Editing Energy level (section)

== Crystalline materials ==
[[Crystal|Crystalline solid]]s are found to have [[energy band]]s, instead of or in addition to energy levels. Electrons can take on any energy within an unfilled band. At first this appears to be an exception to the requirement for energy levels. However, as shown in [[band theory]], energy bands are actually made up of many discrete energy levels which are too close together to resolve. Within a band the number of levels is of the order of the number of atoms in the crystal, so although electrons are actually restricted to these energies, they appear to be able to take on a continuum of values. The important energy levels in a crystal are the top of the [[valence band]], the bottom of the [[conduction band]], the [[Fermi level]], the [[vacuum level]], and the energy levels of any [[defect states]] in the crystal.