The particle that is emitted from an alpha decay isn't actually called a He atom (I edited my root comment so this isn't misleading, apologies) -- I was being loose with terminology -- though it has the right number of protons and neutrons. It's called an alpha particle. Once it steals two electrons -- it carries a +2 charge and is extremely successfully at slicing electrons off of other molecules it comes across -- it is then considered the helium that we know and love, and is now stable with the properties we know.
And by stealing those electrons from other molecules it sets off other chemical reactions, which in things like DNA is highly suboptimal. This all generally happens at the birth of the He atom, presuming it isn't in deep space or something with no electrons to cleave from neighbours, and is only an instantaneous state.
Now I'm properly confused. Are ions not atoms? Like, we call protons protons when we're talking nuclear and call it hydrogen when we're doing chemistry. What makes helium and alpha particles different? (Genuine question.)
Strictly speaking, atoms are neutral and ions are charged. Ions can be monoatomic (with a single nucleus, like He2+) or polyatomic (like H3O+).
H+ is tricky; it’s called hydron (or just H-plus) but it is indeed formed of a single proton, so in practice it is also called that. The names are interchangeable and depend on the field. For example, when looking at irradiation, we call it proton, but either is correct.
If we want to be pedantic, an alpha particle is a He2+ formed during a nuclear reaction, so it typically has quite a bit of kinetic energy as well. In that context they are interchangeable. I am not aware of He2+ being formed in other ways, He is very stable and I am not aware of something that is able to steal its electrons.
And by stealing those electrons from other molecules it sets off other chemical reactions, which in things like DNA is highly suboptimal. This all generally happens at the birth of the He atom, presuming it isn't in deep space or something with no electrons to cleave from neighbours, and is only an instantaneous state.