What are transition elements? Write their general electronic configuration. Explain why Zn, Cd and Hg are not regarded as transition elements but Cu is.

1. 1 · Define in one line. Transition elements have incompletely filled d orbitals in the ground state or in a common oxidation state.
2. 2 · Give the configuration. (n−1)d¹⁻¹⁰ ns¹⁻²; they lie in groups 3 to 12.
3. 3 · Explain group 12. Zn, Cd and Hg are (n−1)d¹⁰ ns², and M²⁺ is d¹⁰: d orbitals full, so not transition elements.
4. 4 · Explain copper. Cu is 3d¹⁰ 4s¹, but Cu²⁺ is 3d⁹: partly filled, so Cu is a transition element.
5. 5 · Close in one line. So the test is a partly filled d subshell in the atom or a common ion.

Scoring tip: Always write the ion configuration (Cu²⁺ 3d⁹, Zn²⁺ 3d¹⁰); that is where the mark is.