An electron is the negatively charged subatomic particle associated with an atom's nucleus. Electrons are elementary particles belonging to the lepton family and carry one negative elementary charge.
In the basic atomic model, electrons occupy regions around the nucleus, often described as shells, orbitals, or an electron cloud. Modern quantum mechanics does not treat them as tiny planets following fixed paths; instead, it predicts the probability of finding an electron in particular regions. Electrons determine much of an atom's chemical behavior because their arrangement controls bonding and reactions.
Electrons are far less massive than protons or neutrons. A proton is positively charged, a neutron has no net electric charge, and a positron is the electron's antimatter counterpart with the same mass but a positive charge. These distinctions are frequent sources of confusion in introductory science questions.
J. J. Thomson's experiments in 1897 provided the first strong evidence for electrons as universal constituents of matter. In a neutral atom, the number of electrons equals the number of protons. If an atom gains or loses electrons, it becomes an ion with a net electric charge.