Neither metal nor nonmetal
Boron is a metalloid, and the odd one out in its row. Pure, it is a black, very hard solid, melting at 2075 °C, that conducts electricity poorly when cold and better when hot, like a semiconductor. Its crystals are built of B₁₂ icosahedra, cages of twelve atoms, joined into networks that give it a hardness second only to diamond among the elements. The reason for its strangeness is arithmetic: a boron atom has three electrons in its outer shell but four orbitals to fill, so it can never complete a normal set of two-electron bonds. It answers by sharing: in the boranes, its compounds with hydrogenElement: Hydrogen. The lightest element, and most of the universe, three atoms are held together by a single pair of electrons. Their bonds earned William Lipscomb a Nobel Prize in 1976, and their use in synthesis Herbert Brown another in 1979.
Of its two stable isotopes, the lighter, ¹⁰B, a fifth of the whole, captures slow neutrons avidly — a property with a career of its own.
Salt lakes and borax
Boron is scarce in the universe — stars do not make it; it comes largely from cosmic rays splitting heavier nuclei — and it makes up only about ten parts per million of the crust. Never found free, and soluble, it collects where water evaporates: as borax, kernite, colemanite and ulexite in the beds of dried-up lakes. Turkey holds the largest deposits, in western Anatolia, and mines the most; the open pit at Boron in California's Mojave Desert is the other great source.
The element is made from its oxide by reduction with magnesium, which gives an impure brown powder, or, for the pure crystalline form, by decomposing boron bromide or chloride with hydrogen on a hot filament.
Glass, crops, chips and reactors
Most boron is used as borates, not as the element. Added to glass it lowers the melting point and the thermal expansion, which is why borosilicate glassware survives sudden heat; spun into fibre it insulates houses and reinforces plastics; it goes into ceramics, enamels and detergents. Farmers spread it on fields that lack it, and boric acid is a mild antiseptic.
The element itself has sharper uses. Boron carbide, one of the hardest materials known, lines body armour and tank plate; boron nitride is a lubricant in one form and, in another, second only to diamond. A few atoms of boron in a crystal of silicon make it a p-type semiconductor, and every microchip relies on that doping. Neodymium–ironElement: Iron. The metal that built the modern world, and the core of the planet beneath it–boron magnets, discovered in 1984, are the strongest permanent magnets there are and turn in hard drives, electric motors and wind turbines. And boron-10 sits in the control rods and emergency shutdown systems of nuclear reactors and in the boric acid dissolved in their cooling water.
Discovery and name
Borax was known long before boron. Carried from the salt lakes of Tibet, it was a goldsmiths' flux for soldering; its name comes through the Arabic buraq from the Persian burah. Wilhelm Homberg made boric acid from it in 1702.
The element was prised out of that acid in 1808, in the race set off by Humphry Davy's isolation of potassium the year before. In June Joseph Louis Gay-Lussac and Louis Jacques Thénard in Paris heated boric acid with potassium and obtained a dark, combustible substance; Davy in London reached the same product within days and named it boracium. Neither had it pure — their brown powder was barely half boron — and the pure element was not prepared until Ezekiel Weintraub reduced its chloride in an electric arc in 1909. Davy's name was shortened to boron, on the pattern of carbon, whose chemistry it resembles.