It is generally true that the electron affinity becomes less exothermic down a group, because of the increase in atomic radius.

There is a well-known exception that the electron affinity of Cl is more exothermic than that of F, because fluorine has a high electron density; adding electrons would be unfavorable because of the electron-electron repulsions.

In Group 2 and Group 5, this is more extreme: the electron affinity becomes more exothermic down the group. Why is this so?

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  • $\begingroup$ I can tell why it's positive for Be, Mg and N, which is quite clear. I can't explain the "trend"... $\endgroup$ – Zenix Mar 4 '20 at 16:18
  • $\begingroup$ There is no clear tendency anywhere. The only column where the electron affinity is regularly modified the way you say when going down the column is the first one. $\endgroup$ – Maurice Mar 4 '20 at 17:13

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