Ionic bonds are formed when the transfer of electrons occurs, whereas covalent bonds are formed when sharing of electrons occurs. In several compounds, the bonds formed are not perfectly ionic. After forming ionic bonds, sharing of electrons starts to take place. Therefore, there are different extents of ionic and covalent characters in different ionic compounds. And to measure the degree of covalent character in an ionic bond, Fajan’s rule is used.

Polarisation

The metal loses electrons during the formation of an ionic bond and forms a cation smaller than the parent atom, whereas an anion gains electrons and forms an anion larger than the parent electron. 

• Due to the positive charge on the cation, it attracts the electron on the anion towards itself. 
• And therefore, the spherical shape of anion is distorted and is said to be polarised by the cation.
• Polarisation occurs when there is a covalent character in ionic compounds. The greater the polarising power of cation, the more will be the covalent character. 

Fajan’s rule depends on the behaviour of cation and anion. Let us see them in detail.

Polarising power of cation: 

• Size of cation: If the cation is large, its positive charge will be distributed. And if the cation is small, its positive charge will be concentrated. And the concentrated positive charge has greater power to attract electrons towards itself than the distributed positive charge. Hence, smaller cations have greater polarising power than larger cations.      So, Li+ > Na+ > K+ > Rb+
• Charge of cation: The greater the charge of cation, the greater its tendency to attract electrons, the greater the polarising power.

So, Al3+ > Mg2+ > Na+  

• Pseudo noble gas configuration: Cations with electronic configuration ns2np6nd10 are highly polarising and impart high covalent character to the compound. 

Ex: Zn = 1s22s22p63s23p63d104s2

      Zn2+ = 1s22s22p63s23p63d104s0

Polarisation of anion: 

• Size of anion: In smaller anions, electrons will be attracted towards the nucleus, whereas in larger anions, electrons will be distributed throughout the atom. So, the larger anion will readily share its electrons with the cation and hence imparts covalent character to the molecule.
• Charge of anion: Greater charge of anion means that it has more electrons to share and hence greater the polarisation and the covalent character.

Application of Fajan’s Rule

Fajan’s rule helps in measuring the extent of covalent character, and measuring the covalent character helps in comparing:

1. Solubility: Ionic compounds dissociate in solvents and are completely soluble in water, but as the covalent character increases the solubility of the compound decreases. 
2. Melting Point and Boiling Point: Covalent bonds are weaker than ionic bonds. Therefore as the covalent character increases, it will be easier to break the bonds in the compound. Thus, compounds’ melting and boiling points decrease with the increase in covalent character.
3. Colour Imparting Nature: Greater the covalent character of the compound, more will be colour imparting nature of the compound.

   Conclusion

   After the formation of the ionic compound by transfer of electrons, anion and cation start to share electrons due to polarising, which imparts covalent character to the compound. Due to the covalent nature of ionic compounds, several properties like melting point, boiling point, solubility, etc., changes. We need to know the extent of the covalent character in the compound using Fajan’s rule.