Question

Why is the first ionisation energy of oxygen lower than that of nitrogen?

Answer 1

1. Nitrogen has a half-filled 2p subshell (2p³), which is more stable and symmetrical.
2. Oxygen has one more electron (2p⁴), causing electron-electron repulsion in the same orbital.
3. This repulsion in oxygen makes it easier to remove an electron, lowering its ionisation energy.
4. Therefore, despite oxygen having a higher nuclear charge, its first ionisation energy is lower than nitrogen’s because nitrogen’s half-filled configuration is extra stable.

This explains the anomaly in the expected trend of ionisation energies between these two elements.