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

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

## Mark scheme (4 marks)

1. Magnesium has a greater nuclear charge than sodium, so the outer electron in magnesium is attracted more strongly to the nucleus.
2. Sodium and magnesium have the same number of inner shells / shielding, so the effective nuclear charge experienced by the outer electron is greater in magnesium.
3. Aluminium's outer electron is in a 3p subshell, which is at higher energy / further from the nucleus than magnesium's 3s subshell.
4. Therefore, less energy is required to remove the 3p electron from aluminium than to remove a 3s electron from magnesium.

## 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-66f36ba5) · Published by Druglandscape Ltd.