Which of the ions have maximum hydration energy?

The correct answer is ²⁺.

To understand why this is the correct answer, let us look at the factors affecting hydration energy step-by-step:

1. Definition of Hydration Energy:
Hydration energy is the amount of energy released when one mole of gaseous ions is dissolved in water to form hydrated ions. It is an exothermic process, and a higher release of energy indicates a stronger electrostatic interaction between the ion and water molecules.

2. Factors Affecting Hydration Energy:
The hydration energy of an ion depends directly on its charge density. Charge density is defined as the ratio of the charge of the ion to its size (ionic radius):

$$\text{Hydration Energy}\propto \text{Charge Density}=\frac{\text{Charge of the ion}}{\text{Size of the ion}}$$

Thus, hydration energy increases with an increase in the charge of the ion and a decrease in the size of the ion.

3. Comparison of the Given Ions:
All the given ions belong to Group 2 (alkaline earth metals) of the periodic table: Beryllium (represented here as ²⁺), Magnesium (Mg²⁺), Calcium (Ca²⁺), and Strontium (Sr²⁺).
- All of these ions carry the same charge of +2.
- As we move down Group 2 from top to bottom, the number of electron shells increases, which increases the ionic radius. The order of ionic size is:

$${\text{Be}}^{2+}<{\text{Mg}}^{2+}<{\text{Ca}}^{2+}<{\text{Sr}}^{2+}$$

4. Conclusion:
Since Be²⁺ (represented as ²⁺) is the smallest ion in this group, it has the highest charge density. Consequently, it exerts the strongest electrostatic attraction on the polar water molecules, releasing the maximum amount of energy upon hydration. Therefore, the ion with the maximum hydration energy is ²⁺.