The decreasing tendency to form salts with water of crystallization (as a group is descended) is again in line with the falling hydration energy. For example, both sodium sulphate and carbonate form hydrates but neither of the corresponding potassium salts do; the sulphates of Group II elements show a similar trend MgSO4 , 7H2O, CaSO4 . 2H2O, BaSO4. For the most part, however, the chemistry of the Group I and II elements is that of the metal and the ions M + for Group I and M2+ for Group II. As already noted the two head
elements, lithium and beryllium, tend to form covalent compounds; the beryllium ion Be2+ , because of its very small radius and double charge, has also some peculiar properties in solution, which are examined later (p. 134).