Trends for electron affinity going across the group increases negatively.

Does this mean Noble gases' electron negativity is more negative than halogens?

I'm confuse here.

Noble gases should have a positive electron affinity as adding an electron would make it unstable and thus, will not form.

But for halogens, their attraction for that 1 electron to fill up the $\mathrm{p}$ subshell is very strong.

But according to the theory that electron affinity increase negatively across the period, does this theory has an exception of noble gases?

Next, what about $\ce{Be}$ and $\ce{Mg}$? Their electron affinities are not negative.


Not exceptions, but see, theories say that going $L→R$ electron affinity increases up to halogens.

For the noble gases, $\Delta_\mathrm{eg}H$ is positive (energy required). While electron affinity is $0$. Which implies no tendency to lose electrons.

For $\ce{Be}$, the small size and nuclear charge are responsible, which is related to $\ce{Mg}$ due to a diagonal relationship.

  • $\begingroup$ Be and Mg are not diagonally related, and in any event diagonal relationships do not work for electron affinities. Mg lacks an exothermic electron affinity because like Be, it has a closed valence shell and an electron would have to be added to a much higher-energy orbital. $\endgroup$ – Oscar Lanzi Oct 19 '20 at 13:02

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