Read Why Chlorine Beats Fluorine for Electron Affinity
Electron gain enthalpy mostly rises across a period and falls down a group like other periodic properties — but chlorine (not fluorine) has the most negative value in its group, a genuine, frequently-tested exception worth memorising directly.
Electron Affinity and Electron Gain Enthalpy Trends is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Chlorine beats fluorine for electron affinity
Fluorine's very small atomic size means the incoming electron experiences unusually strong electron-electron repulsion in its already-compact 2p subshell, partially offsetting the expected trend — chlorine, one size larger, accepts an electron more exothermically (more negative electron gain enthalpy) than fluorine, making Cl the true maximum in its group, not F.
Second electron gain enthalpy is always positive (endothermic)
Adding a SECOND electron to an already-negative ion (O- → O2-, forming an anion from an anion) always requires energy input, since the incoming electron is repelled by the existing negative charge — this is a universal rule for any second electron-gain step, regardless of which element is involved.
Matching electron configuration to electron gain enthalpy value
A fully-filled configuration (like Ne's 1s2 2s2 2p6) has a POSITIVE (unfavourable) electron gain enthalpy, since the incoming electron must enter a new, higher-energy shell; a configuration one electron short of a stable noble-gas or half-filled state (like F's 2p5) has the most negative (favourable) value — matching table-style questions reduce to recognising which configuration is 'one electron away from stable' versus 'already stable.'
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- ✓Cl, not F, has the most negative electron gain enthalpy in its group — small atomic size causes extra electron-electron repulsion that partially offsets F's expected value.
- ✓Second electron gain enthalpy (forming a 2- ion from a 1- ion) is always positive (endothermic), regardless of element.
- ✓A fully-filled configuration accepting an electron is strongly unfavourable (positive ΔHeg); a configuration one electron short of stable is strongly favourable (very negative ΔHeg).
Don't assume fluorine has the most negative electron gain enthalpy in its group just because it's the most electronegative — chlorine actually has the more negative (more favourable) value, due to F's unusually strong electron-electron repulsion in its small 2p subshell.
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