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Since fluorine has its valence electrons in the n=2 energy level, and since chlorine has its valence electrons in the n=3 energy level, one would initially expect that an electron rushing towards fluorine would release more energy, as it would land in the n=2 energy level, whereas in chlorine, the electron would land only in the n=3 energy level, and would then not release as much energy. Thus, one would expect fluorine to have a greater electron affinity than chlorine. However, why is it that chlorine has a higher electron affinity (349 kJ/mol) than fluorine (328.165 kJ/mol)?

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Fluorine, though higher than chlorine in the periodic table, has a very small atomic size. This makes the fluoride anion so formed unstable (highly reactive) due to a very high charge/mass ratio. Also, fluorine has no d-orbitals, which limits its atomic size. As a result, fluorine has an electron affinity less than that of chlorine.

See this.

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    $\begingroup$ I don't think d orbitals contribute to atomic size, since they're always only populated in an internal shell. Calculated atomic radii decrease continuously across a period, and they decrease a little slower in the d-block because the inner d orbitals shield the valence s/p electrons from the nuclear charge better than adding electrons to the outer shell. If anything, the presence of a populated d subshell actually helps contract the atom, allowing it to reach a higher effective nuclear charge than expected in a period with no d subshell. $\endgroup$ – Nicolau Saker Neto Nov 12 '13 at 11:22
  • $\begingroup$ ....but chlorine doesn't have any d-orbitals either? $\endgroup$ – user3932000 Oct 29 '16 at 16:49
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    $\begingroup$ @user3932000 Chlorine have vacant d-orbitals. Since empty orbitals are omitted when writing electronic configuration, it is written as $1s^2 2s^2 2p^6 3s^2 3p^5$. Complete electronic configuration would be $1s^2 2s^2 2p^6 3s^2 3p^5 3d^0$. $\endgroup$ – ashu Nov 6 '16 at 14:42
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    $\begingroup$ @ashu You could say the same for fluorine and say fluorine also has vacant d-orbitals, since its configuration would then be 1s2 2s2 2p5 3s0 3p0 4s0 3d0. $\endgroup$ – user3932000 Nov 6 '16 at 17:39
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    $\begingroup$ @ashu There is absolutely no reason to consider vacant d orbitals in partly filled electron shells. You don’t consider f orbitals for bromine either. In fact, while this answer does start off well I have to give it a $-1$ for invoking a concept that does not help and is introduced incorrectly. $\endgroup$ – Jan Sep 29 '17 at 8:52
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The electron being gained by fluorine would be taken in to a much smaller 2p orbital and requires more electron coupling energy than that of much larger 3p orbital of chlorine. Therefore, energy released during the electron gaining process of fluorine is less than that of chlorine.

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