Fermion << FEHR mee on or FUR mee on >> is any member of a certain class of atomic and subatomic particles. Every particle is classified as either a fermion or a boson. The simplest fermions are leptons and quarks. These are elementary particles—that is, they have no known smaller parts. The electron is one of six types of leptons. There are also six types of quarks. Fermions also include composite objects that are made up of an odd number of fermions. Neutrons and protons are such fermions. Each is composed of three quarks. Other composite fermions include certain nuclei, atoms, and molecules.
Every type of fermion has an antimatter opposite, an antifermion, which is identical in mass but opposite in electric charge or certain other properties. A fermion can be created only in a pair with a corresponding antifermion, and can be destroyed only by encountering such an antifermion. However, certain types of fermions may decay into other fermions.
Fermions differ from bosons in spin, a measure of internal rotation. Fermions have half-integer values of spin (1/2, 3/2, 5/2, and so forth). Bosons have whole-integer values (0, 1, 2, and so forth). Fermions were named after Italian-born physicist Enrico Fermi, who proposed a theory of their behavior in the 1920’s.
See also Supersymmetry.
