<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> The Pauli exclusion principle is the quantum mechanical principle which states that two or more identical fermions (particles with half - integer spin) cannot occupy the same quantum state within a quantum system simultaneously . In the case of electrons in 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 four quantum numbers: n, the principal quantum number, l, the angular momentum quantum number, m, the magnetic quantum number, and m, the spin quantum number . For example, if two electrons reside in the same orbital, and if their n, l, and m values are the same, then their m must 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 physicist Wolfgang Pauli in 1925 for electrons, and later extended to all fermions with his spin--statistics theorem of 1940 . </P> <P> 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 . </P>

Why two different elements cannot occupy the same box in the periodic table
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