In
physics,
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 to planet, and even from place to place on the Earth, but remains relatively constant in time (on a short-term scale).