are: subatomic particles believed, or formerly believed, not to have any constituents. Examples include electrons, protons, neutrons and photons. It is now widely believed that protons and neutrons are in fact composed of constituents called quarks, and a modern listing of 'truly' elementary particles would consist of three families: the leptons (including the electron), the quarks (including the charged constituents of many other 'elementary particles'), and the exchange particles (including the photon and the various other particles that are responsible for the fundamental interactions between quarks and leptons).
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