class='define'>Elementary particles are tiny bits of matter assumed to be the most basic constituents of the universe. Certain elementary particles combine to form an Atom, which is the basic unit of any chemical Element and from which all forms of matter are built. The first elementary 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 Theory predicted the existence of additional elementary particles. A search for the positron, or antiparticle ( Antimatter) of the electron, led to its detection in 1932, but a search for a particle predicted by Yukawa Hideki in 1935 led to the unexpected discovery of the mu meson, or muon, the following year. Yukawa's particle was finally discovered in 1947 and named the pi meson, or pion. 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 the study of the results of particle collisions produced by Particle Accelerators, four basic categories of particles were distinguished, according to their behavior with regard to the four fundamental forces of nature: gravitational, electromagnetic, strong nuclear, and weak nuclear. A given particle experiences certain of these forces but may be immune to others. The gravitational force is experienced by all particles. The electromagnetic force is experienced only by charged particles, although it is transmitted by the Photon, which has no charge. The weak and strong nuclear forces exist only at the atomic level. Of the four classes of particles, the smallest is that of the massless bosons, which include the photon, eight types of gluons, and the hypothetical graviton. The lepton class includes twelve particles: the electron, the positron, the positive and negative muons, the tauon and its antiparticle, and the neutrino or antineutrino associated with each of these particles. The bosons and the leptons are not strongly interacting. Members of the meson class are more massive than the leptons. The mesons are the “glue” that holds nuclei together. By far the largest class of particles is the baryon class, the lightest members of which are the proton and neutron; the heavier members are the hyperons. Baryons and mesons, both strongly interacting, are sometimes considered together as hadrons. A theory independently proposed in 1964 by Murray Gell-Mann and George Zweig explains the properties of all known hadrons according to the assumption that hadrons are built up of other, still more fundamental particles called Quarks.

This definition is in context to Explosive. See more contextual defintions for Elementary Particles.