Natural Sources:

In nature, radioactivity is generated by some isotopes of elements with atomic numbers up to and involving 92 (uranium). These are termed as radioactive isotopes. The isotope of carbon, termed as carbon-14 (¹⁴C), contain eight neutrons. The atoms of ¹⁴C are not stable; over time, they decay into carbon-12 (¹²C) atoms that have six neutrons. The other illustrations of unstable atoms involve hydrogen-3 (³H), also termed as tritium, that has a nucleus having one proton and two neutrons; beryllium-7 (⁷Be), with a nucleus having four protons and three neutrons; and ¹⁰Be, with a nucleus having four protons and six neutrons.

In some illustrations, the most general isotope of a naturally happening element also takes place to be radioactive. Illustrations are radium, radon, and uranium. The stream of cosmic particles from deep space can be consider as a form of radioactivity, though these particles at times can form radioactive isotopes whenever they strike stable atoms in the Earth's upper environment.