# Explain why the first ionisation energy of beryllium is greater than that of boron.

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

## Mark scheme (4 marks)

1. Boron has its outermost electron in a 2p orbital, whereas beryllium has its outermost electron in a 2s orbital.
2. The 2p orbital is at a higher energy (further from the nucleus on average) than the 2s orbital, so the 2p electron is less strongly attracted to the nucleus.
3. The 2p electron in boron is partially shielded by the 2s electrons, reducing the effective nuclear charge experienced by the 2p electron.
4. Therefore less energy is required to remove the outermost electron from boron than from beryllium, giving beryllium a higher first ionisation energy.

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-first-ionisation-energy-45414089) · Published by Druglandscape Ltd.