Hydration of ions is an exothermic process. The energy released when one mole of an ion in the gaseous state is dissolved in water to get it hydrated is called hydration energy. It is the energy released in the hydration of M+(g) ions.
M+(g) + aq → [M(aq)]+ + Energy released
This energy highly depends on:
(i) ionic radius and
(ii) Charge on the ion.
In other words, this energy mainly depends on the charge to size ratio of the ion. Since, the charge to size, the ratio decreases from top to bottom in a group, the hydration energy also decreases from top to bottom in a group.
Thus, this energy decreases from Li+ to Cs+ in group IA. The energy increases significantly by moving from left to right in a period as the charge to size ratio increases, as observed in the metal ions of the 3rd period.
For example, the hydration energy is greater for Mg2+ than for Na+, because Mg ion has a large charge, as well as being a smaller ion than Na+.
The negative sign associated with the hydration energy values indicates the release of energy.
Hydration Energy of ions ( ΔHhyd) in kJ mol-1