Fajans' Rules and Covalent Character

Covalent character in an ionic compound increases when:

Cation is small and highly charged (high polarizing power, high charge density)
Anion is large and easily polarized (high polarizability)
Both conditions are met simultaneously

$Polarizing\ Power \propto \frac{charge}{radius^2}\ (charge\ density)$

Compound	Cation	Anion	Covalent Character
LiCl	$Li^{+}$ (small, high CP)	$Cl^{-}$ (large)	Significant — soluble in organic solvents
NaCl	$Na^{+}$ (moderate)	$Cl^{-}$	Low — ionic, insoluble in organic solvents
$BeCl_{2}$	$Be^{2+}$ (very small, +2)	$Cl^{-}$	Very high — covalent polymer
$CaCl_{2}$	$Ca^{2+}$ (large, +2)	$Cl^{-}$	Low — predominantly ionic
$MgCl_{2}$	$Mg^{2+}$ (intermediate)	$Cl^{-}$	Moderate — partial covalent character

Key consequences of covalent character: