exclusion principle

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exclusion principle, physical principle enunciated by Wolfgang Pauli in 1925 stating that no two electrons in an atom can occupy the same energy state simultaneously. The energy states, or levels, in an atom are described in the quantum theory by various values of four different quantum numbers; the exclusion principle holds that no two electrons can have the same four quantum numbers in an atom. One of these quantum numbers describes one of the two possible directions for the electron's intrinsic spin. As a result of the exclusion principle, two electrons that are in the same energy level as described by the other three quantum numbers are differentiated from each other because they have opposite spins. This principle applies not only to atoms but to other systems containing particles as well, and it applies not only to electrons but also to a large class of particles collectively known as fermions (see elementary particles).

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exclusion principle Basic law of quantum mechanics, proposed by Wolfgang Pauli in 1925, stating no two electrons in an atom can possess the same energy and spin. More precisely, the set of four quantum numbers characterizing certain elementary particles called fermions must be unique.

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exclusion principle A principle introduced in 1934 by the Russian ecologist G. F.Gause (1910–86), that two species cannot coexist in the same locality if they have identical ecological requirements.

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exclusion principle See COMPETITIVE-EXCLUSION PRINCIPLE.

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exclusion principle A principle, introduced by G. F. Gause, that two species cannot coexist in the same locality if they have identical ecological requirements.