NATURAL SOURCES

In nature, radioactivity is produced by certain isotopes of elements with atomic numbers up to and including 92 (uranium). These are known as radioactive isotopes. An isotope of carbon, known as carbon-14 (¹⁴C), has eight neutrons. Atoms of ¹⁴C are unstable; over time, they decay into carbon-12 (¹²C) atoms, which have six neutrons. Other examples of an unstable atoms include hydrogen-3 (³H), also known as tritium, which has a nucleus consisting of one proton and two neutrons; beryllium-7 (⁷Be), with a nucleus containing four protons and three neutrons; and ¹⁰Be, with a nucleus containing four protons and six neutrons.
In some instances, the most common isotope of a naturally occurring element also happens to be radioactive. Examples are radon, radium, and uranium. The barrage of cosmic particles from deep space can be considered a form of radioactivity, but these particles sometimes can create radioactive isotopes when they strike stable atoms in the Earth’s upper atmosphere.