What is a radioisotope?

• A. A stable isotope
• B. A radioactive isotope
• C. A chemical element with no electrons
• D. A high-energy photon

Answer: (B)

A radioisotope is an isotope that is unstable and undergoes radioactive decay, emitting radiation as it changes to a more stable form. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons; some of these combinations are stable, while others are radioactive. The defining idea is instability of the nucleus leading to decay, which is not the case for a stable isotope. A high-energy photon is radiation, not an isotope, and having no electrons describes an ion or bare nucleus, not the concept of isotopes. So the best answer is the one that specifies an isotope that is radioactive.