I often see that the most stable monoatomic ion of nitrogen is N$^{3-}$ (for example on Khan Academy), and I remember being taught something similar, along the lines of atoms wanting complete octets.
However, the $1^\text{st}$, $2^\text{nd}$, and $3^\text{rd}$ electron affinities of nitrogen are, according to [1]:
Nitrogen (kJ/mol)
$-7$, $+673$, $+1070$
The positive (endothermic) $2^\text{nd}$ electron affinity implies that the anions with negative charge greater than one are inherently unstable, in the sense that if I bring an electron near an N$^-$ in the vacuum of outer space, then they would not want to bind. And if I had an N$^{2-}$ atom, out in space, then it would spontaneously decompose to N$^-$ and an electron.
My interpretation of "most stable ion" is that it should not involve any stabilising complexation with a (variable) cationic component.
Is N$^{3-}$ the most stable monoatomic ion? Is there some additional completing octet-related favourability that compensates? I would have thought that if there were, that that would turn up in the energy measurement. If I am tutoring people in chemistry, is there a better way to describe the drive of nitrogen to complete its octet, or should the way we teach the non-metals be revamped by actually looking at the electron affinities for each? (Oxygen for example, has negative both first and second electron affinities.)