What are isotopes?

Isotopes are defined as atoms that have the same number of protons but differ in the number of neutrons within their nuclei. This fundamental distinction leads to variations in atomic mass between isotopes of the same element, as the number of protons in an atom determines its elemental identity, while the number of neutrons influences its mass.

For example, carbon has isotopes such as Carbon-12 (with 6 neutrons) and Carbon-14 (with 8 neutrons). Both isotopes are still carbon since they have the same number of protons (6), but their different neutron counts affect their stability and the way they behave in nuclear reactions.

The other options describe facts that don't capture the essence of what isotopes are. The focus on electrons pertains more to ionization but doesn't define isotopes, while the idea of atoms of different elements sharing properties doesn't relate to isotopes, which are variations of the same element. Furthermore, not all isotopes are radioactive; many stable isotopes exist without any radioactive properties, emphasizing the importance of understanding isotopes as variations within a single element based solely on neutron count.