A level Chemistry · Edexcel 9CH0 · Topic 15A
Transition metal properties
Key points
- A transition metal forms one or more stable ions with an incompletely filled d sub-shell. Zn²⁺ (3d¹⁰) and Sc³⁺ (3d⁰) do not count.
- Electron configurations: remember Cr [Ar]3d⁵4s¹ and Cu [Ar]3d¹⁰4s¹; ions lose 4s electrons first.
- Variable oxidation states arise because 3d and 4s energies are close, e.g. vanadium +2 (violet), +3 (green), +4 (blue, VO²⁺), +5 (yellow, VO₂⁺). Zinc reduces vanadium(V) through all four.
- Colour: ligands split the d orbitals; electrons absorb visible light to jump between levels, and the complementary colour is seen. The energy gap () depends on the metal, its oxidation state, the ligand and coordination number.
- Ions with empty or full d sub-shells are colourless.
- Chromium: dichromate(VI) Cr₂O₇²⁻ (orange) ⇌ chromate(VI) CrO₄²⁻ (yellow) with alkali/acid; zinc and acid reduce Cr(VI) to green Cr³⁺ then blue Cr²⁺.
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What makes a metal a transition metal is not just its place in the d-block but the electron arrangement of its ions. This sub-topic explains the definition, the electron configurations and why so many transition metal compounds are…
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