Lithium salts are commonly hydrated and those of the other alkali ions usually anhydrous.

Explanation:

The hydration enthalpies of alkali metal ions decrease with increase in ionic sizes.

Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺

⇒ Because of smallest size among alkali metals, Li⁺ can polarise water molecules more easily than the other alkali metal ions.

⇒ As a result, water molecules get attached to lithium salts as water of crystallisation.

⇒ Hence, Li⁺ has maximum degree of hydration enthlpy and for this reason lithium salts are mostly hydrated.

⇒ For example, lithium chloride crystallises as LiCl 2H₂O

Note: Hydration enthalpy is defined as the energy released when new bonds formation takes place between the ions and the water molecules.