Aluminum

Aluminum sits at atomic number 13, in period 3 and group 13 of the periodic table, placing it at the boundary between metals and metalloids. Its electron configuration — [Ne] 3s² 3p¹ — leaves a single p-electron ready to be lost, explaining why Al³⁺ dominates its chemistry and why aluminum favors a +3 oxidation state in virtually all of its compounds. With an electronegativity of 1.61, it forms bonds that are meaningfully polar, giving its oxides and hydroxides their well-known amphoteric character: aluminum compounds react with both strong acids and strong bases. That combination of light weight, reactivity, and protective oxide behavior makes aluminum chemically distinctive among common structural metals.

Aluminum's low density of 2.7 g/cm³ and excellent strength-to-weight ratio make it the dominant material in aerospace engineering; a Boeing 737 contains roughly 80,000 pounds of the metal in its fuselage and wing structures. The beverage industry produces about 100 billion aluminum cans per year in the United States alone, taking advantage of its corrosion resistance and infinite recyclability. Electrical transmission lines rely on aluminum rather than copper because, for the same mass, aluminum conducts more current per kilogram — high-voltage power grids use aluminum conductor steel-reinforced (ACSR) cables for this reason. In architecture, aluminum alloys such as 6061 and 7075 form window frames, curtain walls, and structural profiles because they resist weathering without paint or coatings. Automotive manufacturers use aluminum to reduce vehicle weight; a typical modern car body contains 300–400 pounds of the metal, trimming fuel consumption measurably compared to all-steel designs.

Danish chemist Hans Christian Ørsted first isolated a crude form of aluminum in 1825 by reacting aluminum chloride with potassium amalgam, but it was Friedrich Wöhler who refined the process in 1827 and produced the first pure aluminum samples. For decades after that, aluminum was more expensive than gold because reduction was so difficult; Napoleon III famously reserved aluminum cutlery for honored guests at state dinners while lesser guests used silver. In 1886, American Charles Martin Hall and Frenchman Paul Héroult independently and simultaneously developed the electrolytic reduction process — now called the Hall–Héroult process — that made mass production economically viable. The element's name traces to the Latin alumen for alum; Humphry Davy originally proposed 'alumium' and then 'aluminum,' while British chemists later standardized 'aluminium' — the two spellings still divide American and international usage to this day. The Bayer process, developed by Karl Josef Bayer in 1887 for extracting alumina from bauxite ore, completed the industrial foundation that turned aluminum into the second most-used metal on Earth.

Aluminum is the most abundant metal in Earth's crust and the third most abundant element overall, making up roughly 8.2% of crustal mass by weight. It never occurs as a free metal in nature because of its affinity for oxygen; instead, it is locked into hundreds of mineral forms, with bauxite — a mixture of gibbsite Al(OH)₃, boehmite AlO(OH), and diaspore AlO(OH) — serving as the principal commercial ore. Feldspar minerals, which dominate granites and many other igneous rocks, are aluminum silicates that slowly weather into clays such as kaolinite. In stellar nucleosynthesis, aluminum is produced primarily through carbon and neon burning in massive stars, making it relatively common on a cosmic scale. Seawater contains only trace concentrations around 2 ppb because the element rapidly hydrolyzes and precipitates under aqueous conditions.

Aluminum oxide (Al₂O₃), known as alumina in its powdered form and corundum in its crystalline form, is used as an abrasive, a refractory material, and the precursor for nearly all primary aluminum production. Ruby and sapphire are simply corundum with trace chromium or iron impurities, making Al₂O₃ responsible for two of the world's most prized gemstones. Aluminum hydroxide (Al(OH)₃) is the active ingredient in many antacid formulations, where it neutralizes excess stomach acid. Aluminum sulfate (Al₂(SO₄)₃) acts as a flocculant in municipal water treatment, causing suspended particles to clump and settle. Aluminum chloride (AlCl₃) is an essential Lewis acid catalyst in Friedel–Crafts reactions, a workhorse of industrial organic synthesis. Aluminum nitride (AlN) is a semiconductor-grade ceramic prized for its combination of high thermal conductivity and electrical insulation in electronics packaging. Alum — potassium aluminum sulfate, KAl(SO₄)₂·12H₂O — has been used since antiquity as a mordant in textile dyeing and as an astringent in cosmetics.

• Although aluminum is enormously abundant in Earth's crust, it was considered a precious metal in the 1850s — the capstone placed atop the Washington Monument in 1884 was cast from aluminum precisely because it was exotic and expensive at the time.
• Aluminum foil is only about 6 micrometers thick on its shiny side, yet the difference in reflectivity between the two sides is purely a manufacturing artifact: both sides contact rollers during rolling, but the side touching the polished roller ends up shinier.
• Thermite, the incendiary mixture of aluminum powder and iron oxide, burns at roughly 2500 °C and cannot be extinguished with water — pouring water on burning thermite causes explosive steam generation and can scatter the molten iron it produces.
• Aluminum recycling requires only about 5% of the energy needed to smelt new aluminum from ore, which means that recycling a single aluminum can saves enough electricity to run a television for about three hours.
• Despite being chemically reactive enough to dissolve in hydrochloric acid, aluminum survives indefinitely in ordinary air because it instantly forms a dense, self-limiting oxide layer a few nanometers thick that blocks further oxidation — this is why uncoated aluminum does not rust the way iron does.