What does radioactivity refer to?

Radioactivity refers to the spontaneous decomposition of unstable atoms, meaning that certain isotopes of elements possess unstable nuclei that will break down over time. This process results in the release of energy in the form of radiation, which can include alpha particles, beta particles, or gamma rays. This phenomenon is a natural occurrence and is intrinsic to specific isotopes that cannot be artificially controlled or altered until they reach stability through decay.

In contrast, other options provide definitions that do not accurately capture the essence of radioactivity. The controlled emission of energy may refer to processes like controlled nuclear reactions, but it does not embody the idea of spontaneous decay characteristic of radioactive materials. Emission of sound waves from atoms is irrelevant, as sound waves are mechanical waves and not associated with atomic restructuring. Lastly, the disintegration of stable atoms is incorrect since stable atoms do not undergo the decay process that defines radioactivity.