In a single isolated atom discrete energy levels are present. In an aggregate of atoms, as in solids, the individual energy levels are so close together that they can be considered to be continuous, thus forming an energy band. Electrons in the outermost shell (valance shell) of an atom are the valence electrons.

The energy band formed by the valence energy levels is known as valence band. Depending upon the nature of solid, its valence band may be completed or partially filled. Valence band is the highest occupied bands in solids. The lowest unoccupied energy band just above the valence band is called as conduction band and electrons migrating to this band are conduction electrons. Such electrons can move freely in this band. The conduction band may be empty or partially filled. The gap separating valence band and conduction band is called as band gap or forbidden gap. The width of the band gap decides the nature of the solid and no electron can exist in the forbidden band gap. An isolated or free electron moving in vacuum having mass m but when the material is under the action of electric or magnetic field experiences different mass is known as effective mass.

This variation of mass of electron is due to interaction between drifting electrons and atoms in solids.