What's the Pauli-exclusion Principle?

Asked by: Wolfgang M.

Answer

In 1925, Wolfgang Pauli gave physics his exclusion principle as a way to explain the arrangement of electrons in an atom. His hypothesis was that only one electron can occupy a give quantum state. That is, each electron in an atom has a unique set of quantum numbers (the principle quantum number which gives its energy level, the magnetic quantum number which gives the direction of orbital angular momentum, and the spin quantum number which gives the direction of its spin). If this principle did not hold, all of the electrons in an atom would pile up in the lowest energy state (the K shell). In fact, we now know that that the Pauli exclusion principle holds for not just electrons but for any fermions (half-integer spin particles like electrons, protons, neutrons, muons, and many more.)
Answered by: Joseph Kozminski, B.S., Physics Grad Student, MSU, East Lansing, MI