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Subatomic particle

Helium atom (schematic)Showing two protons (red), two neutrons (green) and two electrons (yellow).
Helium atom (schematic)
Showing two protons (red), two neutrons (green) and two electrons (yellow).

In physics, a subatomic particle is a particle smaller than an atom. These include atomic constituents such as electrons, protons, and neutrons (protons and neutrons are actually composite particles, made up of quarks), as well as particles produced by radiative and scattering processes, such as photons, neutrinos, and muons. Most of the particles that have been discovered and studied are not encountered under normal earth conditions; they have to be produced during scattering processes in particle accelerators. The study of subatomic particles is the most active branch of particle physics.


Subatomic particles found in atoms

The only subatomic particles found in ordinary atoms are the electron, proton and neutron. The electron (symbol e-) makes up the bulk of an atom's volume and is responsible for the chemistry of atoms; an electron has 1/1836 the mass of a hydrogen atom and a negative charge. Protons (symbol p+) are found in atomic nuclei; a proton is about the same mass as a hydrogen atom and carries positive charge equal in magnitude but opposite in sign to electron. Neutrons (symbol n) are also found in nuclei and are responsible for differing isotopes of atoms; neutrons are electrically neutral and have slightly greater mass than protons. The proton and neutron are both composed of two kinds of elementary subatomic particles: the up and down quarks (symbols u and d).

Other subatomic particles

Hadrons are particles composed of quarks. They are divided into two classes: baryons and mesons. Baryons are composed of three quarks and have a large rest mass for subatomic particles. The proton and neutron are the only stable baryons (and the neutron is only stable inside atomic nuclei). Every baryon has an antiparticle composed of three antiquarks.

Mesons are composed of a normal quark and an antiquark, which gives them baryon number zero. There are no stable mesons; the most stable have half lives on the order of nanoseconds. They have a rest mass starting with 140 MeV for the lightest mesons, the pion. The antiparticles of mesons are other mesons.

Leptons are not composed of quarks, but are elementary particles (no smaller constituent is currently known). The only known leptons are electrons, muons, tauons and neutrinos. Until recently neutrinos were thought to have zero rest mass; their masses are much smaller than the masses of any other subatomic particles. In addition to the electron, all three types of neutrino are stable. Each lepton also has an antiparticle.

See also

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Last updated: 10-18-2005 22:24:35
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