General approaches to bond energy

The first point to be stressed in any account of metal ion bonding is the great success of simple ionic calculations of bond energies for the IA and IIA metal halides. (At the same time we should note that no covalent model has ever achieved quantitative reliability.) Thus Fig. 2 shows that 

Although the above equation works well for IA cations it fails somewhat for IIA cations, Fig. 3, and much more significantly for IB and IIB cations. A series of modified equations have been proposed in which new terms are added to the ionic energy. 

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