JAMB Chemistry 2010 Paper

Concept:Unpaired electrons are electrons that occupy an orbital alone, without a partner of opposite spin.Explanation:Nitrogen has atomic number $7$, meaning it has $7$ electrons in total.The electron configuration of nitrogen is:$$1s^2\,2s^2\,2p^3$$The $1s$ sub-level has $2$ electrons, which are paired.The $2s$ sub-level also has $2$ electrons, which are paired.The $2p$ sub-level has $3$ electrons and contains three orbitals.According to Hund's rule, electrons fill degenerate orbitals singly before pairing up.Thus, the three $2p$ electrons each occupy a separate $p$ orbital.This gives three unpaired electrons in the nitrogen sub-levels.No $s$ electron is unpaired, so the only unpaired electrons are in the $2p$ sub-level.Answer:There are $\mathbf{3}$ unpaired electrons. The correct option is A. 3.