Bromine

Bromine is one of only two elements that are liquid at room temperature: the other being mercury. It is a dark red-brown, dense liquid with a sharp, unpleasant smell and is corrosive to skin and eyes. Its name comes from the Greek word for stench. Yet bromine compounds are essential to photography, flame retardants, medicines and oil-well drilling.

  • Atomic Number3535 protons, 35 electrons
  • Atomic Mass79.90 uAbout 80× heavier than hydrogen
  • State at Room TempLiquidred-brown LIQUID at room temperature
  • Density3.11 g/cm³About 3× denser than water
  • Melting / Boiling-7.2°C / 58.8°CLiquid from −7°C to 59°C
  • Discovered1826Carl Jacob Löwig & Antoine Balard, 1825-26

Bromine is the only non-metal liquid at room temperature. Compare the halogens.

Halogens progress from gas (F, Cl) to liquid (Br) to solid (I) as mass increases down Group 17.

Atomic Mass Comparison
Chlorine35.5 u
Bromine79.9 u
Iodine127 u
Iron55.8 u
Astatine210 u

Bromine (79.9 u) is about twice as heavy as chlorine (35.5 u) but lighter than iodine (127 u). This increasing mass down Group 17 corresponds to increasing van der Waals forces between molecules, which is why fluorine and chlorine are gases, bromine a liquid, and iodine and astatine solids at room temperature.

What is bromine?

Bromine is a halogen in Group 17 of the periodic table, sitting below chlorine and above iodine. It has 35 protons and seven electrons in its outer shell. At room temperature, bromine (Br₂) is a dense, red-brown liquid that evaporates to produce a suffocating, orange-brown vapour. Like all halogens, it is highly reactive and will attack skin, eyes and mucous membranes on contact. It is less reactive than fluorine or chlorine but more reactive than iodine.

Bromine gets its name from the ancient Greek word bromos, meaning stench or foul smell, entirely appropriate for its choking, distinctive odour. The element was discovered almost simultaneously in 1825 and 1826 by two different chemists: Carl Jacob Löwig in Germany and Antoine Balard in France, both of whom extracted it from brine. The symbol Br comes from the name.

Fact The famous Tyrian purple dye of the ancient world: the colour reserved for Roman emperors and used in the ceremonial robes of kings and popes for centuries, was made from the slime of murex sea snails. This extraordinarily expensive dye was a bromine-containing compound called dibromoindigo. It took approx. 10,000 snails to produce just one gram of the dye.

Where you find bromine

On Earth

Bromine is relatively abundant in seawater and in underground brines (salty water), though rare as a free element.

  • Seawater. Seawater contains approx. 65 parts per million of bromide ions. The Dead Sea and other hypersaline lakes have much higher concentrations. Israel is a major bromine producer, extracting it from Dead Sea brines.
  • Underground brines. Arkansas in the USA has large underground salt brines rich in bromine, making the USA one of the world's largest producers.
  • Bromide salts. Bromine is always found as bromide ions (Br⁻) bonded to sodium, potassium, calcium or magnesium, never as the free liquid element in nature.

How we use bromine

  • Flame retardants. About 40% of all bromine goes into brominated flame retardants, chemicals added to furniture foam, plastics, circuit boards and textiles to slow the spread of fire.
  • Agriculture. Methyl bromide was once widely used as a soil fumigant and pesticide, though it has been largely phased out under the Montreal Protocol because it damages the ozone layer.
  • Medicine. Bromide salts were once widely used as sedatives and anti-epilepsy drugs. "Bromide" became slang for a boring remark because bromides were associated with calming, dull effects.
  • Photography (historically). Silver bromide is light-sensitive and was the active ingredient in photographic film and paper from the 1870s to the digital era.
Did you know? The unusual expression "to take a bromide", meaning to say something dull and unoriginal, comes from the medical use of potassium bromide as a sedative and anti-epilepsy drug in the 19th and early 20th centuries. Bromides made patients drowsy and calm, so a "bromide" came to mean something that had a sedative, dulling effect on the mind.

How it was discovered

Bromine was discovered independently in 1825 and 1826 by two young chemists. Antoine Balard in France extracted an orange liquid from the brine of a saltmarsh in Montpellier and recognised it as a new element, calling it muride (from Latin for brine). Around the same time, Carl Jacob Löwig in Germany independently produced the same orange liquid from a mineral spring. The French Academy of Sciences settled the matter by naming it bromine from the Greek for stench. Both Balard and Löwig share credit for the discovery.

Deeper dive: brominated flame retardants and the ozone layer

Brominated flame retardants (BFRs) are added to plastics, textiles, electronics and furniture to reduce the risk of fire. They work by releasing bromine atoms when heated, bromine atoms interrupt the chain reactions that propagate flames, slowing or stopping burning. This has saved many lives in house fires and aircraft accidents. However, many BFRs are persistent organic pollutants, they accumulate in the food chain and have been found in human blood, breast milk, polar bears and deep-ocean fish. Some have been banned under the Stockholm Convention on persistent organic pollutants.

Bromine atoms, like chlorine atoms, destroy stratospheric ozone. Methyl bromide (CH₃Br), used as an agricultural fumigant, was identified as a significant ozone-depleting substance and phased out under the Montreal Protocol. However, bromine is even more efficient at destroying ozone per atom than chlorine, so even small releases are environmentally significant. Natural sources of methyl bromide from the oceans and kelp beds also contribute to this destruction.

Bromine is the liquid halogen, a red-brown chemical stench with a history stretching from Tyrian purple to flame-proof sofas. Moving to 36 protons brings us to krypton, a noble gas more famous than it deserves to be thanks to a certain superhero's home planet.