Why are halides coloured?

Halides are coloured due to the d-d transitions of electrons within their d-orbitals.

In more detail, halides, specifically transition metal halides, are coloured due to a phenomenon known as d-d transitions. This involves the movement of electrons between different energy levels, or orbitals, within the d-subshell of the atom. The d-subshell is a region of an atom where electrons with similar energy levels are likely to be found.

When light shines on a halide, the energy from the light can be absorbed by the electrons in the d-orbitals. This energy allows the electrons to jump from a lower energy level to a higher one. This jump is what we refer to as a d-d transition. The energy difference between these two levels corresponds to a specific wavelength of light. When this light is reflected back to our eyes, we perceive it as colour.

The specific colour observed depends on the difference in energy between the two d-orbitals involved in the transition. Each colour corresponds to a specific energy difference, with red light corresponding to the smallest energy difference and violet light to the largest. Therefore, the colour of a halide can give us information about the energy levels of its d-orbitals.

It's also worth noting that not all halides are coloured. For example, halides of Group 1 and Group 2 metals are typically colourless because these elements do not have partially filled d-orbitals. The colour in halides is usually observed in transition metals, which have partially filled d-orbitals that allow for d-d transitions to occur.

In summary, the colour of halides is a result of d-d transitions within their d-orbitals. The specific colour observed can provide information about the energy levels of these orbitals.