Graphite is one of the few non-metallic materials that conducts electricity, and its ability to do so is directly related to its unusual atomic structure. Understanding why graphite conducts gives insight into the relationship between atomic bonding and electrical properties.

Carbon's Bonding in Graphite

Carbon atoms have four electrons available for bonding. In graphite, each carbon atom forms covalent bonds with three neighbouring carbon atoms, using three of its four bonding electrons. The atoms arrange themselves in flat hexagonal rings, forming large sheets of carbon called graphene layers. The fourth electron on each carbon atom is not used in the direct covalent bonds with neighbours.

Delocalised Electrons

The fourth electron from each carbon atom is not tied to a specific bond between two atoms. Instead, these electrons are delocalised: they can move freely throughout the graphene layer, not belonging to any particular carbon atom but shared across the whole sheet. This pool of mobile electrons is the key to graphite's electrical conductivity. When a potential difference is applied across a piece of graphite, these delocalised electrons move in response to the electric field, creating an electric current.

This is the same principle that makes metals conduct electricity. Metals have delocalised electrons, sometimes called the electron sea, that are free to move through the metal lattice when a voltage is applied. Graphite's layered structure produces the same result through a different structural arrangement.

Conductivity Within and Between Layers

Graphite's conductivity is anisotropic, meaning it varies with direction. Within the graphene layers, conductivity is high because the delocalised electrons move easily parallel to the layers. Between the layers, conductivity is much lower because the layers are held together only by weak van der Waals forces with no shared electrons between layers. In practice, most applications of graphite as an electrical conductor use it in forms where current flows along the layers.

Practical Applications

Graphite's combination of electrical conductivity, high temperature resistance, and chemical inertness makes it useful in electrodes, electrical contact brushes, and as an additive to improve the conductivity of some polymer composites. The graphite core in pencils also carries a small amount of current, which is why pencil traces on paper can be used to create simple circuits in electronics experiments.

Summary

Graphite conducts electricity because each carbon atom bonds to three neighbours using three electrons, leaving one electron per atom delocalised and free to carry current through the graphene layers. This is the same mechanism that gives metals their conductivity. Conductivity is high along the layers but low between them. Diamond, the other form of pure carbon, is an insulator because all four electrons per atom are used in bonds with no free electrons available.