graphite

Graphite is the soft, slippery, grey form of pure carbon, the mark a pencil leaves on paper. It is the same element as diamond, yet as different as chalk and cheese, and the reason is the way the atoms are stacked. Instead of a rigid 3D cage, graphite is built from flat sheets of carbon, one atom thick, loosely piled on top of each other like a ream of paper.

Within each sheet, carbon atoms sit in a honeycomb of hexagons, and each atom bonds strongly to three neighbours, using three of its four outer electrons. Those in-plane bonds are actually stronger than diamond's, so a single sheet is extremely tough. But the fourth electron roams over the sheet, and between sheets there is only weak van der Waals attraction, no real bonds. So the sheets slide over one another with almost no effort. That is why graphite feels greasy, works as a dry lubricant, and leaves a trail on paper as layers shear off. Those free, roaming electrons also make graphite a good electrical conductor, unlike diamond.

Graphite is the textbook case of anisotropy: strong and stiff along the sheets, weak and slippery across them, so its properties depend on direction. It survives extreme heat without melting (it sublimes above about 3600 degrees C) and resists many chemicals, which is why it is used for electrodes, crucibles, high-temperature furnace parts, and brushes in electric motors. A single one of its sheets, isolated, is the celebrated material graphene. The pencil myth that lead is a metal is exactly that: pencil lead has always been graphite, a form of carbon.