Assertion proposed by Wolfgang Pauli that no two electrons in an atom can be in the same state or configuration at the same time. It accounts for the observed patterns of light emission from atoms. The principle has since been generalized to include the whole class of particles called fermions. The spin of such particles is always an odd whole-number multiple of 1 2 . For example, electrons have spin 1 2 , and can occupy two distinct states with opposite spin directions. The Pauli exclusion principle indicates, therefore, that only two electrons are allowed in each atomic energy state, leading to the successive buildup of orbitals around the nucleus. This prevents matter from collapsing to an extremely dense state
born April 25, 1900, Vienna, Austria died Dec. 15, 1958, Zürich, Switz. Austrian-born U.S. physicist. At the age of 20, he wrote a 200-page encyclopaedia article on the theory of relativity. He taught physics in Zürich (1928-40) and later at the Institute for Advanced Study in Princeton, N.J. In 1924 he proposed that a spin quantum number, + 1 2 or - 1 2 , is necessary to specify electron energy states. In 1930 he proposed that the energy and momentum apparently lost when an electron is emitted from an atomic nucleus in beta decay is carried away by an almost massless, uncharged, and difficult-to-detect particle (the neutrino). He was awarded a 1945 Nobel Prize for his 1925 discovery of the Pauli exclusion principle