American Huilin Institute
Fajans suggested the rules to estimate the extent to which a cation could polarize an anion and thus induce covalent character. This Fajans phenomenal happens to be the IC-potential, the ionocovalency, the effective ionic potential (or the effective polarizing power), n*(Iav/R)½rc-1.
The simple form of the ionic potential considered the valence charge of the ion with respect to its size. The valence charge is numerically equal to the number of valence electrons of the ion. In some cases we may consider the effective nuclear charge Z*. For two ions of the same actual nuclear charge, Hg2+ and Ca2+, the Hg2+ has the higher effective nuclear charge Z* (4.490) and the IC (3.118), it is considerably more polarizing and its compounds are considerably more covalent than those of Ca2+ which has the smaller effective nuclear charge Z* (2.807) and the IC (1.617). So we have melting points HgCl2=276۫, CaCl2=772۫. More comparing compounds are listed in Table 9 (below).
[*] Zhang, Y. Ionocovalency and Applications 1. Ionocovalency Model and Orbital Hybrid Scales. Int. J. Mol. Sci. 2010, 11, 4381-4406