What distinguishes isotopes of an element?

Isotopes of an element are defined by their differing numbers of neutrons, which results in variations in their atomic mass. While isotopes have the same number of protons (which is characteristic of the element itself) and typically the same number of electrons in their neutral state, it is the difference in neutrons that leads to the formation of isotopes.

When an element has isotopes, their atomic masses differ because the atomic mass is the sum of the protons and neutrons in the nucleus. This means that even though isotopes are forms of the same element, their varying numbers of neutrons create distinct atomic weights. For instance, carbon has several isotopes, including carbon-12 (with 6 neutrons) and carbon-14 (with 8 neutrons). Hence, the characteristic that differentiates isotopes is indeed their number of neutrons, leading to variations in atomic mass.

While atomic mass can also vary among isotopes, it is the differing number of neutrons that fundamentally defines the concept of isotopes.