What defines two isotopic atoms?

Isotopes of an element are defined by having the same number of protons. This characteristic means they belong to the same element and have the same atomic number, which is given by the number of protons present in the nucleus.

While isotopes differ in their number of neutrons, this variation in neutrons does not affect their identity as the same element; rather, it affects their mass. Therefore, isotopic atoms will always share the same number of protons, confirming their classification as the same element, despite variations in their mass numbers due to different neutron counts.

In contrast, if two atoms had different numbers of protons, they would represent different elements entirely. Having the same number of electrons would be true for neutral isotopes, but it is not a defining characteristic of isotopic identity, as isotopes can exist in ionic forms and thus have varying electron counts.