When an unstable nucleus emits radiation, what is this process called?

The process when an unstable nucleus emits radiation is known as radioactive decay. This term specifically refers to the spontaneous transformation of an unstable atomic nucleus into a more stable configuration, which often involves the release of particle radiation, electromagnetic radiation, or both.

During radioactive decay, various types of particles such as alpha particles, beta particles, or gamma rays may be emitted. This process is fundamental to nuclear physics and is how many isotopes transform into new elements over time, influencing both environmental conditions and applications in fields like medicine, energy, and geology.

The other options do not accurately describe this specific phenomenon. "Radioactive development" is not a recognized term in nuclear science. "Radon gas mitigation" refers to techniques used to reduce radon levels in the air or water, which is a concern related to the decay of radon isotopes but does not describe the decay process itself. "Radio transmission" pertains to the process of sending signals in the form of electromagnetic waves and has no connection with nuclear decay mechanisms. Thus, radioactive decay is the precise term that defines the emission of radiation from unstable nuclei.