The Pauli principle
The Pauli principle, also known as the Pauli exclusion principle, was proposed by Wolfgang Pauli in 1925. The starting point was the problem of the completion of atomic shells and the structure of the periodic system of chemical elements. On the basis of the multiplet structure of the atomic spectra and the anomalous Zeeman effect, which could not be interpreted for a long time, Pauli realised that the "peculiar, classically non-describable type of ambiguity of the quantum theoretical properties of the luminous electron" demanded a fourth quantum number for it to be described. So he added a spin quantum number, as it became known, to the three known quantum numbers: principal quantum number, orbital quantum number and magnetic quantum number.
This led him to postulate that two electrons in an atomic structure could never have all four quantum numbers in common. If one electron in an atom assumes a state described by all four quantum numbers, then this state is "occupied".
Clarification of the exclusion principle
For a long time the exclusion principle remained an addendum to quantum theory incapable of further explanation. But Pauli never gave up his goal of linking the exclusion principle with other physical principles.
It took almost another 15 years of grappling with the problem until he could finally clarify it. It turned out that the exclusion principle applied to particles with half-integral spin, e.g. electrons and protons. If these particles are described "quantum statistically", then what are termed Fermi-Dirac statistics are employed. Such particles are known as fermions. On the other hand, particles with integral spin, e.g. photons, do not obey the exclusion principle and follow what are termed Bose-Einstein statistics. Such particles are known as bosons.
In 1945 Wolfgang Pauli was awarded the Nobel Prize (external link) for his discovery of the exclusion principle.