Question

Many lithium salts are hygroscopic (absorb water), but the corresponding salts of the other alkali metals are not. Why are lithium salts different from the others?

Short answer

Although hygroscopic, lithium salts are unique from other salts in that they may quickly polarise water molecules.

Step-by-step solution

Hygroscopic substance definition

Given the importance of water as a solvent, it's not unexpected that there is a name for water absorption. A hygroscopic material may absorb or adsorb water from the air it breathes. This usually happens at or around normal room temperature. Salts are the most hygroscopic materials; however, the feature may be found in a variety of other materials.

Difference of Lithium salts from other salts

A substance that rapidly draws water from its surroundings is said to be hygroscopic.${\text{Li}}^{\text{+}}$has the smallest radius among alkali metal ions, due to which it can easily polarize water molecules.${\text{Li}}^{\text{+}}$ions have very large energy of hydration because of its relatively high charge density.

Lithium salts get bonded to the water in process of crystallization. For example, lithium chloride contains two water molecules per mole of LiCl .Other alkali metals have a lower charge density and consequently are less effective in attracting and polarizing water molecules.

Therefore, we can conclude that lithium salts can easily polarize water molecules.