Postulates of Fajans’ Rule

Postulates of Fajans’ Rule

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

(1)Size of the ion: Smaller the size of the cation, the larger the size of the anion and the greater the covalent character of the ionic bond.

(2)The charge of cation: Greater the charge of the cation, the greater the covalent character of the ionic bond.

(3)Electronic configuration: For cations with the same charge and size, the one with (n-1)dn nso which is found in transition elements, has greater covalent character than the cation with ns2 np6 electronic configuration, which is commonly found in alkali or alkaline earth metals.

Explanation of Fazans’ Rule

Rule 1:

The first rule speaks about the polarising power of the cation. If the cation is smaller, then we can say that the volume of the ion is less. If the volume is less, we can conclude that the charge density of the ion would be high.

Since the charge density is high, the polarising power of the ion would be high. This makes the compound to be more covalent.

Rule 2:

The second rule speaks about the polarizability of the anion. The larger the anion, the less the effective nuclear charge that holds the valence electron of the ion in place. Since the last electron is loosely bound in large anions, it can easily be polarised by a cation, thereby making the compound more covalent.

Rule 3:

The third rule is a special case. Let us use an example to explain this point.

Example: If we want to find the more covalent compound among HgCl2 and Calcium Chloride, we cannot use size as a factor to conclude. This is because both Hg2+ and Ca2+ are of almost equal size. To explain this, we employ the third rule.

The electronic configuration of Hg2+ is 6s0 5d10. This configuration is called pseudo-octet because the d-orbital is fully filled, but the element does not have 8 electrons or an octet.

We know that d orbitals are not good at shielding, so we can say that the anion (Cl–) would be more polarised because the d orbital is poor at shielding, making HgCl2 more covalent than CaCl2 because Ca2+ ion has a noble gas configuration.