The d- and f-Block Elements
The d- and f-Block Elements questions centre on electronic configuration, oxidation states, colour and magnetism, lanthanoid contraction, and the chemistry of KMnO₄ and K₂Cr₂O₇.
Core rules
- A transition element has an incomplete d subshell in its atom or in a common ion. Zn, Cd and Hg (d¹⁰) are excluded.
- Cr = [Ar]3d⁵4s¹ and Cu = [Ar]3d¹⁰4s¹, the extra stability of half-filled and fully filled d subshells.
- Characteristic oxidation states: Sc +3 only; Mn +2 to +7; Fe +2 and +3; Cr +3 and +6.
- Colour arises from d–d transitions in a partly filled d subshell; Sc³⁺, Ti⁴⁺ and Zn²⁺ are colourless because d is empty or full.
- Spin-only moment μ = √(n(n+2)) BM, where n is unpaired electrons.
- Lanthanoid contraction: poor shielding by 4f electrons shrinks the atoms La → Lu, explaining the Zr/Hf similarity and the difficulty of separating lanthanoids.
- Ce⁴⁺ and Eu²⁺ are the stable anomalies (Ce⁴⁺ reaches a 4f⁰ arrangement).
Worked example
Find the magnetic moment of Fe³⁺. Fe is [Ar]3d⁶4s², so Fe³⁺ is 3d⁵. With weak-field ligands the five d electrons stay unpaired, n = 5: μ = √(5 × 7) = √35 = 5.92 BM. With strong-field ligands such as CN⁻ a low-spin d⁵ ion has n = 1 and μ = 1.73 BM, so always state which ligand field you are assuming.
Traps that cost marks
- Calling zinc a transition element.
- Assuming every d-block ion is coloured.
- Confusing lanthanoid (4f) with actinoid (5f) contraction.
- Acidifying KMnO₄ with HCl - chloride is oxidised to chlorine; use dilute H₂SO₄.
- Reporting μ without units or with the electron count of the atom rather than the ion.