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Due to their half integer spin as an observer circles a fermion (or a fermion rotates 360 by the observer) the wave function of the fermion changes sign to opposite. 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 Elementary Particle s are either fermions or Boson s (except the hypothetical Plekton ). A composite particle (made up of Elementary Particle s) may either be a fermion or a boson, depending only on the number of fermions inside:
  • Composite particles containing an even number of fermions become '' Bosons '', such as a Meson , or the Nucleus of a Carbon 12 atom.

  • Composite particles containing odd number of fermions remain ''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 composite particle (or system) is only seen at large (compared to size of the system) distance. At close proximity at which spatial structure begins to be important, composite particle (or system) behaves according to its constituent makeup. For example, two atoms of He can not share the same space if it is comparable by size to the size of inner structure of He atom itself (~10^-10 m) - despite bosonic properties of He atoms. Thus, liquid He has finite density comparable to the density of ordinary liquid matter.

The known s and Lepton s. The Elementary Particle s that make up ordinary Matter are fermions, belonging to either the quarks (which form Proton s and Neutron s) or the leptons (such as Electron s). 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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