elementary particles, the most basic physical constituents of the universe. Atoms are the basic units of the chemical elements but are themselves composed of smaller particles. The first subatomic particle to be discovered was the electron, identified in 1897 by Joseph John Thomson. The nucleus of ordinary hydrogen was subsequently recognized as a single particle and was named the proton. The third basic particle in an atom, the neutron, was discovered in 1932. Although models of the atom consisting of just these three particles are sufficient to account for all forms of chemical behavior of matter, quantum mechanics predicted the existence of additional elementary particles.

Decades of painstaking experiments and theoretical insights have led to a surprisingly simple picture of the world of elementary particles and the laws they obey. According to this physical theory, known as the Standard Model, the most fundamental particles fall into three categories: the leptons, the quarks, and the gauge bosons (force carriers). Leptons include the electrically charged electrons, two unstable particles similar, but heavier than electrons, and neutral particles called neutrinos.

Two kinds of quarks, called "up" and "down", make up the protons and neutrons, but heavier, less stable quarks also exist. Gauge bosons give rise to the strong, weak, and electromagnetic forces, which govern the interaction of the quarks and leptons. A more detailed chart summarizes the properties of the particles in the standard model.