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Aluminum (Al), also spelled aluminium, chemical element, a lightweight silvery white metal of main Group 13 (IIIa, or boron group) of the periodic table. Aluminum is the most abundant metallic element in Earth's crust and the most widely used nonferrous metal. Because of its chemical activity, aluminum never occurs in the metallic form in nature, but its compounds are present to a greater or lesser extent in almost all rocks, vegetation, and animals. Aluminum is concentrated in the outer 16 km (10 miles) of Earth's crust, of which it constitutes about 8 percent by weight; it is exceeded in amount only by oxygen and silicon. The name aluminum is derived from the Latin word alumen, used to describe potash alum, or aluminum potassium sulfate, KAl(SO4)2.12H2O.

Aluminum is added in small amounts to certain metals to improve their properties for specific uses, as in aluminum bronzes and most magnesium-base alloys; or, for aluminum-base alloys, moderate amounts of other metals and silicon are added to aluminum. The metal and its alloys are used extensively for aircraft construction, building materials, consumer durables (refrigerators, air conditioners, cooking utensils), electrical conductors, and chemical and food-processing equipment.

Pure aluminum (99.996 percent) is quite soft and weak; commercial aluminum (99 to 99.6 percent pure) with small amounts of silicon and iron is hard and strong. Ductile and highly malleable, aluminum can be drawn into wire or rolled into thin foil. The metal is only about one-third as dense as iron or copper. Though chemically active, aluminum is nevertheless highly corrosion-resistant, because in air a hard, tough oxide film forms on its surface. There are also other aluminum contents like 1050 Aluminum, 1060 Aluminum, 1100 Aluminum, Alloy Aluminum, 2A12 Aluminum, 3003 Aluminum, 5052 Aluminum, 5083 Aluminum, 5754 Aluminum, etc.

Aluminum is an excellent conductor of heat and electricity. Its thermal conductivity is about one-half that of copper; its electrical conductivity, about two-thirds. It crystallizes in the face-centred cubic structure. All natural aluminum is the stable isotope aluminum-27. Metallic aluminum and its oxide and hydroxide are nontoxic.

Aluminum is slowly attacked by most dilute acids and rapidly dissolves in concentrated hydrochloric acid. Concentrated nitric acid, however, can be shipped in aluminum tank cars because it renders the metal passive. Even very pure aluminum is vigorously attacked by alkalies such as sodium and potassium hydroxide to yield hydrogen and the aluminate ion. Because of its great affinity for oxygen, finely divided aluminum, if ignited, will burn in carbon monoxide or carbon dioxide with the formation of aluminum oxide and carbide, but, at temperatures up to red heat, aluminum is inert to sulfur.

Aluminum can be detected in concentrations as low as one part per million by means of emission spectroscopy. Aluminum can be quantitatively analyzed as the oxide (formula Al2O3) or as a derivative of the organic nitrogen compound 8-hydroxyquinoline. The derivative has the molecular formula Al(C9H6ON)3.