iodine

Greyish-black non-metallic element, atomic number 53, relative atomic mass 126.9044. It is a member of the halogen group (Group 7 of the periodic table). Its crystals sublime, giving off, when heated, a violet vapour with an irritating odour resembling that of chlorine. It occurs only in combination with other elements. Its salts are known as iodides and are found in sea water. As a mineral nutrient it is vital to the proper functioning of the thyroid gland, where it occurs in trace amounts as part of the hormone thyroxine. Absence of iodine from the diet leads to goitre. Iodine is used in photography, in medicine as an antiseptic, and in making dyes.

Its radioactive isotope iodine-131 (half-life of eight days) is a dangerous fission product from nuclear explosions and from the nuclear reactors in power plants, since, if ingested, it can be taken up by the thyroid and damage it. It was discovered in 1811 by French chemist B Courtois (1777–1838).

According to the World Health Organization, iodine deficiency is the greatest cause of preventable brain damage in the world.