In a nuclear fission reaction, which has more mass, the initial uranium atom or the sum of its products?
The initial uranium nucleus has more mass than the sum of its fission products.
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The mass of a nucleus is always less than the mass of its protons and neutrons. The difference in mass is called the mass defect and is converted to binding energy according to the formula
The unit for binding energy is the mega-electronvolt (MeV).
The atomic nuclei with the highest binding energies per nucleon are around the size of Fe-56 and Ni-62. They are among the most stable of nuclei.
Thus, when heavy nuclei undergo fission, the daughter nuclei are more tightly bound than the parent nucleus.
Consider the fission of uranium-235 to form barium-141 and krypton-92.
The binding energies are:
U-235 = 1786 MeV
Ba-141 = 1170 MeV
Kr-92 = 782 MeV
Binding energy of products – binding energy of reactants = (1170 + 782 - 1786) MeV = 166 MeV
The difference is the energy released during the fission process. It is equivalent to about four kilotons of TNT per mole of U-235.