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天然存在比
Wikipedia definition
1. Natural abundanceIn chemistry, natural abundance (NA) refers to the abundance 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, and even from place to place on the Earth, but remains relatively constant in time. As an example, uranium has three naturally occurring isotopes: U, U and U.
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