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Niels Bohr’s pioneering quantum model of the atom explained the spectral systems of the hydrogen atom. Logically, physicists, including Bohr, directed their efforts toward extending the theoretical success to more complex atoms. From chemical study, it was concluded that every chemical element corresponds to one type of atom, and all the elements display their chemical properties regularly in the periodic table, beginning with the lightest and simplest: the hydrogen atom. Therefore, one task of atomic physicists of the 1910’s and 1920’s was to explain the periodic table of elements—the orderly array and the increasing complexity of the atoms. Another task was to explain the phenomena of atomic spectra. Bohr’s quantum postulates were found inadequate for more complex atoms or for atoms in an external force field such as a magnetic field. Between about 1915 and 1925, European physicists searched intently for new principles and guidelines for the construction of atomic models that would consistently account for chemical phenomena: the periodic table, physical phenomena, and spectral data.

The Pauli exclusion principle states that no two electrons can exist in the same quantum mechanical state. Not only does this principle effectively reduce chemistry to physics but also, taken as applying to fermions in general, it plays a fundamental role in the development of quantum theory and elementary particle physics.