# Explain why the first ionisation energy of sodium is endothermic and why it is much lower in magnitude than the first ionisation energy of chlorine.

> IB DP Chemistry Standard Level (2023 syllabus) — R1.2 Energy cycles in reactions · Explain · 4 marks

## Mark scheme (4 marks)

1. Ionisation energy is endothermic because energy must be supplied to overcome the electrostatic attraction between the nucleus and the outer electron.
2. Sodium has a larger atomic/ionic radius than chlorine, so the outer electron is further from the nucleus and experiences less attraction.
3. Sodium has greater shielding/screening of the outer electron by inner electron shells compared to the effective nuclear charge experienced by chlorine's outer electron.
4. Chlorine has a higher nuclear charge (17 protons vs 11 in sodium) with a similar level of inner-shell shielding, resulting in a greater effective nuclear charge and therefore stronger attraction on the outer electron.

## Key terms

- [first ionisation energy](https://www.gradenine.co.uk/glossary/first-ionisation-energy)
- [endothermic](https://www.gradenine.co.uk/glossary/endothermic)

## 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)
- [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-fd82cdef) · Published by Druglandscape Ltd.