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 10 miles (16 km) 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
occurs in igneous rocks chiefly as aluminosilicates in feldspars,
feldspathoids, and micas; in the soil derived from them as clay; and upon
further weathering as bauxite and iron-rich laterite. Bauxite, a mixture of
hydrated aluminum oxides, is the principal aluminum ore. Crystalline aluminum
oxide (emery, corundum), which occurs in a few igneous rocks, is mined as a
natural abrasive or in its finer varieties as rubies and sapphires. Aluminum is
present in other gemstones, such as topaz, garnet, and chrysoberyl. Of the many
other aluminum minerals, alunite and cryolite have some commercial importance.
Crude aluminum
was isolated (1825) by Danish physicist Hans Christian Ørsted by reducing
aluminum chloride with potassium amalgam. British chemist Sir Humphry Davy had
prepared (1809) an iron-aluminum alloy by electrolyzing fused alumina (aluminum
oxide) and had already named the element aluminum; the word later was modified
to aluminium in England and some other European countries. German chemist
Friedrich Wöhler, using potassium metal as the reducing agent, produced
aluminum powder (1827) and small globules of the metal (1845), from which he
was able to determine some of its properties.
The new metal
was introduced to the public (1855) at the Paris Exposition at about the time
that it became available (in small amounts at great expense) by the sodium
reduction of molten aluminum chloride. When electric power became relatively
plentiful and cheap, almost simultaneously Charles Martin Hall in the United
States and Paul-Louis-Toussaint Héroult in France discovered (1886) the modern
method of commercially producing aluminum: electrolysis of purified alumina
(Al2O3) dissolved in molten cryolite (Na3AlF6). During the 1960s aluminum moved
into first place, ahead of copper, in world production of nonferrous metals.
For more specific information about the mining, refining, and production of
aluminum, see aluminum processing.
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.
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.
COMPOUNDS:
Ordinarily,
aluminum is trivalent. At elevated temperatures, however, a few gaseous
monovalent and bivalent compounds have been prepared (AlCl, Al2O, AlO). In
aluminum the configuration of the three outer electrons is such that in a few
compounds (e.g., crystalline aluminum fluoride [AlF3] and aluminum chloride
[AlCl3]) the bare ion, Al3+, formed by loss of these electrons, is known to
occur. The energy required to form the Al3+ ion, however, is very high, and, in
the majority of cases, it is energetically more favourable for the aluminum
atom to form covalent compounds by way of sp2 hybridization, as boron does. The
Al3+ ion can be stabilized by hydration, and the octahedral ion [Al(H2O)6]3+
occurs both in aqueous solution and in several salts.
A number of aluminum
compounds have important industrial applications. Alumina, which occurs in
nature as corundum, is also prepared commercially in large quantities for use
in the production of aluminum metal and the manufacture of insulators, spark
plugs, and various other products. Upon heating, alumina develops a porous
structure, which enables it to adsorb water vapour. This form of aluminum
oxide, commercially known as activated alumina, is used for drying gases and
certain liquids. It also serves as a carrier for catalysts of various chemical
reactions.
Anodic
aluminum oxide (AAO), typically produced via the electrochemical oxidation of
aluminum, is a nanostructured aluminum-based material with a very unique
structure. AAO contains cylindrical pores that provide for a variety of uses.
It is a thermally and mechanically stable compound while also being optically
transparent and an electrical insulator. The pore size and thickness of AAO can
easily be tailored to fit certain applications, including acting as a template
for synthesizing materials into nanotubes and nanorods.
Another major
compound is aluminum sulfate, a colourless salt obtained by the action of
sulfuric acid on hydrated aluminum oxide. The commercial form is a hydrated
crystalline solid with the chemical formula Al2(SO4)3. It is used extensively
in paper manufacture as a binder for dyes and as a surface filler. Aluminum
sulfate combines with the sulfates of univalent metals to form hydrated double
sulfates called alums. The alums, double salts of formula MAl(SO4)2 ·12H2O
(where M is a singly charged cation such as K+), also contain the Al3+ ion; M
can be the cation of sodium, potassium, rubidium, cesium, ammonium, or
thallium, and the aluminum may be replaced by a variety of other M3+ ions—e.g.,
gallium, indium, titanium, vanadium, chromium, manganese, iron, or cobalt. The
most important of such salts is aluminum potassium sulfate, also known as
potassium alum or potash alum. These alums have many applications, especially
in the production of medicines, textiles, and paints.
The reaction
of gaseous chlorine with molten aluminum metal produces aluminum chloride; the
latter is the most commonly used catalyst in Friedel-Crafts reactions—i.e.,
synthetic organic reactions involved in the preparations of a wide variety of
compounds, including aromatic ketones and anthroquinone and its derivatives.
Hydrated aluminum chloride, commonly known as aluminum chlorohydrate,
AlCl3∙H2O, is used as a topical antiperspirant or body deodorant, which acts by
constricting the pores. It is one of several aluminum salts employed by the
cosmetics industry.
Aluminum
hydroxide, Al(OH)3, is used to waterproof fabrics and to produce a number of
other aluminum compounds, including salts called aluminates that contain the
AlO−2 group. With hydrogen, aluminum forms aluminum hydride, AlH3, a polymeric
solid from which are derived the tetrohydroaluminates (important reducing
agents). Lithium aluminum hydride (LiAlH4), formed by the reaction of aluminum
chloride with lithium hydride, is widely used in organic chemistry—e.g., to
reduce aldehydes and ketones to primary and secondary alcohols, respectively.
