exeptions in fajan factors

The polarising power, or polarisability and hence formation of covalent bond is favoured by the following factors:

Small Positive Ion (Cation):Due to greater concentration of positive charge on a small area, the smaller cation has high polarising power. This explains why LiCl is more covalent than KCl.

Large Negative Ion (Anion):The larger the anion, the greater is its polarisability, i.e. susceptibility to get polarised. It is due to the fact that the outer electrons of a large anion are loosely held and hence can be more easily pulled out by the cation. This explains why iodides, among halides, are most covalent in nature.

Large Charge on Either of the Two Ions:As the charge on the ion increases, the electrostatic attraction of the cation for the outer electrons of the anion also increases, with the result its ability for forming the covalent bond increases. Thus covalency increases in the order : Na+Cl-, Mg2+(Cl2)2-, Al3+(Cl3)3 -

Electronic Configuration of the Cation : For the two ions of the same size and charge, one with a pseudo noble gas configuration (i.e., 18 electrons in outer-most shell) than a cation with noble gas configuration (i.e. 8 electrons in outermost shell) will be more polarising. Thus copper (I) chloride is more covalent than sodium chloride although Cu+ion (0.96A°) and Na+ ion (0.95A°) have same size and charge.