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 exten-
sively for its semiconductor proper-
ties. It has a diamond-like crystal
structure; an amorphous form also
exists. Chemically, silicon is less reac-
tive than carbon. The element com-
bines 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 car-
bide is made by heating silicon(IV)
oxide with carbon in an electric fur-
nace (depending on the grade re-
quired 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 struc-
tures. There is a clear form known as
moissanite, used as a diamond substi-
tute.
silicon dioxideSee silicon(iv)
oxide.
siliconesPolymeric compounds
containing chains of silicon atoms al-
ternating with oxygen atoms, with
the silicon atoms linked to organic
groups. A variety of silicone ma-
terials exist, including oils, waxes,
and rubbers. They tend to be more
resistant to temperature and chemi-
cal attack than their carbon ana-
logues.
silicon hydrideSee 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 trans-
formed 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 exam-
ple is quartz with iron(III) oxide im-
purities andÛint 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 ultravi-
olet 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
alsoÜnds many applications as a dry-
ing agent in the form of *silica gel.
siloxanesA 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