Iodine
. Iodine, symbol I, chemically reactive element, a blue-black solid at room temperature. In group 17 (or VIIa) of the periodic table , iodine is one of the halogens . Its atomic number is 53. Iodine was first isolated from seaweed residues in 1811 by Bernard Courtois, a French manufacturer of saltpeter. The discovery was confirmed and announced by the French chemists Charles Desormes and Nicholas Clément. The nature of the element was further established in 1813 by the French chemist Joseph Louis Gay-Lussac , who also gave iodine its name. PROPERTIES AND OCCURRENCE The atomic weight of iodine is 126.905. Unlike the lighter halogens, iodine is a crystalline solid at room temperature. The lustrous, blue-black, soft substance sublimes when heated, giving off a violet vapor with a stinging odor like that of chlorine. The vapor rapidly condenses again on a cold surface. Iodine melts at 113.6° C (236° F) and boils at 185° C (365° F). The only isotope that occurs in nature is stable, but seve...