The atom that makes chains
A carbon atom has four electrons in its outer shell and four bonds to make, and it makes them with almost anything — above all with other carbon atoms. Carbon–carbon bonds are strong, so carbon links into chains, rings and cages of any length: more compounds of carbon are known than of all the other elements together, the subject of organic chemistry.
The pure element takes several forms. In diamond each atom is bonded to four others in a rigid tetrahedral lattice, which makes it the hardest natural material and a better conductor of heat than any metal. In graphite the atoms lie in flat hexagonal sheets, strongly bonded within a sheet and barely held between them, so it is soft, slippery, black and conducts electricity. A single sheet of graphite is graphene, isolated by Andre Geim and Konstantin Novoselov in 2004; closed into a ball of sixty atoms it is buckminsterfullerene, found by Harry Kroto, Robert Curl and Richard Smalley in 1985.
Where it is
Carbon is the fourth most abundant element in the universe, made in stars by the fusion of three helium nuclei — a step that works only because a carbon nucleus has an energy level just where it is needed, as Fred Hoyle predicted in 1953. On Earth it is about 0.02 % of the crust, nearly all of it locked in limestone and other carbonates; coal, oil and gas are a small remainder. The air holds it as carbon dioxide, about 0.04 % by volume, and the oceans hold some fifty times more, dissolved.
The carbon cycle turns between them: plants and algae take carbon dioxide from the air and, with waterCompound: Water. Two hydrogens, one oxygen, and most of the living world and sunlight, build it into sugar; everything that eats, breathes or rots sends it back. Diamond forms deep in the mantle and comes up in volcanic pipes; graphite is mined or made in furnaces from petroleum coke.
Life, fire and steel
Every protein, sugar, fat and strand of DNA is a carbon skeleton with other atoms attached. The energy that runs the modern world is carbon too: coal, oil and gas are the remains of ancient life, and burning them puts more than 35 billion tonnes of carbon dioxide into the air every year. The gas traps heat; its rise from about 280 to more than 420 parts per million since 1800 is warming the climate.
Carbon turns ironElement: Iron. The metal that built the modern world, and the core of the planet beneath it into steel: up to about two per cent dissolved in the metal is what makes it hard. Graphite makes electrodes and crucibles, carbon black stiffens tyres, carbon fibre builds aircraft and bicycles, diamond cuts and drills. And the ¹⁴C left in old wood, bone or cloth has dated them since Willard Libby's first measurements in 1949.
Older than history
Charcoal is older than writing: the animals on the walls of the Chauvet cave were drawn with it more than thirty thousand years ago, and coal, soot and diamond were familiar to the ancients. Graphite, dug up at Borrowdale in Cumbria in the sixteenth century, was taken for a kind of lead, which is why pencils still have "leads".
In 1772 Antoine Lavoisier focused the sun through a great lens on a diamond and found that it burnt away entirely into fixed air, the same gas that charcoal gives; in 1779 Carl Wilhelm Scheele showed that graphite did the same. Lavoisier and his colleagues named the element carbone, from the Latin carbo, charcoal, in their Méthode de nomenclature chimique of 1787, and his Traité of 1789 listed it among the simple substances. In 1797 Smithson Tennant weighed the gas from a burnt diamond and settled it: diamond is pure carbon.