Question

Why are halogens coloured?

Answer 1

Fluorine, Chlorine, Bromine, iodine and astatine are members of Group17. They are collectively known as the halogens (Greek ‘halo’ means salt and genes means born i.e. salt producers). When an electron is excited to a high energy level it emits radiations.

Complete step by step solution:
The halogens are located on the left of the noble gases on the table. These five toxic, non-metallic elements structure Group 17 of the table and consist of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Although astatine is radioactive and only has short-lived isotopes, it behaves almost like iodine and is usually included within the halogen group. Because the halogen elements have seven valence electrons, they only require one additional electron to make a full octet. This characteristic makes them more reactive than other non-metal groups.
Almost all halogens are coloured. This is due to the absorption of radiations in the visible region which results in the excitation of outer electrons to higher energy levels. By absorbing different quanta of radiation and the amount of energy required for excitation of outer electrons to higher energy levels differs for each halogen, hence each halogen displays a different colour. On moving from fluorine to iodine, the atomic radii increase, so the energy levels become closer and the low energy light (higher wavelength i.e. towards red) is absorbed and the colour deepens (complimentary is violet) on moving down the group.
Hence, Fluorine being the smallest with maximum effective nuclear charge absorbs high energy violet light and appears pale yellow. Iodine with large size and less effective nuclear charge absorbs low energy yellow light and appears dark violet. Similarly, chlorine shows the greenish-yellow colour and bromine shows red colour.

Notes: Halogens are coloured because of their low ionization energy. Don’t get confused by ionization energy and electron gain enthalpy these are two different things. Ionization energy is the energy required to remove an electron from an atom while electron gain enthalpy is the energy required to gain an electron from an atom. All halogens are of a different colour.