Fajans's Rules

Fajans's rules were given by Kazimierz Fajans in 1923 . He explained the variation of non-polar character in ionic compounds in terms of polarization effect .

Fajans gave the following rules to determine the extent of covalent character induced in an ionic compound :-

1. Induced covalent character in ionic compounds increases with decrease in size of the cation or increase in charge on the cation . This is because very small cations or cations with higher charge have higher charge density which tends to distort or polarize the electron cloud around the anion . As a result the polarization increases due to which induced covalency also increases.

2. Covalent character induced in ionic compound due to polarization of the anion - increases on increase in size and charge on the anion . The electron cloud on the large sized anion is easily polarized by the approaching cation because it (electron cloud) is less strongly held by its nucleous . As a conclusion , we can say that larger is the anion charge , higher is its polarizability .

3. Cations with pseudo noble gas configuration have higher polarizing power than cations with noble gas configuration. Or in another words , we can say that , cations with ns² np⁶ configuration cause less distortion than those with having configuration ns² np⁶ nd¹⁰ . This is because d-electrons shields the nucleous poorly . A cation with 18 electron configuration will lead to higher polarization of the anion .

In Short we can say that - by Fajans's rule , compounds are more likely to be ionic if : there is a small positive charge on the cation , the cation is large and anion is small .

Source :- Textbook and internet .

This was Fajans's Rule . Hope it was helpful to you . 

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