(From the French iode, ultimately from the Greek iodes, "violetlike") A nonmetallic chemical element, one of the halogens, consisting of grayish-black crystals that volatilize into a violet-colored vapor. It is used as an antiseptic, in pharmacology, in the manufacture of dyes, and in photography. Iodine is essential to the human diet; a lack of it cause problems with the thyroid gland. Insufficient iodine in the diet causes goiter, although this condition is now rare because table salt is dosed with iodide.

Iodine was discovered by Barnard Courtois in 1811 in Paris, France; it was isolated from treating seaweed ash with sulfuric acid while recovering sodium and potassium compounds.

Symbol: I
Atomic number: 53
Atomic weight: 126.90447
Density (at room temperature and pressure): 4.93 g/cm3
Melting point: 114°C
Boiling point: 184.4°C
Valence: -1, +1, +5, +7
Ground state electron configuration: [Kr]4d105s25p5

See also: iodine-131