In 1925 Wolfgang Pauli enunciated his eponimous exclusion principle - "under no circumstances whatsoever may two electrons have precisely the same set of quantum numbers"

An interesting feature of the atomic spectrum of helium is that there are no lines in the orthohelium spectrum corresponding to the expected lowest term, namely 13S, the lowest term in the spectrum is 23S, i.e. where the principle quantum number is 2. Pauli in seaching for an explanation proposed the eponymous exclusion principle. Simply "it is impossible for any two electrons in the same atom to have their four quantum numbers identical"; in the language of wave mechanics this means that in an atomic system the complete wave function of every electron, that is the product of the orbital and spin functions, must be antisymmetric. This accounts for the nonexistance of the 13S state for the helium atom. To explain further.

The atom of helium has two electrons, and if the principal quantum number (n) is 1 for both, l and m can only be 0 in each case, so that if the Pauli exclusion principle is to hold the spin quantum numbers of the two electrons must be opposite in sign. The electrons may consequently be described by their quantum numbers :

1 n = 1 l = 0 m = 0 s = + ½
2 n = 1 l = 0 m = 0 s = - ½
Since the two electron spins are antiparallel it follows that only the par helium state is possible when n = 1. The ortho helium state in which the electron spins are parallel, both +½ or both -½ is impossible by the Pauli principle if n is 1. However if n is 2, l can be 0 or 1, and m can be 0, or ±1, so that the two electrons can have parallel spins without violating the exclusion principle. The lowest principle quantum number of the ortho helium state must therefore be 2.