Hi. I read that the mass of nucleons in nucleus of Oxygen-16 (or nucleus of another atom) is less than the sum of its nucleons if they were separated (also the mass of 8 protons and 8 neutrons). But why does nucleons have less energy in nucleus of atoms than if they were separated? What causes that "mass defect" of nucleons? The nucleons on atoms have less mass because part of their mass is converted to nuclear force to bind nucleons together in nucleus of an atom? Or the nucleons release some energy ( and hence mass ) when binding together? If nucleus of an atom is split in two, the mass of the nucleons will increase?