Iodine

Iodine is the heaviest stable halogen in group 17, with configuration [Kr] 4d10 5s2 5p5. One electron short of a noble-gas configuration, it forms the iodide ion, I^−, and covalent I–I bonds in I2 molecules. Because its outer electrons are far from the nucleus, iodine is the least reactive stable halogen and the weakest oxidizing agent among them: chlorine and bromine displace it from iodide solutions. The large, easily distorted electron clouds of I2 molecules attract each other strongly enough that iodine is a solid at room temperature, forming lustrous blue-black crystals.

Heated gently, solid iodine sublimes directly into a violet vapour. Iodine dissolves poorly in water but readily in iodide solutions, where it forms the triiodide ion, I3^−, and it gives an intense blue-black colour with starch. In biology, iodine is essential: the thyroid gland incorporates it into the hormones thyroxine (T4) and triiodothyronine (T3), which regulate metabolism and brain development. Iodized salt, introduced in the 1920s, has prevented goitre and developmental disability in hundreds of millions of people. Iodine compounds also serve as X-ray contrast agents, disinfectants and industrial catalysts.

Discovery

In 1811 the French chemist Bernard Courtois was making saltpetre for Napoleon's gunpowder from seaweed ash. When he added sulfuric acid to the ash residue, a violet vapour rose and condensed into dark crystals. Charles-Bernard Desormes and Nicolas Clément announced the discovery in 1813, and Joseph Louis Gay-Lussac and Humphry Davy both studied the substance. Gay-Lussac named it iode, from the Greek iodes, meaning 'violet'.