Used with
permission of Maiken Naylor, University at Buffalo, Buffalo, NY, USA,
http://ublib.buffalo.edu/libraries/units/sel/exhibits/stamps
Physicist Wolfgang Pauli (1900-1958) empirically made the connection between closed electron shells in an atom with the complex spectra observed in a strong magnetic field (the Zeeman effect of splitting of spectral lines). Already, three quantum numbers had been assigned to the electron in the Bohr-Sommerfeld model of the atom, and to these Pauli added a fourth. Moreover, he generalized that there can never be two or more equivalent electrons with the same four quantum numbers in an atom, and this is known as the Pauli exclusion principle. While we now associate the fourth quantum number with the spin of the electron, this was not known in 1925 when Pauli published his conclusions. Since there are only so many permutations the sets of quantum numbers for a given Bohr orbit may have and still remain unique, the buildup of the periodic table of elements naturally follows. Pauli won the 1945 Nobel prize in physics for his work.