The term nucleon is applied to either of the constituent components of the nucleus of an atom. That means that either a
proton or a
neutron could be a nucleon when they are part of an atomic nucleus. It's really that simple. Further, the number of nucleons in the nucleus of any atom is the number of protons and neutrons in the nucleus. It's the mass number of the atom, and that's because almost all of the mass of any atom is in the nucleus. Use the links below for more information.