Iodine is the heaviest member in a family of elements called halogens. It has an atomic number of 53, an atomic weight of 126.9045 and is denoted by the atomic symbol I. Although iodine is somewhat less reactive than other halogens, it does form compounds, both organic and inorganic, with nearly all elements. At normal temperatures, iodine exists as a lustrous, blue-black solid. When heated, it usually changes directly from a solid into a purple vapor, skipping the liquid phase. The element, which is diatomic (I₂), has one stable isotope, along with 22 artificial isotopes.

Most plants and animals require small amounts of iodine to promote normal growth. In many parts of the United States, drinking water and local foods do not contain enough iodine for humans, so manufacturers of common table salt add iodine to supplement people's diets. But iodine's nutritional properties remained unknown long after the element was discovered...