Pauli exclusion principle

The Pauli exclusion principle states that identical fermions cannot occupy the same quantum state simultaneously. It underlies electron-shell structure in atoms and the stability of ordinary matter.

Fermion: A particle with half-integer spin that obeys Fermi–Dirac statistics. The principle applies to identical particles in this class.
Electron shell: A group of atomic electron states with similar principal quantum numbers. Exclusion limits how many electrons can occupy each shell's available states.
Boson: A particle with integer spin that obeys Bose–Einstein statistics. Unlike fermions, identical bosons may share a quantum state.
Wolfgang Pauli: An Austrian-born theoretical physicist who formulated the exclusion principle in 1925. He introduced the rule to explain patterns in atomic spectra.
Stability of matter: The bounded-energy behavior that prevents large collections of ordinary matter from collapsing without limit. Fermionic exclusion helps keep electrons from collapsing into the lowest available states.
Quantum state: A specification of the measurable properties and probability amplitudes of a quantum system. Exclusion forbids identical fermions from sharing a complete state.
Periodic table: An arrangement of chemical elements by atomic number and recurring chemical properties. Electron filling under exclusion produces the recurring structure of the table.
Bose–Einstein statistics: The statistical distribution governing indistinguishable bosons in thermal equilibrium. Its unrestricted state occupancy contrasts with fermionic exclusion.
Atomic spectrum: The set of discrete frequencies emitted or absorbed by atoms. Unexplained spectral patterns motivated Pauli's proposal.
Fermi energy: The energy of the highest occupied single-particle state at zero temperature in a fermion system. Exclusion fills states up to this energy even in the ground state.

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