A particle which is not made up of smaller particles. The neutron and proton are both made up of smaller particlesâ€” called quarks. On the other hand, quarks are not believed to be made of anything smaller. Like quarks, electrons and photons are also believed to be elementary particles.
Originally a term applied to any particle that could not be further subdivided; now applied only to protons, electrons, neutrons, antiparticles and strange particles, but not to alpha particles and deuterons.
In particle physics, an elementary particle or fundamental particle is a not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic particles of the universe from which all larger particles are made. In the modern theory of particle physics, the Standard Model, the quarks, leptons, and gauge bosons are elementary particles.