Iodine, symbol I, is a member of the halogen group of elements. Halogens are only found in nature as compounds or ions, because of their highly reactive tendencies. The atomic number of iodine is 53, meaning it has 53 protons in its nucleus. It has only one natural isotope. The word iodine is from the Greek for violet, and crystals of iodine, when heated, give off a dense purple vapor. Surprisingly, it is fairly rare in the earth's crust. It does however dissolve readily in water, and so it is common in the oceans. All animals need iodine to exist. Iodine melts at 113.7 degrees Celsius, and boils at 184.3 C.
Bernard Courtois, a French chemist and the son of a calcium carbonate manufacturer, was the first person to isolate iodine, in 1811. As was the custom, he treated residues of burned seaweed, which were then used in the manufacture of calcium carbonate, with sulfuric acid in order to destroy them. However, he used too much acid. Attentively, he noticed that the treated seaweed produced a purple vapor, which condensed to dark crystals on cold surfaces. He gave samples of these crystals to chemist friends, and they were eventually verified as a previously unknown element. Courtois would later be part of a team that first isolated morphine from opium.
The largest share of the world's iodine comes from Chile, where it is produced from caliche, which is a mineral found in the Atacama Desert.
Silver Iodide used to be used frequently in photography, but this method of developing film is becoming less common. Iodine is used as a contrast material (because it is x-ray opaque) in modern diagnostic procedures such as CT scans. Tincture of iodine has long been used as a disinfectant for wounds. Iodine is still used to disinfect drinking water in emergencies and during camping trips. Radioactive iodine is used in medicine to treat thyroid problems.
Humans cannot survive without iodine. It is a component of two essential thyroid hormones. These hormones regulate the metabolism, speeding it up or slowing it down as appropriate. For this reason, salt is available iodized, artificially enriched, in most developed countries. Iodine is also found in seafood, and in plants from soils containing iodine. Iodine is the heaviest element known to be of use in the body.
Iodine deficiency can have serious consequences, including goiter, depression, and weight gain. Iodine deficiency is the leading preventable cause of mental retardation in the developing world.
Another iodine issue is methamphetamine. Iodine can be used in the manufacture of speed. For this reason, purchases of large amounts of iodine-containing compounds are tracked and monitored. Iodine's vapor can be irritating, and it is mildly toxic. On the whole though, iodine is an element we can't live without.