# fermion

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## fermion

(fûr`mēŏn'): see elementary particles**elementary particles,**

the most basic physical constituents of the universe.

**Basic Constituents of Matter**

Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.

**.....**Click the link for more information. ; exclusion principle

**exclusion principle,**

physical principle enunciated by Wolfgang Pauli in 1925 stating that no two electrons in an atom can occupy the same energy state simultaneously. The energy states, or levels, in an atom are described in the quantum theory by various values of four different

**.....**Click the link for more information. ; Fermi-Dirac statistics

**Fermi-Dirac statistics,**

class of statistics that applies to particles called fermions. Fermions have half-integral values of the quantum mechanical property called spin and are "antisocial" in the sense that two fermions cannot exist in the same state.

**.....**Click the link for more information. .

## Fermion

a particle with half-integral spin or an elementary excitation of a quantum system consisting of many particles, that is, a quasiparticle with half-integral spin. Fermions include all baryons—such as the proton, the neutron, and hyperons—and all leptons—the electron, the muon, and neutrinos—and the anti-particles of all baryons and leptons, as well as such quasiparticles as conduction electrons and holes in a solid. Bound systems consisting of an odd number of fermions are also fermions; examples of such systems are atomic nuclei with an odd atomic number and atoms with an odd difference between the atomic number and the number of electrons. The Pauli exclusion principle is valid for fermions. Consequently, systems consisting of identical fermions obey Fermi-Dirac statistics.