Binding energy

One reason for the stability of a nucleus can be seen if we look more closely at a particular nucleus such as helium.

Imagine that you were asked to make such a nucleus. You are given the four pieces (two protons and two neutrons), asked to measure their masses, make them stick them together somehow and then measure the mass of the finished helium nucleus.

You will find that the mass of the completed nucleus is different from that of the total mass of the four particles from which it was made! The mass of the nucleus is less than the total mass of the four particles. This is really quite surprising - it is like taking a cake, weighing it, then cutting up into slices, wieghing them and then finding that the cake had a different mass from the sum of the masses of the slices.

For a nucleus this difference in mass is called the mass defect of the nucleus..
The mass defect for a number of nuclei is shown below.




This can best be explained by looking at how easy it would be to split the alpha particle apart again. If we think of this mass difference as a difference of energy (using E = mc²) then the alpha particle has 28.3 MeV less energy than the four particles. It means that this energy would be needed to split up the helium nucleus. This is called the binding energy of a nucleus.