Iodine, element 53 with a relative atomic mass of 126.90447, was first isolated by Bernard Courtois in 1811 from the ash of seaweed (by treating kelp with H2SO4). It was named by J. L. Gay Lussac in 1813, and its name derives from the Greek word iodes, meaning "violet-colored," reflecting the characteristic lustrous, deep purple color of resublimed crystalline iodine as well as the color of its vapor. Potassium iodide (KI) was used as a remedy for goiter (Derbyshire neck), an enlargement of the thyroid gland, as early as 1819. The thyroid is responsible for the production of thyroxine, a metabolism-regulating hormone. Iodine is an essential trace element for humans and plays an important role in many biological organisms. In modern times, KI is recommended for the treatment of radiation poisoning.
Early sources of iodine were the saltpeter deposits in Chile, whereas contemporary sources also include natural brines and salt wells. It is generally liberated from brine via chlorine gas. Annual production of I2 exceeds 10,000 tons, Japan being the dominant producer. It is the heaviest of the common halogens, and its isotopes range in mass from 117 to 139; the natural isotope, 127, occurs in 100% abundance. Radioactive isotopes include 124, 125, 128, 131, and 132 and can be used as radioactive tracer elements. It readily dissolves in such organic solvents as chloroform, carbon tetrachloride, ethanol, benzene, and ethyl ether to form beautiful purple solutions, but it is only slightly soluble in water.
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