Radioactivity is the

• A. Decay of unstable nuclei and emission of energy particles
• B. Emission of photons by stable nuclei
• C. Chemical reaction that changes one element into another
• D. Spontaneous splitting into two equal fragments

Answer: (A)

Radioactivity is the spontaneous decay of unstable atomic nuclei, accompanied by emission of energetic particles or radiation. An unstable nucleus moves toward a more stable configuration, often transforming into a different isotope or element as it releases energy. The emitted radiation can be alpha particles, beta particles, or gamma rays, and this process reflects changes inside the nucleus itself, not in the surrounding electrons. Emitting photons from a stable nucleus isn’t typical radioactive behavior, and chemical reactions that change one element into another involve electrons or overall chemical processes rather than nuclear decay. Spontaneous splitting into two fragments describes fission, a specific decay mode that happens only in some heavy nuclei, not the general definition of radioactivity.