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 . For example, electrons have spin , 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.