A unit of energy, principally used in atomic and molecular physics. It is defined as the energy acquired by an electron when it is accelerated through a potential difference of 1 volt in a vacuum. 1 eV = 1.602 × 10-19 joules. Electron volts are convenient units for measuring the energies of particles and electromagnetic radiation. The energies of X-rays are expressed in thousands of electron volts keV. The wavelength associated with 1 keV is 0.124 nanometres. Millions MeV and thousands of millions GeV of electron volts are also used as units for the highly energetic atomic particles. An electron with a kinetic energy of a few MeV is travelling at almost the velocity of light.