A delocalized electron is an electron in an atom, ion or molecule not associated with any single atom or a single covalent bond.
In a ring structure, delocalized electrons are indicated by drawing a circle rather than single and double bonds. This means the electrons are equally likely to be anywhere along the chemical bond.
Delocalized electrons contribute to the conductivity of the atom, ion, or molecule. Materials with many delocalized electrons tend to be highly conductive.
Delocalized Electron Examples
In a benzene molecule, for example, the electrical forces on the electrons are uniform across the molecule. The delocalization produces what is called a resonance structure.
Delocalized electrons are also commonly seen in solid metals, where they form a "sea" of electrons that are free to move throughout the material. This is why metals are typically excellent electrical conductors.
In the crystal structure of a diamond, the four outer electrons of each carbon atom participate in covalent bonding (are localized). Contrast this with bonding in graphite, another form of pure carbon. In graphite, only three of the four outer electrons are covalently bonded to other carbon atoms. Each carbon atom has a delocalized electron that participates in chemical bonding but is free to move throughout the plane of the molecule. While the electrons are delocalized, graphite is a planar shape, so the molecule conducts electricity along the plane, but not perpendicular to it.