<P> The Pauli exclusion principle is the quantum mechanical principle which states that two or more identical fermions ( particles with half - integer spin ) can not 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 spins 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>
who said no two objects can occupy the same space at the same time
Wolfgang Pauli