Property of the alkaline earth metals that increases with their atomic number
(a) solubility of their hydroxides in water
(b) solubility of their sulphates in water
(c) ionisation energy
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(a) Electronegativity as well as ionisation energy both usually decrease on moving downward a group with increase in atomic number. The hydroxides and sulphates of alkaline earth metals are ionic solids and the solubility of ionic solids is governed by two factor, viz, lattice energy and hydration energy. For solubility, hydration energy>lattice energy
Hydration energy varies inversely with size, ie, decreases with increase in size. However, lattice energy in case of sulphates, remains almost same with increase in the atomic number of alkaline earth metals, due to large size of sulphate ion. Hence, hydration energy only governs the solubility in this case. Thus, solubility of alakaline earth metal sulphates decreases as the hydration energy decreases on moving downward the IIA group.
On the other hand, in case of hydroxides, the lattice energies are different because of medium size of hydroxide ions and decreases on moving from Be to Ba. This tends to increase the solubility and to overcome the counter-effect produced by the decrease in hydration energy. hence, the solubility of alkaline earth metal hydoxides increases with increase in the atomic number of alkaline earth metals.