Particle Physics the lightest elementary particle with an electric charge: it is a lepton with a negative charge of c. 1.602 × 10 coulomb and a rest mass of c. 9.109 × 10 kg (c. 0.511 MeV/c), which is c. of the mass of a proton: ordinarily an atom has the same number of negative electrons around the nucleus as the number of positive protons in the nucleus