No two electrons in an atom can ever occupy the same state

In 1925, Wolfgang Pauli proposed that no two electrons in an atom can share the same set of quantum numbers, forcing electrons to stack into distinct shells rather than piling into the lowest energy level. That single rule explains the structure of the periodic table and why solid matter doesn't collapse in on itself.

— Wolfgang Pauli, On the Connection between the Completion of Electron Groups in an Atom with the Complex Structure of Spectra — Zeitschrift für Physik, vol. 31, 1925