These have identical physical properties, except for their crystal structure. In 1969 a new allotropic form of carbon was produced during the sublimation of pyrolytic graphite at low pressures. Little information is presently available about this allotrope. In combination, carbon is found as carbon dioxide in the atmosphere of the earth and dissolved in all natural waters. Coal, petroleum, and natural gas are chiefly hydrocarbons.
Carbon is unique among the elements in the vast number and variety of compounds it can form.
With hydrogen, oxygen, nitrogen, and other elements, it forms a very large number of compounds, carbon atom often being linked to carbon atom. There are close to ten million known carbon compounds, many thousands of which are vital to organic and life processes. Whitout carbon, the basis for life would be impossible. While it has been thought that silicon might take the place of carbon in forming a host of similar compounds, it is now not possible to form stable compounds with very long chains of silicon atoms. Carbon has seven isotopes.