Natural abundance

In <a href="/facts/Physics/a7aH2y08">physics</a>, natural abundance (NA) refers to the abundance of <a href="/facts/Isotope/KD9B1y6o">isotopes</a> of a <a href="/facts/Chemical_element/0jducgWD">chemical element</a> as naturally found on a <a href="/facts/Planet/rbPjE6Bx">planet</a>. The relative atomic mass (a weighted average, weighted by <a href="/facts/Mole_fraction/nW4EepmJ">mole-fraction</a> abundance figures) of these isotopes is the <a href="/facts/Atomic_weight/A3vbuz2n">atomic weight</a> listed for the element in the <a href="/facts/Periodic_table/umdGIfUb">periodic table</a>. 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).
As an example, <a href="/facts/Uranium/n2sGWBzU">uranium</a> has <a href="/facts/Isotopes_of_uranium/zWIQ6MVL">three naturally occurring isotopes</a>: 238U, 235U, and 234U. Their respective natural mole-fraction abundances are 99.2739–99.2752%, 0.7198–0.7202%, and 0.0050–0.0059%. For example, if 100,000 uranium atoms were analyzed, one would expect to find approximately 99,274 238U atoms, approximately 720 235U atoms, and very few (most likely 5 or 6) 234U atoms. This is because 238U is much more stable than 235U or 234U, as the <a href="/facts/Half-life/kGVH8BAB">half-life</a> of each isotope reveals: 4.468 × 109 years for 238U compared with 7.038 × 108 years for 235U and 245,500 years for 234U.
Exactly because the different uranium isotopes have different half-lives, when the Earth was younger, the isotopic composition of uranium was different. As an example, 1.7×109 years ago the NA of 235U was 3.1% compared with today's 0.7%, and that allowed a <a href="/facts/Natural_nuclear_fission_reactor/G7FIGsEV">natural nuclear fission reactor</a> to form, something that cannot happen today.
However, the natural abundance of a given isotope is also affected by the probability of its creation in <a href="/facts/Nucleosynthesis/F9MIeADF">nucleosynthesis</a> (as in the case of <a href="/facts/Samarium/09K3kl93">samarium</a>; radioactive <a href="/facts/Samarium-147/e5wQw5B0">147Sm</a> and 148Sm are much more abundant than stable 144Sm) and by production of a given isotope as a daughter of natural radioactive isotopes (as in the case of radiogenic <a href="/facts/Isotopes_of_lead/V9JFUTRo">isotopes of lead</a>).

Natural abundance open-in-new

Natural abundance