Electron affinity of nitrogen and exchange energy

When studying the effects of electron filling in chemistry, I was studying why energy is needed to add an electron to a nitrogen atom i.e. electron affinity.

Nitrogen already has an electron structure of $$\ce{1s^2 2s^2 2p^3}$$ and using my knowledge of exchange energy, the energy released as exchange energy is

$$\frac{3 \times 2}{2} = 3K$$

But adding an electron means there is 1 electron $$\ce{1s^2 2s^2 2p^4}$$ with opposite spin to the rest of the other electrons so the total exchange energy is still

$$\frac{3 \times 2}{2} + \frac{1 \times 0}{2} = 3K$$

so why is it unfavourable i.e. endothermic for nitrogen to became a nitrogen anion $$N^-$$?

• @AvnishKabaj He's talking about atomic nitrogen... – Mithoron Dec 31 '18 at 20:51

$$\rm N^{-}$$ has the outer electron structure of $$\rm 2s^2 2p^4$$. This means we have coulombic repulsion in one of the p orbitals which means adding a single electron will be endothermic. This accounts for the decrease in ionization energy between N and O.
When you get to $$\rm N^{3-}$$ we have 6K units of exchange energy which is enough to offset the coulombic repulsion.