Sulfur

Rings of eight

Sulfur is a pale yellow, brittle, tasteless solid with no smell of its own — the reek people associate with it belongs to its compounds. Below oxygen in group 16, with six outer electrons, [Ne] 3s² 3p⁴, it forms rings and chains as readily as oxygen forms O₂ molecules. The everyday form is a crown-shaped ring of eight atoms, S₈, packed into orthorhombic crystals; above 95.3 °C the same rings repack into monoclinic needles.

Melt it and it turns to a runny amber liquid; heat it past about 159 °C and the liquid suddenly thickens and darkens, because the rings break open and join into chains millions of atoms long; pour it into cold water and it sets as plastic sulfur, rubbery and stretchy, which slowly crystallises back. It burns in air with a blue flame to sulfur dioxide, the choking gas of struck matches and volcanic vents, and with hot metals it gives sulfides.

Brimstone, ore and sea

Around volcanoes sulfur is found as itself, yellow crusts left where hot gases cool; Sicily supplied most of Europe's from such deposits until the twentieth century, and at Kawah Ijen in Java it is still dug by hand. Vast beds also lie above the salt domes of the Gulf of Mexico, where bacteria made it from gypsum and which from 1894 Herman Frasch's process melted underground with superheated water and pumped up. Bound to metals it is the sulfide ores — pyrite, the fool's gold that is a sulfide of iron, galena, sphalerite, cinnabar and the copper sulfides — and bound to oxygen it is gypsum and the sulfate that makes up nearly 0.3 % of seawater.

Today most sulfur is not mined at all. Oil and natural gas contain it, and since it must be removed to prevent acid rain, refineries strip it out as hydrogen sulfide and convert that to elemental sulfur by the Claus process; world production, some 80 million tonnes a year, is mostly this by-product.

Acid, rubber and life

Most sulfur is burnt. Sulfur dioxide, oxidised over a vanadium catalyst and absorbed in water, gives sulfuric acid, of which the world makes some 250 million tonnes a year, more than of any other chemical. More than half of it goes to dissolve phosphate rock into fertiliser; the rest to metals, pigments, detergents and car batteries. Charles Goodyear found in 1839 that heating rubber with sulfur turns it from a sticky gum into a springy, durable solid — vulcanisation, on which every tyre depends. Sulfur was one of the three ingredients of the gunpowder the Chinese invented in the ninth century, and it has been burnt as a fumigant since antiquity; dusted on vines against mildew since the 1840s, it remains one of the most used fungicides.

Living things need it. Two amino acids, cysteine and methionine, contain it, and the bridges between cysteines hold proteins in shape — insulin, and the keratin of hair, which a perm breaks and re-forms. Garlic, onion, skunk and rotten egg all smell of sulfur compounds.

History and name

Sulfur has no discoverer. It is the brimstone that rained on Sodom in Genesis and that Odysseus burnt to purify his hall, and to the Arabic and medieval alchemists it was one of the two principles of all metals, the fiery, combustible one, paired with mercury. The name is the Latin sulpur, of unknown older origin; brimstone means burning stone. Lavoisier placed it among the simple substances in 1789; Humphry Davy suspected in 1809 that it contained hydrogen and oxygen, but Gay-Lussac and Thénard showed these to be impurities, and the element stood. The spelling wavered for centuries between sulphur and sulfur; IUPAC chose sulfur in 1990.