Postulates of Fajans’ Rule

Fajans’ rule is used to determine the nature of the ionic or the covalent bond. This depends upon the relative sizes of the anions and cations and the charge on the cations. 

Fajans’ rule can be summarised as follows:

Ionic Character Covalent Character

 Large Cation  ~ Small Cation 

 Small Anion  ~ Large Anion

 Small Charge  ~ Large Charge 

It has been observed that sometimes ionic bonds show some characteristics of a covalent bond, and sometimes covalent bonds also show some characteristics of an ionic bond. 

Fajans’ rule is associated with the covalent characteristics in the ionic bonds or the ionic compounds. Experimentally, it has been observed that polar covalent bonds are more stable than pure covalent bonds or pure ionic bonds. The greater the degree of ionic polarisation, the greater the stability of polar covalent bonds. 

The extent of polarisation depends upon the cation’s polarising power and the polarisability of the anion. The rules regarding polarisation are known as Fajans’ rule. 

Postulates of Fajans’ Rule with examples

Fajan has given some postulates or conditions to explain the gradual increment of covalent characters in an ionic bond or ionic compound. The main postulates given by Fajan are as follows:

Postulate I: Size of the cations

The smaller the size of the cation, the greater the polarising power. This is because they cause polarisation of electron charge clouds of an anion. Therefore, with the decrease in the size of the cation, the covalent character increases for a particular anion of the concerned compounds. 

For example, in group 2 alkaline metal elements, the cationic size gradually increases while going from top to bottom. Hence, the covalent character of these metals decreases on moving from top to bottom.

Postulate II: The size of the anions

The covalent character of an ionic compound directly depends on the size of the anion. The anions of larger size have high polarizability, so a cation can easily deform its electron charge cloud. Therefore, with the increase in the size of the anion, the covalent character also increases in the related compound. 

For example, while observing the elements from top to bottom in the groups, the size of the halide ions increases gradually. Hence, while coming from top to bottom, the covalent character of the halides of halogens also increases. The covalent characteristics of the different halides of calcium increase from fluorine (F-) to Iodine (I-). 

Postulate III: Charge of a cation or an anion or both

With the enhancement in the extent of charge on a cation or an anion, the degree of polarisation also improves. So, the anion’s degree of polarisation by a cation enhances with the enhancement in the charge extent on a cation or an anion or both.

The electrostatic forces causing the polarisation increase with the increase in the charges on the cations or anions. With an increment in the charge on anions or cations, the covalent characteristics of the related compound increase. 

For example, AlCl3 shows way more covalent characters than MgCl2. MgCl2 is also more covalent than NaCl.

Postulate IV: The number of electrons in the outermost inert shell of the cations –

It has been observed that the cations with a pseudo inert gas configuration [ ns2 p6 d10 ] or with an inert pair configuration [ (n-1)d10 ns2 ] have a higher polarising power, while the cations with a noble gas configuration [ns² np⁶] have lower polarising power. 

That is because of the greater effective nuclear charge (lower shielding effect of the f-orbital) in the case of pseudo inert gas configuration or an inert pair configuration and the smaller effective nuclear charge in the case of noble gas configuration. 

For example, chloride compounds of Cu (copper), Ag (silver), and Au (gold) [with an electronic configuration of ns2 p6 d10 ] are more covalent than chloride compounds of Na (sodium), K (potassium) [ with an electronic configuration of ns² np⁶ ]. 

Postulate V: The dielectric constant of the medium

In a polar medium, the value of the dielectric constant of that medium is high. So, due to the high-value dielectric constant in a polar medium, the polarisation doesn’t happen. But, if the medium is non-polar, then polarisation can occur with ease due to the lower value of the dielectric constant. So, therefore, the covalent character of an ionic compound increases if the medium is non-polar. 

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. The greater the polarising power of a compound, the greater the increase of covalent character in that ionic compound. The covalent character of an ionic compound also depends upon the dielectric constant of the medium as a non-polar medium, the covalent character of the ionic compound increases.