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Why do the noble gases not follow any trend in electron affinity. I understand that they would require energy to add an electron to an already stable octet (adding to a new shell), but shouldn't a trend WITHIN the gases still hold?
I think that a trend still does hold true for the noble gases. You are right that it would require energy put into the system in order to attach another electron because the atoms already have a full octet. All of the halogens as well as the elements Beryllium and Magnesium have electron affinities above zero for this particular reason. The trend in the electron affinities of the halogens is that as you go down group 18, the energy that you have to put in to attach an electron to the atom increases. It makes sense that the increase in energy is from the increase in electrons and the increasing in shielding that these electrons provide.
The trend in electron affinity is best applied to elements that do not have a full octet and thus benefit from additional electrons that allow them to be more stable. Thus the halogens have extremely high electron affinity because the only require one more electron to fulfill their octet, but the trend doesn't continue for the noble gases that have an extremely low electron affinity.
I read in another discussion topic that the trends don't really apply to the noble gases because they are stable, therefore they are much less reactive than the atoms in other groups and don't need to follow the same trends.
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