Fermion

In particle physics, fermions are particles with half-integer spin; the particle type is named after Enrico Fermi. In the Standard Model are two types of elementary fermions, quarks and leptons. Since fermion numbers are often approximately conserved, they are sometimes said to be the constituents of matter.

Due to their half-integer spin, as an observer circles a fermion (or a fermion rotates 360°) the wavefunction of the fermion changes sign. So, fermions have an antisymmetric wavefunction. As a result of the antisymmetric wavefunction, fermions obey Fermi-Dirac statistics, whose consequence is the Pauli exclusion principle — no two fermions can occupy the same quantum mechanical state at the same time.

All observed elementary particles are either fermions or bosons. A composite particle (made up of more fundamental particles) may either be a fermion or a boson, depending only on the number of fermions it contains:

Composite particles containing an even number of fermions are bosons, such as a meson, or the nucleus of a carbon-12 atom.
Composite particles containing odd number of fermions are fermions, such as a baryon, or the nucleus of a carbon-13 atom.

A composite particle may contain any number of bosons with no effect on whether it is a boson or a fermion.

Of course, fermionic or bosonic behavior of a composite particle (or system) is only seen at large (compared to size of the system) distance. At proximity, where spatial structure begins to be important, a composite particle (or system) behaves according to its constituent makeup. For example, two atoms of helium can not share the same space if it is comparable by size to the size of the inner structure of the helium atom itself (~10⁻¹⁰ m) — despite bosonic properties of the helium atoms. Thus, liquid helium has finite density comparable to the density of ordinary liquid matter.

The known elementary fermions are divided into two groups: quarks and leptons. The elementary particles that make up ordinary matter are fermions, belonging to either the quarks (which form protons and neutrons) or the leptons (such as electrons). The Pauli exclusion principle obeyed by fermions is responsible for "rigidness" of ordinary matter and for the stability of the electron shells of atoms, making complex chemistry possible. It also results in the pressure exerted by degenerate matter.

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