Natural abundance refers to 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.
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.
See also
The content of this page is retrieved from http://en.wikipedia.org/wiki/Natural_abundance under GFDL
