Energy Levels
(From Wikipedia)

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

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 (see continuous spectrum for the more general case).

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.