Radioactivity
The spontaneous emission of particles or electromagnetic radiation from unstable atomic nuclei.
Unit
Bq
In depth
Discovered by Becquerel in 1896 and investigated by the Curies, radioactivity occurs when a nucleus has an unfavourable neutron-to-proton ratio or excess energy. The process is entirely random for any single nucleus, yet perfectly predictable for large numbers.
Examples in the real world
Uranium fogging a photographic plate through black paper; a smoke alarm's americium-241 source.