Atomic number 35

Bromine

Br
+ phase
− phase

Shells Orbitals 3d 4s 4p

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35 electrons in 4 shells: 2, 8, 18, 7.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Halogen
Period: 4
Group: 17
Block: p
Standard atomic weight: 79.904(3) Da
Electron configuration: [Ar] 3d¹⁰ 4s² 4p⁵
Phase at 25 °C: liquid
Melting point: −7.2 °C
Boiling point: 58.8 °C
Density: 3.11 g/cm³
Electronegativity (Pauling): 2.96
Discovered: 1826, Antoine-Jérôme Balard

Neighbours in the table

Se Kr

OpenStax sorts the nonmetals by state: under normal conditions more than half are gases, the rest are solids, and one is a liquid. That one is bromine, on Los Alamos’s page the only nonmetallic liquid element. It melts at −7.20 °C and boils at 58.80 °C, and between those it is a dense red-brown liquid of 3.11 g/cm³. Los Alamos has it turning readily to red vapour at room temperature.

On stage, 35 electrons form shells of 2, 8, 18 and 7, or [Ar] 3d¹⁰ 4s² 4p⁵. OpenStax counts the ten 3d electrons as core, not valence, which leaves seven valence electrons, as in fluorine and chlorine. That shared count is why the three behave alike. Each atom in Br₂ makes just one bond; the shared pair supplies the one electron it lacked for an octet, eight outer electrons.

Its electronegativity, the pull it exerts on a bond’s shared electrons, is 2.96 on the Pauling scale. OpenStax ties oxidising power to that pull and calls bromine the weaker oxidising agent beside chlorine: it takes electrons less forcefully. In bromides it is the more electronegative partner and takes the oxidation state −1, the charge assuming wholly ionic bonds. Beside oxygen or fluorine it goes positive, to +5 in BrF₅ and +7 in perbromate, whose salts OpenStax calls difficult to prepare.

Where it is found

Bromine is a brine element. The US Geological Survey (USGS) locates it principally in seawater, salt lakes and underground brines beside petroleum deposits. Seawater holds about 65 parts per million, USGS says, and the Dead Sea alone an estimated 1 billion tons. The crust is poorer: 2.4 milligrams per kilogram in PubChem’s element record. In the sea it is the bromide ion, Br⁻, since OpenStax counts the halogens too reactive to occur free in nature.

Getting the element out is chlorine’s job. Chlorine passed through the brine takes an electron from each bromide ion and leaves as chloride, and every two bromide ions become one Br₂ molecule. It works because chlorine is the stronger oxidising agent: the heavier halide ions, bromide among them, give up electrons to the lighter halogens. OpenStax credits the method with almost all American bromine.

Two US companies pump it from Arkansas brines, USGS’s 2025 summary says; Michigan wells also yield it, Los Alamos and Jefferson Lab say. Output outside the United States in 2024 was an estimated 400,000 metric tons of bromine content, the US figure withheld: Israel about 140,000, Jordan 120,000, China 100,000.

Why it matters

Bromine was first made by a student, Carl Löwig, in the summer before his first year at Heidelberg. He showed the red, smelly liquid to his professor, Leopold Gmelin, who recognised something new and asked for more. Winter exams and the holidays held him up, and in 1826 Antoine-Jérôme Balard published a paper describing the new element. CIAAW’s history in PubChem’s record dates Löwig’s work to 1825 and credits Balard, who announced it first.

USGS’s 2025 summary puts two applications at the head of world demand: brominated flame retardants and clear brine fluids for drilling and completing oil and gas wells. For the well-completion and packer fluids it finds no comparable substitute. The bromine in a zinc-bromine battery can be recovered in full when the battery is spent, USGS adds.

Most US bromine once went into ethylene dibromide, a leaded-petrol additive that stopped lead compounds building up in engines; unleaded fuel cut that demand greatly. Bromide salts served as sedatives from the 19th century until the risk of bromism, chronic bromide poisoning, removed them from most US pharmaceuticals by 1978. In July 2024 the Food and Drug Administration revoked its authorisation for brominated vegetable oil, a drink-flavouring stabiliser since the 1920s, when no safe dietary level could be established.

Properties

Properties of bromine Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	35
Standard atomic weight	79.904(3) Da	Da	IUPAC/CIAAW 2024
Melting point	−7.2 °C	°C	Los Alamos: −7.2 °C
Boiling point	58.8 °C	°C	Los Alamos: 58.8 °C
Density	3.11 g/cm³	g/cm³
Electronegativity (Pauling)	2.96
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵
Phase at 25 °C	liquid

The standard atomic weight of this element is an interval in nature, from 79.901 to 79.907 Da; the value shown is IUPAC's conventional abridged value.

Safety

Elemental bromine is hazardous as a liquid and as a vapour heavier than air, and the hazard sits with the plants, tankers and laboratories that handle it. PubChem’s GHS classification gives the signal word Danger; the harmonised European entry reads: causes severe skin burns and eye damage, fatal if inhaled, very toxic to aquatic life. One of 725 European Chemicals Agency reports finds no hazard; 99.9% of the rest carry each code. NIOSH’s entry for molecular bromine gives the exposure routes as inhalation, ingestion and skin or eye contact; it and the PubChem Laboratory Chemical Safety Summary give the detail.

Questions

Is bromine the only liquid element?

No, but it is the only nonmetal that is one, on Los Alamos's page; mercury is a liquid at room temperature too, OpenStax notes. Bromine's group tells the story: chlorine above it is a gas, iodine below it a solid, and bromine sits between. OpenStax puts the change of state down to London forces, the attractions between neighbouring molecules, which strengthen as molecular mass rises and as the molecule becomes more polarisable, its electron cloud easier to distort.

Why is bromine less reactive than chlorine?

OpenStax calls bromine the weaker oxidising agent of the two and ties oxidising power to electronegativity, which falls with each step down a group; bromine's 2.96 on the Pauling scale is below chlorine's. With sulfur, chlorine gives SCl₂, bromine only S₂Br₂, and iodine does not react with sulfur at all. Bromine still oxidises iodide: OpenStax gives its reaction with hydrogen iodide a potential of 0.55 V.

Why is bromine called bromine?

For its smell. Antoine-Jérôme Balard, who announced the element in 1826 and holds the credit for it, named it after bromos, the Greek word for stench, as Jefferson Lab tells it in PubChem's element record; CIAAW's history glosses the word as bad stench or bad odour. Los Alamos describes the smell as strong and disagreeable and like chlorine's, and NIOSH's card calls the fumes suffocating and irritating.

Is bromine used to treat water?

Yes, it is. The US Geological Survey lists water treatment among bromine's applications, and Los Alamos lists water purification compounds and sanitisers among its uses. The Survey adds that chlorine and iodine can stand in for bromine for sanitation purposes.

Why does bromine's atomic weight have a range?

Bromine has two stable isotopes, bromine-79 and bromine-81, in nearly equal amounts: CIAAW gives bromine-79 between 50.5 and 50.8 per cent of atoms and bromine-81 the rest. IUPAC's isotope guide in PubChem's record explains that atoms and molecules of the two differ slightly in physical and chemical behaviour, so natural processes sort them, and different terrestrial materials end up with different proportions and therefore different atomic weights. IUPAC's conventional value for bromine's atomic weight is 79.904 Da.

Compounds containing bromine

HBr Hydrogen bromide 2 atoms, 1 single bond.

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