Natural abundance

In physics, natural abundance refers to the degree of presence of isotopes of a chemical element found naturally on a given planet. The relative atomic mass (the mean mole fraction weighted by abundance percentages) of these isotopes is the atomic weight of the element on the periodic table. The abundance of an isotope varies from planet to planet, and even from place to place on Earth, but remains relatively constant over time (on a short-term scale).

For example, uranium has three naturally occurring isotopes: ²³⁸U, ²³⁵U, and ²³⁴U. Their respective abundances in the natural mole fraction are in the percentage intervals [99.2739-99.2752%], [0.7198-0.7202%] and [0.0050-0.0059%]. If you analyzed 100,000 uranium atoms, you would expect to find about 99,274 ²³⁸U atoms, about 720 ²³⁵U atoms, and very few (most likely 5 or 6) ²³⁵U atoms. sup>234U. This is because ²³⁸U is much more stable than ²³⁵U or ²³⁴U, as the half-life of each isotope reveals: 4.468 × 10⁹ years for the ²³⁸U compared to 7,038 × 10⁸ years for the ²³⁵U and 245,500 years for the ²³⁴U.

Just because different isotopes of uranium have different half-lives, when the Earth was younger, the isotopic composition of uranium was different. As an example, 1.7 million years ago the natural abundance of ²³⁵U was 3.1%, compared to 0.7% today. This abundance allowed the existence of natural nuclear fission reactors, something impossible with the current isotopic abundance.

However, the natural abundance of a given isotope is also affected by the probability of its creation by nucleosynthesis (as in the case of the radioactive isotopes of samarium ¹⁴⁷Sm and ¹⁴⁸Sm, which are much more abundant than the stable isotope ¹⁴⁴Sm) and by the production of a given isotope by natural radiation processes (as in the case of radiogenic lead isotopes).

Deviations from natural abundance

It is now known from the study of the Sun and primitive meteorites that the solar system was initially nearly homogeneous in its isotopic composition. Deviations of the envelope from the galactic average, looking locally at the time when the Sun's nuclear burning began, can generally be explained by mass-dependent fractionation (see article on mass-independent fractionation) plus a limited number of nuclear decays and transmutation processes. There is also evidence of short-lived (now extinct) infiltration of isotopes from a nearby supernova explosion that could have triggered the collapse of a solar nebula. Hence, deviations from natural abundance on earth are often measured in parts per thousand (per thousand or in ‰), because they are less than one percent (%). The only exception to this rule is the presolar granules found in primitive meteorites. These granules, confined since their formation, have not undergone homogenization processes, and often carry the nuclear signature of the specific nucleosynthesis processes in which their elements were formed. In these materials, deviations from the "natural abundance" of isotopes are sometimes measured by a factor of 100.