Why are transition metals often coloured, but group 1 metals aren't?

Transition metals are often coloured due to their ability to absorb light and re-emit it, while group 1 metals lack this ability.

Transition metals are known for their distinctive colours, which are a result of their unique electronic configurations. These metals have partially filled d-orbitals that can absorb and re-emit light of certain wavelengths, resulting in the perception of colour. When a transition metal ion is in a compound, the d-orbitals split into different energy levels. When light shines on the compound, electrons in the lower energy d-orbitals absorb the energy and jump to the higher energy levels. The energy difference between these levels corresponds to the energy of the light absorbed. The remaining light is then transmitted or reflected, and this is the colour we see.

On the other hand, group 1 metals, also known as alkali metals, do not exhibit this characteristic. They have a full s-orbital and an empty p-orbital, with no partially filled d-orbitals. Therefore, they cannot absorb and re-emit light in the same way as transition metals. Instead, they tend to reflect all wavelengths of light, appearing silvery-white or grey.

Furthermore, the colour of transition metals can change depending on their oxidation state, as this alters the energy gap between the d-orbitals. For example, vanadium can appear yellow, green, blue or purple depending on its oxidation state. This is another feature that distinguishes transition metals from group 1 metals, which typically only exist in one oxidation state (+1) under normal conditions.

In summary, the colour of transition metals is a result of their unique electronic configurations, specifically the presence of partially filled d-orbitals, which allow them to absorb and re-emit light. In contrast, group 1 metals lack this ability due to their different electronic configurations, resulting in their typical silvery-white or grey appearance.