This rule was given by a famous scientist named Kazimierz Fajans. In Fajan’s rule, we find that the chemical bond is covalent or ionic based on the size of the cation or anion. Ionic bonds will have a low positive charge and large cations. Covalent is the opposite of Ionic. Covalent bonds have a high positive charge and small cations.

What is the Fajans’ Law?

In Fajans’ law, we must detect whether chemical bonds are converted into cations or anions. Fajans used various techniques to detect ionic or covalent bonds like x-ray and crystallography techniques.

For example, Aluminium Iodide shows a covalent character, but it’s ionic.

Postulates of Fajans’ Rule

There are three postulates of Fajans’ Rule.

• The “Size of ion:” If the size of the cation is smaller then the size of the anion is larger.
• The “Charge of the Cation”: If the charge of cation is greater, then it shows a more covalent character.
• The “Electronic Configuration”: This explains the greater covalent character of transition elements.

Polarisation

 In an ionic molecule, when two oppositely charged ions come closer to each other, the cation attracts the electron cloud of the anion towards itself. This process is called the polarisation of the anion.

Factors suggested by Fajans

1. a) Size of the Cation: Smaller the size of the cation, the more strongly it attracts the outermost electrons of the anion. This means the polarising power depends on the size of the element. If the size is small, then it will have more polarising power.
2. b) Size of the Anion: If the size of the anion is larger, then cation will be attracted by the outermost electron more quickly.
3. c) Charge on the Cation: If we have two elements, one has charge +1 and another one +2, since +2 charge has more polarising power,  it will attract the electron cloud of the anion more easily.

This means the more significant the charge on cation, the more strongly it attracts the electron cloud of the anion.

1. d) Charge on the Anion: If we have two elements, Carbon and Fluorine. Carbon has a -4 charge, and Fluorine has a -1 charge. Carbon has a larger size than Fluorine. Due to this, carbon polarising power is high and it will easily be attracted by the outer shell electron cloud and then attracted by cation. 

Polarising Power / Polarisability

1) Polarising power: The word “Polarise” means to divide into completely opposing groups. Polarising power is used to polarise cation and anion. It depends on the charge density. The polarising power is also greater if the charge density is more significant. Cation is slightly polarised in the presence of anion.

2) Polarisability: The word Polarisability means the tendency of anion to get distorted/polarised by the cation. This leads to polarisation.

Consequences of Polarisation

1. Ionic Compounds are soluble in non-polar solvents. 
2. The melting point of the ionic compound is less.
3. The stability of Anion is achieved.

Detailed Illustration of Fajans’ Rule.

1) Aluminium Iodide: It’s the Ionic bond in which the transfer of electrons takes place. Iodine is more significant in size but has a lesser nuclear charge. So because of its charge, electrons will be less attracted to it. Aluminium has more charge than Iodine so electrons will be attracted towards it.

2) Carbon Fluoride: In this Ionic bond, Fluorine is smaller, so it will attract more shared electrons towards itself.

Conclusion

Fajan has given some rules regarding the presence of the covalent characters in the ionic bonds or ionic compounds. These are known as Fajans’ rules. He also has given some postulates like the effect of size of cation or anion on the covalent characters etc. In Fajans’ rule, we find whether the element is ionic or covalent. No compound is 100% Ionic or 100% covalent. They both are relative terms. Polarising power of cation and polarizability are two factors that govern Fajans’ rule.