Question

According to Fajan’s rule, covalent bond is favoured by:
A. Small cation and large anion
B. Small cation and small anion
C. Large cation and large anion
D. Large cation and small anion

Answer 1

Hint: Fajan’s rule is a set of factors which help us classify whether a bond is covalent or ionic. It depends on the size, charge and electronic configuration of ions such as larger anion tend to have greater covalent character and opposite to cation.

Complete step by step answer:
There are generally two types of bonds, ionic and covalent. But to do that we use Fajan’s rule. Fajan’s rule is a set of factors of ions, cations and anions, which help us decide whether the bond the ions will form is ionic or covalent. Fajan’s rule has following categories:
1. Higher the charge on the cation, greater the chemical bond to be covalent character
2. If the anion size is larger or cation size is smaller, then the chemical bond will be more covalent. This point tells that charge density will increase if the ion size decreases and thus its polarising power will be high.
3. Certain electronic configuration makes the cation more favourable for covalent bond. If the configuration is closer to octet, less is the covalency.
So, according to the above rule, option A with small cation and larger anion favours the covalent bond.
So, the correct answer is “Option A”.

Note: The second point is about the polarising power of the cation or anion. More is the polarising power of cation, more is the tendency to form the covalent bond. When we mean with polarisation, the cation distorts the charge distribution over the anion.