EXCLUSION
The
exclusion principle is an assertion that no two electrons in an atom can be at the same time in the same
quantum state. It was proposed in 1925 by
Wolfgang Pauli to
account for the observed patterns of light emission from atoms. The frequency of the emitted light
is linked to the electrons' energy inside the atoms (a
photon is
emitted when an electron undergoes a transition to a lower energy
level). And the energy of an electron is in turn determined by its
quantum state.
The
exclusion principle has been subsequently generalized as to include a whole class of particles of which the electron is only one member. These are the
fermions, which include, for example, neutrons, protons and quarks.