Isotopes are defined as atoms of the same element that have the same number of protons but a different number of what?

Isotopes are variants of a particular chemical element that have the same number of protons, which determines the element's identity, but differing numbers of neutrons. This difference in neutron count leads to variations in atomic mass while maintaining the same chemical properties because protons define the element's identity and electrons govern chemical behavior.

For instance, carbon has several isotopes, including Carbon-12 (with 6 neutrons) and Carbon-14 (with 8 neutrons). Both are still carbon because they have 6 protons, but they differ in their neutron counts, making them isotopes of carbon. Thus, the identification of isotopes is fundamentally linked to the variation in neutrons while the number of protons remains unchanged, confirming that the correct answer is linked to neutrons.