Transition metals can form compounds with different colours. Explain why transition metals are able to form coloured compounds, and state one example of a coloured transition metal compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Transition metals are found in the d-block of the periodic table. Unlike Group 1 metals, transition metals have incompletely filled d sub-levels in their atoms or ions, which gives them distinctive properties.
Model answer (5 marks)
Transition metals have partially filled d orbitals. When visible light strikes the compound, electrons in the d orbitals absorb specific wavelengths of light, promoting them to higher‑energy d orbitals. The wavelengths not absorbed are transmitted or reflected, giving the compound its colour. Example: copper(II) sulfate is blue.
Examiner tips
- Mention partially filled d orbitals and d–d transitions; explain absorption and transmission; give a correct example.
- Use UK spelling and correct chemical formula; include oxidation state if possible.
Common mistakes
- Saying only s‑orbital electrons are involved; forgetting to mention d–d transitions; giving a non‑transition metal example.
Mark scheme (5 marks)
- Transition metals have incompletely filled / partially filled d orbitals (d sub-level)
- When light falls on the compound / ion, electrons in the d orbitals absorb energy from certain frequencies / wavelengths of visible light
- Electrons are promoted / excited to a higher energy d orbital
- The remaining (unabsorbed) wavelengths are transmitted / reflected, and this is the colour we observe
- A correct named example of a coloured transition metal compound, e.g. copper(II) sulfate (blue), potassium manganate(VII) (purple), iron(III) oxide (red/brown), cobalt(II) chloride (pink/blue), chromium(III) oxide (green)
Key terms in this question
transition metal · coloured compound
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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