Transition metals form coloured ions or compounds due to the partially filled d-orbitals. When an electron from a lower energy d orbital is excited to a higher energy d orbital, the energy of excitation corresponds to the frequency of light absorbed. This frequency generally lies in the visible region. 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 ion absorbs light of one colour from the visible region, and so it appears in the complementary colour of the light absorbed.
The frequency of the light absorbed is determined by the nature of the ligand. In aqueous solutions, where water molecules are the ligands, Cu2+ is blue, Ni2+ is green and Mn2+ is pink. Some metal ions exhibit different colours in different oxidation states.
The ions of transition elements are coloured due to
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → All of the above
The passage states that transition metal ions are coloured because of their partially filled d orbitals, and that the colour is due to the d–d excitation of an electron.
Incompletely filled d orbitals: only an ion with a partly filled d subshell has an electron that can move from one d orbital to another. Ions with d0 or d10 configurations have no such electron.
d–d transition: an electron is excited from a lower energy d orbital to a higher energy d orbital.
Absorption of visible light: the energy of this excitation corresponds to light of a frequency that generally lies in the visible region. The ion absorbs that part of white light and shows the complementary colour of the light absorbed.
All three describe the same cause, so all of the above is correct.