Transition metal compounds are usually coloured because

  • A. electrons move between d orbitals split by the ligands, absorbing part of the visible spectrum
  • B. the metals reflect all wavelengths equally
  • C. they contain water of crystallisation
  • D. their ions are very large

Explanation

Ligands split the five d orbitals into groups of slightly different energy, and the small energy gap corresponds to visible light, so the complex absorbs one part of the spectrum and appears in the complementary colour. Ions with an empty or completely full d subshell, such as Sc3+ and Zn2+, have no such transition available and their compounds are white. Changing the ligand changes the gap, and hence the colour.

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