Unstable nuclei

What makes a nucleus unstable?

The strong nuclear force has a very small range, which, for small isotopes, is fine. But when the nucleus gets bigger, the strong nuclear force can’t hold the nucleus together as well, and it becomes unstable.

It can’t hold it together because of its short range - some protons are more than 3 \times 10^{-15} metres apart, which is its maximum range.

Question	Answer
What makes a nucleus unstable?	The strong nuclear force has a short range (3 \times 10^{-15} m). For large nuclei, protons exceed this range so the force cannot hold the nucleus together, making it unstable.