What is Pauli's exclusion principle?

1.	What is Pauli's exclusion principle?
Answer» ThePauli exclusion principleis thequantum mechanicalprinciple which states that two or moreidenticalfermions(particles with half-integerspin) cannot occupy the samequantum statewithin aquantum systemsimultaneously. In the case ofelectronsin atoms, it can be stated as follows: it is impossible for two electrons of a poly-electron atom to have the same values of the fourquantum numbers:n, theprincipal quantum number,ℓ, theangular momentum quantum number,mℓ, themagnetic quantum number, andms, thespin quantum number. For example, if two electrons reside in the sameorbital, and if theirn,ℓ, andmℓvalues are the same, then theirmsmust be different, and thus the electrons must have opposite half-integer spin projections of 1/2 and −1/2. This principle was formulated by Austrian physicistWolfgang Pauliin 1925 for electrons, and later extended to all fermions with hisspin–statistics theoremof 1940.

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