electron

An electron is a type (i.e., flavor) of lepton that has an electric charge of -1 and is stable. (A lepton is an elementary particle that is a fermion, i.e., with 1/2 integer spin, but unlike a quark, does not respond to the strong force.) Through the electromagnetic forces subject to quantum-mechanical restrictions, electrons can be bound to an atomic nucleus forming atoms and assist in binding atoms together into molecules. Electrons can exist free from atoms and such free electrons can make up some of the particles of a plasma (and most baryonic matter is in the form of plasma and most electrons in the universe are free of atoms). An electron is very light compared to protons and neutrons, on the order of 1/1800 of their mass, but the energy-equivalent of its mass (its rest energy) is considerable, 0.511 MeV, tens of thousands of times the energy needed to ionize an atom. Electrons are created by beta decay and by pair production, the latter which produces both an electron and a positron (the antimatter equivalent of an electron), requiring twice the rest energy of an electron.

The term photoelectron refers to an electron freed by the energy of a photon, from a molecule (this action being termed the photoelectric effect) or from an atom (termed photoionization).