Carbon

Carbon has four valence electrons ([He]2s2 2p2), too many to lose easily and too few to gain, so it shares them, forming four covalent bonds. Carbon-carbon bonds are strong (about 346 kJ/mol for a single bond), and carbon can form single, double and triple bonds, so its atoms link into chains, branches and rings of almost unlimited variety. Tens of millions of carbon compounds are known, more than for all other elements combined, and this versatility is why an entire branch of chemistry, organic chemistry, is devoted to it.

The same bonding flexibility gives elemental carbon several very different forms. In diamond, every atom bonds to four neighbours in a rigid three-dimensional network, making it the hardest natural material and an electrical insulator. In graphite, atoms form flat hexagonal sheets with delocalized electrons that conduct electricity, while the sheets slide over one another. Carbon also forms hollow fullerene cages, nanotubes and single-atom-thick graphene. Through the carbon cycle, the element moves between the atmosphere (about 420 ppm CO2), oceans, rocks, fossil fuels and living things.

Discovery

Charcoal and soot have been used since prehistoric times, so carbon has no discoverer. In 1772 Antoine Lavoisier showed that diamond burns to give the same gas as charcoal, and in 1797 Smithson Tennant proved diamond is pure carbon. Lavoisier listed carbone among the elements in 1789, from the Latin carbo, charcoal. The name graphite was coined in 1789 by Abraham Gottlob Werner, from the Greek graphein, to write.