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A fermion is a category of elementary particles. They are very small and very light. Fermions can be thought of as the building blocks of matter because atoms are made up of fermions. Paul Dirac named them fermions in honor of a famous scientist called Enrico Fermi.

An electron (a charged particle) is a fermion, but a photon (the particle of electromagnetic radiation) is not. The spin numbers of fermions are 1/2, 3/2, 5/2, etc. Because their spin is not a whole number, they follow the Pauli exclusion principle: no two fermions can share a quantum state (which includes position in space), if they have the same quantum numbers, such as spin. In other words, fermions can collide with each other. This behavior is not in the other class of elementary particles called bosons: you can have many bosons, for example photons, in the same place at the same time. The motion of fermions follows the Fermi-Dirac statistics.

Most well known fermions have spin of 1/2. An example of a type of fermion with a spin of 1/2 is the electron. The electron belongs to a group of fermions called leptons.

Fundamental fermions (fermions that are not made up of anything else) are either quarks or leptons. There are 6 different types of quarks (called "flavours") and 6 different types of leptons. These are their names:

  • Quarks — up, down, charm, strange, top, bottom
  • Leptons — electron, muon, tau, electron neutrino, muon neutrino, tau neutrino

Each of these fermions also has an anti-particle associated with it, so there are a total of 24 different fundamental fermions. The anti-particle is similar to the original particle, but with opposite electrical charge. The "up", "charm", and "top" quarks have electrical charge of +2/3. Their anti-particles have charge -2/3 (anti-up, anti-charm, anti-top). The other three quarks (down, strange and bottom) have charge -1/3, and their anti-particles have charge +1/3. The electron, muon, and tau leptons all have charge of -1, and their anti-particles (anti-electron or "positron", anti-muon, anti-tau) have charge +1. All the neutrinos and anti-neutrinos have charge 0. The main difference between quarks or leptons with the same charge is their mass.

The supersymmetric counterpart of any fermion is called a "sfermion."