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A fermion is one of the things that everything is made of. Fermions are really small and do not weigh much. Fermions can be thought of as the building blocks of matter because atoms are made up of fermions.

An electron (a particle of electricity) is a fermion, but a photon (a particle of light) is not. Fermions are particles with spin numbers that are 1/2, 3/2, 5/2, etc. (Spin is a made-up name that scientists use to describe a phenomenon that they can not totally understand–it does not literally spin). Paul Dirac named them fermions in honor of a famous scientist called Enrico Fermi.

Fermions are special because you cannot put two of them in the same place at the same time, if they have the same quantum numbers, such as spin. Scientist have given the name Pauli exclusion principle to this behavior. Fermions obey Fermi-Dirac statistics. This behavior is different to particles in the opposite class called bosons. An example of a boson is a photon. Unlike fermions, you can have many bosons in the same place at the same time.

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 in how much they weigh.

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