Hydration represents the dissolution of a substance in water by absorbing water molecules by weak valency force. The energy released when 1g mol of an ion in the gaseous state is dissolved in water to get it hydrated is called hydration energy. M+(g)+aq⟶M+(aq) ∆H=− Hydration energy (i) Smaller the cation, greater is the degree of hydration Li+‌>Na+>K+>Rb+>Cs+ Be2+‌Mg2+>Ca2+>Sr2+>Ba2+ (ii) The hydration energy of ions increases with increase in their valencies. Al3+>Mg2+>Na+ Thus, Mg2+ ion has higher hydration energy than Na+ion, but lower hydration energy than Al3+ and Be2+.