What does the atomic number of an element tell you?
Identifies the number of protons a single atom of the element contains.
While having a neutral charge, it also provides the number of electrons the element has (in one atom).
While isotopes are a thing, it doesn't completely change the atom.
Have a different number of neutrons, you have an isotope, but if the number of protons differ, you are dealing with an entirely different element (in neutral state - reactions are out of the picture).
Hope this helps :)