noun

radioactivity

ray-dee-oh-ak-TIV-uh-tee/ˌreɪdiəʊækˈtɪvəti/

Spontaneous emission of radiation from unstable atomic nuclei.

The nucleus's radioactivity releases a cluster of two protons and two neutrons.

A magnified scientific visualization of alpha decay: a large atomic nucleus emits a compact cluster of two protons and two neutrons, visibly moving away from it.

Etymology

radioLatinray

+activFrenchactive

+ityFrenchstate or quality

The “ray” and “active” pieces describe the capacity to emit radiation. The noun names both this property of unstable nuclei and the particles or energy released as they decay.

In context

StarTalk5:54

there are particles that decay you've heard of like radioactivity right yes it's one part it decays and becomes another part it releases often deadly energy yes okay

Neil deGrasse Tyson Explains Time Dilation

More examples

Uranium's natural radioactivity releases energy as its unstable nuclei decay.
Some smoke detectors use the radioactivity of a tiny americium source.
The sample's radioactivity decreases over time.

Forms

radioactive adjective — Emitting radiation spontaneously from unstable nuclei.

Similar words

radioactive decay: Names the nuclear change that produces the emitted radiation.
radiation: Includes energy emitted by both radioactive and nonradioactive sources.