Bromine (Br)

Bromine and mercury are the only two elements that are liquid in an ordinary room, and bromine is the only nonmetal in that pair. It is a dense, dark red-brown fluid that gives off a coloured vapour continuously, because its boiling point sits close enough to room temperature that a sample is always partly in the air above it. Chlorine above it in the group is a gas; iodine below it is a solid that sublimes. Bromine occupies the narrow band in between, and everything about how it is shipped, stored and traded follows from that awkward position.

A name that describes the smell

Antoine Jérôme Balard, working in Montpellier in 1825 and 1826 on the residues left after salt was crystallised from marsh brine, treated the mother liquor with chlorine and got a red-brown layer he could not identify as anything known. He proposed calling it muride, from Latin muria, brine. The commission of the French Academy that reviewed his work rejected the name and substituted brome, from Greek bromos — stench. The element is one of very few named for how unpleasant it is.

Two students, one spring, and a bottle Liebig kept

Balard is credited, but he was not first to hold the substance. Carl Löwig, a student at Heidelberg, had already extracted the same red liquid in 1825 from the waters of a salt spring at Bad Kreuznach. His professor, Leopold Gmelin, asked him to prepare a larger quantity before publishing. By the time Löwig had done so, Balard's paper was out. The disagreement between the two is genuine, and the usual summary is fair: Löwig found it first and Balard characterised and announced it first.

There is a third figure with a worse outcome. Justus von Liebig had been sent a sample of brine from the same Bad Kreuznach spring some years earlier, examined it, decided the red material was a compound of iodine and chlorine, and shelved the bottle. He is said to have kept it afterwards as a reminder of the cost of concluding too quickly from a plausible analogy. Whether or not the cabinet in that anecdote existed, the misidentification is documented, and it cost him an element.

The snail dye that needed bromine to be purple

Tyrian purple, the imperial dye of the ancient Mediterranean, is 6,6'-dibromoindigo. It was extracted from the hypobranchial gland of Murex and related sea snails at a yield so poor that thousands of animals produced a few grams, which is why the colour signified rank rather than taste.

Structurally it is indigo — the same molecule as denim dye — with a bromine atom added at each end. Those two atoms are the entire difference between a deep blue and the reddish purple that Rome legislated over. The bromine comes from seawater, concentrated by the snail's own enzymes, and it was the first documented case of an organism deliberately making an organobromine compound.

Not seawater, but the brine left behind

Seawater carries roughly 65 parts per million of bromide, which is far too dilute to work with. Commercial production goes instead to places where evaporation has already done the concentrating. The Dead Sea is the richest, with bromide around a hundred times the ocean concentration, and plants on both the Israeli and Jordanian shores dominate world output. The other major source is the Smackover Formation brine beneath southern Arkansas, pumped from deep wells; China draws on inland brines and salt lakes.

Because the raw material is a liquid that already exists underground, bromine has no mine, no ore grade and no tailings — a supply chain shaped more like natural gas than like a metal.

Flame retardants, and two markets that vanished

The dominant use today is flame retardancy. Brominated compounds interrupt combustion in the gas phase, releasing bromine radicals that quench the chain reactions in a flame, and they do it at lower loadings than mineral alternatives. Tetrabromobisphenol A, reacted into the epoxy resin of printed circuit boards, is the largest single product; roughly speaking, most of the world's circuit boards contain bound bromine.

Two much larger bromine markets simply disappeared:

• Leaded petrol. 1,2-dibromoethane was added alongside tetraethyl lead as a scavenger, converting lead deposits into volatile lead bromide that left with the exhaust. This consumed more bromine than anything else in the mid-twentieth century and went to essentially zero as leaded fuel was withdrawn.
• Photographic film. Silver bromide was the light-sensitive salt in film and paper. Digital imaging removed that demand within about fifteen years.

Smaller but genuinely useful roles remain: dense calcium and zinc bromide solutions serve as solids-free drilling fluids where mud would damage a formation, and brominated activated carbon is injected into coal plant flue gas to capture mercury.

Fifty times worse than chlorine on ozone

Bromine radicals destroy stratospheric ozone by the same catalytic cycle as chlorine, but roughly fifty times more efficiently per atom, because bromine is less readily locked up in reservoir compounds that take it out of circulation. Two important bromine products were curtailed as a result. Methyl bromide, an outstanding soil fumigant, was phased out under the Montreal Protocol with narrow exemptions; halon fire suppressants, which are unmatched at protecting electronics and aircraft engine nacelles, were banned from new production and are now supplied from recycled stocks held for critical uses.

Two isotopes that weigh almost the same

Bromine has two stable isotopes in nearly equal proportion — bromine-79 at about 51% and bromine-81 at about 49%. That near-even split is why the standard atomic weight is published as an interval rather than a single value, and it gives organic chemists one of their most useful diagnostics: any bromine-containing molecule shows twin peaks of almost identical height, two mass units apart, in a mass spectrum. Nothing else produces quite that pattern, so a bromine atom in an unknown compound announces itself before any other analysis is done.

Recognised as essential in 2014

Bromine was long assumed to be biologically incidental. In 2014 a group at Vanderbilt led by Billy Hudson showed that peroxidasin uses bromide to form sulfilimine cross-links in collagen IV, the protein mesh underlying every basement membrane in the animal body, and that fruit flies deprived of bromide die. It is the most recent element to be added to the list of those required by animals, and the requirement is structural rather than catalytic — a rare thing for a trace element.