A light metal that protects itself
Aluminium is a soft, silvery, ductile metal a third as dense as steel. Its atom, [Ne] 3s² 3p¹, gives up three electrons to become Al³⁺, and it holds on to oxygen so avidly that a fresh surface oxidises in seconds. That is its salvation: the oxide, Al₂O₃, forms a transparent, hard skin only a few nanometres thick that seals the metal from further attack. IronElement: Iron. The metal that built the modern world, and the core of the planet beneath it rusts through because its oxide flakes off; aluminium keeps its shine for decades.
Beneath it the metal is reactive. Powdered and mixed with iron oxide, it burns as thermite, hot enough to weld rails; it dissolves in acids and, being amphoteric, in alkalis too. It conducts electricity about two thirds as well as copper by volume and twice as well by weight, so overhead power lines are aluminium around a steel core.
From bauxite to metal
Aluminium is the most abundant metal of the crust, 8.2 % of it by mass, but it is never found free. It is locked in the feldspars, micas and clays of ordinary rock, and in corundum, whose coloured crystals are ruby and sapphire. The one ore worth working is bauxite, a red-brown earth left by tropical weathering.
In the Bayer process (1888) the bauxite is digested in hot caustic soda, which dissolves the aluminium and leaves the iron as red mud; aluminium hydroxide is then precipitated and calcined to pure alumina, Al₂O₃. In the Hall–Héroult process (1886) the alumina is dissolved in molten cryolite, Na₃AlF₆, at about 960 °C and electrolysed between carbon electrodes, and molten aluminium collects at the bottom of the cell. The electricity is the cost, some 13 to 15 kWh for each kilogram, so smelters stand beside hydroelectric dams and cheap coal; melting scrap takes about 5 % of that energy. Production is around 70 million tonnes a year, second only to iron.
What it is for
Aluminium flies. The Wright Flyer's engine had an aluminium crankcase in 1903, and after Alfred Wilm found in 1906 that an alloy with copper hardens on standing — duralumin — the metal became the airframe itself. Airliners are still mostly aluminium alloys, and cars increasingly. It is rolled into foil, drawn into cans recycled by the billion, extruded into window frames and strung as the cables of the grid. Its compounds serve too: alumina as abrasive and ceramic, the hydroxide as an antacid, and the alums, which have fixed dyes to cloth since Egyptian times.
The body has no use for it: the commonest metal in the ground is absent from all known biochemistry, and in lakes acidified by acid rain it dissolves out of the soil and kills fish.
Discovery and name
In 1754 Andreas Marggraf showed that the earth of alum, alumina, was distinct from lime. Humphry Davy failed to reduce it in 1807 but named the metal he was sure it held — alumium, then aluminum — and British reviewers made it aluminium, to sound like sodium and potassium. Hans Christian Ørsted obtained the first impure specimen in 1825, by heating aluminium chloride with potassium amalgam; Friedrich Wöhler improved the experiment in 1827 and described the metal's properties.
It was a curiosity dearer than goldElement: Gold. The yellow metal that never tarnishes. Henri Sainte-Claire Deville's sodium process of 1854 brought the price down enough for Napoleon III, so the story goes, to serve honoured guests on aluminium plates, and for the Washington Monument to be capped in 1884 with an aluminium pyramid weighing about 2.8 kg. In 1886 Charles Martin Hall in Ohio and Paul Héroult in France, both twenty-two and unknown to each other, found the same electrolytic route; within a few years the price collapsed. IUPAC settled on the spelling aluminium in 1990.