A light metal that burns
Magnesium is a silvery-white metal, the lightest of those strong enough to build with: its density, 1.74 g/cm³, is two thirds of aluminium's and less than a quarter of steel's. It stands in group 2 between beryllium and calcium, with two electrons in its outer shell, [Ne] 3s², which it gives up readily to become the ion Mg²⁺. In dry air a thin skin of oxide protects it; with cold waterCompound: Water. Two hydrogens, one oxygen, and most of the living world it reacts slowly, with steam briskly, and in dilute acid it dissolves at once, giving off hydrogenElement: Hydrogen. The lightest element, and most of the universe.
Set alight, it burns with a white flame so fierce that it lit the first flash photographs and still fills flares and fireworks. The fire is hard to put out: burning magnesium takes the oxygen it needs from water and even from carbon dioxide, so only sand or a dry powder will smother it.
In the rock, the sea and the leaf
Magnesium is never found free. It makes up about 2 % of the crust — in dolomite, magnesite, talc and olivine — and far more of the mantle beneath, whose olivine and pyroxene are magnesium silicates; taken whole, the Earth holds more magnesium than any element except iron, oxygen and silicon. The sea carries 1.3 g in every kilogram, and since 1941 the metal has been won from seawater by precipitating the hydroxide, converting it to the chloride and electrolysing the melt. Most of today's production, about a million tonnes a year and largely Chinese, uses the Pidgeon process instead: calcined dolomite is heated with ferrosilicon under vacuum, and the magnesium distils off and condenses.
Life depends on it. At the centre of every chlorophyll molecule sits one magnesium ion; without it plants could not catch light. An adult body holds about 25 g, half of it in bone, and some three hundred enzymes need the ion — every reaction that spends ATP handles the molecule as its magnesium complex.
Alloys, salts and reagents
Alloyed with aluminium and zinc, magnesium is cast into parts that must be light: the wheels of racing cars, the bodies of cameras and laptops, the seats and gearboxes of aircraft. The Volkswagen Beetle's crankcase and gearbox were magnesium castings. A large share of production goes not into magnesium parts at all but into aluminium, which it strengthens: the drinks can is made of aluminium–magnesium alloys.
Its compounds are older and homelier. Magnesia, the oxide, lines furnaces because it melts above 2800 °C; the hydroxide is milk of magnesia, an antacid; the sulfate is Epsom salt, named for the Surrey spring where it was found in the seventeenth century. In 1900 Victor Grignard found that magnesium turnings in ether react with organic halides to give reagents that build carbon–carbon bonds at will; the discovery won him the Nobel Prize in 1912.
Discovery and name
The name is older than the element. Magnesia, a district of Thessaly in Greece, lent its name in antiquity to minerals dug there — a black one that became manganese, and a white earth, magnesia alba, an eighteenth-century purgative. In 1755 Joseph Black of Edinburgh showed by careful weighing that magnesia alba was an earth distinct from lime, and in doing so discovered the gas it gives off on heating, carbon dioxide, which he called fixed air.
The metal came in 1808. Humphry Davy passed a current through moist magnesia mixed with mercury oxide and distilled a little new metal from the amalgam; he first proposed magnium, to avoid confusion with manganese, then settled on magnesium. The first coherent lump was made in 1831 by Antoine Bussy in Paris, who reduced magnesium chloride with potassium.