# Explain why the first ionisation energy of magnesium is greater than that of aluminium.

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

## Mark scheme (4 marks)

1. Magnesium's outer electron is removed from a 3s subshell/orbital, whereas aluminium's outer electron is removed from a 3p subshell/orbital.
2. The 3p subshell is higher in energy / further from the nucleus than the 3s subshell, so the 3p electron in aluminium experiences greater shielding (from the 3s electrons).
3. Therefore the effective nuclear charge experienced by the outermost electron is lower in aluminium than in magnesium.
4. Consequently, less energy is required to remove the outermost electron from aluminium than from magnesium, so Al has a lower first ionisation energy.

## 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)
- [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-27e60f64) · Published by Druglandscape Ltd.