I've got a question on nuclear physics, specifically binding energy. I understand that binding energy is the energy needed to separate a nucleus into its constituents. But something has been bugging me. For example, Two deuterium nuclei fuse together to form a Helium-3 nucleus, with the release of a neutron. The binding energies per nucleon for deutrium is 1.09 MeV and for Helium-3, 2.54 MeV. If I'm not mistaken, binding energy is also defined as the energy that would be released should a nucleus be formed from its separate nucleons. If my calculations are correct, the Helium-3 nucleus has a higher binding energy than the 2 deuterium nuclei. Why is this so? Also, the energy released would be 3.26 MeV. That would mean that the Helium-3 nucleus has a higher mass than the 2 deuterium nuclei put together. But why? Shouldn't the mass of the 2 deuterium nuclei be more than the Helium-3 nucleus? Therefore, the mass defect would result in the energy released.