The heaviest stable halogen is iodine. Iodine forms dark purple crystals, confirming its name, Greek for violet colored. It was first obtained in 1811 from the ashes of seaweed. Iodine is purified by heating the solid, which sublimes, or goes directly to the gas state. The pure solid is obtained by cooling the vapors. The vapors are irritating to eyes and mucous membranes.
Iodine was obtained commercially from mines in Chile in the 1800s. In the twentieth century brine from wells has been a better source. Especially important are brine wells in Japan, and, in the United States, in Oklahoma and Michigan.
Iodine is necessary in the diet because the thyroid gland produces a growth-regulating hormone that contains iodine. Lack of iodine causes goiter. Table salt usually has about 0.01% of sodium iodide added to supply the needed iodine. Other compounds function in chemical analysis and in synthesis in a chemistry laboratory of organic compounds. Iodine was useful in the development of photography. In the daguerreotype process, an early type of photography, a silver plate was sensitized by exposure to iodine vapors.