īˈədīn, –dĭn [Gr.,=violet], nonmetallic chemical element; symbol I; at. no. 53; at. wt. 126.9045; m.p. 113.5 degrees Celsius; b.p. 184.35 degrees Celsius; sp. gr. 4.93 at 20 degrees Celsius; valence −1, +1, +3, +5, or +7. Iodine is a dark-gray to purple-black, lustrous, solid element with a rhombic crystalline structure. It is the least active of the halogens, which are found in group VIIa of the periodic table. It is normally diatomic, i.e., it has 2 iodine atoms in each molecule, in the solid, liquid, and vapor states. When heated it passes directly from the solid to the vapor state (sublimation), the vapor having an intense violet color and a characteristic irritating odor. Iodine is only slightly soluble in water but dissolves readily in a solution of sodium or potassium iodide. Tincture of iodine is a solution of iodine and potassium iodide in alcohol. Iodine also dissolves in carbon disulfide, carbon tetrachloride, and chloroform, giving a deep violet solution. Iodine forms many compounds. With hydrogen it forms hydrogen iodide, which in water solution becomes hydriodic acid. It forms compounds with certain nonmetals (e.g., carbon, nitrogen, phosphorus, and oxygen) and with most metals. Iodine is displaced from its compounds by the other halogens. The element is obtained from salt deposits, as from the saltpeter beds in Chile, where it occurs in small quantities as an iodate, and from the salt brines associated with some oil wells in California and Louisiana. It is also found as an iodide in the ash of certain seaweeds. Iodine may be prepared by displacement from its compounds with chlorine. Treating an iodide with manganese dioxide and sulfuric acid sublimes the iodine. Iodine is important in medical treatment; tincture of iodine and iodoform are widely used. Iodine is employed in the preparation of certain drugs and in the manufacture of some dyes. Silver iodide, a yellow salt, is used in photography; it is water insoluble and turns black when exposed to light. Starch turns deep blue (almost black) in the presence of a small amount of iodine; this reaction serves as a test for either starch or iodine. Iodine in small amounts is essential to human nutrition; in the thyroid gland it becomes a part of the iodine-containing hormones. Goiter, a swelling of the thyroid, is often a symptom of inadequate iodine in the diet. Iodine has only one stable isotope, iodine–127; it is the only isotope of iodine occurring in nature, although 24 iodine isotopes are known. Iodine–131 is a radioactive isotope with a half-life of 8 days. It is used medically to diagnose abnormalities of the thyroid gland. It is also a component of fallout produced by nuclear explosions. Iodine was discovered in 1811 by Bernard Courtois.

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