a stable elementary particle present in all atoms, orbiting the nucleus in numbers equal to the atomic number of the element in the neutral atom; a lepton with a negative charge of 1.602 176 462 × 10–19 coulomb, a rest mass of 9.109 381 88 × 10–31 kilogram, a radius of 2.817 940 285 × 10–15 metre, and a spin of1⁄2