Pauli exclusion principle, also known as Pauli principle and exclusion principle, is one of the basic laws of the movement of microscopic particles. It points out that in a system composed of fermions, there cannot be two or more particles in the same state. It takes four quantum numbers to completely determine the state of an electron in an atom, so the Pauli exclusion principle in an atom is shown as follows: no two or more electrons can have exactly the same four quantum numbers, or at most two electrons can be accommodated in a proton orbit determined by the orbital quantum numbers m, l, n, and the spin direction of these two electrons must be opposite. This becomes one of the criteria for the periodic arrangement of electrons outside the nucleus to explain the periodic table of elements.
