20.4 Radioactivity
Radioactivity is the process in which unstable atoms release energy to become stable. This energy is given off as radiation, which can be in the form of particles or waves. Henri Becquerel discovered radioactivity in 1896.
Some nuclei become unstable due to an imbalance of protons and neutrons or excess energy inside. To become stable, the nucleus breaks down and gives off radiation in a process called radioactive decay. This happens naturally and cannot be controlled. It is spontaneous and random.