# Explain why the third ionisation energy of magnesium is significantly greater than its second ionisation energy, whilst the successive ionisation energies from the first to the second are relatively close together.

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

## Mark scheme (4 marks)

1. The first and second ionisation energies involve removal of electrons from the same outer (3s) subshell/shell, so the increase is relatively small.
2. The third electron must be removed from a full inner shell (2p subshell / n = 2 shell), which is closer to the nucleus.
3. Electrons in a lower principal quantum level (n = 2) experience less shielding from inner electrons / are more strongly attracted to the nucleus.
4. Therefore significantly more energy is required to remove the third electron, causing the large jump in ionisation energy.

## Key terms

- [ionisation energy](https://www.gradenine.co.uk/glossary/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-third-ionisation-energy-ee0e7bc7) · Published by Druglandscape Ltd.