The potential energy curve of a spherical ion pair

Comparison of the force constants in Tables 4.1 and 5.1 shows that even though the ionic bonds in MX(g) are stronger and relatively shorter than the covalent bonds in both M2(g) and X2(g), the force constants are smaller than in the dihalogens ionic bonds are short and strong, but soft  

We shall now return to the spherical ion model and use it to construct the potential energy curve of an ion pair. 

When the separated ions are formed from the separated atoms M(g)+X(g) M+(g) + X-(g), 

At this point the energy of the ion pair is exactly equal to the energy of the separated atoms. For NaCl this occurs at a distance of about 1000 pm. See Fig. 5.2. 

Problem 5.3 Calculate the distance at which equation (5.8) is satisfied for NaCl. For which alkali metal halide is the distance the shortest, and for which is the distance the longest Calculate these distances. 

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