`MgCl_(2)` and `CaCl_(2)` are easily hydrated, while NaCl and KCl are not hydrated. Why?

Explanation

Compounds of alkaline earth metals are more extensively hydrated than those of alkali metals because the hydration enthalpies of alkaline earth metal ions are larger than those of alkali metal ions.
e.g., `MgCl_(2)` and `CaCl_(2)` exist as `MgCl_(2).6H_(2)O` and `CaCl_(2).6H_(2)O` while NaCl and KCI do not form such hydrates.