How does mass deficiency affect an atom?

Answer 1

The "mass defect" -- the fact that the mass of an isotope is less that the sum of its nucleons is due to the fact that when nucleons combine (either by fusion, for elements with atomic number less than iron[A=26]; or fission for elements with atomic number greater than iron) there is a release of energy due to the nuclear binding force, which source of the "mass defect" you correctly identified. The energy released as a result of the binding of the nucleons is equivalent to a small amount of mass of the matter -- Einstein's famous equation: E=mc^2, so the energy release E

Answer 2

Mass deficiency is the difference between the mass of an atom and the sum of the masses of its individual particles (protons, neutrons, electrons). This difference is converted into energy according to Einstein's equation E=mc^2 during nuclear reactions, such as fusion or fission. This released energy is what powers stars, nuclear reactors, and atomic bombs.