IODE

Iode, in English commonly called iodine, is a chemical element with the symbol I and atomic number 53. It is a nonmetal in the halogen group (Group 17) and occurs primarily as diatomic molecules (I2). In its solid form it is a dark violet-black crystalline substance, and it sublimates readily to a violet vapor. Its melting point is 113.7°C and its boiling point is 184.3°C.

Iode is relatively scarce in the Earth’s crust but is concentrated in seawater as iodide ions and in certain mineral deposits. Commercial production derives from iodide-rich brines, caliche deposits, or seaweed ashes, with processes that oxidize iodide to iodine and then separate the element from other components. It forms various iodide compounds with many elements and commonly exhibits the −1 oxidation state in these salts.

Iode has several important applications. It is an essential micronutrient for humans and other animals, needed for the synthesis of thyroid hormones; iodine is commonly supplied through iodized salt and seafood. In medicine, iodinated compounds are used as contrast agents in imaging and as antiseptics, such as povidone-iodine. In chemistry, iodine serves as a reagent and a source of iodide in synthesis, and iodide-containing compounds have roles in catalysis and materials science.

Safety and history are notable. Exposure to iodine vapor can irritate the eyes, skin, and respiratory tract; excessive intake or exposure can disturb thyroid function. Iode was discovered in 1811 by Bernard Courtois and named for the violet color of its vapors, with the name derived from Greek words meaning violet.