I was also thinking that if an alpha particle managed to snag an electron, it wouldn't slow it down much because the mass of an electron is so tiny compared to the mass of an alpha particle. If the alpha particle did pick up an electron it would just charge on it's merry way.
There were two things I neglected:
- The force required to break the electron free from it's previous nucleus, and
- once the alpha particle acquires an electron, it is no longer a particle, it is now an atom, with all the honors and responsibilities that go with that title.
Even if this snagging operation does not slow down our errant particle, it is now an atom, so instead of being able to blithely zoom through the electron clouds of other atoms, it is now going to be repelled by them. It's like there was one kid's balloon per acre to watch out for, now there's a blimp every acre. They are going to be a little harder to miss. And every time you run into one, some of your energy (velocity) is going to be transferred to the target (blimp, atom) and you will rebound with slightly less energy (velocity).