# Explain why the first ionisation energy of phosphorus is greater than that of sulfur, even though sulfur has a higher nuclear charge.

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

## Mark scheme (4 marks)

1. Phosphorus has a half-filled 3p sub-level (3p³) with one electron in each 3p orbital
2. This half-filled arrangement confers extra stability due to the exchange energy / symmetrical electron distribution
3. Sulfur has configuration [Ne]3s²3p⁴, so one 3p orbital contains a pair of electrons
4. Electron–electron repulsion between the paired electrons in sulfur makes it easier to remove one electron, lowering the first ionisation energy relative to phosphorus

## Key terms

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

## 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-f1484356) · Published by Druglandscape Ltd.