Question

The ionisation energy of nitrogen is larger than that of oxygen because of

• A. greater attraction of electrons by the nucleus
• B. the size of nitrogen atom being smaller
• C. the half-filled p-orbitals possess extra stability
• D. greater penetration effect

Answer 1

Answer: (C)

The ionisation energy of nitrogen is larger than that of oxygen because nitrogen has half-filled orbitals and thus, a more stable configuration.
$N(7)=1s^2,2s^2,2p_x^1,2p_y^2,2p_z^1$
(half-filled, thus stable)
$O(8)=1s^2,2s^2,2p_x^2,2p_y^1,2p_z^1$
(less stable)
Exactly half-filled and completely filled configuration have extra stability.