Question

Why the hydration enthalpy of the alkali metals decrease with in increase in size

Answer 1

Hydration enthalpy is a measure of the energy released when attractions are set up between positive or negative ions and water molecules.

The size of the hydration enthalpy is governed by the amount of attraction between the ions and the water molecule.

The attractions are stronger the smaller the ion. For example, hydration enthalpies fall as you go down a group in the Periodic Table. The small lithium ion has by far the highest hydration enthalpy in Group1, and the small fluoride ion has by far the highest hydration enthalpy in Group 7. In both groups, hydration enthalpy falls as the ions get bigger.