Compounds of transition metals are generally coloured. Give reason.

Ans.

Compounds of transition metals are generally colored due to the presence of partially filled d-orbitals in the transition metal ions and the phenomenon of d-d electronic transitions.

The color of transition metal compounds arises from the absorption of certain wavelengths of light by the electrons in the d-orbitals of the metal ions. In the ground state, the d-orbitals of transition metal ions are partially filled with electrons. When light falls on the compound, it can promote one or more electrons from the lower energy d-orbitals to higher energy d-orbitals. This absorption of light corresponds to the excitation of electrons from one d-orbital to another within the same d-subshell, known as d-d electronic transitions.

The energy difference between the d-orbitals corresponds to specific wavelengths of light that are absorbed by the compound. The wavelengths of light that are not absorbed are transmitted or reflected, giving rise to the observed color of the compound. The color that is perceived by the human eye is the complementary color of the absorbed light.

The energy required for these electronic transitions depends on factors such as the nature of the transition metal ion, its oxidation state, and the ligands surrounding the metal ion. Ligands can influence the energy levels of the d-orbitals and the magnitude of the energy gap between them, thereby affecting the color of the compound.

In summary, the presence of partially filled d-orbitals in transition metal ions allows for d-d electronic transitions, leading to the absorption of certain wavelengths of light and the manifestation of color in transition metal compounds.