Isotope is a RELATIONAL term. There is NOT one “normal” version of the atom and then everything else is an isotope. Instead, ALL ATOMS with the same number of protons but different numbers of neutrons are isotopes. Think about it this way. Suppose you had a group of men in the room, and I asked you which ones were brothers. You would not look for a “normal” son and then say all of the others with the same father are his brothers. Instead, you would look for all men with the same father and say, these are ALL brothers.

When faced with a list of atoms, you look for all of the ones with the same number of protons but different numbers of neutrons. Those are all isotopes. Thus, in the practice problem for module 3, question 2, the list contains 5 atoms. Of those atoms, 3 have the same number of protons but different numbers of neutrons. Thus, from this list, the isotopes are:

²²Na , ²³Na , and ²⁴Na

Those all have the same number of neutrons and different numbers of protons. ²²Ne is not an isotope of anything else in the list because Ne has 10 protons and no other atom in the list has 10 protons. ²²Mg is not an isotope of anything else in the list because it has 12 protons, and no other atom on the list has 12 protons. However, the three listed above are all isotopes because they have the same number of protons but different numbers of neutrons.