Transition metals form coloured ions or compounds due to the partially filled d-orbitals. In the presence of solvent molecules (in solutions) or ligands (in complexes) or counter ions (in crystals), the d-orbitals split into two sets. The electrons in transition metal ions which occupy one set of d-orbitals having lower energy can be excited to another set of d-orbitals having high energy by absorbing energy from visible light. Since the energy difference \((\Delta E)\) is small between the two sets of d-orbitals, the light in the visible region only is absorbed by the electron during its excitation. The colour of the transition metal ion is due to the d–d excitation or the d–d transition of the electron. During d–d excitation the electron absorbs one colour in the visible light and thus it appears in the complementary colour of the absorbed light. The complimentary colours can be identified using the Munsell colour wheel (as depicted below).

The number of electrons undergoing d–d transition and the energy difference between the two sets of orbitals decide the colour. The colour of particular transition metal ion, e.g., copper ion which is blue in aqueous solution changes to dark blue in the presence of sufficient ammonia and to green if sufficient chloride ions are added. \(\Delta\)E value depends on the nature of metal ion, the nature of ligands and several other factors. Some metal ions exhibit different colours in different oxidation states.

Transition elements are coloured due to

All are correct

The correct answer is option 4. All are correct.

Transition elements exhibit color due to various factors associated with their electronic configurations and d orbitals:

1. Unpaired d-electrons: Transition metal ions often have unpaired electrons in their d orbitals, which can absorb specific wavelengths of light through electronic transitions. The presence of unpaired electrons contributes to the observed color of transition metal complexes.

2. Incompletely filled d-orbitals: The d orbitals of transition metals are incompletely filled in many cases, allowing for electronic transitions between different energy levels within the d orbitals. This incompleteness contributes to the color observed in transition metal complexes.

3. d–d transitions or d–d excitation: The absorption of light by transition metal complexes occurs through d–d transitions, where electrons in the d orbitals are excited to higher energy levels. This excitation process results in the absorption of specific wavelengths of light, leading to the observed color of transition metal complexes.

Therefore, all three statements are correct in explaining why transition elements exhibit color. Unpaired d-electrons, incompletely filled d-orbitals, and d–d transitions collectively contribute to the color observed in transition metal complexes.