Given that electron affinity is positive, why don't these atoms attract free electrons and "prefer" to exist as anions? Never made sense to me — I could be missing something obvious.
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3$\begingroup$ There are no free electrons, much like there is no such thing as a free lunch. Every electron belongs to some entity, which may be more or less unwilling to share it. $\endgroup$– Ivan NeretinCommented Dec 24, 2015 at 21:30
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Yes, the electron affinity for group 17 elements (group VII by older IUPAC nomenclature) is generally exothermic, so the uptake of an electron should be favourable.
But as Ivan mentioned in a comment, there is no such thing as a free or lonesome electron. Each electron must first be displaced from another atom — ionised; the corresponding thermodynamic property is the ionisation enthalpy. For every element, the ionisation enthalpy is positive, thus it is always unfavourable to displace an electron from a neutral atom. No exceptions.
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$\begingroup$ isn't the first electron affinity of any element (except perhaps noble gases) exothermic? $\endgroup$ Commented Dec 25, 2015 at 3:14
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$\begingroup$ @Dissenter No. Manganese, beryllium and nitrogen and possibly others have endothermic EAs $\endgroup$– JanCommented Dec 25, 2015 at 15:30