What defines a radioisotope?

A radioisotope is specifically defined as an unstable isotope that emits radiation in the form of alpha particles, beta particles, or gamma rays as it decays to a more stable form. This characteristic of instability leads to the emission of radiation, which can be detected and measured. These emissions can be utilized in various applications, including medical imaging, cancer treatment, and scientific research.

The other options do not accurately define a radioisotope. For instance, isotopes with the same number of protons are merely different forms of an element and do not necessarily indicate their stability or radioactivity. Additionally, not all isotopes are unstable, so stating that isotopes are always stable contradicts the definition of what a radioisotope is. Lastly, while different isotopes can exhibit varying chemical properties under certain conditions, this feature alone does not define a radioisotope, especially since both stable and unstable isotopes can have different chemical behaviors. Thus, the defining feature of a radioisotope is its instability, which results in radiation emission.