CHARLES D. WINTERS / SCIENCE PHOTO LIBRARY CHARLES D. WINTERS / SCIENCE PHOTO LIBRARY
Iodine. Conical flask containing crystals and purple vapour of iodine. Iodine, chemical symbol I, is a violet-black, non-metallic element belonging to the halogen group of the periodic table. Iodine is present in seawater and was formerly extracted from seaweed; it is now obtained from oil-well brines. Iodine is required as a trace element by most living organisms. In mammals it is concentrated in the thyroid gland as a constituent of thyroid hormones. Tincture of iodine (iodine dissolved in ethanol) is used in medicine as a mild antiseptic. There are 14 radioactive isotopes of iodine, and one stable isotope (iodine-127).
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