Why do transition elements form coloured compounds?

Transition elements form coloured compounds due to the presence of unpaired d-electrons that absorb specific wavelengths of light.

Transition elements, also known as transition metals, are elements that have partially filled d or f orbitals. They are unique in their ability to form coloured compounds, and this is primarily due to the presence of unpaired d-electrons in their atomic structure. When white light, which is a mixture of all colours of light, shines on a transition metal compound, the unpaired d-electrons absorb specific wavelengths of light. The remaining light, which is not absorbed, is then transmitted or reflected, and this is the colour that we see.

The specific colour observed depends on the difference in energy between the d-orbitals. This energy difference corresponds to a particular wavelength of light in the visible spectrum. When an electron absorbs this light, it jumps from a lower energy level to a higher one, a process known as electronic transition. The energy of the absorbed light is equal to the energy difference between the two levels. Therefore, different transition elements, with different energy gaps, will absorb different wavelengths of light and thus appear different colours.

For example, copper(II) sulphate is a blue compound because the copper(II) ions absorb light in the red region of the spectrum, and the light that is not absorbed and therefore transmitted is blue. Similarly, chromium(III) ions form green compounds because they absorb light in the red-orange region, and the transmitted light is green.

In summary, the formation of coloured compounds by transition elements is a fascinating demonstration of the interaction between light and matter, specifically the absorption of light by unpaired d-electrons in the atomic structure of these elements. This absorption process, and the subsequent transmission or reflection of the remaining light, is what gives these compounds their characteristic colours.