Which long-lived actinoid isotopes are considered primordial?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — actinoid element Modern version of the periodic table of the elements (printable).

In the actinoid series, there are three isotopes considered primordial because they possess extremely long half-lives—on the order of billions of years—and are believed to have been present when Earth accreted. These isotopes are:1

Thorium-232 ( $^{232}$ Th)1
Uranium-235 ( $^{235}$ U)1
Uranium-238 ( $^{238}$ U)1

While all actinoid elements are radioactive and lack stable isotopes, these three are the only members of the series found in nature with such significant longevity. Other radioactive isotopes found in nature are typically shorter-lived products resulting from the decay chains of these primordial parents.12