Atomic and Nuclear Physics
DOE-HDBK-1019/1-93
CHART OF THE NUCLIDES
Rev. 0
Page 15
NP-01
Natural Abundance of Isotopes
The relative abundance of an isotope in nature compared to other isotopes of the same element
is relatively constant. The Chart of the Nuclides presents the relative abundance of the naturally
occurring isotopes of an element in units of atom percent. Atom percent is the percentage of
the atoms of an element that are of a particular isotope. Atom percent is abbreviated as a/o.
For example, if a cup of water contains 8.23 x 10 atoms of oxygen, and the isotopic abundance
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of oxygen-18 is 0.20%, then there are 1.65 x 10 atoms of oxygen-18 in the cup.
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The atomic weight for an element is defined as the average atomic weight of the isotopes of the
element. The atomic weight for an element can be calculated by summing the products of the
isotopic abundance of the isotope with the atomic mass of the isotope.
Example:
Calculate the atomic weight for the element lithium. Lithium-6 has an atom percent
abundance of 7.5% and an atomic mass of 6.015122 amu. Lithium-7 has an atomic
abundance of 92.5% and an atomic mass of 7.016003 amu.
Solution:
The other common measurement of isotopic abundance is weight percent (w/o). Weight percent
is the percent weight of an element that is a particular isotope. For example, if a sample of
material contained 100 kg of uranium that was 28 w/o uranium-235, then 28 kg of uranium-235
was present in the sample.
Enriched and Depleted Uranium
Natural uranium mined from the earth contains the isotopes uranium-238, uranium-235 and
uranium-234. The majority (99.2745%) of all the atoms in natural uranium are uranium-238.
Most of the remaining atoms (0.72%) are uranium-235, and a slight trace (0.0055%) are
uranium-234. Although all isotopes of uranium have similar chemical properties, each of the
isotopes has significantly different nuclear properties. For reasons that will be discussed in later
modules, the isotope uranium-235 is usually the desired material for use in reactors.
A vast amount of equipment and energy are expended in processes that separate the isotopes of
uranium (and other elements). The details of these processes are beyond the scope of this
module. These processes are called enrichment processes because they selectively increase the
proportion of a particular isotope. The enrichment process typically starts with feed material
that has the proportion of isotopes that occur naturally. The process results in two types of
