# Explain why the first ionisation energy of oxygen is lower than that of nitrogen, even though oxygen has a higher nuclear charge.

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

## Mark scheme (4 marks)

1. Nitrogen has a half-filled 2p sub-level (one electron in each of the three 2p orbitals), which is a particularly stable arrangement.
2. This half-filled arrangement gives nitrogen extra stability, so more energy is required to remove an electron from nitrogen than expected.
3. In oxygen, the fourth 2p electron must pair up in an already occupied orbital, causing electron–electron repulsion.
4. This repulsion between paired electrons in oxygen makes it easier to remove one of them, lowering oxygen's first ionisation energy relative to nitrogen.

## Key terms

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

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-first-ionisation-energy-e22cf399) · Published by Druglandscape Ltd.