Pauli exclusion principle

The Pauli exclusion principle says that two identical fermions cannot occupy the exact same quantum state at the same time. For electrons in an atom, that means no two electrons can share the same four quantum numbers.

Wolfgang Pauli proposed the principle in 1925. He later received the 1945 Nobel Prize in Physics for this discovery.

It explains why electrons fill atomic orbitals in shells and subshells rather than all collapsing into the lowest state. That structure gives elements their chemical behavior and periodic trends.

No. It applies to fermions such as electrons, protons, and neutrons, which have half-integer spin. Photons are bosons, so many photons can occupy the same quantum state, as in a laser.