Now it is clear that aqueous Ca+ is energetically unstable with respect to Ca++(aq) and calcium metal, and the analogous conclusion can be reached for any of the other alkaline-earth systems. Examination of the enthalpy changes involved shows clearly that it is the enthalpy of hydration of the dipositive ion that is responsible for its stability in aqueous solution. The rationalization of the occurrence. of the +2 state in the ionic crystals of the alkaline-earth compounds is aimilar to the above argument, and has been given in Section 11.2 In effect, the gain in the crystal-lattice energy obtained by forming a dipositive ion more than compensates for the extra energy reuired to remove the second electron from an alkaline-earth atom