Fajans' Rule - Disclosing Covalent Characteristics in Ionic Bonds

True ionic bond (with 100% ionic characteristics) or covalent bond (with 100% covalent characteristics) is a myth. It is an ideal situation which doesn’t exist. In reality, every bond has some covalent as well as ionic characteristics. In this article, we are going to discuss covalent characteristics in ionic bonds.

What is the Fajan Rule?

Few ionic bonds have partial covalent characteristics which were first discussed by a scientist, Kazimierz Fajans in 1923. In the time with the help of X-ray crystallography, he was able to predict ionic or covalent bonding with the attributes like ionic and atomic radius.

He gave the rule to predict whether a chemical bond will be covalent or ionic which is now commonly known as Fajan’s rule.

Fajan’s rules

The rule can be stated on the basis of 3 factors, which are:

  1. Size of the ion: Smaller the size of cation, the larger the size of the anion, greater is the covalent character of the ionic bond.
  2. The charge of Cation: Greater the charge of cation, greater is the covalent character of the ionic bond.
  3. Electronic configuration: For cations with same charge and size, the one, with (n-1)dnso which is found in transition elements have greater covalent character than the cation with nsnp6 electronic configuration, which is commonly found in alkali or alkaline earth metals.

The covalent character is found in compounds which have high polarizing cation and high polarizable anion.

Fajans’ Rule

Fajans’ Rule

This can be summarized as:

Ionic Characteristic Covalent Characteristic
Large Cation Small Cation
Small Anion Large Anion
Small-charge Large Charge

Explanation through an example

Let us consider AlI3; this is an ionic bond which was formed by transfer of electrons.

  • The iodine being bigger in size has a lesser effective nuclear charge. Thus, the bonding electrons are attracted lesser towards the Iodine nucleus.
  • On the contrary, the aluminum having three positive charges attracts the shared pair of electron towards itself.
  • This leads to insufficient charge separation for it to be ionic and so it results in the development of covalent character in AlI3.

Let us consider AlF3, this is an ionic bond which was also formed by transfer of electron.

  • But here the fluorine being smaller in size attracts the shared pair of electron more towards itself and so there is sufficient charge separation to make it ionic.

But as Pauling brought forward the idea of Electronegativity, Fajans’ rules have been displaced. For more information about Fajans’ rule and more topics, register with BYJU’S.

Practise This Question

Enzyme-catalysed reactions have the order