Transition metals form coloured compounds, whereas compounds of Group 1 metals are white solids that dissolve to give colourless solutions. Explain why transition metal compounds are coloured and describe two other characteristic properties of transition metals that distinguish them from Group 1 metals.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Transition metals occupy the central block of the periodic table between Group 2 and Group 3. Unlike Group 1 metals such as sodium and potassium, transition metals display a distinctive set of chemical and physical properties.
Model answer (5 marks)
Transition metals have partially filled d‑orbitals. When light hits a complex, d‑electrons can be promoted to a higher d‑orbital that is slightly higher in energy. The energy difference corresponds to a wavelength in the visible region, so the complex absorbs that colour and the complementary colour is perceived.
1. Variable oxidation states – e.g. Fe²⁺ and Fe³⁺ or Cu⁺ and Cu²⁺.
2. Catalytic activity – e.g. Fe in the Haber process, MnO₂ in H₂O₂ decomposition, V₂O₅ in the Contact process.
These properties contrast with Group 1 metals, which have only one stable oxidation state, are not catalytic, and have lower melting points and densities.
1. Variable oxidation states – e.g. Fe²⁺ and Fe³⁺ or Cu⁺ and Cu²⁺.
2. Catalytic activity – e.g. Fe in the Haber process, MnO₂ in H₂O₂ decomposition, V₂O₅ in the Contact process.
These properties contrast with Group 1 metals, which have only one stable oxidation state, are not catalytic, and have lower melting points and densities.
Examiner tips
- Show the d‑orbital explanation first to secure the colour marks. Mention a specific example for each of the two other properties. Use the exact wording from the mark scheme (e.g. "variable oxidation states", "catalytic activity").
Common mistakes
- Confusing Group 1 metals with transition metals in the colour explanation. Failing to give a concrete example for the variable oxidation state. Using generic terms like "high melting point" without comparing to Group 1.
Mark scheme (5 marks)
- Transition metals have partially filled d-orbitals / d sub-shells (accept: d electrons that can absorb certain frequencies/wavelengths of visible light)
- The absorption of certain frequencies of visible light by d electrons causes the compound to appear coloured (the complementary colour is seen)
- Transition metals can form ions with more than one stable oxidation state / variable oxidation states (correct example required for the mark, e.g. iron can form Fe²⁺ and Fe³⁺ / copper can form Cu⁺ and Cu²⁺)
- Transition metals and their compounds act as catalysts (correct named example required, e.g. iron in the Haber process, manganese dioxide in the decomposition of hydrogen peroxide, vanadium pentoxide in the Contact process)
- Transition metals have high melting points / high densities / high tensile strength compared to Group 1 metals (any one of these, correctly stated as a comparison or contrast with Group 1)
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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