CHEM-I-PAGE

Aluminum   

Standard Sate: solid at 298k

Color: Silvery                                                                                                                   

Pure aluminium is a silvery-white metal with many desirable characteristics. It is light, nontoxic (as the metal), nonmagnetic and nonsparking. It is somewhat decorative. It is easily formed, machined, and cast. Pure aluminium is soft and lacks strength, but alloys with small amounts of copper, magnesium, silicon, manganese, and other elements have very useful properties. Aluminium is an abundant element in the earth's crust, but it is not found free in nature. The Bayer process is used to refine aluminium from bauxite, an aluminium ore.

also spelled ALUMINIUM (Al), chemical element, a lightweight,
silvery-white metal of main Group IIIa (boron group) of the periodic
table. Aluminum is the most abundant metallic element in the 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 the Earth's crust, of which it
constitutes about 8 percent by weight; it is exceeded in amount only
by oxygen and silicon.

Occurrence, uses, and properties.


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 Hans Christian �rsted by
reducing aluminum chloride with potassium amalgam. 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. A German chemist, Friedrich W�hler,
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 H�roult 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 Industries,
Extraction and Processing: Aluminum.

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. (See alloy.)

Pure aluminum (99.996 percent) is quite soft and weak; commercial
aluminum (99.0 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.


Compounds.


Ordinarily, aluminum is trivalent. At elevated temperatures, however,
a few gaseous monovalent and bivalent compounds have been
prepared (AlCl, Al2O, AlO).

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.

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 most important of such salts is aluminum
potassium sulfate, also known as potassium alum or potash alum,
KAl(SO4)12H2O. 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, AlCl3; 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, AlCl3H2O, 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. (See
Friedel-Crafts reaction.)

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. atomic number 13 atomic weight
26.9815 melting point 660 C boiling point 2,467 C specific gravity
2.70 (20 C) valence 3 electronic config. 2-8-3 or 1s22s22p63s23p1

"aluminum" Encyclop�dia Britannica Online.

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