When Plutonium-239 undergoes alpha decay, what does it become?
It becomes uranium-235.
An alpha particle is a helium-4 nucleus, it contains gwo protons and two neutrons. So, alpha decay, giving off an alpha particle, does the following:
1)reduces the atomic number (number of just protons) by two. Plutonium has atomic number of 94, take two protons away and you are left with 92 which corresponds to uranium.
2) reduces the mass number (protons plus neutrons) by 4. If you start with 239 and subtract two protons and two neutrons, you are left with 235.