A unit of energy that is used to describe the energies of subatomic particles and photons, or the energy levels of atoms. One electronvolt (symbol eV), which is the kinetic energy gained by an electron when it is accelerated through a potential difference of one volt, is equivalent to 1.602×10−19 J. Commonly used multiples are keV (103 eV), MeV (106 eV), and GeV (109 eV).