Compare with isotopic abundance. The average fraction of atoms of a given isotope of an element on Earth. 
In 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, weighted by mole-fraction abundance figures) of these isotopes is the atomic weight listed for the element in the periodic table. The abundance of an isotope varies from planet...
Percentage of an element occurring on earth in a particular stable isotopic form.
Percentage of an element occurring on earth in a particular stable isotopic form.