Hydration enthalpy of which of the following ions is highest?

Step-by-step solution

Hydration enthalpy is the amount of energy released when one mole of ions are dissolve in excess of water. Hydration enthalpy increases as the ionic size decreases and its charge increases. Na ⁺ and Mg ²⁺ belong to third period and therefore have smaller size than K ⁺ and Ca ²⁺ of fourth period. Mg ²⁺ is smaller than Na ⁺ and also has higher charge than Na ⁺ , therefore Mg ²⁺ will have highest hydration enthalpy.