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Vol. 25 No. 5
September-October 2003

Making an imPACt | Recent IUPAC technical reports and recommendations that affect the many fields of pure and applied chemistry.
See also www.iupac.org/publications/pac
 

The Atomic Weights of the Elements: Review 2000 (IUPAC Technical Report)

J.R. de Laeter, J. K. Böhlke, P. De Bi èvre, H. Hidaka, H.S. Peiser, K.J.R. Rosman, and P.D.P. Taylor

Pure and Applied Chemistry
Vol. 75, No. 6, pp. 683-799 (2003)

A consistent set of internationally accepted atomic weights is an essential aim of the scientific community because of the relevance of these values to science, technology, and commerce. Accurate determinations of the atomic weights of certain elements also influence the values of fundamental constants such as the Avogadro, Faraday, and Universal Gas constants.

Various committees or commissions have held responsibility for evaluating and recommending atomic weights of the elements since the late 19th century. This responsibility has resided with IUPAC since it was constituted in 1920. For the last several decades, the Commission on Atomic Weights and Isotopic Abundances (CAWIA) has published updated tables of recommended ("standard") atomic weights and their uncertainties in PAC approximately every two years. In The Atomic Weights of the Elements: Review 2000, members of CAWIA provide a comprehensive overview of this process in two parts. In the first part, the concept of standard atomic weights, the methods used to determine them, and the basis for making changes are described in a historical review covering the 20th century. In the second part, a detailed summary is provided for each element describing CAWIA decisions that have lead to changes in that element's standard atomic weight and its uncertainty since the 1960s.

Atomic weights were once considered to be constants of nature and were determined by mass-ratio measurements coupled with an understanding of chemical stoichiometry, but they are now based almost exclusively on knowledge of the isotopic composition (derived from isotope-abundance ratio measurements) and the atomic masses of the isotopes of the elements. Technological advances in mass spectrometry and nuclear-reaction energies have permitted measurements of atomic masses with a relative uncertainty of better than 1 X 10-7 and of isotope-abundance ratios of better than 1 X 10-3 in many cases. The improving accuracy and precision of such measurements led to the discovery that many elements exhibit variation in their isotope-abundance ratios (and atomic weights) in different specimens. These variations are caused by a variety of physicochemical and biochemical processes in both natural and industrial systems, place severe constraints on the uncertainties with which some standard atomic weights can be stated, and were once considered a hindrance to the accuracy of chemical measurements. Subsequently, however, these variations have been recognized as powerful tools for investigating important phenomena in physics, chemistry, biology, cosmology, geology, archeology, industry, forensics, and many other fields of study. The Atomic Weights of the Elements: Review 2000 documents the evolution of two major perspectives in atomic-weight science during the 20th century: increasingly precise measurements of isotope-abundance ratios and atomic weights with ties to the SI (metrology), and discovery and application of isotope-abundance variations in science and technology.

www.iupac.org/publications/
pac/2003/7506/7506x0683.html


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