An electron is an elementary particle that carries a negative elementary charge −e. It is a spin-½ lepton of mass 9.109 382 15 × 10−31 kg. Because of this small mass the motion of an electron must often be described by quantum mechanics or quantum electrodynamics. However, classical electrodynamics, describing the behavior of electrons in electromagnetic fields by the (classical) Maxwell equations, has still its use in electrical engineering and many branches of physics. Together with atomic nuclei, electrons constitute atoms and molecules. Their (quantum mechanical) interaction with adjacent nuclei causes chemical bonding and bonding in crystals.
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