Hydration enthalpy () is the change in enthalpy when one mole of gaseous ions is dissolved in a particular amount of water under standard conditions to make it a dilute solution.
Hydration enthalpy generally decreases with increase in size of the ion and so while moving from top to bottom in a group or left to right in a period, hydration enthalpy decreases.
Thus the ions which are smaller in size have higher hydration enthalpy. liberates the most energy among all the elements upon hydration, forming a cage like structure surrounded by water molecules.