Mass Defect, Binding Energy, Fission and Fusion

One of the most surprising facts in nuclear physics is that the mass of a nucleus is always less than the total mass of its separate protons and neutrons. If you add up the masses of the individual nucleons and compare that with the measured mass of the assembled nucleus, there is a shortfall. This missing mass is called the mass defect, $\Delta m$. For example, a helium-4 nucleus has slightly less mass than two free protons plus two free neutrons. The mass defect is defined as $\Delta m = Zm_p + Nm_n - m_{\text{nucleus}}$, where $m_p$ and $m_n$ are the proton and neutron masses.

The explanation comes from Einstein's mass-energy equivalence, expressed by the famous equation $E = mc^2$. Mass and energy are two aspects of the same underlying quantity, and one can be converted into the other.