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 d–d excitation or d–d transition of the electron. During d–d excitation the electron absorb one colour in the visible light and thus it appears in the complimentary colour of the absorbed light. The complimentary colours can be identified using 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.

Cuprous ion is colourless while cupric ion is coloured because

Cuprous ion has a completed d-orbital and cupric ion has an incomplete d-orbital

The correct answer is option 4. cuprous ion has a completed d-orbital and cupric ion has an incomplete d-orbital.

Cuprous ion (Cu⁺) has the electron configuration [Ar]3d¹⁰. This means that it has a completely filled d-orbital, which is why it is colourless. Cupric ion (Cu²⁺) has the electron configuration [Ar]3d⁹. This means that it has an incomplete d-orbital, which is why it is coloured.

The colour of a transition metal ion is due to the d-d transitions that can occur in the d-orbitals. When an electron in a d-orbital absorbs energy, it can jump to a higher energy level. The difference in energy between the two energy levels corresponds to a specific wavelength of light, which is why transition metals appear coloured.

In the case of cuprous ion, there are no d-d transitions that can occur because the d-orbital is completely filled. This is why cuprous ion is colourless.

In the case of cupric ion, there are d-d transitions that can occur because the d-orbital is incomplete. This is why cupric ion is coloured.

Therefore, the correct answer is (4).