Radioactivity is defined as:

• A. Nucleus loses energy by emitting radiation
• B. Absorption of radiation by the nucleus
• C. Emission of photons during chemical reactions
• D. Ground state energy release in chemical bonds

Answer: (A)

Radioactivity is the spontaneous decay of an unstable atomic nucleus, where the nucleus releases energy and often particles or photons as it transforms toward a more stable state. Saying the nucleus loses energy by emitting radiation captures this process—the energy carried away by the emitted radiation reflects the nucleus moving to a lower-energy, more stable configuration. The other ideas don’t describe radioactivity: absorbing radiation by the nucleus isn’t the defining process, radioactivity isn’t about photons emitted during ordinary chemical reactions, and chemical bonds releasing energy involve chemistry, not nuclear changes.