What are isotopes?

Isotopes are defined as variations of the same chemical element that share the same number of protons but possess a different number of neutrons in their atomic nuclei. This variance in neutron count means that isotopes of a given element can have different mass numbers. For example, carbon has several isotopes, including carbon-12 and carbon-14; both have 6 protons, which defines them as carbon, but carbon-12 has 6 neutrons, while carbon-14 has 8 neutrons.

This concept is essential in understanding the behavior of elements in chemical reactions and their stability, particularly in the context of radioactive isotopes, which can decay and emit radiation. Thus, the correct choice accurately captures the fundamental definition of isotopes in nuclear chemistry and physics.