allotropy

Carbon has three allotropes: diamond, graphite, and the fullerenes. Diamond is strong because each carbon atom is linked to four other carbon atoms. Graphite is made up of layers that slide across one another (giving graphite its qualities as a lubricator); each layer is a giant molecule. In the fullerenes, the carbon atoms form spherical cages. Buckminsterfullerene (shown here) has 60 atoms. Other fullerenes, with 28, 32, 50, 70, and 76 carbon atoms, have also been identified.

Property whereby an element can exist in two or more forms (allotropes), each possessing different physical properties but the same state of matter (gas, liquid, or solid). The allotropes of carbon are diamond, fullerene, and graphite. Sulphur has several allotropes (flowers of sulphur, plastic, rhombic, and monoclinic). These solids have different crystal structures, as do the white and grey forms of tin and the black, red, and white forms of phosphorus.

Oxygen exists as two gaseous allotropes: the O₂ molecule which makes up 21% of the air we breathe, and the highly reactive ozone, O₃, which occurs in trace amounts as a pollutant and in the stratosphere.