Atomic weights of the elements. Review 2000 (IUPAC Technical Report)
J. R. de Laeter, J.
K. B�hlke,* P. De Bi�vre, H. Hidaka, H. S. Peiser, K. J. R. Rosman,
and P. D. P. Taylor
Abstract: A consistent set of internationally accepted atomic
weights has long been an essential aim of the scientific community because
of the relevance of these values to science and technology, as well
as to trade and commerce subject to ethical, legal, and international
standards. The standard atomic weights of the elements are regularly
evaluated, recommended, and published in updated tables by the Commission
on Atomic Weights and Isotopic Abundances (CAWIA) of the International
Union of Pure and Applied Chemistry (IUPAC). These values are invariably
associated with carefully evaluated uncertainties. Atomic weights were
originally determined by mass ratio measurements coupled with an understanding
of chemical stoichiometry, but 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. Atomic weights
and atomic masses are now scaled to a numerical value of exactly 12
for the mass of the carbon isotope of mass number 12. Technological
advances in mass spectrometry and nuclear-reaction energies have enabled
atomic masses to be determined with a relative uncertainty of better
than 1 ×107 . Isotope abundances for an increasing
number of elements can be measured to better than 1 ×103
. The excellent precision of such measurements led to the discovery
that many elements, in different specimens, display significant variations
in their isotope-abundance ratios, caused by a variety of natural and
industrial physicochemical processes. While such variations increasingly
place a constraint on the uncertainties with which some standard atomic
weights can be stated, they provide numerous opportunities for investigating
a range of important phenomena in physical, chemical, cosmological,
biological, and industrial processes. This review reflects the current
and increasing interest of science in the measured differences between
source-specific and even sample-specific atomic weights. These relative
comparisons can often be made with a smaller uncertainty than is achieved
in the best calibrated absolute (=SI-traceable) atomic-weight
determinations. 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. This review is in
two parts: the first summarizes the development of the science of atomic-weight
determinations during the 20th century; the second summarizes the changes
and variations that have been recognized in the values and uncertainties
of atomic weights, on an element-by-element basis, in the latter part
of the 20th century.
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