Isotopes and elements are human made labels based on nucleon and proton counts. Hydrogen and deuterium are both the same element, but are obviously different nuclear structures. Even at a chemical level deuterium is appreciably different. That difference between isotopes and elements diminishes but never disappears as atomic mass rises.
Isotopes are only imperfect in the context of one labeling system. But not from a quantum viewpoint.
Isotopes are only imperfect in the context of one labeling system. But not from a quantum viewpoint.