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

Answer 1

Answer: (C)

The electronic configuration of nitrogen is
${}_{7}N=1s^2,2s^2,2p^3$

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
${}_{8}O=1s^2,2s^{2}2p^4$

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 -orbital which counterbalance the increase in effective nuclear charge from nitrogen to oxygen