Exclusion principle

Bohr and Sommerfeld worked out the energy levels of the electrons in an atom and expressed them as quantum numbers, three of which were known at the time. Pauli suggested a fourth, and the exclusion principle, which states that no two electrons in a system can have identical sets of quantum numbers.
 Wolfgang Pauli United States 1925 Exclusion principle

Bohr and Sommerfeld worked out the energy levels of the electrons in an atom and expressed them as quantum numbers, three of which were known at the time. Pauli suggested a fourth, and the exclusion principle, which states that no two electrons in a system can have identical sets of quantum numbers.