White, red and black
Phosphorus has five outer electrons, [Ne] 3s² 3p³, and stands below nitrogen, but it behaves nothing like the inert gas above it. Freshly made, it is white phosphorus: a soft, waxy solid of P₄ molecules, four atoms at the corners of a tetrahedron with their bonds bent under strain — which is why it is so reactive. It ignites in air a little above room temperature, so it is stored under waterCompound: Water. Two hydrogens, one oxygen, and most of the living world; it glows faintly green in the dark as it oxidises, the chemiluminescence that gave the element its name; and it is very poisonous — a fraction of a gram kills.
Heated to about 250 °C, or left in the light, white phosphorus turns into red phosphorus, an amorphous polymer, odourless, not luminous, not poisonous and stable in air — the safe form on the side of a matchbox. Under pressure it becomes black phosphorus, flaky and graphite-like, the most stable form of all.
Rock, bone and cell
Phosphorus is never found free. About a tenth of a percent of the crust, it is locked in phosphate, mostly in the mineral apatite, and the phosphate rock worth mining — chiefly in Morocco, China, the United States and Russia — is ancient sea floor on which phosphate rained down from marine life. Some 220 million tonnes of rock are dug each year. Nearly all of it is treated with sulfuric acid to make phosphoric acid and the phosphate fertilisers on which the world's harvests depend, and there is no substitute: nothing grows without phosphorus, and the deposits do not renew on any human timescale.
For life carries phosphorus at its core. The two strands of DNA are held on a backbone of phosphate; ATP, the molecule every cell spends, stores its energy in phosphate bonds; the membrane of every cell is a double layer of phospholipids; and bone and teeth are calcium phosphate.
Fire, food and poison
The first matches, the lucifers of the 1830s, had white phosphorus in the head and struck on anything; they also gave the girls who made them phossy jaw, a rotting of the jawbone, and were banned by international convention in 1906. The safety match, perfected in Sweden in 1855, moved the phosphorus — red, harmless — to the striking surface. The element is still made, about a million tonnes a year, by heating phosphate rock with coke and sand in an electric furnace: for pure phosphoric acid, for flame retardants, for the organophosphate insecticides and, in the 1930s, the nerve agents that grew out of them. White phosphorus bombs burnt Hamburg in 1943, the city where the element was found. Phosphorus is also what makes ironElement: Iron. The metal that built the modern world, and the core of the planet beneath it and steel brittle. Phosphates soften water in detergents — and, running off into lakes, feed the algal blooms that choke them.
Discovery and name
Phosphorus is the first element whose discovery is on record. In 1669 Hennig Brand, a Hamburg alchemist searching for the philosopher's stone, boiled down some fifty buckets of urine, let the residue rot, and distilled it at great heat; what came over was a waxy substance that shone in the dark. He named it for the morning star, phosphorus, the light-bearer. Brand sold the secret; it reached Johann Kunckel and Johann Daniel Kraft, and Kraft showed the glowing substance to Robert Boyle in London in 1677. Boyle worked out the process for himself, deposited it sealed with the Royal Society in 1680 and published his observations that year as The Aerial Noctiluca. In 1769 Johan Gottlieb Gahn found phosphorus in bone, and Carl Wilhelm Scheele devised a way of making it from bone ash. Lavoisier listed it among the simple substances in 1789.