Coloured Ions

The colour of transition metal ions is caused by d-d electronic transitions. In the presence of ligands or water, the five d-orbitals split into two energy levels, and an electron in a lower d-orbital can absorb a specific wavelength of visible light to jump to a higher d-orbital. The colour observed is the complement of the absorbed wavelength. This requires partly filled d-orbitals with unpaired electrons. Ions such as Sc^3+ (3d^0) and Zn^2+ (3d^10) are colourless because they have either no d-electrons or completely filled d-orbitals, so no d-d transition is possible. The option of larger atomic size does not cause colour. The option of higher atomic mass is irrelevant to colour. The option of complete absence of d-electrons in coloured ions is the opposite of the truth. This origin of colour is a central NCERT property of the d-block. Such questions reward conceptual clarity, since a student who truly grasps coloured ions can solve many superficially different variants with the same approach. Plausibility check: Cu^2+ with one unpaired d-electron is blue, while Zn^2+ with a full d-shell is colourless, confirming the role of d-d transitions.