Binding energy
Definition
The energy that holds a nucleus together — equivalently, the energy you would need to supply to pull all its protons and neutrons completely apart.
Symbol: · Unit: mega-electronvolts (MeV)
Technical definition
The energy equivalent of the mass defect of a nucleus: the difference between the mass of the assembled nucleus and the sum of its separate nucleons, converted via E = mc^2. It measures nuclear stability; iron-56 sits near the maximum binding energy per nucleon.
Example
A helium-4 nucleus weighs about 0.03 atomic mass units less than its two protons and two neutrons separately. That missing mass, times c^2, is roughly 28 MeV of binding energy — the reason helium nuclei are so stable.
Binding energy is not fuel stored inside the nucleus waiting to explode. It is the energy that was released when the nucleus formed (the mass defect). A larger binding energy means a more stable nucleus, not a more dangerous one.