system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some isotopes, such as potassium-40, undergo beta decay while others do not?

If a neutron can decay into a proton, and some other subatomic particles, via the weak nuclear force, why does this not happen in all atoms?
gpt
In order for a decay to be possible, it must be energetically allowed (meaning there is a final state with a lower energy than the initial state), and there must be a path to reach that lower state without violating any relevant conservation law (charge, angular momentum, parity, etc.).

About 90% of experimentally known nuclides are unstable, so it’s not the ones that beta decay with are special, it’s the ones that don’t decay at all that are very special. They are the lucky ones that simply don’t have a way to decay to a more favorable state in any reasonable amount of time (some of the heavy ones may have positive Q-values for alpha or cluster emission, but the time it takes for the cluster to form and tunnel out is effectively infinite, so they’re observationally stable).

Alpha decay, cluster emission, and spontaneous fission are most important for very heavy nuclides, and nucleon emission takes over at extreme N/Z, but in between the extremes of high mass and high |N - Z|, the weak decays (beta and electron capture) are the dominant ground state decay modes.