They 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
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 the models of the
atom consisting of just these three particles are
sufficient to account for all forms of chemical behaviour
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.