# Explain why the first ionisation energy of oxygen is lower than that of nitrogen, despite oxygen having a greater nuclear charge.

> IB DP Chemistry Higher Level (2023 syllabus) — S1.3 Electron configurations · Explain · 4 marks

## Mark scheme (4 marks)

1. Nitrogen has a half-filled 2p subshell (one electron in each 2p orbital), which is a particularly stable arrangement.
2. In oxygen, two electrons must occupy the same 2p orbital, resulting in electron–electron repulsion between the paired electrons.
3. This repulsion makes it easier to remove one of the paired electrons from oxygen, lowering its first ionisation energy relative to nitrogen.
4. The effect of electron–electron repulsion in oxygen outweighs the effect of the increased nuclear charge, so oxygen's first ionisation energy is lower than nitrogen's.

## Key terms

- [first ionisation energy](https://www.gradenine.co.uk/glossary/first-ionisation-energy)
- [nuclear charge](https://www.gradenine.co.uk/glossary/nuclear-charge)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-first-ionisation-energy-019b0077) · Published by Druglandscape Ltd.