Radioactivity
A property exhibited by certain types of matter emitting energy and subatomic particles spontaneously. It is, in essence, an attribute of individual atomic nuclei. An unstable nucleus will decompose spontaneously or decay into a more stable configuration. Still, it will only do so in a few specific ways by omitting certain particles or forms of electromagnetic energy. Radioactive decay is a property of several naturally occurring elements and artificially produced isotopes of the elements. The rate at which a radioactive element decays is expressed in terms of its half-life, i.e., the time required for one-half of any given quantity of the isotope to decay. Half-lives range from more than 1024 years for some nuclei to less than 10−23 seconds. The product of a radioactive decay process—called the daughter of the parent isotope—may be unstable, in which case it will also decay. The process continues until a stable nuclide has been formed [Online Encyclopedia Britannica].
The spontaneous disintegration of specific atomic nuclei is accompanied by the emission of alpha-particles (helium nuclei), beta-particles (electrons or positrons), or gamma radiation (short-wavelength electromagnetic waves) [Law, Rennie, p.480].
It is the act of emitting radiation spontaneously. This is done by an atomic nucleus that, for some reason, is unstable; it "wants" to give up some energy to shift to a more stable configuration. During the first half of the twentieth century, much of modern physics was devoted to exploring why this happens, with the result that nuclear decay was relatively well understood by 1960. Too many neutrons in a nucleus lead it to emit a negative beta particle, which changes one of the neutrons into a proton. Too many protons in a nucleus lead it to emit a positron (positively charged electron), changing a proton into a neutron. Too much energy leads a nucleus to emit a gamma ray, which discards great energy without changing any of the particles in the nucleus. Too much mass leads a nucleus to emit an alpha particle, discarding four heavy particles (two protons and two neutrons) [EHSS Welcome Report].
Radioactivity. Online Encyclopedia Britannica. Retrieved from: https://www.britannica.com/science/radioactivity
Law, J., Rennie, R. (2015). Oxford Dictionary Of Physics 7th ed. Oxford: University Press.
Radioactivity. EHSS Welcome Report. Retrieved from: https://ehss.energy.gov/ohre/roadmap/achre/intro_9_2.html