curring in various forms of silicon(IV)
oxide (e.g. *quartz) and in *silicate
minerals. The element is extracted
by reducing the oxide with carbon in
an electric furnace and is used extensively for its semiconductor properties. It has a diamond-like crystal
structure; an amorphous form also
exists. Chemically, silicon is less reactive than carbon. The element combines with oxygen at red heat and is
also dissolved by molten alkali. There
is a large number of organosilicon
compounds (e.g. *siloxanes) although
silicon does not form the range of
silicon–hydrogen compounds and
derivatives that carbon does (see
silane). The element was identiÜed
by Antoine *Lavoisier in 1787 and
Ürst isolated in 1823 by Jöns
*Berzelius.
A
• Information from the WebElements site
silicon carbide (carborundum) A
black solid compound, SiC, insoluble
in water and soluble in molten alkali;
r.d. 3.217; m.p. c. 2700°C. Silicon carbide is made by heating silicon(IV)
oxide with carbon in an electric furnace (depending on the grade required sand and coke may be used). It
is extremely hard and is widely used
as an abrasive. The solid exists in
both zinc blende and wurtzite structures. There is a clear form known as
moissanite, used as a diamond substitute.
silicon dioxide See silicon(iv)
oxide.
silicones Polymeric compounds
containing chains of silicon atoms alternating with oxygen atoms, with
the silicon atoms linked to organic
groups. A variety of silicone materials exist, including oils, waxes,
and rubbers. They tend to be more
resistant to temperature and chemical attack than their carbon analogues.
silicon hydride See silane.
silicon(IV) oxide (silicon dioxide;
silica) A colourless or white vitreous
solid, SiO 2 , insoluble in water and
soluble (by reaction) in hydroÛuoric
acid and in strong alkali; m.p.
1713°C; b.p. 2230°C. The following
forms occur naturally: cristobalite
(cubic or tetragonal crystals; r.d.
2.32); tridymite (rhombic; r.d. 2.26);
*quartz (hexagonal; r.d. 2.63–2.66);
lechatelierite (r.d. 2.19). Quartz has
two modiÜcations: α-quartz below
575°C and β-quartz above 575°C;
above 870°C β-quartz is slowly transformed to tridymite and above
1470°C this is slowly converted to
cristobalite. Various forms of
silicon(IV) oxide occur widely in the
earth’s crust; yellow sand for example is quartz with iron(III) oxide impurities and Ûint is essentially
amorphous silica. The gemstones
amethyst, opal, and rock crystal are
also forms of quartz.
Silica is an important commercial
material in the form of silica brick, a
highly refractive furnace lining,
which is also resistant to abrasion
and to corrosion. Silicon(IV) oxide is
also the basis of both clear and
opaque silica glass, which is used on
account of its transparency to ultraviolet radiation and its resistance to
both thermal and mechanical shock.
A certain proportion of silicon(IV)
oxide is also used in ordinary glass
and in some glazes and enamels. It
also Ünds many applications as a drying agent in the form of *silica gel.
siloxanes A group of compounds
containing silicon atoms bound to
oxygen atoms, with organic groups
linked to the silicon atoms, e.g.
R 3 SiOSiR 3 , where R is an organic
group. *Silicones are polymers of
siloxanes.
483
siloxanes
s
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oxide (e.g. *quartz) and in *silicate
minerals. The element is extracted
by reducing the oxide with carbon in
an electric furnace and is used extensively for its semiconductor properties. It has a diamond-like crystal
structure; an amorphous form also
exists. Chemically, silicon is less reactive than carbon. The element combines with oxygen at red heat and is
also dissolved by molten alkali. There
is a large number of organosilicon
compounds (e.g. *siloxanes) although
silicon does not form the range of
silicon–hydrogen compounds and
derivatives that carbon does (see
silane). The element was identiÜed
by Antoine *Lavoisier in 1787 and
Ürst isolated in 1823 by Jöns
*Berzelius.
A
• Information from the WebElements site
silicon carbide (carborundum) A
black solid compound, SiC, insoluble
in water and soluble in molten alkali;
r.d. 3.217; m.p. c. 2700°C. Silicon carbide is made by heating silicon(IV)
oxide with carbon in an electric furnace (depending on the grade required sand and coke may be used). It
is extremely hard and is widely used
as an abrasive. The solid exists in
both zinc blende and wurtzite structures. There is a clear form known as
moissanite, used as a diamond substitute.
silicon dioxide See silicon(iv)
oxide.
silicones Polymeric compounds
containing chains of silicon atoms alternating with oxygen atoms, with
the silicon atoms linked to organic
groups. A variety of silicone materials exist, including oils, waxes,
and rubbers. They tend to be more
resistant to temperature and chemical attack than their carbon analogues.
silicon hydride See silane.
silicon(IV) oxide (silicon dioxide;
silica) A colourless or white vitreous
solid, SiO 2 , insoluble in water and
soluble (by reaction) in hydroÛuoric
acid and in strong alkali; m.p.
1713°C; b.p. 2230°C. The following
forms occur naturally: cristobalite
(cubic or tetragonal crystals; r.d.
2.32); tridymite (rhombic; r.d. 2.26);
*quartz (hexagonal; r.d. 2.63–2.66);
lechatelierite (r.d. 2.19). Quartz has
two modiÜcations: α-quartz below
575°C and β-quartz above 575°C;
above 870°C β-quartz is slowly transformed to tridymite and above
1470°C this is slowly converted to
cristobalite. Various forms of
silicon(IV) oxide occur widely in the
earth’s crust; yellow sand for example is quartz with iron(III) oxide impurities and Ûint is essentially
amorphous silica. The gemstones
amethyst, opal, and rock crystal are
also forms of quartz.
Silica is an important commercial
material in the form of silica brick, a
highly refractive furnace lining,
which is also resistant to abrasion
and to corrosion. Silicon(IV) oxide is
also the basis of both clear and
opaque silica glass, which is used on
account of its transparency to ultraviolet radiation and its resistance to
both thermal and mechanical shock.
A certain proportion of silicon(IV)
oxide is also used in ordinary glass
and in some glazes and enamels. It
also Ünds many applications as a drying agent in the form of *silica gel.
siloxanes A group of compounds
containing silicon atoms bound to
oxygen atoms, with organic groups
linked to the silicon atoms, e.g.
R 3 SiOSiR 3 , where R is an organic
group. *Silicones are polymers of
siloxanes.
483
siloxanes
s
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