What differs among isotopes of atoms?

Isotopes of atoms are differentiated primarily by the number of neutrons they contain. While all isotopes of a given element have the same number of protons, defining their chemical properties, it is the variation in the number of neutrons that distinguishes one isotope from another. This difference in neutrons results in different atomic masses for the isotopes. For instance, the most common isotope of carbon has six neutrons, while another isotope, carbon-14, has eight neutrons. This variation in neutrons can influence nuclear stability and radioactivity, leading to distinct physical properties, but does not affect the element's chemical behavior.

The other choices relate to fundamental aspects of atoms: protons determine the element type, electrons govern chemical interactions, and "atoms" as a term encompasses all of these particles together, but none of these is what specifically varies in isotopes. Understanding isotopes is crucial in fields such as nuclear physics and chemistry, where stability and decay rates are of interest.