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# Stability and average binding energy per nucleon

## The depth of the well, not what's in it

If you throw stones down a well then the total amount of energy transferred depends on two things

1. How deep the well is
2. How many stones you throw down

But whether a stone is likely to find its way out of the well again only depends on how deep the well is.

This is why the stability of a nucleus (i.e. whether it's going to start falling to pieces) depends on the average binding energy per nucleon (a measure of the depth of the well) rather than the total binding energy (a measure of how much energy was transferred dropping everything into it).

## The depth of the well changes

Unlike real wells, the imaginary potential energy well of a nucleus changes its depth depending on how big the nucleus is.

As you make a nucleus bigger from scratch it gets deeper quite rapidly then after about 56 nucleons gets gradually shallower the more you add.

back to Lesson 13: Binding Energy and Mass Defect