In nuclear reactions, a small amount of mass is converted into energy. This is described by Einstein's famous equation, E=mc², where:

* E is the energy released

* m is the mass converted

* c is the speed of light (a very large constant)

This means that even a tiny amount of mass loss can result in a massive amount of energy release, as seen in nuclear bombs and nuclear power plants.

Here's a breakdown of what's happening:

1. Nucleons (protons and neutrons) are bound together in the nucleus. This binding energy is what holds the nucleus together.

2. During nuclear reactions, the arrangement of nucleons changes. For example, in fission, a heavy nucleus splits into lighter nuclei. In fusion, light nuclei combine to form a heavier nucleus.

3. The binding energy of the products is different from the binding energy of the reactants. If the binding energy of the products is higher, some mass is converted into energy and released. If the binding energy of the products is lower, energy is absorbed and mass is gained.

So, while the total number of nucleons remains the same, a small amount of mass is converted into energy, and the difference in mass is what we call the mass defect.