In nature, pure carbon is found in two well-known forms, or allotropes: diamond and graphite. In 1985, however, scientists discovered that pure carbon could be induced to form an even more unusual structure, the buckminsterfullerene, or buckyball. Esoteric as they may seem, buckyballs possess a number of unusual properties that make them of interest to chemists.
The fullerene buckyball is the roundest molecule known to science. Each buckyball contains exactly sixty atoms of carbon in a structure that resembles a hollow soccer ball. Each carbon in the framework is bonded to three others. The bonds between the carbon atoms form 20 six-sided hexagons and 12 five-sided pentagons. It is the smallest fullerene in which no two pentagons share an edge. Scientists have also been able to produce larger or smaller fullerenes containing different numbers of carbons; these adopt different shapes, but are much less stable. The most stable kind of fullerene is the C60 variety.
Solids made from pure buckyballs are crystals that act either as insulators that do not conduct electricity or semiconductors whose conductivity increases with increasing temperature. If the buckyballs are doped with certain metals such as potassium or rubidium, the buckyball solid can conduct electricity. If they are doped with organic reducing agents, they can become ferromagnetic (that is, permanently magnetised by a magnetic field), which is a bizarre behaviour for a nonmetal.
The crystal form of buckminsterfullerene is soft like graphite, in which different layers can slide past each other. If the buckminsterfullerene solid is compressed, however, it becomes very hard. It can exist in hexagonal or cubic crystal forms. The index of refraction for a buckyball crystal (that is, the ratio of the speed of light in the crystal to the speed of light in vacuum) is 2.2, compared with 2.42 for diamond. The density of solid buckminsterfullerene is 1.65 grams per cubic centimetre.
The interior diameter of the molecular "cage" formed by the buckyball is 7.11 angstroms, where an angstrom is a 10-billionth of a meter --- big enough, in fact, that in some experiments scientists have been able to trap atoms of other elements inside this "cage." The carbons in the pentagonal rings are not doubly bonded, so electrons are not completely delocalised over the whole framework. Consequently, buckyballs behave like double-bonded carbon compounds deficient in electrons, and they react readily with many electron-rich compounds.