Chemistry Reference
In-Depth Information
fullerene
. Only the first two, graphite and diamond, occur in nature;
fullerene
, which is artificially made by humans, was discovered quite
recently, at the end of the twentieth century.
Graphite
occurs naturally as
a dark gray, greasy-feeling mineral, which has been used, since antiquity,
as a black pigment. Its greasy feel is due to the crystal structure of graphite:
each carbon atom in the graphite structure is attached to another three,
forming large, flat sheets of hexagonally bonded carbon atoms. The sheets
have smooth surfaces that can easily slide and move relative to one
another and also provide the greasy feeling.
Charcoal
, often found in
archaeological excavations, consists of a porous mixture of graphite and
partially carbonized organic compounds. It is formed when organic matter,
particularly wood, burns in an atmosphere lacking or devoid of oxygen. In
diamond
, a gemstone, each carbon atom is attached to others within the
ordered crystalline, tetrahedral structure. Since the carbon atoms are rel-
atively small and the bonds between them are very strong, diamond is very
hard, the hardest material known. It is because of its hardness, for
example, that diamond powder has long been used as an abrasive for
drilling holes and polishing surfaces. The crystals of diamond are also very
tough and resistant to pressure; if tapped in just exactly the right place,
however, they cleave along crystal faces.
Tin and Tin Pest
Another element that exhibits allotropy because of variations in the
crystal structure is tin. The common allotrope is
tin metal
, also known as
a
(
alpha
)
tin
, which is stable at ambient temperatures. The other allotrope,
which generally occurs as a gray powder and is known as
b
(
beta
)
tin
, but
also as
tin pest
, is formed only at very low temperatures: when tin cools
down to temperatures below
−
18°C, the ordinary allotrope,
α
tin, is con-
verted to
tin, and the transformation is irreversible under ordinary tem-
peratures. Tin objects exposed to temperatures below
β
18°C in very cold
regions of the world, for example, are generally severely damaged when
part of the tin converts to tin pest. In extreme cases, when exposure to low
temperatures extends for long periods of time, the allotropic conversion
may result in the transformation of tin objects into heaps of gray
−
β
-tin
powder.
Black pigments of vegetable origin have generally been made from
various kinds of charred plant matter, mostly wood, but also leaves or seeds;
the
charcoal
formed during the charring process is then washed, to remove
soluble matter, and finally ground to powder. Over 95% of well-burned char-
