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Here is another post that discuss about co-Fajan-s-Rulevalency in ionic compound

Fajan's Rule by Jim Livingston on Scribd

Fajan’s rule

Fajans’ rule predicts whether a chemical bond will be covalent or ionic. A few ionic bonds have partial covalent characteristics which were first discussed by Kazimierz Fajans in 1923. 

Postulates of Fajans’ Rule

The rule can be stated on the basis of three factors, which are:

1. Size of the ion: Smaller the size of cation, the larger the size of the anion, greater is the covalent character of the ionic bond.
2. The charge of Cation: Greater the charge of cation, greater is the covalent character of the ionic bond.
3. Electronic configuration: For cations with same charge and size, the one, with (n-1)dn nso which is found in transition elements have greater covalent character than the cation with ns2 np6 electronic configuration, which is commonly found in alkali or alkaline earth metals.

Explanation of Fajans’ Rule

Here is the explanation of Fajans’ rules:
Rule 1: It is about the polarising power of the cation. When the cation is smaller, the charge density of the ion will be high and the polarizing power of the ion will be high this makes the compound to be more covalent.

Rule 2: The second rule is about the polarizability of the anion. Since in the large aions the electron is loosely bonded, it can easily be polarized by an anion. It can easily be polarized by a cation thereby making the compound more and more covalent.  

Rule 3: If we want to find the more covalent compound among HgCl2 and Calcium Chloride we cannot use size as a factor to conclude. This is because both Hg2+ and Ca2+ are of almost equal size. To explain this, we employ the third rule. The electronic configuration of Hg2+ is 6s0 5d10. This configuration is called pseudo-octet because d-orbital is fully filled, but the element does not have 8 electrons or an octet. We know that d orbitals are not good at shielding, so we can say that the anion (Cl–) would be more polarised because the d orbital is poor at shielding making HgCl2 more covalent than CaCl2 because Ca2+ ion has a noble gas configuration.