most important thing that has happened in this field is the enormous expansion of people's sense of possibilities. Carbon atoms can be joined to each other and to other molecules, atoms, and clusters in ways that had not previously been imagined. Perhaps this shouldn't have come as such a surprise to beings whose own existence depends so heavily on the marvelous chemical versatility of carbon."

A CARBON PRIMER

Just how versatile is carbon? Of all the elements, not one forms more compounds than carbon; they number in the hundreds of thousands and are the basis for an entire subdivision of chemistry—organic chemistry.

Carbon is exceptional for a number of reasons but most of all for its versatility in bonding to other atoms (especially to other carbon atoms) by sharing its four available electrons in single, double, or triple bonds. Add to this the ease with which carbon can shift its bonding to make a more favorable arrangement. Other elements are not this flexible. For example, the formula for acetylene, a welding gas, is C2H2. The two carbon atoms are linked by a strong triple bond, and each carbon's remaining single bond attaches to a hydrogen atom. However, if tempted with two more hydrogen atoms, the carbons will gladly lessen their grip on each other and change their triple bond to a double one, so that each atom can take on another hydrogen, creating ethylene, C2H4, which is used in the manufacture of plastics. The carbons can accommodate yet another pair of hydrogen atoms by changing their double bond to a single one—the weakest of all—to form ethane, C2H6, a fuel and refrigerant.

Before the fullerene discovery, it was gospel among chemists that pure carbon had only two distinct crystalline forms: diamond and graphite. (Other carbon material, such as charcoal and coke, has an amorphous quality but appears to consist primarily of tiny graphite crystals.) Diamond is the hardest substance known, while graphite is so soft and slippery that it is the basis for pencil lead. Such different characteristics result from the very different ways in which the carbon atoms are arranged. Diamond is a lattice whose basic

FIGURE 9.2 Computer simulation of a soccer-ball-shaped buckyball (C60). (Courtesy of University of Arizona/Dennis Lichtenberger.)



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