Since the helium atom has only two electrons, it provides an ideal model to study how the mutual influence of coupled fermions affects the energy levels available to them. The spectrum of helium displays very clearly the effect of the so-called exchange forces. Exchange forces are determined by the symmetry properties of the electrons' quantum state (see origins). For example, if the spin part of the electrons' state is symmetric, then their global wave function must be anti-symmetric, which means that the electrons are unlikely to be found close to each other, i.e. they tend to keep apart from one another. This affects the corresponding energy level, since the Coulomb interaction between the two electrons, as well as that between each electron and the nucleus of the atom, depend on their mutual average distance. The spectrum of helium reveals therefore the symmetry constraints expressed by the exclusion principle.

Historically, one of the first achievements based on the exclusion principle was the successful explanation of the structure of helium spectrum. When a gas is illuminated, it absorbs the radiation energy and reemits it as light at certain frequencies. These frequencies are associated with the transitions of the atom's electrons from one energy level to another, so that the spectrum of the emitted radiation works as a map of the atom's energy levels.