7/23/2019 Pauli Exclusion

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The Pauli exclusion principle is the quantum mechanical principle that states that

two identical fermions (particles with half-integer spin) cannot occupy the same quantum

state simultaneously. In the case of electrons, it can be stated as follows: it is impossible for two

electrons of a poly-electron atom to have the same values of the four quantum numbers: n,

the principal quantum number , ℓ  , the angular momentum quantum number , mℓ , the magnetic

quantum number , and ms, the spin quantum number. For two electrons residing in the

same orbital, n, ℓ , and mℓ  are the same, so ms, the spin, must be different, and thus the electrons

have opposite half-integer spins, 1/2 and -1/2. This principle was formulated by Austrian

physicist Wolfgang Pauli in 1925.

 A more rigorous statement is that the total wave function for two

identical fermions is antisymmetric with respect to exchange of the particles. This means that the

wave function changes its sign if the space and  spin co-ordinates of any two particles are

interchanged.

Particles with an integer spin, or bosons, are not subject to the Pauli exclusion principle: any number

of identical bosons can occupy the same quantum state, as with, for instance, photons produced by

a laser  and Bose–Einstein condensate.