Diamond, Graphite, Graphene, and Fullerenes: Substances That Contain Only Carbon Atoms
The difference that hybridization can make is illustrated by diamond and graphite. Diamond is the hardest of all substances, whereas graphite is a slippery, soft solid most familiar to us as the lead in pencils. Both materials, in spite of their very different physical properties, contain only carbon atoms. The two substances differ solely in the hybridization of the carbon atoms.
Diamond consists of a rigid three-dimensional network of carbon atoms, with each carbon bonded to four others via sp3 orbitals. The carbon atoms in graphite, on the other hand, are sp2 hybridized, so each bonds to only three other carbons. This trigonal planar arrangement causes the atoms in graphite to lie in flat, layered sheets. Because there are no covalent bonds between the sheets, they can shear off from neighboring sheets.
Diamond and graphite have been known since ancient times—but a third substance found in nature that contains only carbon atoms was discovered in 2004. Graphene is a one-atom-thick planar sheet of graphite. It is the thinnest and lightest material known. It is transparent and can be bent, stacked, or rolled. It is harder than diamond, and it conducts electricity better than copper. In 2010, the Nobel Prize in Physics was given to Andre Geim and Konstantin Novoselov of the University of Manchester for their ground-breaking experiments on graphene.
Fullerenes are also naturally occurring compounds that contain only carbon. Like graphite and graphene, fullerenes consist solely of sp2 carbons, but instead of forming planar sheets, the carbons join to form spherical structures.