What is the primary characteristic that makes an isotope radioactive?

The primary characteristic that makes an isotope radioactive is the instability of its nucleus. Radioactive isotopes have nuclei that are unstable due to an imbalance in the number of protons and neutrons or due to excess energy. This instability causes the nucleus to undergo radioactive decay, which involves transforming into a different element or a different isotope of the same element while emitting radiation in the process.

When an isotope's nucleus is unstable, it may break down spontaneously, releasing particles or electromagnetic radiation. This phenomenon is what distinguishes radioactive isotopes from stable ones. The decay continues until a stable configuration is achieved, potentially resulting in a series of transformations known as a decay chain.

While the presence of neutrons and high atomic weight can be related to radioactivity in some isotopes, they are not definitive characteristics that solely determine radioactivity. On the other hand, the presence of electrons does not contribute to the radioactivity of an isotope since it is the nucleus that primarily dictates stability and decay behavior. Thus, the instability of the nucleus is the definitive factor in determining whether an isotope is radioactive.