Project Details Assessment of fundamental understanding of isotopic abundances and atomic weights of the chemical elements

Project No.:
Start Date:
01 October 2006
End Date:
28 February 2011


Review fundamental issues and concerns that have been raised bymembers of the Commission on Isotopic Abundances and Atomic Weights(CIAAW). Discussthe issues and present the recommendations to CIAAW on the solutionto the problems that will help to provide a future direction forthe work of CIAAW on isotopic abundances and atomic weights of thechemical elements.


During the 20th century, the CIAAW evaluated 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.

In the 21st century, are there multiple audiences for the work of the CIAAW? Are Tables of Standard Atomic Weights and of Isotopic Compositions and other data published by CIAAW different for these audiences? Determine details needed in data products that will be published by the CIAAW. Will publication time scales vary for the different groups of audiences? Provide the CIAAW with some recommendations on these issues.

Do the synthetic elements require an atomic weight value and if so, how should this atomic weight be determined?

Does the atomic weight value require a revised definition?

How should the CIAAW treat uncertainties in the recommended values for atomic weights and isotopic abundances? How should the isotopic variations in nature be treated for the various CIAAW audiences? Provide some recommendations to the CIAAW.

Investigate the feasibility for a CIAAW website and web page to market the CIAAW and its activities and provide the latest information to the scientific community on the atomic weights and isotopic abundances. If feasible, web page designs should be provided to CIAAW.

To clarify the isotopic measurement vocabulary, should CIAAW consider preparation of a sub-document of the “ISO” “Vocabulary of Basic and General Concepts in Metrology” (VIM) in the field of isotope measurements as used in many applied scientific fields?


* project announcement published in Chem. Int. Mar-Apr 2007, p. 18

In July 2007, the task group met in Sèvres, France to review fundamental issues and concerns that have been raised and topics for discussion included:
1.  Discussion of multiple audiences for the work of CIAAW.
2.  Are data products different for potentially different CIAAW audiences?
3.  Potential publication time scales and other recommended details.
4.  Do radioactive elements require an atomic weight value?
5.  Does the definition of atomic weight require a revision?
6.  Uncertainty treatment for recommended atomic weights and abundances.
7.  Treatment of natural variations for potentially different CIAAW audiences.
8.  Status and recommendations for a CIAAW website and design.
9.  Should CIAAW prepare an ISO VIM subdocument for isotope measurements?

Specific recommendations accepted by the Commission follow.  A few recommendations were not accepted and these will be reviewed at a subsequent meeting. Accepted recommendations include:

1. create a web site for the scientific public with data and publications from published Commission reports.  In response to this recommendation, the web site at was approved by the Commission July 31, 2007.

2. evaluate the Table of Standard Atomic Weights Abridged to Five Significant Figures and update and republish biennially as needed. In response to this recommendation, a new Table of Standard Atomic Weights Abridged to Five Significant Figures appears in the technical report titled ‘Atomic weights of the elements 2007’, published in PAC 2009 (doi:10.1351/PAC-REP-09-08-03).

3. re-evaluate the Table of Standard Atomic Weights Abridged to Four Significant Figures biennially. In response to this recommendation, a new Table of Standard Atomic Weights Abridged to Four Significant Figures appears in the technical report titled ‘Atomic weights of the elements 2007’, published in PAC 2009 (doi:10.1351/PAC-REP-09-08-03).

4. address specific issues related to atomic-weight variations for poly-isotopic elements; in response, project proposals on variations of isotopic composition of heavy elements and on radiogenic element were prepared.

5. forward the most recent (and future) evaluation reports of the Standard Atomic Weights of the Elements to CCQM (The Consultative Committee for Amount of Substance – Metrology in Chemistry) at BIPM (International Bureau of Weights and Measures). CCQM needs to be kept aware of the atomic weights work of the Commission because the atomic weights are the link between the amount of substance and mass, and this recommendation was carried out.

6. work with IUPAC’s Committee on Chemistry Education and develop a strategy to create CDs, posters, etc, to disseminate a Periodic Table of the Isotopes and information on isotopic abundances and atomic weights.

7. improve the process used to assign “standard atomic weight” values for elements with atomic weight variations and to define “standard atomic weight”.  Discussion of a project proposal on vocabulary in isotopic abundance measurements and atomic weights is ongoing.

8. consider the use of asymmetric uncertainties for “standard atomic weights” of elements that exhibit atomic weight variation instead of symmetric uncertainties that currently are used. For elements with atomic weight variation (e.g., H, Li, B, C, N, O, S, and Cl), the “standard atomic weights” currently are given as a single value with symmetric uncertainties covering all or most of the evaluated range.  The single value is selected to be near the middle of the range of variation and therefore may not correspond to any measured value.

9. publish best measurements (column 6 of the Commission’s Table of Isotopic Compositions of the Elements) even when they do not yield a new standard atomic weight. The Commission’s Table of Isotopic Compositions of the Elements is not published biennially.  Publishing these best measurements in biennial reports will alert the scientific public of Commission updates in evaluated data in a timely manner.

10. initiate a survey and evaluation of calibrated isotopic reference materials to identify elements without such reference materials. A project proposal was prepared and submitted to IUPAC.

11. consider the creation of a vocabulary of terms specific to standard atomic weights and isotopic abundances and their measurement.


September 2009 – The Task Group met again at the International Atomic Energy Agency (IAEA) in Vienna, Austria on July 20 and 21, 2009. Specific recommendations from the sessions include the following:

1. The Commission should publish the Table of Standard Atomic Weights (TSAW 2009) with ranges of atomic weights and recommended (reference) values for poly-nuclidic elements, when the range exceeds the measured uncertainty. (relevant to project 2007-028-1-200)

2. It is recommended that the preferred value of an atomic weight for elements with an evaluated range that exceeds the measurement uncertainty be the TSAW 2007 standard atomic weight value without an uncertainty value attached.

3. It is recommended that non-mass dependent variations of isotopes be identified in the Table of Isotopic Composition of the Elements (TICE) by a footnote, which is explained in the rubric.

4. It is recommended that elements with radiogenic isotopes be identified in TICE by a footnote, which is explained in the rubric.

5. It is recommended that a unique footnote be used to identify elements with anomalous isotopic compositions from the two billion year old natural nuclear reactor at Oklo in Gabon, Africa.

6. It is recommended that footnotes and annotations be made consistent between the published TSAW and TICE documents.

7. In order to address the problem of what is meant by a calibrated system for isotopic measurements, it is recommended that a project proposal be prepared to form a Task Group (headed by John deLaeter) to prepare a document on guidelines on atomic weights and isotopic abundance measurements. The Guidelines document should be published in Pure and Applied Chemistry and educational journals.

8. The Commission needs to provide a priority list of isotopic reference materials for elements that do not presently have isotopic reference materials, taking into consideration the physical form of the material. (This recommendation might be an outcome of the Project on Isotopic Reference Materials).

9. The scope of the future work of the Commission should incorporate isotopic reference material value assessment.

10. The Commission should prepare a TICE 2009 document for publication and the Commission should publish TSAW and TICE documents concurrently, whenever the TICE data changes.

11. The Commission should incorporate current atomic mass data into each new TICE publication after the 2009 publication.

12. The Commission should begin a discussion about the definition of the standard atomic weight. Such a discussion could impact several disciplines (Divisions) within IUPAC.

13. The Commission should include a statement on measurement uncertainty in its regular reports that all measurement uncertainties are measurement uncertainties with coverage factor k = 2 unless otherwise noted (i.e., the standard deviation = 2 sigma). This implies that the annotations C, F, L and N might be eliminated in column 6 (best measurement) of the TICE table for future updates, retaining the historical information.

14. The Commission should make its publications compliant with GUM, which means with metrological concepts.

15. The Sub-committee / Task Group should consider the evaluation of the uncertainty in the atomic mass tables and its impact of the standard atomic weight values of mono-nuclidic elements, which account for approximately ¼ of the first 92 chemical elements. The proposal to redefine the kilogram might have an impact on the uncertainty of the atomic mass unit and the carbon mass standard and impact the above mentioned atomic weights.


March 2011 – project completed –  see final report (pdf file – 16KB)


Last Update: 9 March 2011