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Pauli exclusion principle

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, , the angular momentum quantum number, *m*, the magnetic quantum number, and *m*_{s}, the spin quantum number. For two electrons residing in the same orbital, *n*, , and *m* are the same, so *m*_{s}, 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.

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