Lesson 2 Background Notes

The Periodic Table versus the Chart of the Nuclides

Some of the activities in this unit require students to use “stairstep” charts to plot the nuclear transitions occurring in the natural radioactive decay series and in spent fuel. These charts introduce students to a device they may not be familiar with that presents information on radionuclides and radioactive decay. They are adapted from sections of a more general presentation called the Chart of the Nuclides. The Chart of the Nuclides presents the most pertinent information on all known radioactive and non-radioactive nuclides in a way that makes it relatively easy to follow the nuclear transitions resulting from radioactive decay. It also gives the half-lives of the radionuclides and the energies associated with the decay.

The Chart of the Nuclides is similar to the Periodic Table of the Elements in that it organizes and arranges information to give scientists both a quick overview and important detailed information. The periodic table was developed before radioactivity was discovered and before the isotopic nature of the chemical elements had become known. The periodic table presents in convenient form the periodicity or repetition of chemical behavior with increasing atomic number. For example, lithium, sodium, and potassium with atomic numbers 3, 11, and 19, respectively, are similar in their chemical properties. Likewise, fluorine, chlorine, and bromine with atomic numbers 9, 17, and 35, respectively, have common characteristics, differing mainly in degree of reactivity. The periodic table aided in the search for chemical elements then unknown by predicting their properties. It also encouraged research in the structure of atoms and how electrons are arranged around nuclei in a manner that makes the chemical behavior of the elements understandable and predictable. Although there have been many refinements to Mendeleev’s original 1869 formulation, the fundamental principles leading to the tabulation remain the same today. The periodic table is of primary interest to chemists, both practical and theoretical.

The Chart of the Nuclides is a product of the atomic age. The necessity for such a tabulation arose from the production and isolation of large numbers of radionuclides following the development of the atomic bomb and a concurrent large increase in our knowledge of radioactivity. The need for an ordered formulation of isotopic information increased with the initiation of nuclear power development, the increased interest in various nuclides (both radioactive and nonradioactive), and the realization of the enormous benefits that could be achieved in all areas of human endeavor from the use of various nuclides. The Chart of the Nuclides has a more universal appeal than the Periodic Table of the Elements and is of practical use in nearly all technical disciplines.

The chart was developed in the late 1940s at the Knolls Atomic Power Laboratory, then operated by the General Electric Company under the direction of what was then the U.S. Atomic Energy Commission and is now the U.S. Department of Energy. The chart has gone through many revisions since it was first developed. The present arrangement is similar to that suggested in the beginning by Emilio Segre. The current chart reflects known isotopic data up to 2005.

The chart is called the Chart of the Nuclides rather than the “Chart of the Isotopes” because nuclide is a more general term than isotope. Nuclide is a term applicable to all atomic forms of all the chemical elements. Isotope is a more restrictive term and refers to the various atomic forms of a single chemical element. The isotopes of a single chemical element may be thought of as a “family” of nuclides.

The Knolls Atomic Power Laboratory (now operated by Lockheed Martin for the U.S. Department of Energy’s National Nuclear Security Agency) has published the 16th edition of its classic Chart of the Nuclides. Available in both Wallchart and Textbook versions, this document presents the key nuclear properties of every known stable and radioactive form of each element.

Evaluated nuclear data is given for about 3100 known nuclides and 580 known isomers* in a format similar to a periodic table. The nuclides are arranged with atomic number Z (number of protons) along the vertical axis and neutron number N along the horizontal axis. For each nuclide the half-life, atomic mass, decay modes, relative abundances and other nuclear properties are detailed. Color coding is used to emphasize half-lives and neutron absorption properties.

The updated chart includes approximately 300 new nuclides and 100 new isomers not found in the 15th (1996) edition. There has been at least one change in more than 95% of the squares on the chart.

The Chart of the Nuclides is available as a 36" x 60" single sheet (known as a Wallchart) or a bound 88-page soft-cover book (known as a Textbook ). The Wallchart comes with a 48-page booklet of explanatory information. The explanatory information includes:

• A short history of the development of the periodic table
  
  
• Detailed descriptions of the types of information given on the Chart
  
  
• A discussion of trends in stability and instability of nuclides on the Chart
  
  
• Unit conversion factors and values of fundamental physics constants

KAPL is operated for Naval Reactors Program of the Department of Energy by KAPL, Inc., a wholly owned subsidiary of the Lockheed Martin Corporation. The Chart has been compiled, edited, and periodically revised by Knolls Atomic Power Laboratory scientists since 1956. Order chart from: http://www.chartofthenuclides.com

*Some atoms are known to have isomeric, or metastable, forms of themselves. A nuclear isomer is an atom having exactly the same number of protons and neutrons as another atom of the same element (i.e., the same isotope), but with an excited nucleus holding a much higher energy state than the ground, or normal, state. An isomer’s nucleus has an unusually excited proton or neutron. The isomer’s nucleus retains its higher energy state much longer than does an atom with that same nucleus in its ground state.

Over time, isomers release their extra energy as gamma rays, decaying to the ground energy state of the atom. In some cases, a released gamma ray strikes one of the atom’s own orbital electrons, causing the electron to absorb the energy and be knocked out of the atom. This is called internal conversion, a form of self-ionization.

Isomeric atoms are either shape or spin isomers. That is, they are slow to lose their extra energy because the atom’s nucleus is atypical in spatial structure (shape isomer) or in angular momentum (a measure of the spin of the nucleus). A significant change in shape or spin must occur for the energy to be released. (Note: Nuclear isomers are different from chemical isomers, a more common term used for compounds that have the same chemical formula but different structures, which has the effect of changing the way they interact with plane-polarized light.)