Bromine is a chemical element with symbol and atomic number #35. It is the third-lightest , and is a fuming red-brown liquid at room temperature that evaporates readily to form a similarly coloured gas. Its properties are thus intermediate between those of chlorine and iodine. Isolated independently by two chemists, Carl Jacob Löwig (in 1825) and Antoine Jérôme Balard(in 1826), its name was derived from the Ancient Greek βρῶμος ("stench"), referencing its sharp and disagreeable smell.
Elemental is very reactive and thus does not occur free in nature, but in colourless soluble crystalline mineral salts, analogous to table salt. While it is rather rare in the Earth's crust, the high solubility of the bromide ion (Br−) has caused its accumulation in the oceans. Commercially the element is easily extracted from brinepools, mostly in the United States, and . The mass of bromine in the oceans is about one three-hundredth that of chlorine.
At high temperatures, organobromine compounds readily dissociate to yield free bromine atoms, a process that stops free radical chain reactions. This effect makes organobromine compounds useful as fire retardants, and more than half the bromine produced worldwide each year is put to this purpose. The same property causes sunlight to dissociate volatile organobromine compounds in the atmosphere to yield free bromine atoms, causing depletion. As a result, many organobromide compounds—such as the pesticide methyl bromide—are no longer used. Bromine compounds are still used in well drilling fluids, in photographic film, and as an intermediate in the manufacture of organicchemicals.
Large amounts of bromide salts are toxic from the action of soluble bromide ion, causing . However, a clear biological role for bromide ion and hypobromous acid has recently been elucidated, and it now appears that bromine is an essential trace element in humans. The role of biological organobromine compounds in sea life such as algae has been known for much longer. As a pharmaceutical, the simple bromide ion (Br−) has inhibitory effects on the central nervous system.