system
Given the following post, respond as if you're commenting on Reddit:
human
How does adding a neutron to an atom change it so drastically?

For example, how does adding a neutron to deuterium turn it into tritium and make it so dangerous?
gpt
Nuclear stability is a complicated balance between many things. There are around 3000 known nuclides and less than 300 of them are stable.

Nucleons in a nucleus live in bound orbitals similar to how electrons are arranged in atoms, except the interactions between nucleons are a little more complicated than the interactions between electrons, and there is a separate shell structure for protons and neutrons.

Just like atoms with certain numbers of electrons are more favorable than others, nuclei with certain numbers of protons and/or neutrons tend to be more favorable as wel.

Adding a single particle to a closed-shell nucleus can be enough to make the new nucleus unstable. Your example of deuterium and tritium is a counterexample, because the deuteron doesn’t have a closed proton or neutron valence shell. The triton has a closed neutron shell, but not a closed proton shell.

However there is another nuclide along the A = 3 isobar that is stable (^(3)He), so the triton can beta decay to it.