Any of various self-contained units of matter or energy. Discovery of the electron in 1897 and of the atomic nucleus in 1911 established that the atom is actually a composite of a cloud of electrons surrounding a tiny but heavy core.
By the early 1930s it was found that the nucleus is composed of even smaller particles, called protons and neutrons. In the early 1970s it was discovered that these particles are made up of several types of even more basic units, named quarks, which, together with several types of leptons, constitute the fundamental building blocks of all matter. A third major group of subatomic particles consists of bosons, which transmit the forces of the universe. More than 200 subatomic particles have been detected so far, and most appear to have a corresponding antiparticle (&see; antimatter).
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