With the exception of hydrogen and helium that were generated during the Big Bang, the rest of the chemical elements have been formed in stars. Nuclear fusion joins the nuclei of light elements to create heavier ones.

Atoms with heavy nuclei (atomic number greater than 83) are unstable and tend to emit particles to transform into lighter and more stable elements. Not only are the heavy elements radioactive, the isotope of carbon (carbon-14) disintegrates emitting beta particles (electrons) that transform it into nitrogen-14. This property is used in the dating of samples derived from living beings.