How are isotopes of an element primarily distinguished from each other?

Isotopes of an element are primarily distinguished from each other by their mass number. The mass number of an isotope is the total number of protons and neutrons in the nucleus of an atom. While all isotopes of a given element have the same number of protons (which is represented by the atomic number) and thus share similar chemical properties, they differ in the number of neutrons they contain.

This difference in neutrons results in a variation in mass number. For example, carbon has isotopes such as carbon-12 and carbon-14. Both isotopes have 6 protons (which makes them carbon), but carbon-12 has 6 neutrons, while carbon-14 has 8 neutrons, leading to different mass numbers. Therefore, the key factor in distinguishing isotopes is their mass number, which directly reflects the variation in neutron count.

In comparison, while the atomic number characterizes the element itself, it remains constant among isotopes of the same element. Chemical properties and electron configurations may show slight variations due to differences in mass, but these are not the primary distinguishing factors for isotopes.