Science
ISOTOPES
Science > The Atom and Atomic Structure
Atoms with the same number of protons but a different number of neutrons are called isotopes, and are named by citing both the element and the atomic weight. All isotopes of the same element are chemically identical, because unlike electrons or protons, neutrons have no positive or negative charge. However, there are important differences in isotopes' nuclear properties. Uranium-235 (which has 92 protons and 143 neutrons) and uranium-238 (with 3 additional neutrons) are both isotopes of uranium, with the extra neutrons making U-238 slightly heavier. The heavier isotope is not subject to fission, while the lighter one is, but uranium-235 is very rare, comprising of less than one percent of naturally occurring uranium. Thus, in order to fuel the gun-type atomic bomb dropped on Hiroshima, scientists had to find a way to a separate out the U-235 from the chemically identical, vastly more common U-238 in the raw ore. Isotope separation projects sought to use the slight difference in mass to accomplish this task, and scientists and technicians at Oak Ridge and other facilities developed techniques using centrifuges, thermal diffusion, gaseous diffusion, and electromagnetic separation. To view the next science section of the Manhattan Project, proceed to Nuclear Physics. |
The text for this page is original to the Department of Energy's Office of History and Heritage Resources. Major sources consulted include the following. Useful for understanding basic atomic science is Henry DeWolf Smyth, Atomic Energy for Military Purposes: The Official Report on the Development of the Atomic Bomb under the Auspices of the United States Government, 1940-1945 (Princeton, NJ: Princeton University Press, 1945), page 5. Also useful is John F. Hogerton, ed., "Isotope," and "Isotope Seperation" The Atomic Energy Deskbook (New York: Reinhold Publishing Corporation, 1963; prepared under the auspices of the Division of Technical Information, U.S. Atomic Energy Commission), page 257. For current information on the physical properties of uranium isotopes see the U.S. Department of Energy funded data chart at, http://education.jlab.org/itselemental/iso092.html. The graphic illustrating the two main isotopes of uranium is adapted from images that originally appeared in The Harnessed Atom: Nuclear Energy and Electricity (DOE/NE-0072; Washington: Office of Program Support, Department of Energy, 1986), 18.