Question

Which among the alkali metals have the highest degree of hydration?

Answer 1

Hydration energy is the amount of energy released on the hydration of a mole of a substance. So, hydration enthalpy can be defined as the energy released on the bond formation between the ions and the water molecules.

Complete answer:
Alkali metals belong to the group one of the periodic table and are known to be the soft metals.
Hydration energy of a particular ion is also a characteristic of its structural shape.
The hydration energy of a particular ion depends upon the force of attraction or the strength of bond formed between the ions and the water molecules.
This in turn depends upon the size of cation formed. The smaller the cation size, the greater the strength of bond and thus in turn greater the hydration energy.
Among the alkali metals, the smallest size is Lithium, so Lithium has the highest degree of hydration.
Due to its smaller size, lithium has greater nuclear charge over its electron and thus easily and strongly attracts the electron of the other ion or atom to form the bond and thus has high hydration enthalpy or high degree of hydration.

Note:
As we know that hydration enthalpy depends upon the size of the ion, so as we go down the alkali metals, the hydration enthalpy decreases down the group such that Lithium has the highest degree of hydration and Caesium has the lowest degree of hydration. Caesium has lowest hydration energy because of its larger size and less nuclear charge dispersion.