Q. The first ionisation energy of oxygen is less than that of nitrogen. Which of the following is the correct reason for this observation?
Solution:
The electronic configuration of nitrogen is $ _{7}N=1{{s}^{2}},2{{s}^{2}},2{{p}^{4}} $
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 ionization energy of nitrogen is greater than that of oxygen. The electronic configuration of oxygen is $ _{8}O=1{{s}^{2}},2{{s}^{2}},2{{p}^{4}} $
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.
