What distinguishes isotopes from one another in a given element?

Isotopes of a given element are distinguished from one another primarily by their different atomic masses. This variation arises because isotopes have the same number of protons (which defines the element) but different numbers of neutrons in their nuclei. The total number of protons and neutrons combined gives the atomic mass of the isotope, hence the distinction based on mass.

While isotopes of an element can exhibit slight differences in physical and chemical properties, these differences are generally not as significant as the differences in atomic mass. For example, the chemical properties of isotopes are usually similar because they still have the same electron structure. Thus, the fundamental characteristic that sets isotopes apart is their atomic mass due to variations in neutron count.