Answer to Question #2994 Submitted to "Ask the Experts"

Uranium, with atomic number 92, is the last naturally occurring element on the periodic table. This is because all nuclides above atomic number 83 are radioactive (Turner 1995), and all above uranium that might have been present in Earth's early geological history have relatively short half-lives and have decayed away (Faure 1986). The reason that uranium is found naturally is that, even though radioactive, it has two isotopes (²³⁵U and ²³⁸U) with half-lives (700 million years and 4.5 billion years, respectively) that are on the order of Earth's age. A third naturally occurring uranium isotope, ²³⁴U, is formed as a result of radioactive decay of ²³⁸U.

You can get more information about naturally occurring radioactive materials at The TENORM Page and in the "Naturally Occurring Radioactive Material (NORM)" section on this Web site in Ask the Experts Environmental and Background Radiation Category.

James S. Bogard, PhD, CHP

References

Turner JE. Atoms, radiation and radiation protection. Second Edition: John Wiley & Sons, Inc.; 1995: 95-96.

Faure G. Principles of isotope geology. Second Edition: John Wiley & Sons, Inc.; 1986: 14-20.