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Fajans Rule

Fajans Rule in chemistry helps to determine whether a chemical bond is covalent or ionic. This depends on the charge of the cation and the relative sizes of the anions and cations.

These rules were formulated by Kazimierz Fajans in 1923.

We can summarize Fajans rule in the table given below:

Ionic CharacterCovalent Character 
Large Cation Small Cation 
Small Anion Large Anion 
Small Charge Large Charge 

What is Fajans Rule

Before we get to know about Fajan’s principles, let us be comfortable with the accompanying terms:

Polarizing Power: It is the extent to which the cation can polarize an anion. 

Polarizability: It is the extent to which an ion can be polarized.

Due to polarization, sharing of electrons occurs between two ions to some extent and the bond shows covalent character.

Postulates of Fajans Rule

3 factors on which these rules can be stated are

  1. Size of the Ion: Smaller the size of the cation, the larger the size of the anion, and hence greater is the covalent character of the ionic bond.
  2. The Charge of the Cation: Greater the charge of cation, greater is the covalent character of the ionic bond.
  3. Electronic Configuration: It has been found that, cations with pseudo inert gas configuration, such as, ns2 p6 d10, or with inert pair configuration, such as, (n-1)d10 ns2, have high polarizing power, while cations with noble gas configuration, such as, ns2 np6, have low polarizing power.
Polarization of Anion by Cation - Fajans Rule
Polarization of Anion by Cation

Fajans Rules and Explanation

Rule 1: It tells us about the polarizing power of the cation. When the cation is smaller, we can say that the volume of the ion is less.
If the volume is less we can say that the charge density of the ion would be high. Since the charge density of the ion would be high, this makes the compound to be more covalent.

Rule 2: This tells us about the polarizability of the anion. If the anion is larger, then the effective nuclear charge that holds the valence electrons of the ions in place, is less.
As in large anions the last electron is loosely bound, it can be easily polarized by the cation, and hence making the compound more covalent.

Rule 3: We can explain this with the help of an example.

Let us take two compounds CaCl2 and HgCl2. We cannot use size as a factor to find out which  of these two compounds is more covalent, because both the cations (Hg+2 and ca+2) of these compounds are almost same in size. So, to explain this, the third rule comes into play.

The electronic configuration of Hg+2 is 6s0 5d10 . This configuration is Pseudo octet.

As d orbitals are not good at shielding so we can say that the anion( Cl) would be more polarized due to the poor shielding effect of the d-orbital. This makes HgCl2  more covalent than CaCl2 as the Ca+2 ion has a noble gas configuration.

Detailed Illustration

Let us look into a detailed illustration with some examples to understand Fajans Rule.

Consider the elements NaF, NaCl, NaBr and NaI.

Let us arrange them in the increasing order of covalency;

Since the cation(Na+) is the same in all the compounds, let us compare the anions. In anions larger the size more the covalency.

So the order would be NaF < NaCl < NaBr < NaI.

FAQs

Who gave Fajans rule?

This was introduced by Polish-born US chemist Kazimierz Fajans in 1923.

What is Fajans rule in chemistry?

Fajans rule states that a compound with low positive charge, large cation and small anion has ionic bond whereas a compound with high positive charge, small cation and large anion are covalently bonded.

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