Iodine is a chemical element with symbol I and atomic number 53. The name is from Greek ioeids, meaning violet or purple, due to the color of elemental iodine vapor. 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 are also used in medical applications. Iodine is found on Earth mainly as the highly water-soluble iodide 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.
Contributions by Karlhahn, Sbharris, and Vsmith.