Some nuclides deviate sufficiently from the ‘line of stability’ that they emit ionising radiation.
Some nuclei are fissile (but only a few of the most massive), some are neutron-rich and tend to emit
Taken from: https://en.m.wikipedia.org/wiki/Stable_nuclide
A relatively large number emit
The final point I’d like to make is that very often a nucleus, having emitted a decay product or undergone fission, is left in an excited state, it will then emit a
Sorry it’s been such a long answer!