Carbon

Diamond and graphite are both pure carbon. Diamond is the hardest natural material, transparent, and an electrical insulator that conducts heat better than any metal. Graphite is black, opaque, soft enough to leave a mark on paper, and conducts electricity. Nothing distinguishes them but the arrangement of identical atoms: in diamond each carbon binds to four neighbours in a rigid three-dimensional lattice, in graphite to three, forming flat sheets that slide over one another.

That two such different substances are the same element was not obvious and had to be shown. Antoine Lavoisier burned a diamond in oxygen in the 1770s and collected the product; it was the same gas that charcoal gives. Smithson Tennant confirmed the quantitative result in 1797.

The list of forms has since grown. Fullerenes, closed cages of sixty or more atoms, were found by Harold Kroto, Robert Curl and Richard Smalley in 1985 and named for Buckminster Fuller. Carbon nanotubes followed. Graphene, a single sheet of graphite one atom thick, was isolated by Andre Geim and Konstantin Novoselov at Manchester in 2004 using adhesive tape, and won them the Nobel Prize in 2010.

Carbon does not melt at atmospheric pressure. Heated hard enough it sublimes, passing straight from solid to vapour above roughly 3600 degrees C, which is why the claim that tungsten has the highest melting point of any element always carries a quiet qualification.

Alchemy used carbon constantly without naming it. Charcoal is the reducing agent that pulls metal out of ore, and every smelting furnace from the Bronze Age onward is a carbon reaction. Diamond was known as a gem and prized for hardness. But charcoal, soot, graphite and diamond were four unrelated substances to every practitioner before Lavoisier, and none of them was ever assigned a planet.

Carbon makes four bonds, and it makes them readily to itself. Chains, branches, rings and sheets of carbon atoms are all stable at the temperatures at which liquid water exists, and each carbon left over can carry hydrogen, oxygen, nitrogen, sulfur or phosphorus. No other element combines that versatility with that stability. Silicon, directly below it and often proposed as an alternative basis for life, forms weaker bonds to itself, and its dioxide is a solid rather than a gas, which blocks anything resembling a respiratory cycle before it starts.

The consequence is that the chemistry of carbon compounds is a subject in its own right, larger by number of known substances than all the rest of chemistry together. Proteins, sugars, fats and nucleic acids are all carbon frameworks. The distinction between organic and inorganic chemistry was originally a claim that the first required a vital force; Friedrich Wohler's synthesis of urea from ammonium cyanate in 1828 removed the claim and left the name behind.

Carbon is only about 0.02 per cent of the Earth's crust, far less abundant than oxygen, silicon, aluminium or iron. Living things concentrate it enormously. The carbon cycle moves it between atmosphere, ocean, rock and biology on timescales running from days to hundreds of millions of years, and the current alteration of that cycle by burning fossil carbon is the central environmental fact of the present.

Carbon-14, made continuously in the upper atmosphere by cosmic rays and taken up by everything alive, decays with a half-life of about 5,730 years. Willard Libby's radiocarbon dating, developed from 1946, made it possible to put absolute dates on organic remains for the first time, and reordered a great deal of archaeology in the process.