The unit of energy adapted to the atomic scale is the electronvolt or eV. One electronvolt is the energy gained by a corpuscle carrying an elementary electric charge (such as the electron or the proton) in the presence of a potential difference of 1 Volt. The unit is very small (0.16 billionth of a billionth of a joule). The energies required to expel external electrons from an atom or the energies released during a chemical reaction, are commonly of a few eV. The energies involved in nuclear processes are about one million times greater than those observed in chemical phenomena. For this reason, it is convenient to express these energies in million of electronvolts or MeV. The intermediate unit is one thousand electron volts, or keV.