iodine (I),chemical element, a member of thehalogen elements, or Group 17 (Group VIIa) of theperiodic table.

History

In 1811 the French chemistBernard Courtois obtained a violet vapor by heatingseaweed ashes withsulfuric acid as a by-product of the manufacture ofsaltpeter. This vapor condensed to a black crystalline substance, which he called “substance X.” In 1813 British chemistSir Humphry Davy, who was passing through Paris on his way to Italy, recognized substance X as an elementanalogous tochlorine; he suggested the nameiodine from the Greek wordioeides, “violet colored.”

Occurrence and distribution

Iodine is never found in nature uncombined, and it is not concentrated sufficiently to form independent minerals. It is present inseawater, but sparingly, as the iodideion, I⁻, to the extent of approximately 50 mg per metric ton (0.0016 ounce per ton) of seawater. It is also formed inseaweeds,oysters, andcod livers. Sodium iodate (NaIO₃) is contained in crudeChilesaltpeter (sodium nitrate, NaNO₃). Thehuman body contains iodine in thecompoundthyroxine, which is produced in thethyroid gland.

The only naturally occurringisotope of iodine is stableiodine-127. An exceptionally usefulradioactive isotope isiodine-131, which has a half-life of eight days. It is employed in medicine to monitor thyroid gland functioning, to treatgoitre and thyroidcancer, and to locatetumors of thebrain and of theliver. It is also used in investigations to trace the course ofcompounds in metabolism. Several iodine compounds are used as contrast mediums in diagnosticradiology. In aqueous solution even minute amounts of iodine in the presence ofstarch produce a blue-black color.