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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (4 marks)

Fluorine’s 2p orbitals are very compact, so when an extra electron is added the inter‑electron repulsion in the 2p subshell is large. This repulsion reduces the energy released, making the electron affinity less exothermic. Chlorine’s 3p orbitals are more diffuse and the atom is larger, so the added electron experiences much less repulsion. Consequently, chlorine releases more energy on gaining an electron, giving it a more exothermic first electron affinity despite being less electronegative.

Examiner tips

  • Mention the small radius/compact 2p orbitals of F and the resulting high electron‑electron repulsion.
  • Explain that larger radius/3p orbitals of Cl reduce repulsion, increasing exothermicity.
  • Use the terms ‘electron affinity’, ‘exothermic’, ‘electron‑electron repulsion’, ‘atomic radius’ and ‘electronegativity’ as in the mark scheme.
  • Keep the answer concise – 4 points only.

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 in this question

electron affinity

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