Project Details Assessment of fundamental understanding of isotopic abundances and atomic weights of the chemical elements (2016-2017)

Project No.:
Start Date:
01 December 2015
End Date:
Division Name:
Inorganic Chemistry Division
Division No.:


Review fundamental issues and concerns that have been raised by members of the Commission on Isotopic Abundances and Atomic Weights (CIAAW). Discuss the issues and present the recommendations to CIAAW on the solution to the problems that will help to provide a future direction for the work of CIAAW on isotopic abundances and atomic weights of the chemical elements.


The Commission on Isotopic Abundances and Atomic Weights evaluates the atomic weights, isotopic abundances and their variations and made recommendations to a general chemical community. This project will evaluate a number of questions that have arisen, and these are posed below.

Review of the Commission’s recent publications by the ICTNS (Interdivisional Committee on Terminology, Nomenclature and Symbols) indicated that a review of the term “normal material” needs an updated definition. This term is fundamental to both the Table of Standard Atomic Weights (TSAW) and the Table of Isotopic Compositions (TICE).

To improve users’ understanding of the basic connection between the isotopic abundances and the atomic weights of the elements, the possible consolidations of TSAW and TICE will be considered by the task group.

The task group will also consider an atomic-weights table for general science and for educational communities that may have fewer digits with uncertainties than the full table. This table should change less frequently that the full unabridged table of standard atomic weights.


Sep 2016 update – A IUPAC Technical Report, entitled “Review of footnotes and annotations to the 1949–2013 tables of standard atomic weights and tables of isotopic compositions of the elements” has been published in Pure and Applied Chemistry, Volume 88, Issue 7, Pages 689–699, DOI: 10.1515/pac-2016-0203.

Feb 2017 update – A IUPAC Technical Report, entitled “Isotope-abundance variations and atomic weights of selected elements: 2016” has been published in Pure and Applied Chemistry (2016), Volume 88, Issue 12, Pages 1203–1224,

Aug 2017 update – A PowerPoint and a Technical document titled “Clarifying the meaning of ‘normal material’, standard atomic weight value and uncertainty” has been prepared and will be presented to members of the Commission on Isotopic Abundances and Atomic Weights meeting in Groningen, The Netherlands, in September 2017. These materials should be of benefit to new and younger members of the Commission, and they will address the following:

(1) Why does the Commission continue to produce a Table of Standard Atomic Weights when the values are not weights, but are dimensionless? Why not produce a Table of Standard Relative Atomic Masses?
(2) Historically, how has the Commission set the lower and upper standard atomic weight bounds of elements?
(3) How did the one-digit uncertainty for standard atomic weights arise?
(4) Why is a mineral survey important for consideration of a new standard atomic weight?
(5) SIAM is responsible for evaluation of best measurements of isotopic composition. How does a best measurement of a material relate to a new standard atomic weight?
(6) From where did the adoption of the 6s uncertainty in the standard atomic weights of some elements arise?
(7) Why did the Commission decide not to assign standard atomic weights to many radioactive elements?

July 2018 update – A Technical Report title “Clarification of the term “normal material” used for standard atomic weights” is published in PAC: Coplen, T., Holden, N., Wieser, M., et al. (2018) Pure and Applied Chemistry, 90(7), pp. 1221-1224;

Dec 2019 update – The work on a second manuscript continued during 2019. A draft entitled “Update on expression of uncertainty in standard atomic weights” was submitted to PAC as a Technical Report. The task group received referees comments, some positive and some negative and revision is on-going.

June 2020 update – Published AOP 19 June 2020, The Table of Standard Atomic Weights—an exercise in consensus, Tyler B. Coplen, Norman E. Holden, Tiping Ding, Harro A.J. Meijer, Jochen Vogl, and Xiangkun Zhu, Rapid Communications in Mass Spectrometry ;

Page last updated 21 June 2020