Explain why halogens are coloured.

Halogens are coloured because they absorb certain wavelengths of light and reflect others, which our eyes perceive as colour.

Halogens, which include fluorine, chlorine, bromine, iodine, and astatine, are a group of non-metal elements found in Group 17 of the periodic table. They are known for their vibrant colours, which are a result of their unique electronic configurations.

The colour of a substance is determined by the wavelengths of light it absorbs and reflects. When white light, which contains all wavelengths of visible light, shines on a substance, the substance absorbs certain wavelengths and reflects others. The wavelengths that are reflected reach our eyes and are perceived as colour.

In halogens, the absorption of light occurs when an electron absorbs a photon of light and jumps to a higher energy level. The energy of the absorbed photon corresponds to the difference in energy between the electron's initial and final energy levels. This energy difference determines the wavelength of the absorbed light.

Each halogen has a different electronic configuration, which means that each one absorbs and reflects different wavelengths of light. For example, chlorine is a pale green colour because it absorbs light in the red part of the spectrum and reflects light in the green part of the spectrum. Bromine, on the other hand, is a deep red colour because it absorbs light in the blue part of the spectrum and reflects light in the red part of the spectrum.

It's also worth noting that the colour of a halogen can change depending on its physical state. For example, iodine is a grey solid at room temperature, but when it is heated and turns into a gas, it becomes a vibrant purple colour. This is because the energy levels of the electrons change when the iodine changes state, which changes the wavelengths of light it absorbs and reflects.

In conclusion, the colours of the halogens are a fascinating demonstration of the interaction between light and matter, and they provide a visual representation of the unique electronic configurations of these elements.