Pauli exclusion principle
Basic principle of quantum theory stating that no two fermions can be in exactly the same state – for instance: no two fermions with identical properties can be at the same location. Formulated by the physicist Wolfgang Pauli.
Electrons are fermions, and the Pauli exclusion principle plays a crucial role in bringing about the properties of matter as we know them: It is responsible for the fact that the electrons of atoms do not all cluster together in the lowest-energy state close to the atomic nucleus, but instead spread out, occupying different states. This is what gives atoms their shell structure, responsible for different atoms’ different chemical properties.