Pauli Exclusion Principle
The 1920s were a decade of enormous upheaval in atomic physics. Niels Bohr's model of the atom, proposed in 1913, had been a historic breakthrough in scientists' attempts to understand the nature of matter. But even from the outset, it was obvious that the Bohr model was inadequate to explain the fine points of atomic structure.
In some ways, the most important contribution made by Bohr was his suggestion that electrons can appear in only specific locations outside the atomic nucleus. These locations were designated as shells and were assigned integral "quantum" numbers beginning with 1. Electrons in the first shell were assigned the quantum number 1, those in the second orbit, the quantum number 2, those in the third shell, quantum number 3, and so on.
Eventually it became evident to scientists that additional quantum numbers would be needed to fully describe an electron in its orbit around the atomic nucleus. For example, the German physicist Arnold Sommerfeld (1868-1951) announced in 1915 that electrons traveled not in circles, but in ellipses around an atomic nucleus. The eccentricity of the elliptical orbit could be expressed, Sommerfeld said, by a second quantum number. By the mid-1920s, two additional quantum numbers, one defining the magnetic characteristics of an electron and one defining its spin, had been adopted.
Science EncyclopediaScience & Philosophy: Evolution to FerrocyanidePauli Exclusion Principle - Historical Background, The Exclusion Principle, Electronic Configurations, Rationalizing The Periodic Law