Nonmetallic chemical element, chemical symbol I, atomic number 53. The heaviest nonradioactive halogen, it is a nearly black crystalline solid (diatomic molecule I) 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.

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