Whether a particular element is stable or not depends on the number of neutrons in the nucleus.
For example, any nucleus with six protons is a carbon nucleus. If the carbon nucleus also contains six or seven neutrons, it will be stable: hence carbon-12 and carbon-13 are not radioactive. If, however, the nucleus contains eight neutrons, it will be unstable: hence carbon-14 is radioactive.
Using particle accelerators, elements and isotopes can be made that do not occur in nature. Here we will only consider naturally occurring nuclei with half-lives of more than 1 year.
First, here are examples of nuclei produced by the action of cosmic rays on Earth's atmosphere - these are called cosmogenic isotopes.
List of Naturally Occurring
Cosmogenic Radioactive Isotopes
| Isotope | Protons + Neutrons | Half Life/years |
|---|---|---|
| hydrogen-3 | 1 + 2 | 12.32 |
| beryllium-10 | 4 + 6 | 1.39 million |
| carbon-14 | 6 + 8 | 5,700 |
| sodium-22 | 11 + 11 | 2.605 |
| silicon-32 | 14 + 18 | 150 |
| chlorine-36 | 17 + 19 | 301,000 |
| argon-39 | 18 + 21 | 269 |
| krypton-81 | 36 + 45 | 229,000 |
| iodine-129 | 53 + 76 | 15.7 million |
Next, here are the radioactive nuclei that have survived since the formation of the Solar System. These isotopes, called primordial isotopes, have, of course, extremely long half-lives.
List of Naturally Occurring
Primordial Radioactive Isotopes
| Isotope | Protons + Neutrons | Half Life/years |
|---|---|---|
| potassium-40 | 19 + 21 | 1.25×109 |
| calcium-48 | 20 + 28 | 2.30×1019 |
| vanadium-50 | 23 + 27 | 1.4×1017 |
| germanium-76 | 32 + 44 | 1.8×1021 |
| selenium-82 | 34 + 48 | 1.1×1020 |
| krypton-78 | 36 + 42 | 9.2×1021 |
| rubidium-87 | 37 + 50 | 4.97×1010 |
| zirconium-96 | 40 + 56 | 2.0×1019 |
| molybdenum-100 | 42 + 58 | 7.80×1018 |
| cadmium-113 | 48 + 65 | 7.7×1015 |
| cadmium-116 | 48 + 68 | 3.10×1019 |
| indium-115 | 49 + 66 | 4.4×1014 |
| tellurium-128 | 52 + 76 | 2.2×1024 |
| tellurium-130 | 52 + 78 | 8.81×1018 |
| xenon-136 | 54 + 82 | 2.16×1021 |
| barium-130 | 56 + 74 | 1.2×1021 |
| lanthanum-138 | 57 + 81 | 1.02×1011 |
| neodymium-150 | 60 + 90 | 7.90×1018 |
| neodymium-144 | 60 + 84 | 2.29×1015 |
| samarium-147 | 62 + 85 | 1.06×1011 |
| samarium-148 | 62 + 86 | 7.00×1015 |
| europium-151 | 63 + 88 | 5.00×1018 |
| gadolinium-152 | 64 + 88 | 1.1×1014 |
| lutetium-176 | 71 + 105 | 3.76×1010 |
| hafnium-174 | 72 + 102 | 2.002×1015 |
| tungsten-180 | 74 + 106 | 1.80×1018 |
| rhenium-187 | 75 + 112 | 4.12×1010 |
| osmium-186 | 76 + 110 | 2.00×1015 |
| platinum-190 | 78 + 112 | 6.5×1011 |
| bismuth-209 | 83 + 126 | 1.9×1019 |
| thorium-232 | 90 + 142 | 1.41×1010 |
| uranium-235 | 92 + 143 | 7.04×108 |
| uranium-238 | 92 + 146 | 4.47×109 |