As the name implies, isotopes are two elements occupying the same position in the periodic table of elements. The arrangement of the periodic table of elements is arranged according to their nuclear charges from small to large. Since isotopes occupy the same position, it means that their nuclear charges are the same, that is, the number of protons is the same, but the number of neutrons is different, so their mass numbers (or relative atomic masses) are different.

For example, ordinary hydrogen, heavy hydrogen and overweight hydrogen all have chemical symbols of H, D and T, which are called deuterium and tritium. They are isotopes, and their protons are 1, but H has no neutrons, D has one neutron and T has two neutrons.

If you want to subdivide the structural differences, it may be the content of the university, and you don't need to know.