Question

Why do the hydration enthalpy in alkali metals decrease down the group?

Answer 1

hello friend .
as we know that water is a polar molecule where the oxygen atom has a partial negative charge.  Also as lithium has the smallest ionic size its positive field(due to its high effective nuclear charge it attracts negative charges effectively and also due to the absence of numerous inner elcetron its screening effect is also very low compared to the other alkali metals) is very strong. It atrracts the electronegative aprt of the water molecule effectively than the other ions of alkali metals(i.e. they have a larger size and their ability to attract the electronegative part of the water molecules decreases -large size due to which the nucleus of the ion is far away from the water molecule leading to a reduction in attractive force).