Question

Why are lithium salts commonly hydrated and those of the other alkali ions usually, anhydrous?

Answer 1

Lithium is the smallest in size among the alkali metals. Hence, $L{i}^{+}$ ion can polarize water molecules more easily than other alkali metals.

As a result, water molecules get attached to lithium salts as water of crystallization. The hydration enthalpies of alkali metal ions decrease with increase in ionic sizes.

$\text{Order of ionic radius}$ $\Rightarrow L{i}^{+}>N{a}^{+}>K^{+}>R{b}^{+}>C{s}^{+}$

Hence, lithium salts such as lithium chloride $\left(LiCl.2H_{2}O\right)$ are commonly hydrated.

As the size of the ions increases, their polarizing power decreases. Hence, the other alkali metals being bigger in size cannot easily polarize water molecules.
So, there salts do not get bonded to water of crystallization.