Carbon

Carbon is the element the whole of organic chemistry and all known life rest on. What makes it exceptional is not abundance: it is less than a percent of the Earth's crust. It is the ability of its atoms to join to one another in chains, rings and branched frameworks of essentially unlimited length.

More compounds of carbon are known than of all the other elements combined, and the count runs into tens of millions.

Bound, in carbonates — limestone, chalk and marble are all CaCO₃ — in atmospheric carbon dioxide, dissolved in the ocean, in oil, gas and coal, and in everything alive.

Free, carbon exists in several forms, and the differences between them are the textbook demonstration that structure decides properties:

• Diamond — each atom bonded to four neighbours in a rigid three-dimensional framework. Hence its record hardness and its transparency.
• Graphite — atoms bonded into flat sheets that hold on to each other only weakly and slide easily. Hence softness, metallic lustre and electrical conductivity.
• Fullerenes and nanotubes — closed cages and rolled-up sheets, discovered at the end of the twentieth century.
• Graphene — a single layer of graphite, whose isolation earned the 2010 Nobel Prize.

All of them are the same atoms.

Charcoal and soot were in use before writing was, so carbon has no date of discovery. A different discovery mattered more: that diamond and graphite are the same substance.

In 1772 Lavoisier burned a diamond at the focus of a lens and found it gave the same gas as burning charcoal. In 1797 Smithson Tennant completed the proof, showing that equal masses of diamond and graphite yield equal amounts of carbon dioxide.

Carbon sits in the middle of the second period, and that is the key to it. Giving up four electrons is as hard as taking on four, so it shares them instead — forming four strong covalent bonds.

The carbon–carbon bond is comparable in strength to the carbon–oxygen bond, which is unusual: for most elements the bond to itself is markedly weaker. That is precisely why long carbon chains are stable and long chains of, say, silicon are not.

Carbon forms single, double and triple bonds, and in aromatic systems delocalised ones. The whole variety of organic chemistry grows out of that small repertoire.

Metallurgy: coke reduces iron from its ore, and the carbon content of steel decides its hardness. Energy: coal, oil and gas still supply most of the world's. Materials: carbon fibre, graphite electrodes, activated charcoal, carbon black in tyres.

Radiocarbon dating is a field of its own. The isotope ¹⁴C forms continuously in the atmosphere under cosmic radiation and decays at a fixed rate in an organism once it dies; what remains gives the age of organic finds up to roughly fifty thousand years.