Q. The first ionisation energy of oxygen is less than that of nitrogen. Which of the following is the correct reason for this observation?
A) Lesser effective nuclear charge of oxygen than nitrogen
B) Lesser atomic size of oxygen than nitrogen
C) Greater interelectron repulsion between two electrons in the same p-orbital counter balances the increase in effective nuclear charge on moving from nitrogen to oxygen
D) Greater effective nuclear charge of oxygen than nitrogen
Explanation
(c)
The electronic configuration of nitrogen is
_{7}N$= 1s^{2},2s^{2},2p^{3}$
[image:3_image4.emf]
Due to presence of half-filled $p$-orbital, (more stable) a large amount of energy is required to remove an electron from nitrogen. Hence, first ionisation energy of nitrogen is greater than that of oxygen.
The electronic configuration of oxygen is
_{8}O=$1s^{2},2s^{2}2p^{4}$
[image:3_image5.emf]$\$
The other reason for the greater IP of nitrogen is that in oxygen, there is a greater interelectronic repulsion between the electrons present in the same $p$-orbital which counter-balance the increase in effective nuclear charge from nitrogen to oxygen.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.