I.

Introduction

Fermion, one of the two main classes of fundamental particles that make up matter and energy. Fermions play an important role in the structure of matter. The particles that make up atoms (electrons, protons, and neutrons) are all fermions. Fermions are named for Italian-born American physicist Enrico Fermi. In the 1920s Fermi calculated a set of rules that define the behavior of fermions. See also Elementary Particles.