Heavy nuclei gain stability by undergoing nuclear fission.The following figure shows the binding energy as a function of the mass number.The mass changes and associated energy changes in nuclear reactions are significant.For example, the energy released from the nuclear reaction of 1 kg of uranium is equivalent to the energy released during the combustion of about four billion kilograms of coal.Corresponding to the mass defect, the mass of the nucleus is always less than the sum of the masses of the individual protons and neutrons that comprise it.Conversely, energy is needed to break apart a nucleus into its nucleons.The first nuclear fission reaction discovered involved uranium-235.
In the reaction we just saw, the total mass on the right side of the equation is 0.186 amu smaller than that on the left.
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Less stable atoms have lower binding energies per nucleon.
In other words, it is harder to break apart a nucleus with a high binding energy than a nucleus with a low binding energy.