Purely covalent crystals are relatively few in number. In addition to diamond, some
examples are silicon, germanium, and silicon carbide, all of which have the same tetra-
hedral structure as diamond; in SiC each atom is surrounded by four atoms of the
other kind. Cohesive energies are usually greater in covalent crystals than in ionic ones.
As a result covalent crystals are hard (diamond is the hardest substance known, and
SiC is the industrial abrasive carborundum), have high melting points, and are insol-
uble in all ordinary liquids. The optical and electrical properties of covalent solids are
discussed later.A
n unexpected form of carbon was accidentally discovered in 1985 at Rice University in
Texas. The commonest version consists of 60 carbon atoms arranged in a cage structure of
12 pentagons and 20 hexagons whose geometry is like that of a soccer ball (Fig. 10.10). This
extraordinary molecule was called “buckminsterfullerene” in honor of the American architect R.
Buckminster Fuller, whose geodesic domes it resembles; the name is usually shortened to
buckyball.
Buckyballs, which are stable and chemically unreactive, can be made in the laboratory from
graphite and are present in small quantities in ordinary soot and in a carbon-rich rock found in
Russia. The original C 60 buckyball is not the only form of fullerene known: C 28 , C 32 , C 50 , C 70 , and
still larger ones have been made. Fullerene molecules are held together to form solids by van der
Waals bonds like those that hold together the layers of C atoms in graphite. Since their discovery,
the fullerenes and their offshoots have shown some remarkable properties. For instance, the com-
bination of C 60 with potassium to form K 3 C 60 yields a superconductor at low temperatures.
Carbon nanotubes, cousins of buckyballs, consist of tiny cylinders of carbon atoms arranged
in hexagons, like rolled-up chicken wire. Depending on whether their rows of hexagons are
straight or wind around in a helix, such nanotubes act either as electrical conductors or as
semiconductors and their use is being explored in such electronic applications as transistors and
flat-panel displays. If carbon nanotubes can be made long enough, they will form exceedingly
strong fibers, ten times stronger than steel while six times lighter, that are flexible as well. Fibers
like this would be ideal in composite materials to reinforce epoxy resins. Nanotubes also have
promise for storing the hydrogen needed for the fuel cells of future electric cars, which would
make heavy steel containers unnecessary.344 Chapter Ten
Figure 10.10In a buckyball, carbon atoms form a closed cagelike structure in which each atom is bonded
to three others. Shown here is the C 60 buckyball that contains 60 carbon atoms. The lines represent
carbon-carbon bonds; their pattern of hexagons and pentagons closely resembles the pattern made by
the seams of a soccer ball. Other buckyballs have different numbers of carbon atoms.Buckyballs and Nanotubes
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