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 $N^-$?


$ \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.

You see this effect when considering the electron configurations of the transition elements.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.