Magnesium oxide, bond energy

From observations made during a flame spectra study of the bond dissociation energy of magnesium oxide, Cotton and Jenkins... 

Fig. 4.9 Energies of free cations and of ionic compounds as a function of the oxidation state of the cation. Top Lines represent the ionization energy necessary to form the +1. +2, +3, and + 4 cations of sodium, magnesium, and aluminum. Note that although the ionization energy increases most sharply when a noble gas configuration is broken, isolated cations are always less stable in Itiifher oxidation states. Bottom Lines represent the sum of ionization energy and ionic bonding energy for hypothetical molecules MX, MXj, MXj, and MX in which the interatomic distance, r, has been arbitrarily set at 200 pm. Note that the most stable compounds (identified by arrows) arc NaX, MgXj, and AlXj. (All of the.se molecules will be stabilized additionally to a small extent by the electron affinity of X.)...

In the case of sodium oxide, the solid is held together by electrostatic attractions between 1+ and 2- ions. In the magnesium oxide, the electrostatic attractions are between 2+ and 2-. It takes more energy to break this ionic bonding. 

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