Uranium

Isotope	Atomic mass (Da)	Isotopic abundance (amount fraction)
²³⁴U	234.040 950(8)	0.000 054(5)
²³⁵U	235.043 928(8)	0.007 204(6)
²³⁸U	238.050 79(1)	0.992 742(10)

In 1969, the Commission recommended A_r(U) = 238.029(1) for the atomic weight of U based on mass-spectrometric determinations and a careful analysis of the variability of x(²³⁵U) in nature. In 1979, the Commission took note of later studies of the variations of the ²³⁵U abundance in normal sources, which justified a more precise value for the standard atomic weight, thus leading to A_r(U) = 238.0289(1). The atomic weight and uncertainty of uranium were changed to 238.028 91(3) in 1999 on the basis of new calibrated mass-spectrometric measurements.

That value applies to uranium as found in normal terrestrial sources, except as discovered in one locality in south-west Africa (Gabon at Oklo), hence the annotation "g". Uranium is used in the nuclear fuel cycle either enriched or depleted in ²³⁵U, hence the annotation "m".

All uranium isotopes are α-emitters. Isotopes ²³⁵U and ²³⁸U are primordial with the ²³⁵U abundance declining very gradually in geological time because of its faster decay. ²³⁴U, itself a decay product of ²³⁸U, is in equilibrium established by the ratio of the half-lives. ²³⁵U decays by a branched series ending with ²⁰⁷Pb, ²³⁸U (and ²³⁴U) by a similar series ending in ²⁰⁶Pb. The ²³⁸U–²⁰⁶Pb and the ²³⁵U–²⁰⁷Pb decay systems are of fundamental importance in geochronology.

SOURCE Atomic weights of the elements: Review 2000 by John R de Laeter et al. Pure Appl. Chem. 2003 (75) 683-800
© IUPAC 2003

CIAAW

Uranium
A_r(U) = 238.028 91(3) since 1999

The name derives from the planet Uranus, which in Roman mythology was "Father Heaven". The German chemist Martin-Heinrich Klaproth discovered the element in 1789, following William Hershel's discovery of the planet in 1781. The metallic uranium was first isolated by the French chemist Eugène-Melchior Peligot in 1841.

Isotopic reference materials of uranium.