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Carbon, that most catholic of the elements, occupies the sixth station in the periodic table — a position of singular importance, for upon its tetravalent nature rests the entire edifice of organic chemistry, and with it the very fabric of biological life. Possessed of four valence electrons in its outermost shell, the carbon atom forges bonds with a versatility unmatched by any other element, joining itself to itself in chains of arbitrary length and to a vast company of other elements in countless combinations.

Were one to assemble all the compounds of all the elements together upon the floor of the great Library, those of carbon alone would fill more shelves than all the rest combined. The discipline that treats of these is termed Organic Chemistry — a name that betrays the historical conviction, since happily disposed of, that carbon compounds were producible only by living things.

The four outer electrons — the agents of all carbon's chemistry — may arrange themselves in three principal hybridisations: sp³, yielding tetrahedral geometry as in diamond and methane; sp², yielding trigonal-planar arrangement as in graphite and benzene; and sp, yielding linear conformation as in acetylene. From these three modes, all of carbon's structural diversity proceeds.

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The chemistry of carbon is the chemistry of complexity itself — a single element rehearsing, in its many guises, the entire vocabulary of form.

Beneath the apparent simplicity of a single element there lies a kingdom of forms. Carbon expresses itself in allotropes whose physical attributes could scarcely be more divergent — diamond, the hardest of natural substances, and graphite, soft enough to mark paper, are each composed of nothing but carbon atoms, differing only in the geometrical arrangement of those atoms in space.

The traveller through the carbon cabinet shall encounter eight principal specimens: from the humble lump of coal to the lattice of graphene. Each is a singular variation upon the elemental theme.

Nothing is created, nothing is destroyed; all is transformed. The atoms of carbon presently constituting the leaf of an oak were, perhaps a year ago, dispersed in the atmosphere as carbon dioxide, and may, a century hence, lie compressed in a stratum of coal. The Carbon Cycle — that perpetual transmigration of carbon between atmosphere, ocean, biota, and lithosphere — is the great metabolic engine of the planet.

Photosynthesis withdraws carbon from the air and deposits it, transformed, into the tissues of plants; respiration and combustion return it whence it came. Across geological time, vast quantities are sequestered in carbonate rocks and fossilised hydrocarbons, only to be released again — slowly by volcanism and weathering, or, of late, with great rapidity by the intervention of mankind.

The carbon atom achieves the noble-gas configuration of neon by accumulating four further electrons through the formation of covalent bonds. So firmly is this principle established that we may speak, without exaggeration, of carbon as the architect of molecular form: it is the geometry of carbon's bonds — tetrahedral, planar, linear — that dictates the three-dimensional shape of the entire organic kingdom.

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That a single element should serve as the foundation for so vast and intricate an architecture is, of all the wonders of nature, perhaps the most quietly astonishing.