An allotrope of carbon with many interesting physical properties. Graphite consists of carbon atoms, chemically bound into covalent lattice sheets, exactly one carbon atom thick. Substantial graphite deposits consist of many of these lattice sheets placed one atop of the other and scrunched together. The only thing holding these sheets together are intermolecular forces, much weaker than an actual covalent bond. Thus, it is very easy to shift these sheets in relation to each other with little friction. This ease in slipping the sheets of carbon relative to each other makes graphite an excellent lubricant. This ability also makes graphite easy to write with. The intermolecular forces between most substances and the lattice sheets are stronger than between the lattice sheets themselves. When graphite is rubbed against a substance, sheets come off of the structure and deposit themselves on the surface that is being written on.
Graphite exhibits many interesting chemical properties as well. Unbound carbon atoms exhibit 4 valence electrons. To become graphite, these carbon atoms undergo sp2 hybridization to form three sp2 orbitals that rest in a roughly triangular fashion around the atom. Additionally, one 2p orbital still exists perpendicular to the plane of the sp2 orbitals. This configuration can be visualized as a flattened tripod with a camera sticking up on it. The legs, flat against the floor, are like the sp2 orbitals. The base of the tripod is akin to the carbon atom"s nucleus and inner electron shell. The camera sticking up is like the remaining 2p orbital. When the carbon atoms bond to each other, the sp2 orbitals mesh together forming sigma bonds. At the same time, the 2p orbitals overlap at their boundaries, forming non-localized resonating pi bonds. The whole shebang looks something like this:
\ / \\ /
C-C C-C
// \\ / \\
-C C-C C-
\ / \\ /
C=C C-C
/ \ / \\
This is just one possible resonance structure of the carbon sheets; the pi bonds are delocalized, so the double bonds resonate. The delocalized nature of these pi bonds makes graphite an electrical conductor, although a weak one: electrons are free to flow from atom to atom through their overlapped 2p orbitals.