How do isotopes differ from one another?

Isotopes are variants of the same chemical element that share the same number of protons but differ in the number of neutrons within their atomic nuclei. This difference in neutron count leads to variations in atomic weight.

For example, carbon has isotopes such as carbon-12 (with 6 protons and 6 neutrons) and carbon-14 (with 6 protons and 8 neutrons). Although both isotopes are carbon and have the same chemical properties due to the identical number of protons, their atomic weights differ because of the varying number of neutrons.

This distinction in atomic weight is significant in various scientific fields, including chemistry and physics, as it affects the behavior of the isotopes in reactions and their stability. Thus, the correct answer highlights that isotopes vary in atomic weight rather than other aspects such as proton number, chemical reactivity, or electron distribution.