WAEC Chemistry 2021 Paper

Concept:A coordinate covalent bond forms when one atom donates a lone pair to another atom that has an empty orbital.Explanation:In $\mathrm{NH}_3$, nitrogen has a lone pair of electrons. In $\mathrm{BCl}_3$, boron is electron-deficient and has an empty orbital. So, nitrogen in $\mathrm{NH}_3$ donates its lone pair to boron in $\mathrm{BCl}_3$. This forms a coordinate bond: $\mathrm{H_3N \rightarrow BCl_3}$. The other options do not have one donor with a lone pair and one acceptor with an empty orbital.Answer:C. $\mathrm{BCl}_3$ and $\mathrm{NH}_3$.