Bromine
Bromine (from Ancient Greek:βρῶμος, brómos, meaning "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.
Identity
- Atomic mass
- [79.901, 79.907] u
- Category
- Halogens
- Group
- 17
- Period
- 4
- Block
- p
- Oxidation States
- −1, +1, +3, +5, +7
- Phase (25 °C)
- Liquid
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5
- Short configuration
- [Ar] 3d10 4s2 4p5
- Electrons per shell
- 2, 8, 18, 7
- Electronegativity (Pauling)
- 2.96
- Electron affinity
- 325 kJ/mol
- 1st ionization energy
- 1.14e+3 kJ/mol
- Ionization energies
- 1140 · 2103 · 3470 · 4560 · 5760… kJ/mol
Physical properties
- Melting point
- -7.3 °C (266 K)
- Boiling point
- 58.9 °C (332 K)
- Density
- 3.1028 g/cm³
- Molar heat capacity
- —
Chemical behavior of bromine
Bromine has seven valence electrons ([Ar] 3d¹⁰ 4s² 4p⁵). Its marked tendency is to gain an electron to attain the stable configuration of the krypton noble gas, adopting the characteristic oxidation state of -1 in bromides, although with more electronegative elements such as fluorine or oxygen it can present positive oxidation states (+1, +3, +5 and +7). As a halogen of period 4, it is a dense, volatile liquid non-metal, with intermediate oxidising power: it is a more energetic oxidant than iodine, but less than chlorine and fluorine. It forms covalent bonds with non-metals and ionic bonds with electropositive metals.
Reactivity
Br₂ + 2 KI → 2 KBr + I₂
It displaces iodine from its aqueous iodide solutions due to its greater electron affinity and reduction potential, releasing elemental iodine that stains the solution dark brown.
2 Na + Br₂ → 2 NaBr
It reacts vigorously and exothermically with alkali metals such as sodium to form crystalline sodium bromide.
Br₂ + H₂O ⇌ HBr + HBrO
In water it undergoes a partial disproportionation or dismutation reaction, generating hydrobromic acid and hypobromous acid, with disinfectant and bleaching properties.
H₂C=CH₂ + Br₂ → BrCH₂-CH₂Br
It rapidly adds to carbon-carbon double bonds in unsaturated organic compounds, decolourising the reddish bromine solution in a classic test of organic chemistry.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Silver bromide | AgBr | Fundamental light-sensitive compound in the emulsion of films and papers of traditional analogue photography |
| Sodium bromide | NaBr | Historical sedative and anticonvulsant in pharmacology, disinfectant of swimming pool and spa waters |
| Hydrobromic acid | HBr | Strong mineral acid used in organic chemical synthesis and in the production of inorganic bromides |
| Potassium bromate | KBrO₃ | Oxidising agent in volumetric analysis and former flour improving agent |
Electron shell distribution
View in the atom builderIsotopes
Natural bromine is formed by two stable isotopes in almost equimolar proportions: bromine-79 (⁷⁹Br) with 44 neutrons (50.69%) and bromine-81 (⁸¹Br) with 46 neutrons (49.31%). This almost 50% binary distribution produces an unmistakable spectral doublet with M and M+2 mass peaks of the same height in mass spectrometry. More than thirty synthetic radioisotopes are known; bromine-82 (⁸²Br, half-life of 35.3 hours) is used as a radioactive tracer in hydrological and industrial applications to detect leaks in underground pipelines.
Occurrence and production
It is the 64th most abundant element in the Earth's crust (about 2-3 ppm), but is very concentrated in the form of bromide anion (Br⁻) in seawater (approximately 65 mg/L) and above all in highly evaporated hypersaline brines such as those of the Dead Sea or deep wells in the United States. It does not exist in free state due to its high reactivity. Industrially it is extracted by treating these brines with gaseous chlorine, which oxidises bromide ions to liquid bromine, which is then dragged by water vapour or air.
History and origin of the name
It was discovered independently by the French chemist Antoine Jérôme Balard in 1826 while analysing mother brines of salt marshes in Montpellier, and by the German student Carl Jacob Löwig in 1825 from mineral waters of Bad Kreuznach. Balard was the first to publish the finding and characterise it as a new halogen chemical element. The name 'bromine' was proposed by the Académie des sciences from the Greek term 'brômos' (bile or pestilence), due to the asphyxiating, penetrating and irritating odour of its reddish vapours.
Applications and relevance of bromine
Uses of the element
Elemental bromine is mainly used as a synthesis intermediary in the fine chemical and pharmaceutical industry, as well as in the production of high-density drilling fluids (zinc and calcium bromide brines). It is also used as halogenating reagent in organic synthesis and in disinfectant formulations for industrial cooling towers and water treatments.
Uses of its compounds
Historically, brominated compounds dominated chemical photography (silver bromide) and leaded gasoline antiknock additives (dibromoethane). Today its main market is brominated flame retardants (BFR) for plastics and electronic textiles, along with industrial biocides, agricultural pesticides (methyl bromide, in withdrawal) and sedative and bronchodilator drugs.
Health, safety and environment
Biological role
Bromine does not perform a primary biological function in mammals, although recent research has suggested that traces of bromine are essential as a cofactor for the enzyme peroxidasin in the synthesis of collagen IV and tissue development. At higher doses, bromides act as depressants of the central nervous system.
Safety
Liquid bromine is extremely dangerous and corrosive: contact with skin causes severe chemical burns and painful ulcers that take a long time to heal. Its vapours are intensely toxic and irritating to the eyes and upper respiratory tract and can cause fatal pulmonary oedema. It must always be handled under an extraction hood and with airtight protective clothing.
Environment
Many brominated flame retardants and pesticides such as methyl bromide are persistent organic compounds (POPs) that bioaccumulate in marine and terrestrial food webs. Methyl bromide contributes notably to stratospheric ozone depletion, which is why its production is restricted by the Montreal Protocol.
Frequently asked questions about Bromine
Why is bromine liquid while fluorine and chlorine are gases?
At room temperature, fluorine and chlorine are gases, bromine is liquid and iodine is solid. This difference is due, in large part, to the fact that the molecules of these elements attract each other more as their size increases. In bromine, those attractions are enough to keep the molecules together in liquid state, but not enough to form a solid under normal conditions.
Why were bromides used as medical sedatives in the 19th and 20th centuries?
Potassium and sodium bromide salts were the first effective drugs discovered to calm epileptic seizures and treat anxiety and nervousness in 19th-century medicine. The bromide ion crosses the blood-brain barrier and partly replaces the chloride ion in neurons, hyperpolarising their membranes and inhibiting the excitability of the central nervous system. However, its accumulation caused chronic toxicity (bromism) with serious psychiatric alterations, so it was replaced by safer drugs.
Why is the Dead Sea the main source of bromine extraction in the world?
The Dead Sea is an endorheic basin with no outlet to the ocean subjected for millennia to extreme solar evaporation in a desert climate. While ordinary ocean water contains about 65 milligrams of bromide per litre, the deep waters of the Dead Sea concentrate more than 4,000 milligrams per litre. This colossal mineral wealth allows concentrated brines to be pumped to factories where pure bromine is extracted with extremely reduced energy consumption and operating costs.
Why is bromine so dangerous for the ozone layer?
Although there is less bromine than chlorine in the atmosphere, each bromine atom released by certain pesticides and fire extinguishers is up to sixty times more destructive. Bromine participates in continuous chemical cycles that break ozone molecules without being easily trapped in harmless compounds, causing much faster damage to the layer that protects us from the sun's ultraviolet radiation.
How do chemists detect the presence of bromine in a sample?
Natural bromine contains two main isotopes, bromine-79 and bromine-81, in similar proportions. When a sample is analysed with a mass spectrometer, molecules containing bromine usually produce two characteristic signals separated by two mass units. This pattern helps chemists recognise compounds containing the element, although it is usually combined with other tests to confirm its identity.
Why are bromine compounds added to plastics of electronic devices?
Electronic devices generate heat and contain flammable plastic materials. By incorporating substances with bromine in casings and circuits, if a fire starts, the heat decomposes these additives releasing bromine atoms. These atoms immediately trap the oxygen radicals that feed the flames, extinguishing the fire or slowing its advance to give time to evacuate.
How was silver bromide used to take photographs?
Traditional photographic films contained tiny crystals of silver bromide sensitive to light. When taking a photograph, light produced microscopic changes in those crystals and left an invisible image, called latent image. During development, the areas that had received more light darkened and the image became visible. This process was fundamental to analogue photography before the expansion of digital cameras.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.