Radioactivity is a physical phenomenon, where unstable atomic nuclei of certain chemical elements decay and emit radiation. This radiation is also referred to as ionizing radiation.

The building blocks of an atom are protons and neutrons in the nucleus, and electrons orbiting the nucleus. Some atomic nuclei have too many or too few neutrons. This makes them unstable and they "want" to attain a more stable state. They "decay" and emit particles and/or energy.