Energy level

This article is about orbital (electron) energy levels. For compounds' energy levels, see chemical potential.

Energy level is simply defined as the different states of potential energy for electrons in an atom. A quantum mechanical system can only be in certain states, so that only certain energy levels are possible. The term energy level is most commonly used in reference to the electron configuration in atoms or molecules. In other words, the energy spectrum can be quantized.

As with classical potentials, the potential energy is usually set to zero at infinity, leading to a negative potential energy for bound electron states.

Energy levels are said to be degenerate, if the same energy level is obtained by more than one quantum mechanical state. They are then called degenerate energy levels.

The following sections of this article gives an overview over the most important factors that determine the energy levels of atoms and molecules.