Question

The first electron affinity of C, N and O will be of the order:
(A) $N$ < $C$ < $O$
(B) $N$ < $C$ < $O$
(C) $C$ < $O$ < $N$
(D) $O$ < $N$ < C$

Answer 1

Write the electron configuration of carbon, nitrogen and oxygen in their ground state. It is important to understand the term electron affinity and its difference from electronegativity which is also associated with the electrons and size of atoms.

Complete Solution :
The electron affinity of an atom or molecule can be defined as the amount of energy released when an electron is attached or gained by an electrically neutral atom or molecule in gaseous state thus leading to the formation of an anion.
As suggested in the hint we will write the electronic configuration for the elements carbon, nitrogen and oxygen in their respective ground states.
Carbon:
E.C = $1{{s}^{2}}2{{s}^{2}}2{{p}^{2}}$
Nitrogen:
E.C = $1{{s}^{2}}2{{s}^{2}}2{{p}^{3}}$
Oxygen:
E.C = $1{{s}^{2}}2{{s}^{2}}2{{p}^{4}}$

It is observed that the electron affinity increases across a period. This is because there is an increase in nuclear charge which is a deciding factor for calculating electron affinity.
Keeping the above point in mind we can easily say that carbon has least electron affinity and oxygen the most. However, we have to take the electronic configuration into consideration as well.
Nitrogen has a stable half-filled p orbital. This stability reduces the effective nuclear charge. As a result, the electron affinity decreases as well.
Thus, the correct order of first electron affinity of C, N and O will be $N$ < $C$ < $O$.
So, the correct answer is “Option B”.

Note: Do not confuse electron affinity with electron gain enthalpy. Both denote the energy released when an atom gains an electron. However, electron gain enthalpy has the opposite sign of electron affinity. This means that if electron affinity is positive, then the electron gain enthalpy will be negative with the same modulus value.