Which statement best defines radioactive decay?

• A. A chemical reaction producing energy.
• B. The process of heat transfer through a solid.
• C. The emission of radiation from a stable nucleus due to thermal processes.
• D. The process of releasing radiation when an atom splits or decays.

Answer: (D)

Radioactive decay is the spontaneous transformation of an unstable atomic nucleus, accompanied by the emission of radiation such as alpha, beta, or gamma rays. This process happens because the nucleus is in an unstable arrangement and moves toward a more stable configuration, releasing energy as radiation in the process. It is not a chemical reaction, and it does not depend on temperature. The idea that radiation comes from a nucleus changing or decaying captures the essence of radioactive decay, whereas the other statements describe chemical energy, heat transfer, or radiation from a stable nucleus driven by heat, which are not how radioactive decay works.