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Iodine is a chemical element with symbol I and atomic number 53. The name is from Greek ''ioeidēs'', meaning violet or purple, due to the color of elemental iodine vapor.〔Online Etymology Dictionary, s.v. (''iodine'' ). Retrieved 7 February 2012.〕 Iodine and its compounds are primarily used in nutrition, and industrially in the production of acetic acid and certain polymers. Iodine's relatively high atomic number, low toxicity, and ease of attachment to organic compounds have made it a part of many X-ray contrast materials in modern medicine. Iodine has only one stable isotope. A number of iodine radioisotopes, such as 131I, are also used in medical applications. Iodine is found on Earth mainly as the highly water-soluble iodide ion I−, which concentrates it in oceans and brine pools. Like the other halogens, free iodine occurs mainly as a diatomic molecule I2, and then only momentarily after being oxidized from iodide by an oxidant like free oxygen. In the universe and on Earth, iodine's high atomic number makes it a relatively rare element. However, its presence in ocean water has given it a role in biology. It is the heaviest essential element utilized widely by life in biological functions (only tungsten, employed in enzymes by a few species of bacteria, is heavier). Iodine's rarity in many soils, due to initial low abundance as a crust-element, and also leaching of soluble iodide by rainwater, has led to many deficiency problems in land animals and inland human populations. Iodine deficiency affects about two billion people and is the leading preventable cause of intellectual disabilities. Iodine is required by higher animals for synthesizing thyroid hormones, which contain the element. Because of this function, radioisotopes of iodine are concentrated in the thyroid gland along with nonradioactive iodine. If inhaled, the radioisotope iodine-131, which has a high fission product yield, concentrates in the thyroid, but is easily remedied with non-radioactive potassium iodide treatment. ==Characteristics== Under standard conditions, iodine is a bluish-black solid with a metallic lustre, appearing to sublimate into a noxious violet-pink gas, the colour due to absorption of visible light by electronic transitions between the highest occupied and lowest unoccupied molecular orbitals. Melting at , it forms compounds with many elements but is less reactive than the other members of its group, the halogens, and has some metallic light reflectance. Elemental iodine is slightly soluble in water, with one gram dissolving in 3450 ml at 20 °C and 1280 ml at 50 °C; potassium iodide may be added to increase solubility via formation of triiodide ions. Nonpolar solvents such as hexane and carbon tetrachloride provide a higher solubility. Polar solutions are brown, reflecting the role of these solvents as Lewis bases, while nonpolar solutions are violet, the color of iodine vapor. Charge-transfer complexes form when iodine is dissolved in polar solvents, modifying the energy distribution of iodine's molecular orbitals, hence changing the colour. A metal ion may replace the solvent, in which case the two species exchange electrons, the ion undergoing π backbonding. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「iodine」の詳細全文を読む スポンサード リンク
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