Bromine – facts and properties
- Symbol
- Br
- Atomic number
- 35
- Atomic mass
- 79.904 u
- Period
- 4
- Group
- 17
- Category
- halogen
- Electron configuration
- [Ar] 3d¹⁰ 4s² 4p⁵
- Electronegativity
- 2.96
- Atomic radius
- 94 pm
- Valence electrons
- 7
- State at standard conditions
- liquid
- Melting point
- -7.2 °C
- Boiling point
- 58.8 °C
- Density
- 3.11 g/cm³
- Discovered
- 1826
Bromine is a halogen with atomic number 35 and the only nonmetal that is liquid at room temperature. The dark red-brown liquid evaporates in air into pungent red-brown fumes, and the element owes its name to this stench. Bromine is strongly corrosive and toxic. Its compounds, by contrast, are versatile: they are found in flame retardants, drilling fluids and the history of photography.
Position in the periodic table
Occurrence and extraction
Bromine occurs in nature almost only as bromide, a negatively charged ion, mainly in the sea and in salt deposits. Seawater contains a small amount of bromide, and the concentration is much higher in some salt lakes and brines. The Dead Sea is especially rich. The main producers are Israel, Jordan, China and the United States.
To extract it, chlorine gas is passed into bromide-containing brine. Chlorine is more reactive and displaces bromine from the bromide: 2 Br⁻ + Cl₂ → Br₂ + 2 Cl⁻. The liberated bromine is blown out with steam or air and condensed.
The element was discovered in 1826 by the young French chemist Antoine-Jérôme Balard, who was studying residues from salt marshes in Montpellier. At almost the same time the Heidelberg student Carl Löwig had come across the same element, but he published later. The name comes from the Greek word bromos, meaning stench.
Properties and reactions
Bromine is in period 4 and group 17, so it is a halogen between chlorine and iodine. Its electron configuration is [Ar] 3d¹⁰ 4s² 4p⁵, with 7 valence electrons. Its electronegativity is 2.96. Bromine has a density of 3.11 g/cm³, which is more than three times that of water. It melts at -7.2 °C and boils at 58.8 °C. Because the boiling point is so low, bromine evaporates easily even at room temperature. The vapour is heavier than air and sinks to the floor.
Bromine exists as the diatomic molecule Br₂. It is a strong oxidising agent, but weaker than chlorine and stronger than iodine. In the halogen series chlorine displaces bromine from bromides, and bromine in turn displaces iodine from iodides. With metals it forms bromides, with hydrogen hydrogen bromide. Bromine water, the yellow-brown solution of bromine in water, loses its colour on contact with alkenes and so reveals double bonds in organic substances. In school, the teacher usually shows this test with very dilute bromine water in a fume cupboard or replaces it with safer tests.
Silver bromide is a pale yellow precipitate that darkens in light. This sensitivity made it the basis of classical photography, in film and photographic paper.
Bromine strongly irritates skin, eyes and airways and causes burns that heal poorly. Handling requires gloves, safety goggles and a fume cupboard. Bromine does not belong in student experiments; work with it is reserved for trained professionals.
Uses in everyday life and technology
A lot of bromine goes into flame retardants. Brominated compounds in plastics, circuit boards and insulation capture radicals in a fire and slow combustion. Some of these substances, however, accumulate in the environment and in living things. For that reason several of them, such as certain polybrominated diphenyl ethers, are now banned or heavily restricted.
A second major use is drilling fluids: heavy calcium and zinc bromide solutions hold back pressure in deep boreholes. Bromine is also used as a disinfectant in water treatment, for example in swimming pools and cooling towers. Methyl bromide was long used as a fumigant against pests, but it damages the ozone layer and is largely banned under the Montreal Protocol. Bromine compounds were once added to petrol to remove lead from the engine.
In medicine, potassium bromide was a sedative and an anti-epileptic drug in the nineteenth century. In humans it has been replaced, but in veterinary medicine it is still used against epilepsy in dogs. Traces of bromide occur in food and the body. In 2014 researchers showed in fruit flies that bromine is needed to build collagen IV, a component of the basement membrane. The mechanism occurs in all animals; whether bromine is indispensable for humans is not yet conclusively settled.
Elemental bromine is a material for industry. Consumers meet it mostly in the form of compounds.
Molecules containing this element
- Bromine Br2
- Hydrogen bromide HBr
- Ammonium bromide NH4Br
Quick question
In which group are the halogens located?
FAQ
Is bromine a liquid or a gas?
At room temperature bromine is a dark red-brown liquid that evaporates constantly. Besides mercury it is the only element that is liquid under normal conditions, and it is the only nonmetal of this kind. The vapour is red-brown and heavier than air.
Is bromine toxic?
Yes. Liquid bromine causes severe burns on the skin, and the fumes damage eyes and airways even in small amounts. Only trained professionals may work with bromine, in a fume cupboard. Bromides, the salts of bromine, are unproblematic in small amounts.
What is bromine used for?
Mainly for flame retardants, for heavy drilling fluids in oil production and for disinfectants in water treatment. Historically, silver bromide was important in photography and potassium bromide was used as a medicine.