Which of the following ions will show maximum hydration?

Explanation

Enthalpy change of hydration depends upon charge density Charge density depends on two factors: 1.) Size of metal ion :- smaller the size of metal ion greater will be the hydration. For smaller ions the polar water molecules will be closer to the metal ion nucleus hence larger will be the hydration energy. 2.) Charge on ion :- greater the charge on metal ion greater will be the hydration. Since with high charge the polar water molecule will be pulled to a greater extent and hence large hydration energy. Since Al has the highest charge it possesses a very small size so a very high charge density,that is why having a very high hydration energy.