carbon

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.

Non-metallic element, atomic number 6, relative atomic mass 12.011. It occurs on its own as diamond, graphite, and as fullerenes (the allotropes), as compounds in carbonaceous rocks such as chalk and limestone, as carbon dioxide in the atmosphere, as hydrocarbons in petroleum, coal, and natural gas, and as a constituent of all organic substances.

In its amorphous form, it is familiar as coal, charcoal, and soot. The atoms of carbon can link with one another in rings or chains, giving rise to innumerable complex compounds. Of the inorganic carbon compounds, the chief ones are carbon dioxide (CO₂), a colourless gas formed when carbon is burned in an adequate supply of air; and carbon monoxide (CO), formed when carbon is oxidized in a limited supply of air. Carbon disulphide (CS₂) is a dense liquid with a sweetish odour when pure. Another group of compounds is the carbon halides, including carbon tetrachloride (tetrachloromethane, CCl₄).

When added to steel, carbon forms a wide range of alloys with useful properties. In pure form, it is used as a moderator in nuclear reactors; as colloidal graphite it is a good lubricant and, when deposited on a surface in a vacuum, reduces photoelectric and secondary emission of electrons. Carbon is used as a fuel in the form of coal or coke. The radioactive isotope carbon-14 (half-life 5,730 years) is used as a tracer in biological research and in radiocarbon dating. Analysis of interstellar dust has led to the discovery of discrete carbon molecules, each containing 60 carbon atoms. The C₆₀ molecules, which can also be synthesized, were named buckminsterfullerenes (or fullerenes) because of their structural similarity to the geodesic domes designed by US architect and engineer Richard Buckminster Fuller. Variation of the production method led to the discovery of tube-shaped carbon molecules known as carbon nanotubes.

The element has the following characteristic reactions.

With air or oxygen It burns on heating to form carbon dioxide in excess air, or carbon monoxide in a limited supply of air.

C + O2 → CO2  ΔH = −394 kJ mol−1

2C + O2 → 2CO

With metal oxides It reduces many metal oxides at high temperatures. Fe2O3 + 3C → 2Fe + 3CO

With steam It forms water gas (a cheap, useful, industrial fuel) when steam is passed over white-hot coke.

C + H2O → CO + H2

With concentrated acids With hot, concentrated sulphuric or nitric acids it forms carbon dioxide.