The hydration energies (strictly, hydration enthalpies) fall, as expected, as we descend either Group, and are larger for Group II than for Group I ions. The solubilities of the salts of Groups I and II are determined by a balance between lattice energy, hydration
energy and the entropy change in going from solid to solution, and only a few generalisations are possible. Thus high charge and low ionic radii tend to produce insolubility (for example salts of lithium, beryllium and magnesium, especially those with doubly charged anions such as carbonate CO32-). At the other end of the scale, low charge and large radii also produce low solubility (for example salts of potassium, rubidium and caesium containing large anions such as the tetraphenylborate anion (p. 136). In between, solubility is the rule for all Group I salts, and for most Group II salts containing singly-charged negative ions; for many Group II salts with doublyor triply-charged anions (for example CO32-, SO42-, PO43- ) insolubility is often observed.