Bromine is the chemical element with symbol Br and atomic number 35. It is a halogen and one of only two elements that are liquids near ordinary room temperature. Elemental bromine exists as Br₂, a dense red-brown, fuming liquid with a strong irritating odour.
Bromine quick facts
| Property | Value |
| Symbol | Br |
| Atomic number | 35 |
| Relative atomic mass | 79.904 |
| Group | 17 |
| Period | 4 |
| Block | p-block |
| State at 20°C | Liquid |
| Density | 3.1028 g/cm³ |
| Melting point | −7.2°C |
| Boiling point | 58.8°C |
| Electron configuration | [Ar] 3d¹⁰ 4s² 4p⁵ |
| Common oxidation state | −1 |
| CAS number | 7726-95-6 |
| Stable natural isotopes | ⁷⁹Br, ⁸¹Br |
Position and electron configuration
Bromine is in group 17, period 4 and the p-block. Its electron configuration is [Ar] 3d¹⁰ 4s² 4p⁵. Like other halogens, bromine has seven valence electrons and readily forms bromide ions, Br⁻.
See the complete CleverlySmart periodic table.
Physical and chemical properties
Bromine is less reactive than chlorine but more reactive than iodine. Its vapour is red-brown and corrosive. Bromine reacts with many metals to form bromides and with hydrogen to form hydrogen bromide, which dissolves in water to produce hydrobromic acid.
Important bromine compounds
- Hydrogen bromide (HBr): forms hydrobromic acid in water and is important in chemical synthesis.
- Silver bromide (AgBr): historically central to photographic film because of its light sensitivity.
- Brominated flame retardants: used in some polymers and electronics, although several compounds face restrictions because of environmental persistence and toxicity concerns.
- Bromides and organobromine compounds: important intermediates in pharmaceutical, agricultural and specialty chemical synthesis.
Occurrence and production
Bromine is too reactive to occur freely in nature. It is found mainly as bromide ions dissolved in seawater, salt lakes and underground brines. Commercial bromine is produced by oxidising bromide-rich brines, commonly using chlorine, and separating the liberated bromine.
Uses of bromine
- Chemical synthesis: brominated intermediates are widely used to build pharmaceuticals and fine chemicals.
- Flame retardants: certain brominated compounds inhibit combustion, though environmental concerns have reduced or changed many applications.
- Oil and gas operations: dense bromide brines are used in some drilling and completion fluids.
- Water treatment: bromine-based disinfectants are used in some spas, cooling systems and industrial water applications.
- Photography: silver bromide was historically one of the key light-sensitive materials in photographic emulsions.
Bromine isotopes
Natural bromine contains two stable isotopes, ⁷⁹Br and ⁸¹Br, in nearly equal proportions. This produces a distinctive two-isotope pattern in mass spectra of bromine-containing molecules.
For broader isotope information, see the CleverlySmart isotope table.
Biological role and safety
Bromide ions occur naturally in living organisms and the environment, but elemental bromine is highly hazardous. Bromine liquid causes severe chemical burns, and its vapours strongly irritate and can damage the respiratory system. It must be handled in closed systems or fume hoods with appropriate protective equipment.
Some brominated organic compounds can persist in the environment or affect ozone chemistry, so their environmental behaviour depends strongly on molecular structure and volatility.
Discovery and name
French chemist Antoine Jérôme Balard discovered bromine in 1826 while studying brine residues. The name derives from the Greek bromos, meaning stench, referring to the element’s strong odour.
Frequently asked questions
Why is bromine a liquid?
Br₂ molecules are larger and more polarizable than Cl₂ molecules, giving bromine stronger intermolecular forces and a higher boiling point that places it in the liquid state near room temperature.
Is bromine dangerous?
Elemental bromine is toxic and corrosive. It should not be handled outside properly equipped laboratory or industrial settings.
Bromine chemistry in water and the atmosphere
Bromine compounds are important oxidants and halogenating agents. In water treatment, bromine-based disinfectants can remain effective over a useful pH range, which is why they are used in some spas, cooling towers and industrial systems. In bromide-containing water, however, strong oxidation can also form brominated disinfection by-products, so treatment chemistry must be carefully controlled.
Reactive bromine species also participate in atmospheric chemistry. Certain long-lived brominated compounds historically used in fire suppression can reach the stratosphere, where released bromine radicals catalytically destroy ozone. This is why many ozone-depleting halons have been phased out or tightly controlled.
Why bromine is liquid at room temperature
Bromine molecules are larger and more polarizable than chlorine molecules. Stronger London dispersion forces between Br₂ molecules raise the melting and boiling points, placing elemental bromine in the liquid state near room temperature while chlorine remains a gas.
Frequently asked questions
Where does commercial bromine come from?
Most is recovered from bromide-rich underground brines and salt-lake waters rather than mined as elemental bromine.
Why must bromine be handled in a fume hood?
Its vapour is highly irritating, corrosive and toxic, so engineering controls are essential to prevent inhalation and contact.
