WAEC Chemistry 2023 Paper

Concept:A co-ordinate covalent bond forms when an ion accepts a lone pair of electrons from a donor atom.Explanation:Copper(II) ion, $\mathrm{Cu^{2+}}$, has the electron configuration $[\mathrm{Ar}]\,3d^9$.The $3d$ subshell is not completely filled, so there is a vacant $d$-orbital available.A donor atom, such as a ligand, can donate its lone pair of electrons into this vacant $d$-orbital.By accepting this electron pair, $\mathrm{Cu^{2+}}$ forms a co-ordinate covalent bond.Colour and unpaired electrons relate to other properties, not to accepting an electron pair.The positive charge helps attract electrons, but the vacant $d$-orbital is what allows bond formation.Answer:C. have vacant d-orbital