Nonmetallic chemical element , chemical symbol I, atomic number 53.
The heaviest nonradioactive halogen , it is a nearly black crystalline solid (diatomic molecule I 2 ) that sublimes (see sublimation ) to a deep violet, irritating vapour. It is never found in nature uncombined. Its sources (mostly in brines and seaweeds) and compounds are usually iodides; iodates (small amounts in saltpeter) and periodates also occur. Dietary iodine is essential for thyroid gland function; in areas of the world where food contains insufficient iodine, an iodine compound such as potassium iodide (KI) is added to table salt ( sodium chloride ) to prevent iodine deficiency . Elemental iodine is used in medicine, in synthesizing some organic chemicals, in manufacturing dyes, in analytical chemistry (see analysis ) to measure fat saturation (see hydrogenation ) and to detect starch , and in photography. The radioactive isotope iodine-131 (see radioactivity ), with an eight-day half-life , is very useful in medicine (see nuclear medicine ) and other applications.