An energy level is one of the discrete, allowed energies that an electron may possess in an atom. Because these levels are quantised, an electron can occupy one level or another but not the energies in between. An electron moving between two levels absorbs or emits a photon whose energy equals the gap between them, which is why atoms give line spectra.

能级是电子在原子中可拥有的离散允许能量之一。由于这些能级是量子化的,电子只能处于某一能级或另一能级,而不能取它们之间的能量。电子在两能级间移动时,会吸收或放出能量等于二者间隔的光子,这正是原子产生线状光谱的原因。