Fermions are particles which have half-integer spin like 1/2, 3/2, 5/2. Fermions obey the Pauli exclusion principle and must not have the same set of quantum numbers in the same quantum system and therefore are constrained by the Pauli exclusion principle. Electrons, protons and neutrons are fermions.
The fact that electrons are fermions explains the structure of the periodic table of the elements since there can be only one electron for each state in an atom (only one electron for each possible set of quantum numbers). Many electrons are forced into higher energy levels in the shells structures of atoms. Protons and neutrons inside the nucleus also occupy shells and many protons and neutrons are forced into higher energy levels by the Exclusion Principle.

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