WAEC Chemistry 2016 Paper

Concept:Coloured ions in aqueous solution are generally produced by transition metals that have partially filled $d$-orbitals.Explanation:The $s$-block elements calcium, magnesium, and sodium form ions with noble gas configurations.Their aqueous ions $\mathrm{Ca}^{2+}$, $\mathrm{Mg}^{2+}$, and $\mathrm{Na}^{+}$ are colourless because they do not have partially filled $d$-orbitals.Copper is a transition metal.Its aqueous ion $\mathrm{Cu}^{2+}$ has the electronic configuration $[\mathrm{Ar}]\,3d^9$.The single unpaired electron and the partially filled $3d$ subshell permit $d$-$d$ transitions.These transitions absorb certain wavelengths of visible light.The remaining light is reflected or transmitted, giving the solution a characteristic blue colour.Thus, among the options, copper is the element that produces a coloured ion in aqueous solution.Answer:B. Copper