Pauli exclusion principle
A rule, complementary to the aufbau principle
, of building up the electronic configuration
of atoms and molecules: a maximum of two electrons can occupy an orbital and then only providing that the spins of the electrons are paired, i.e. opposed. The principle demands that the wavefunction for a many-electron system must be asymmetric
with respect to the permutation of the space-spin coordinates for every pair of electrons.