|This article or section is in need of attention from an expert on the subject.
|It has been suggested that this article or section be merged into [[::Abundance of the chemical elements|Abundance of the chemical elements]]. (Discuss)|
In chemistry, natural abundance (NA) refers to the prevalence of isotopes of a chemical element as naturally found on a planet. The relative atomic mass (a weighted average) of these isotopes is the atomic weight listed for the element in the periodic table. The abundance of an isotope varies from planet to planet but remains relatively constant in time.
As an example, uranium has three naturally occurring isotopes: U-238, U-235 and U-234. Their respective NA is 99.2745%, 0.72% and 0.0055%. For example, if 100,000 uranium atoms were analyzed, one would expect to find approximately 99,275 U-238 atoms, 720 U-235 atoms, and no more than 5 or 6 U-234 atoms. This is because U-238 is much more stable than U-235 or U-234, as the half-life of each isotope reveals: 4.468×109 years for U-238 compared to 7.038×108 years for U-235 and 245,500 years for U-234.
af:Natuurlike verspreiding ar:توافر طبيعي للنظائر الكيميائية ast:Abondanza natural ca:Abundància natural de:Natürliche Häufigkeitko:자연존재비 it:Abbondanza isotopica hu:Természetes előfordulás ms:Kelimpahan semulajadi nl:Relatieve aanwezigheidnds:Vörkamen in de Natursl:Pogostost izotopa