent character of an ionic bond increases with the polarizing power of the cation. Polarizing power increases with:

1. Small Cationic Size.
2. High Cationic Charge.

• Compare: $N{a}^{+},M{g}^{2+},A{l}^{3+},S{i}^{4+}$.
• $S{i}^{4+}$ has the highest charge ($+4$) and the smallest ionic radius among the series.
• Therefore, it strongly polarizes the electron cloud of the $C{l}^{-}$ ions, resulting in maximum covalent character.