Question

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

Answer 1

Due to small size of Li it has high hydration energy. Because of this property Li furnish the water easily.But as we move down in the group ionic mobility increases and hydration energy decreases due to which bigger elements salts like of Rb and Cs can not hold the water molecule thats why they are anyhydrous.

Answer 2

as a result,water molecules gets attached to lithium salts as water of crystallisation. hence lithium salts such as trihydrated lithium chloride are commonly hydrated.
as the size of ions increases,the polarising power decreases.hence other alkali metal ions usually taken as anhydrous salts.