Commonly Encountered Radionuclides Approximately 2,300 nuclides
have been identified; most of them are radioactive. Which
nuclides are radioactive? Unstable nuclides of any element can
exist. However, almost all elements that are heavier than
bismuth, which has 83 protons, have an unstable nucleus; they
are radioactive and are known as "heavy nuclides. Nuclides with
fewer than 83 protons are known as "light nuclides. While there
are hundreds of radionuclides, many of them are rarely
encountered. People are much more likely to encounter a few that
are used routinely for medical, military, or commercial
purposes. The list below and accompanying fact sheets discuss
twelve radionuclides. They are the ones most commonly used and
most commonly found in Superfund Sites. Please note: Where an
element is listed rather than an individual radionuclide, the
element has several radioactive isotopes of interest: |Fact
Sheets on Commonly Encountered Radionuclides| |* tritium is a
specific isotope, H-3.|