Common units of nuclear energy are megaelectronvolts (MeV). In nuclear and particle physics the energetics of nuclear reactions is determined by the Q-value of that reaction. The Q-value of the reaction is defined as the difference between the sum of the masses of the initial reactants and the sum of the masses of the final products, in energy units (usually in MeV). Electronvolts are a traditional unit of energy, particularly in atomic and nuclear physics. An electronvolt is equal to energy gained by a single electron when it is accelerated through 1 volt of electric potential difference. For example, the total energy released in a reactor is about 210 MeV per 235U fission