# Explain why the electron affinity (first) of chlorine is more exothermic than that of fluorine, despite fluorine being more electronegative.

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

> Electron affinity is defined as the enthalpy change when one mole of gaseous atoms each gains one electron to form a mole of gaseous anions. For the halogens, the first electron affinity of fluorine is −328 kJ mol⁻¹ and that of chlorine is −349 kJ mol⁻¹.

## Mark scheme (4 marks)

1. Fluorine has a very small atomic radius / the 2p orbitals are very compact, leading to high electron–electron repulsion in the 2p subshell when an extra electron is added.
2. This inter-electron repulsion in the small 2p subshell of fluorine reduces the energy released when an electron is added, making the process less exothermic than expected.
3. Chlorine has a larger atomic radius / uses 3p orbitals that are more diffuse / spatially spread out, so inter-electron repulsion when gaining an electron is much less significant.
4. Therefore, even though fluorine is more electronegative (stronger nuclear attraction per electron), the anomalously high repulsion in its small 2p orbitals means chlorine overall releases more energy on gaining an electron, giving it the more exothermic electron affinity.

## Key terms

- [electron affinity](https://www.gradenine.co.uk/glossary/electron-affinity)

## 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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