What is the primary characteristic of radioactivity?

The primary characteristic of radioactivity is that it involves the release of energy. This process occurs when unstable atomic nuclei lose energy through the emission of radiation, which can take the form of particles (such as alpha or beta particles) or electromagnetic waves (such as gamma rays). As these unstable isotopes decay, they release significant amounts of energy, which is a fundamental aspect of radioactivity.

The other options do not accurately describe the essence of radioactivity. For instance, while it might seem passive in some contexts, it is actually a spontaneous process driven by the internal instability of certain isotopes. Additionally, radioactivity is not limited to unstable elements; it occurs specifically in unstable isotopes rather than stable ones. Lastly, although radioactivity has important applications in the medical field, including in diagnostic imaging and cancer treatment, it is not solely used for these purposes and plays significant roles in fields such as energy generation and scientific research.