The covalent bonding in ¶diamond consists of electrons that are intimately shared between the carbon atoms. We already saw that these strong covalent bonds are usually represented by drawing them as sticks between the atoms. Diamond is important because it is the hardest substance known, and can be used for making sharp cutting tools, such as used in drilling for oil. Other important materials, such as silicon and germanium used for computer chips also have the diamond structure.

There is a common alternative to diamond for the structure of carbon - ¶graphite. The carbon atoms in graphite are also strongly joined by covalent bonds, but only within a plane, unlike the 3D network of bonds in diamond. These planes of carbon atoms simply stack together one on top of the other, with only very weak forces between them. The planes of carbon atoms can then easily slip over each other, and graphite is therefore an important lubricant ! ¶Talcum powder feels smooth for similar reasons.

Drawn like this, diamond and graphite look very different, and of course so they are. But if we look down the cube body-diagonal direction of diamond, which is perpendicular to the planes of packing, we see the trigonal symmetry, which gives a somewhat different picture.

Now if we look down the corresponding direction for graphite, which is again perpendicular to the planes of packing, we see the hexagonal symmetry, and some similarity between the structures of these two very different materials.

Recently a large number of new carbon structures with exciting properties were discovered. The famous ¶buckyballs consist of 60 carbon atoms bonded together to form a hollow sphere. These C60 structures look like tiny geodisic domes of the type made famous by the architect Buckminster-Fuller (hence the common name buckyball).

Larger spheres and ellipsoids can also be constructed, and even hollow nanotubes of carbon, as if graphite layers were rolled up to form microscopic pipes. These new materials, called Fullerenes, have exciting physical and chemical properties that are only now being explored. This last picture was taken from Rice University's gallery of fullerene structures.

So crystal structures have something in common with architecture. Let's look at some other structures that also form beautiful networks of atoms. Because of their structure they can be used as microscopic filters, and also to break up molecules, or to join them together. These molecular sieves and catalysts are called zeolites.