In quantum physics, what principle says identical fermions cannot share the same quantum state?

The Pauli exclusion principle says identical fermions cannot share the same quantum state. Wolfgang Pauli formulated the principle in 1925 to explain patterns in atomic electron shells.

Electrons are fermions, so no two electrons in one atom can have the same complete set of four quantum numbers. This rule forces electrons to occupy different available states and produces the structure of the periodic table. It also explains why electron shells fill in layers rather than collapsing into one state.

The principle is a consequence of the antisymmetric quantum state required for identical fermions. Fermions have half-integer spin, including electrons, protons, and neutrons. Bosons, such as photons, follow different statistics and can occupy the same state in unlimited numbers.

Pauli’s rule is not simply an electrical repulsion effect. It remains important even when particles are treated as identical and their interactions are separated from the description of their quantum statistics.