For the case of U-235, which undergoes alpha decay, when an atom radioactively decays 2 protons and 2 neutron, essentially a Helium atom, are ejected from the nucleus. What remains is Th-231. So if you have 10 lbs of U-235 and wait out it's half life, what you would have left is 5 lbs of U-235 and 4.915 lbs of Th-231 all mixed up and the remaining 0.085 lbs off somewhere as the He particles.