How does alpha decay change the nucleus of a radioactive atom?
Let's look at what an alpha particle is, then we will think about how the emitting atom would be changed by losing the alpha particle.
An alpha particle is actually a helium-4 nucleus: 2 protons and 2 neutrons.
So, any atom that emitted an alpha particle would lose 2 protons and 2 neutrons.
Let's look at Americium-241 (which is used with beryllium as a source of neutrons in a neutron probe for measuring water in soil). Am-241 emits an alpha particle. Now Am-241 has 95 protons in the nucleus and (241-95 = 146) neutrons. It loses two protons and two neutrons. So it becomes element 93, Neptunium, and the total number of nucleons is now (241-4 = 237).
The equation looks like this:
You can do the same thing with Uranium-238, and you should get Thorium-234.