Bismuth (Bi)

For most of the twentieth century bismuth held a specific title: the heaviest stable element. Beyond it, everything is radioactive. The line had to be drawn somewhere and bismuth-209, sole survivor of an element with exactly one naturally occurring isotope, was where it fell.

In 2003 a French group working deep under the Alps at the Modane underground laboratory took the title away. Pierre de Marcillac and colleagues built cryogenic scintillating bolometers out of bismuth germanate crystals, cooled them to milli-kelvin temperatures, and watched. Alpha decays turned up — a handful of them, unmistakably — giving bismuth-209 a half-life of about 1.9 × 10¹⁹ years.

What a billion universes' worth of half-life looks like

That number is roughly a billion times the present age of the universe, so the practical consequences are nil. A kilogram of bismuth registers something on the order of one alpha decay every few minutes, which is far below the natural radioactivity of a kilogram of your own body and immeasurably below that of a kilogram of granite.

The bookkeeping consequences are not nil. Lead-208 inherits the title of heaviest genuinely stable nuclide. Bismuth-209 becomes the heaviest primordial nuclide — the heaviest thing still present on Earth from the material the solar system formed out of. And every reference table that lists bismuth as stable is, strictly, out of date, which is why the decay had to be observed with a detector buried under a mountain: only in a place shielded from cosmic rays could an event rate that low be distinguished from background at all.

Weisse Masse, wismut, bisemutum

Bismuth was mined in Saxony from the fifteenth century and was consistently mistaken for something else — a variety of lead, or of tin, or of antimony. Georgius Agricola wrote about it in 1546 and treated it as its own material, though within a classification scheme that made it a relative of lead. The demonstration that it is a separate element came from Claude François Geoffroy in 1753.

The name's origin is genuinely unresolved. The German Wismut has been derived from weisse Masse, white mass, and alternatively from Wiese, a meadow, after the mining district where it was dug. Agricola latinised whatever he heard as bisemutum, and that is where the symbol comes from — one of the few element symbols traceable to a sixteenth-century mining vernacular rather than to Latin or Greek.

The metal that magnets push away

Every material responds to a magnetic field. Most respond so weakly that the effect is invisible. Bismuth's response is the strongest negative one of any metal: it is diamagnetic, it develops a field opposing the one applied to it, and it is repelled by both poles of a magnet.

That is one of a family of unusual electronic behaviours that all trace to the same cause. Bismuth is a semimetal — its conduction and valence bands overlap by only a few tens of millielectronvolts, so it has very few charge carriers, and those it has are unusually light and mobile. Consequences follow throughout its physics: an enormous Hall coefficient, which made bismuth spirals the standard magnetic field sensor for half a century; a thermal conductivity lower than any metal except mercury; and an extreme magnetoresistance. The Shubnikov–de Haas and de Haas–van Alphen oscillations, now standard tools for mapping the electronic structure of any conductor, were both discovered in bismuth in 1930 because bismuth shows them more clearly than anything else.

Modern condensed-matter physics kept the habit. Bismuth antimonide was the first three-dimensional topological insulator confirmed experimentally, in 2008, and bismuth selenide and telluride became the workhorse materials of the field that followed.

Bismuth telluride is in your wine cooler

The commercially dominant compound is bismuth telluride, and it is dominant because nothing else does the job. Bi₂Te₃ and its alloys have the best thermoelectric figure of merit of any material near room temperature, which means that essentially every Peltier device — the solid state coolers in portable refrigerators, in laser diode mounts, in CPU chillers and in scientific instruments — is built from it. There is no serious competitor in that temperature range and there has not been one for sixty years.

The freezing that expands

Bismuth is one of the very small set of substances whose solid is less dense than its liquid; it expands by around 3.3% as it solidifies. Water does this, and so do silicon, germanium, gallium and antimony, and almost nothing else.

For a caster this is a gift. Molten alloy that expands slightly as it sets is pressed into every corner of the mould rather than shrinking away from it, which is why bismuth appeared in type metal for printing and still appears in alloys used for precision castings.

The bigger use of that behaviour is in fusible alloys. Bismuth combined with lead, tin, indium or cadmium produces mixtures that melt at temperatures a boiling kettle could reach — Wood's metal at 70 °C, cadmium-free Field's metal at 62 °C. Fire sprinkler heads are held shut by links of such alloy, and pressure vessels carry fusible plugs of it, so that a fire or an overheat releases the system mechanically without needing electronics or a sensor.

Lead's replacement, extracted from lead

Bismuth's twenty-first-century growth market is substitution. As lead has been legislated out of plumbing fittings, solders, shot, fishing weights, ceramic glazes and machining brass, bismuth has repeatedly been the element that took its place — it is similarly dense, similarly soft, similarly low-melting, and it is not a cumulative neurotoxin.

Medicine has used it far longer. Bismuth subsalicylate is the active ingredient of the pink antacid suspension found in bathroom cabinets across the English-speaking world, and it temporarily blackens the tongue and stool because bismuth sulfide forms in the gut. Bismuth quadruple therapy remains a first-line regimen against Helicobacter pylori in regions where antibiotic resistance has made the alternatives unreliable. Cosmetics use bismuth oxychloride as the pearlescent pigment in nail polish and eyeshadow.

There is a neat irony in the supply chain. Bismuth is almost never mined for its own sake: the great majority is recovered as a byproduct of refining lead, with smaller contributions from tungsten and copper operations. The metal that is replacing lead is chiefly obtained by purifying lead.

Production is heavily concentrated in China, which accounts for most global output, and in February 2025 bismuth was added to Chinese export controls alongside tungsten, tellurium, molybdenum and indium — a reminder that a substitute material can inherit a supply risk of its own.

Stair-step crystals

The bismuth most people have actually seen is not industrial. Slowly solidified bismuth grows skeletal hopper crystals, in which the edges of each cube advance faster than the faces and leave a stepped, terraced hollow behind. The rainbow colouring on them is not the metal: freshly grown crystals are silvery, and the oxide layer that forms in air produces thin-film interference, the same effect that colours an oil slick, with the hue set by the thickness of the film.