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

> IB DP Chemistry Higher Level (2023 syllabus) — S3.1 The periodic table: classification of elements · Explain · 4 marks

## Mark scheme (4 marks)

1. Both sulfur and phosphorus are in the same period, so they have the same number of electron shells / similar shielding.
2. Phosphorus has a half-filled 3p sub-level (3p³), which is a particularly stable arrangement.
3. Sulfur has a 3p⁴ configuration in which one 3p orbital contains two paired electrons.
4. Inter-electronic / electron–electron repulsion between the paired electrons in sulfur's 3p orbital makes one electron easier to remove, lowering the first ionisation energy relative to phosphorus.

## 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-0fe82e8d) · Published by Druglandscape Ltd.