|
Vol.
25 No. 5
September-October 2003
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
Page
last modified 3 September 2003.
Copyright © 2002-2003 International Union of Pure and
Applied Chemistry.
Questions regarding the website, please contact [email protected]
|