system
Given the following post, respond as if you're commenting on Reddit:
human
Why is alpha decay always in the form of helium-4? Why does the particle never break apart into larger particles, like lithium?

Is it to do with the amount of electrons on the outer shell? Because that would make sense as to why it's always helium. Just curious.
gpt
Alpha decay is **defined** to be the emission of a ^(4)He nucleus.

But the alternative kind of decay you're describing *does* exist. It's called *cluster decay*. It's when a nucleus decays by emitting some cluster of particle larger than an alpha particle. It's sort of in between alpha decay and spontaneous fission.

Cluster decays are rare, because alpha particles are extremely tightly-bound. It's so energetically it's very favorable for a heavy nucleus to proceed toward lighter masses by emitting alpha particles.

There is also the fact that these kinds of decays proceed via quantum tunneling. It's generally easier for an alpha particle to tunnel out of a heavy nucleus than it would be for a ^(12)C cluster, for example.