Nuclear fussion

In the actual fact, the "mass number" of an isotope is not the real "mass" of the nucleus. It only indicates the number of nucleons (protons and neutrons) in the nucleus.
Due to the strong electrostatic repulsion between like charges, protons present in a nucleus are subject to repulsion from one another. To overcome this repulsion, there exists a binding force (this force is known as strong nuclear interaction force) in the nucleus holdings all nucleons together to form a nucleus. The origin of this force "appears" to come from the slight reuction in mass of each mucleon. Hence, although the number of nucleons before and after the fusion reaction remains the same, the actual masses of these nucleons apprea to be different because of the action of binding energy in the nucleus. The difference in binding energies before and after the fusion reaction is released during the fusion process.

When a proton and neutron combine, there will be a negligible loss of mass (0.00239 atomic mass unit). This is due to the strong binding energy that holds a nucleus together. For the simplicity of calculation, proton and neutron are considered to have 1 amu in all configurations, which cannot reflect the actually loss in mass.

http://en.wikipedia.org/wiki/Nuclear_fusion

http://science.jrank.org/pages/4735/Nuclear-Fusion.html