An electrical current is the movement of electrons, so a conductive material allows the free movement of electrons. Metals are good conductors of electricity because of their electronic structure. There are basically two big groups of orbitals in metals—the valence band and the conduction band. The valence band is the group of orbitals that are normally filled in a metal, while the conduction band is empty. These are called bands because they are made up of sets of closely spaced energy levels. Metals, and other good conductors of electricity, can have very small or no band gaps at all. Semiconductors have small band gaps, and electrons can be promoted from the valence band to the conduction band either by heat or light. Insulators, materials that do not conduct electricity, have large band gaps.