What is an isotope?

An isotope is defined as an atom of the same element that has the same number of protons but a different number of neutrons. This change in neutrons results in a variance in the atomic mass of the element, while the chemical properties generally remain the same since those properties are primarily determined by the number of protons and electrons.

When considering the other choices, it's clear how they differ: an atom with a different number of protons would constitute a different element entirely, not just an isotope. An atom that has gained electrons pertains to the formation of an ion, particularly a negative ion, and does not involve changes in neutrons or isotopes. Lastly, an ion with a positive charge describes a cation, which is also unrelated to the concept of isotopes, as it focuses on changes in electron count rather than neutrons.

Thus, the correct answer about isotopes effectively captures their unique characteristics tied to variations in neutrons without altering the elemental identity.