The most basic constituents of the universe. ATOMS are the basic units of the Chemical Elements (see Element )but are themselves composed of smaller particles. The first Subatomic Particle to be discovered was the Electron, identified in 1897 by Thomson, Joseph John. 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 Theory predicted the existence of additional elementary particles. A search for the
positron, or Antiparticle (see Antimatter) of the electron, led to its detection in 1932, but a search for a particle
predicted by Hideki, Yukawa in 1935 led to the unexpected discovery of the muon the following year.
Yukawa's particle, the pion, was finally discovered in 1947. Both the muon and the
pion were first observed in Cosmic Rays. As the list of particles and antiparticles grew, through further study of
cosmic rays and study of the results of particle collisions produced by Particle Accelerator, three basic categories of elementary particles were ultimately
distinguished: leptons, quarks, and Bosons. Leptons and quarks are Fermions, the basic constituents of nuclear and atomic structure, or Matter; Bosons are the particles that transmit the fundamental Forces of nature between fermions. The smallest class of elementary particles is
that of the massless bosons, which comprises the Photon(s), gluon, W and Z Particles, and the hypothetical graviton. The lepton class contains twelve particles:
the electron, muon, tauon, and their antiparticles, and the Neutrino or antineutrino (see neutrino) associated with each. The Quark, the third class, also number twelve: the whimsically named up, down, charm,
strange, top (or truth), and bottom (or beauty) quarks and their antiparticles.
Quarks are always found in pairs or triplets with other quarks or antiquarks
to form particles called hadrons. More than 200 hadrons have been identified.
Hadrons were earlier classified as either mesons, which include the muon and pion
and consist of quark-antiquark pairs, or baryons, which consist of quark
triplets. The lightest baryons are the proton and neutron; heavier baryons are known
as hyperons. Elementary particles interact with one another through the four
fundamental forces: Gravitation, electromagnetism, the Weak Interaction, and the Strong Interaction. Gravitation is experienced by all particles, but electromagnetism is
experienced only by charged particles, such as the electron, proton, and muon. Hadrons
and leptons, including the electron, muon, and the neutrinos, participate in
the weak interaction associated with particle decay. The strong interaction is
responsible for the structure of the atomic nucleus, and only hadrons
participate in it.