Question

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
• E. Higher electronegativity of oxygen than Nitrogen

Answer 1

Answer: (C)

The electronic configuration of nitrogen is ${}_{7}N=1s^2,2s^2,2p^4$
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 ${}_{8}O=1s^2,2s^2,2p^4$
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.