In graphite, the individual carbon atoms link up to form sheets of carbon atoms. Within each sheet every carbon atom is bonded to three adjacent carbon atoms that lie at the apices of equilateral triangles. This produces hexagonal rings of carbon atoms. Each carbon atom has four valence electrons available to participate in the formation of chemical bonds. Three of these electrons are used in forming strong covalent bonds with the adjacent atoms in the sheet.
Covalent bonds are a type of chemical bond in which electrons are shared between atoms.
The fourth electron is free to wander over the surface of the sheet making graphite an electrical conductor. The spacing between the sheets of carbon atoms is greater than the diameter of the individual atoms. Weak bonding forces called van der Waals forces hold the sheets together. Because these forces are weak, the sheets can easily slide past each other. The four valence electrons of each carbon atom participate in the formation of very strong covalent bonds.