Bromine (from Greek: βρῶμος, brómos, meaning "strong-smelling" or "stench") is a chemical element with symbol Br, and atomic number 35. It is a halogen. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826. Elemental bromine is a fuming red-brown liquid at room temperature, corrosive and toxic, with properties between those of chlorine and iodine. Free bromine does not occur in nature, but occurs as colorless soluble crystalline mineral halide salts, analogous to table salt.
Bromine is rarer than about three-quarters of elements in the Earth's crust. The high solubility of bromide ions has caused its accumulation in the oceans, and commercially the element is easily extracted from brine pools, mostly in the United States, Israel and China. About 556,000 tonnes were produced in 2007, an amount similar to the far more abundant element magnesium.
At high temperatures, organobromine compounds readily convert to free bromine atoms, a process which has the effect of stopping free radical chemical chain reactions. This effect makes organobromine compounds useful as fire retardants; more than half the bromine produced industrially worldwide each year is put to this use. Unfortunately, the same property causes sunlight to convert volatile organobromine compounds to free bromine atoms in the atmosphere, and an unwanted side effect of this process is ozone depletion. As a result, many organobromide compounds that were formerly in common use—such as the pesticide, methyl bromide—have been abandoned. Bromine compounds are still used for purposes such as in well drilling fluids, in photographic film, and as an intermediate in the manufacture of organic chemicals.
Bromine has been long believed to have no essential function in mammals, but recent research suggests that bromine is necessary for tissue development. In addition, bromine is used preferentially over chlorine by one antiparasitic enzyme in the human immune system. Organobromides are needed and produced enzymatically from bromide by some lower life forms in the sea, particularly algae, and the ash of seaweed was one source of bromine's discovery. As a pharmaceutical, the simple bromide ion, Br–, has inhibitory effects on the central nervous system, and bromide salts were once a major medical sedative, before being replaced by shorter-acting drugs.