WAEC Chemistry 2023 Paper

Concept:The number of unpaired electrons is found from the electron configuration of the ion using Hund's rule.Explanation:The atomic number of iron is 26, so a neutral $\mathrm{Fe}$ atom has 26 electrons.Its configuration is $[\mathrm{Ar}]3d^6 4s^2$.When $\mathrm{Fe}$ loses 3 electrons, it first loses the two $4s$ electrons and then one $3d$ electron.So the configuration of $\mathrm{Fe}^{3+}$ becomes $[\mathrm{Ar}]3d^5$.Hund's rule states that electrons occupy degenerate orbitals singly before pairing.Thus, all five $3d$ electrons stay unpaired.Therefore, $\mathrm{Fe}^{3+}$ contains 5 unpaired electrons.Answer:5 unpaired electrons are present in $\mathrm{Fe}^{3+}$, so the correct option is D.