8.8
COVALENT SOLIDS
(a) 1.4 A (b)3.4 A
Figure 8.16 Ball-and-stick and space-filling models of graphite (a) Face of a portion of one sheet
that shows the two-dimensional
σ-bonding framework and the 1.4 Å bond length. (b) Three adjacent sheets of gr
aphite showing how the sheets
stack in the solid material with
a 3.4 Å separation between sheets.
Note that the atoms of adjacent
sheets just touch in the space-
filling model.
Figure 8.17 C
or “Buckyball” 60
Covalent solids are very large networks of atoms held together by covalent bonds, which are very strong interactions. Consequently,
covalent solids have very high melting points
(600 - 2000
oC) and are very hard. Ionic bonds are largely non-directional as the opposite
charges simply pack around one another as ef
ficiently as possible, while covalent bonds
are directional and lead to molecular stru
ctures as discussed in Chapter 6. Thus, the
structures of extended solids formed from covalent bonds differ from those formed from ionic bonds. Some elements can exist in several structural forms called
allotropes
, which
can have very different properties. For ex
ample, graphite and diamond are two allotropes
of carbon, and we begin our discussion of covalent solids by examining some structure-property relationships in these well known materials and in some other less known but interesting forms of carbon.
The carbon atoms in
graphite
are sp
2 hybridized and form a two-dimensional
- σ
bonding framework as shown in Figure 8.16a. The sheets then stack as shown in Figure 8.16b to produce the solid material. Graphite is used in electrodes, lubricants, and pencils. These uses can all be understood in terms of the bonding and structure of the material. Its conducting ability (electrodes) arises because the p orbital of each atom that is not used in hybridization is part of a
system that is delocalized over the entire sheet. The number of π
atoms involved in the system is quite large, so the delocalized
system produces bands. π
One of the bands is half-filled, so graphite is
a metallic conductor, but only in the plane of
the sheet, not between sheets. The distance betw
een sheets is fairly large (3.4Å), which is
indicative of relatively weak intermolecular interaction between layers. Consequently, the layers are easily moved over one another, wh
ich makes graphite an excellent lubricant.
The layers can even be removed completely
with rubbing, which makes it ideal for the
“lead” in pencils.
Fullerenes
are carbon compounds with structures based on the graphite structure. The
most common fullerene is C
(Figure 8.17), which is called buckminster-fullerene (or 60
buckyball) after the architect of the geodesic dome. Buckyball was discovered in the early 1980’s. It contains 20 hexagons and 12 penta
gons and is the shape of a soccer ball. The
hexagons are the same units that form the basi
s of graphite, but the pentagons are required
for closure. A great deal of research was done on fullerenes
to develop new technologies.
For example, efforts were made to use it as
a molecular ball-bearing lubricant, and to
encapsulate smaller molecules in its cavity, so
the molecules could be released slowly into
Chapter 8 Solid Materials
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