| Abundance in context of the Periodic Table Isotope pages refers to Natural Abundance; the prevalence of different isotopes of an element as found in Earth naturally. The weighted (by natural abundance) average mass of these isotopes is the atomic weight listed for the element in the periodic table. The abundance of an element varies from planet to planet.
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| Percent natural abundances refer to the relative proportions, expressed as percentages by number, in which the isotopes of an element are found in natural sources.
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| "Synthetic" refers to a synthetic isotope, that is, a radionuclide that is not found in nature: no natural process or mechanism exists which produces it, or it is
so unstable that it decays away in a very short period of time. Examples include Technetium-95 and Promethium-146. Many of these are found in, and harvested from, spent nuclear fuel assemblies. Some must be manufactured in particle accelerators.
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