Question

Discuss the general trend in the following property of the 3d transition elements (21-29):

Formation of coloured ions

Answer 1

1. The 3d transition elements (21-29) form coloured ions due to electronic transitions called d-d transitions, where electrons in split d-orbitals absorb visible light and get promoted to higher energy d-orbitals. This absorption gives complementary colours to the ions.
2. The colour depends on the number of unpaired d-electrons and the nature of ligands, causing crystal field splitting.
3. Ions with empty (e.g., Sc³⁺) or completely filled d-orbitals (e.g., Zn²⁺, Cu⁺) are colourless because no d-d transitions occur.
4. Across the series from Sc to Cu, generally ions form coloured compounds except for those with d⁰ or d¹⁰ configurations.
5. Colour intensity and type change with changes in the oxidation state and ligand environment.