Band gaps can be found in insulators and semiconductors. In graphs of the electronic band structure of solids, the band gap is the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band. This is the same as the energy needed to free an outer shell electron from its orbit about the nucleus to become a mobile charge carrier. The free electron is able to move freely within the solid material. So the band gap is a major factor determining the electrical conductivity of a solid. Substances with large band gaps are generally insulators, those with smaller band gaps are semiconductors. Conductors either have very small band gaps or no band gap if the energy levels of valence and conduction bands overlap.