Mass defect and nuclear binding energy — common mistakes

Remember to distinguish between the total binding energy and binding energy per nucleon. Total binding energy tells you how much energy holds the whole nucleus, while binding energy per nucleon tells you about its stability relative to its size.

When protons and neutrons combine to form a nucleus, some of their mass is converted into energy to bind them together. This "missing" mass is called the mass defect, and the equivalent energy is the binding energy, which is released during the formation of the nucleus.

The formula E=mc² directly links the mass defect (Δm) to the nuclear binding energy (ΔE). It means that the energy released when a nucleus forms (or absorbed to break it apart) is precisely equivalent to the change in mass, multiplied by the square of the speed of light.

Iron-56 (Fe-56) is one of the most stable nuclei, as it has one of the highest binding energies per nucleon. This is important because nuclear reactions (fusion and fission) tend to move towards forming more stable nuclei. Lighter nuclei fuse to get closer to iron, and heavier nuclei fission to also get closer to iron, both processes releasing energy.