The basic "nuclear physics for dummies" explanation of nuclear physics goes something like this: There are two dominant forces at play in atomic nuclei: the residual strong force (aka the nuclear force) which binds nucleons together and the electromagnetic force (or, more simply, the electrostatic force aka Coulomb repulsion) which causes positively charged protons to repel each other. The nuclear force is strong but only effective over short distances, while Coulomb repulsion is weaker but active over longer distances. Hence, for small nuclei the strong force is dominant and it is more energetically favourable to fuse together into larger nuclei. For larger nuclei, the repulsive force that wants to rip the nucleus apart dominates and its more energetically favourable to split apart into smaller nuclei. The "break even" point is around iron/nickel where isotopes with the highest nuclear binding energy per nucleon are found. There are simplifications here, of course, but if anything I've said is egregiously wrong please tell me. My question, then, is what do we mean when talk we about nuclides heavier than iron being "stable". They're clearly not purely energetically stable because they don't have the maximum possible binding energy per nucleon. Wikipedia defines a stable nuclide as one that "are not radioactive and so (unlike radionuclides) do not spontaneously undergo radioactive decay". Is it really case that so-called stable nuclides heavier than iron do not undergo spontaneous decay period, or do they merely have such long half-lives that they may be considered stable for all intents and purposes? Clearly, there is more to it than just the "for dummies" explanation in terms of binding energy, or else we would expect isotopes with close the same molecular weight to have almost the same half lives, which is obviously not the case.