A stable elementary particle with a mass of 9.1091×10−31 kg (equivalent, in energy terms, to 0.511 MeV) and a negative charge of 1.602×10−19 coulomb. The electron has a spin of 1/2 (in units of the Planck constant, h, divided by 2π) and is a member of the lepton family. Electrons can exist either as free particles or as the negatively charged components of atoms