When a nucleus emits an alpha particle, by how much does the atomic number of the remaining nucleus decrease?
Okay, let’s say we start out with a very large atomic nucleus, lots of protons and lots of neutrons. We’ll say it’s this nucleus that emits an alpha particle. Now, an alpha particle is a particle that has two protons, those are blue, and two neutrons, those are green. We want to know, after this emission, how much does the atomic number of the remaining nucleus decrease. It’s helpful at this point to recall what atomic number means.
The atomic number of an atom is the number of protons in its nucleus. So that means the atomic number of this original larger nucleus will decrease by the number of protons that are in an alpha particle. Since the number of protons in an alpha particle is two, that’s the same as the atomic number decrease in the remaining nucleus.
So we say, after this emission, the atomic number of the nucleus decreases by two.