Carbon is unique among the elements because it forms a vast number of compounds, more than the total of all other elements combined with the exception of hydrogen. It exists in three allotropic forms, namely, diamond, graphite and amorphous carbon. Recently new carbon molecules such as C-60 were discovered. The C60shape is an example of a new class of substances called fulerenes.

Diamond and graphite are naturally occurring crystalline solids with widely divergent properties, whereas amorphous carbon is a term applied to a comparatively large variety of carbonaceous substances not classified as either diamond or graphite.