Bromine - Br

Bromine (Br) is a reactive halogen used in chemical manufacture and disinfection. For water treatment, the distinction between elemental bromine, bromide and bromate is essential: they have different properties, risks and treatment requirements.

Chemical properties · Uses and occurrence · Health effects · Environmental effects · Water and treatment

Chemical properties of bromine

Selected properties of bromine
Symbol / atomic number	Br / 35
Group / period / block	17 / 4 / p
Standard atomic weight interval	[79.901, 79.907]
Electron configuration	[Ar] 3d¹⁰ 4s² 4p⁵
Elemental molecular form	Br₂
Density (liquid, near room temperature)	3.1028 g cm⁻³
Melting point	−7.2 °C
Boiling point	58.8 °C
Electronegativity (Pauling)	2.96
Non-bonded atomic radius	0.185 nm
Covalent radius	0.117 nm
First ionisation energy	1139.9 kJ mol⁻¹
Naturally occurring isotopes	⁷⁹Br and ⁸¹Br
Discovery	Carl Löwig prepared it in 1825; Antoine-Jérôme Balard published its discovery in 1826
CAS registry number	7726-95-6

Atomic weight follows CIAAW; parentheses indicate uncertainty in the final digits. Physical properties follow the Royal Society of Chemistry unless noted. Atomic radii depend on their definition. The atomic-weight interval reflects natural variation in isotope abundance.

Sealed flask containing reddish-brown bromine — Bromine contained in a sealed flask.

Uses, occurrence and chemical forms

Bromine is a halogen, alongside fluorine, chlorine and iodine. Elemental bromine is a volatile, reddish-brown liquid at room temperature and gives off irritating vapour.

In nature, bromine is commonly present as bromide ions in seawater and concentrated brines. Elemental bromine is produced from bromide-rich resources. Bromine-containing substances have applications in chemical manufacture, pharmaceuticals, flame retardants and disinfection; uses and restrictions depend on the particular substance.

Three forms are especially important in water engineering:

Bromine (Br₂): the reactive elemental substance.
Bromide (Br⁻): a dissolved ion that can occur naturally in source water.
Bromate (BrO₃⁻): an oxidised by-product that can form during treatment.

These names describe different chemicals. A concentration measured as bromide must not be interpreted as the same concentration of active bromine disinfectant or bromate.

Health effects of bromine

Elemental bromine can cause skin and eye burns. Inhalation can irritate and injure the respiratory tract, causing coughing and difficulty breathing; serious exposures can have lasting effects. Exposure severity depends on concentration, duration and route. Significant exposure requires prompt medical assessment and removal from the source.

Bromide salts, bromate and organic bromine compounds have different toxicological profiles. It is inaccurate to assign identical health effects to every bromine-containing chemical or to assume that all organic bromine compounds are synthetic and equally hazardous. Identify the substance before interpreting an exposure or analytical result.

Environmental effects of bromine compounds

Bromide is naturally present in saline waters, while industrial discharges and other human activities can alter local concentrations. The environmental significance of a bromine-containing material depends on its identity, concentration, persistence and exposure pathways.

Water disinfection can transform bromide into more reactive bromine species and brominated by-products. Monitoring should therefore address the relevant individual chemicals, rather than treating total bromine as a complete measure of ecological or health risk.

Bromine, bromide and bromate in water treatment

Bromine-based disinfectants produce active species including hypobromous acid in water. They are used in applications such as swimming pools and some industrial water systems. See bromine as a disinfectant for further background.

During ozonation, bromide-containing source water can form bromate. Formation depends on bromide concentration, ozone dose, pH and other water characteristics. Bromide can also contribute to brominated organic disinfection by-products when oxidants react with natural organic matter.

WHO's provisional guideline for bromate in drinking water is 10 µg/L (0.010 mg/L). This value applies to bromate, not to all bromine species, and is not a universal legal limit.

Preventing bromate formation is generally preferable to removing it after it forms. Assess source-water bromide, optimise the disinfection process, and verify both microbial treatment and by-product control. Reducing disinfection simply to lower bromate must not compromise microbiological safety.

Where removal is necessary, membrane or ion-exchange options require assessment for the specific contaminant and water composition. A process effective for bromide should not automatically be assumed to have the same performance for bromate. Confirm performance through testing and ongoing monitoring.

Bromine page last reviewed: 24 October 2026 by Lenntech.