Question

Which of the following is the best explanation for the unusual decrease in first ionization energy moving from nitrogen $\left(N\right)$ to oxygen $\left(O\right)$?

• A. The oxygen atomic radius is smaller than nitrogen, making it easier to remove an electron from oxygen as compare to nitrogen.
• B. The electron being removed from nitrogen is a member of a half-filled sublevel, which is more stable than the partially filled oxygen sublevel, making it easier to remove an electron from oxygen as compared to nitrogen.
• C. The electron being removed from nitrogen is one that occupies a $2p$ orbital singularly, whereas the electron being removed from oxygen is one of pair of electrons within a $2p$ orbital of oxygen. Removing the electron from the paired orbital reduces electron-electron repulsions, thereby is easier to remove compared to the electron of nitrogen.
• D. Oxygen has a greater atomic number of nitrogen, thus a greater effective nuclear charge $\left(Z_{eff}\right)$, which will result in a greater hold on its' electrons, thereby requiring more effort to remove the electron from oxygen as compared to nitrogen.

Answer 1

The correct option is B The electron being removed from nitrogen is a member of a half-filled sublevel, which is more stable than the partially filled oxygen sublevel, making it easier to remove an electron from oxygen as compared to nitrogen.

Electronic configuration of Oxygen is

$1s^{2}2s^{2}2p^4$

Electronic configuration of Nitrogen is

$1s^{2}2s^{2}2p^3$

You must be knowing that an atom containing half filled or fully filled orbitals are very stable. Nitrogen has a half filled $2p$ orbital. So when it is already stable, why would it want to lose an electron upon supplying energy and become unstable? This is why one requires large amounts of energy to ionized the nitrogen atom.

On the other hand, since Oxygen is already unstable relative to Nitrogen, by losing one electron it attains a stable half filled $2p$ orbital. So oxygen undergoes ionization at a relatively lower energy. This is why nitrogen has a higher ionization energy than oxygen.