State the full electronic configurations of a Cu atom and a \({\text{C}}{{\text{u}}^ + }\) ion.

Cu:

\({\text{C}}{{\text{u}}^ + }\):

Explain the origin of colour in transition metal complexes and use your explanation to suggest why copper(II) sulfate, CuSO₄(aq), is blue, but zinc sulfate, ZnSO₄(aq), is colourless.

\({\text{C}}{{\text{u}}^{2 + }}{\text{(aq)}}\) reacts with ammonia to form the complex ion \({{\text{[Cu(N}}{{\text{H}}_{\text{3}}}{{\text{)}}_{\text{4}}}{\text{]}}^{2 + }}\). Explain this reaction in terms of an acid-base theory, and outline how the bond is formed between \({\text{C}}{{\text{u}}^{2 + }}\) and \({\text{N}}{{\text{H}}_{\text{3}}}\).

Cu:

\({\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{d}}^{{\text{10}}}}{\text{4}}{{\text{s}}^{\text{1}}}\);

\({\text{C}}{{\text{u}}^ + }\):

\({\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{d}}^{{\text{10}}}}\);

Ignore relative order of 3d and 4s.

Penalize only once if noble gas core is given.

d orbitals are split (into two sets of different energies);

frequencies of (visible) light absorbed by electrons moving from lower to higher d levels;

colour due to remaining frequencies/complementary colour transmitted;

\({\text{C}}{{\text{u}}^{2 + }}\) has unpaired electrons/partially filled d sub-level;

\({\text{Z}}{{\text{n}}^{2 + }}\) has filled d sub-shell;

electronic transitions/d-d transitions possible for \({\text{C}}{{\text{u}}^{2 + }}\) / no electronic/d-d transitions possible for \({\text{Z}}{{\text{n}}^{2 + }}\);

Allow wavelength as well as frequency.

\({\text{N}}{{\text{H}}_{\text{3}}}\): Lewis base / \({\text{C}}{{\text{u}}^{2 + }}\): Lewis acid;

each \({\text{N}}{{\text{H}}_{\text{3}}}\)/ligand donates an electron pair (to \({\text{C}}{{\text{u}}^{2 + }}\));

\({\text{N}}{{\text{H}}_{\text{3}}}\) replace \({{\text{H}}_2}{\text{O}}\) ligands around \({\text{C}}{{\text{u}}^{2 + }}\) ion/around central ion;

forming coordinate (covalent)/dative covalent bond;

Many candidates identified the electronic configuration of Cu as an exception but the 3d electron was often removed in forming the ion instead of the 4s.

Precision of language proved to be an issue in (e) with some candidates referring to Cu and Zn and not their ions and some students explained the colour as a result of “reflection” or “emission”.

In (f), many candidates mentioned proton donors and proton acceptors and made no reference to Lewis theory.