If the electron affinity for a neutral element is positive (i.e. endothermic process), is the anion more stable than the neutral atom (at STP)?

Question

This might be a bad question, but I have to ask because I'm finding conflicting answers online. Let's say that I have the following reaction: $$O + e^-\rightarrow O^-$$ While I understand that the electron affinity value for this process is positive (i.e. this is an exothermic process), I can't believe that the $O^-$ ion would be more stable than $O$ since $O^-$ not only assumes the electron configuration of a halogen, but now it is also negatively charged. Instead, I am more keen on believing that $O^-$ is more stable than $O$ plus a free electron, but, I don't know if this implies that $O^-$ is more stable than $O$.

By the same token though, if I consider the reaction: $$Cl + e^-\rightarrow Cl^-$$ It only seems logical to me that $Cl^-$ should be more stable than $Cl$ since a noble gas configuration has been achieved. And yet, I am having a hard time believing that an anion can be more stable than a neutral atom (in a condition where it isn't being supported by other intereactions like hydrogen bonding in water).

This is probably not the most well worded question, but if someone could help clear my confusion, I would really appreciate it! Also, is there a way to calculate just the free energy of the neutral $O$ atom and the charged $O^-$ anion (that is, not the net energy of the whole electron addition reaction but just of each of the reactants and products individually)? Thanks!

Answer 1

Stability is resistance to spontaneous change by itself, or in context of the given default environment.

Reactivity is thermodynamic or kinetic tendency of substance to react with particular explicit reactants or their groups.

Gaseous fluorine is perfectly stable, but extremely reactive. Peroxyacetone is not so much reactive, but very unstable.

If isolated in high degree of vacuum, $\ce{Cl}$, $\ce{Cl-}$, $\ce{O}$, $\ce{O-}$- are stable.

But $\ce{Cl}$ and $\ce{O}$ manifest thermodynamic and kinetic reactivity toward free electrons, forming above ions, or toward their counterpart to form molecules $\ce{Cl2}$ and $\ce{O2}$, or $\ce{ClO}$.