Chemistry - A Molecular Science

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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