Fermions and bosons
All fundamental particles in nature can be divided into one of two categories: Fermions or Bosons. The table below enumerates the differences.
| Fermions | half-integral spin (1/2, 3/2,...) | only one per state | Examples: electrons, protons, neutrons, quarks, neutrinos |
| Bosons | integral spin (0, 1, 2,...) | Many can occupy the same state | Examples: photons, 4He atoms, gluons |
Bosons have intrinsic angular momenta in integral units of h/(2p). For instance the spin of a photon is 1 and the spin of a 4He atom is zero. Many bosons can occupy a single quantum state. This allows them to behave collectively and is responsible for the behavior of lasers and superfluid helium. Only one fermion can exist in a given quantum state. This is known as the Pauli exclusion principle, which is the subject of the next page.
Any object that is composed of an even number of fermions is a boson, while any particle that is composed of an odd number of fermions is a fermion. For example, a proton is made of three (spin 1/2) quarks; hence it is a fermion. A 4He atom is made of 2 protons, 2 neutrons and 2 electrons; hence it is a boson.