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Atomic Weights of the Elements: Review 2000 Pure Appl. Chem., Vol. 75, No. 6, pp. 683–800, 2003. © 2003 IUPAC INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY INORGANIC CHEMISTRY DIVISION COMMISSION ON ATOMIC WEIGHTS AND ISOTOPIC ABUNDANCES* ATOMIC WEIGHTS OF THE ELEMENTS: REVIEW 2000 (IUPAC Technical Report) Prepared for publication by J. R. DE LAETER1, J. K. BÖHLKE2,‡, P. DE BIÈVRE3, H. HIDAKA4, H. S. PEISER2, K. J. R. ROSMAN1, AND P. D. P. TAYLOR3 1Department of Applied Physics, Curtin University of Technology, Perth, Australia; 2United States Geological Survey, 431 National Center, Reston, VA 20192, USA; 3Institute for Reference Materials and Measurements, European Commission – JRC B-2440, Geel, Belgium; 4Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan *Membership of the Commission for the period 2000–2001 was as follows: Chairman: L. Schultz (Germany); Secretary: R. D. Loss (Australia); Titular Members: J. K. Böhlke (USA); T. Ding (China); M. Ebihara (Japan); G. I. Ramendik (Russia); P. D. P. Taylor (Belgium); Associate Members: M. Berglund (Belgium); C. A. M. Brenninkmeijer (Germany); H. Hidaka (Japan); D. J. Rokop (USA); T. Walczyk (Switzerland); S. Yoneda (Japan); National Representatives: J. R. de Laeter (Australia); P. De Bièvre (Belgium); C. L. do Lago (Brazil); Y. K. Xiao (China/Beijing). ‡Corresponding author Republication or reproduction of this report or its storage and/or dissemination by electronic means is permitted without the need for formal IUPAC permission on condition that an acknowledgment, with full reference to the source, along with use of the copyright symbol ©, the name IUPAC, and the year of publication, are prominently visible. Publication of a translation into an- other language is subject to the additional condition of prior approval from the relevant IUPAC National Adhering Organization. 683 684 J. R. de LAETER et al. Atomic weights of the elements: Review 2000 (IUPAC Technical Report) 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 eth- ical, legal, and international standards. The standard atomic weights of the ele- ments 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 in- variably 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 car- bon isotope of mass number 12. Technological advances in mass spectrometry and nuclear-reaction energies have enabled atomic masses to be determined with a rel- ative uncertainty of better than 1 × 10–7. Isotope abundances for an increasing number of elements can be measured to better than 1 × 10–3. The excellent preci- sion 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 vari- ations increasingly place a constraint on the uncertainties with which some stan- dard atomic weights can be stated, they provide numerous opportunities for inves- tigating a range of important phenomena in physical, chemical, cosmological, biological, and industrial processes. This review reflects the current and increas- ing 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 con- stants 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. © 2003 IUPAC, Pure and Applied Chemistry 75, 683–800 Atomic weights of the elements: Review 2000 685 CONTENTS PREFACE Purpose of the review 687 Acknowledgments 688 List of acronyms 688 PART 1: HISTORY, ASSESSMENT, AND CONTINUING SIGNIFICANCE OF ATOMIC-WEIGHT DETERMINATIONS 689 Introduction 689 History 692 Commission on Atomic Weights and Isotopic Abundances 693 Discovery of radioactivity and isotopes 695 Atomic masses 696 Determination of atomic weights 697 Atomic-weight scale 698 “Absolute” isotope abundances yield “absolute” atomic weights 700 Variations in isotope abundances yield variations in atomic weights 702 Radioactive decay 703 Isotope fractionation from natural processes 704 Artificial isotopic variations 707 Atomic weights of the monoisotopic elements 708 Atomic weights for fundamental constants 710 Atomic weights and metrology in chemistry 712 Tables of standard atomic weights 713 Tables of the isotopic compositions of the elements 719 Continuing significance of atomic weights and isotopic abundances 730 PART 2: ELEMENT-BY-ELEMENT REVIEW OF THE STANDARD ATOMIC WEIGHTS 732 Introduction 732 1H Hydrogen 733 2He Helium 734 3Li Lithium 735 4Be Beryllium 736 5B Boron 736 6C Carbon 737 7N Nitrogen 738 8O Oxygen 739 9F Fluorine 740 10Ne Neon 741 11Na Sodium (Natrium) 741 12Mg Magnesium 742 13Al Aluminium (Aluminum) 742 14Si Silicon 743 15P Phosphorus 744 16S Sulfur 744 17Cl Chlorine 745 18Ar Argon 746 19K Potassium (Kalium) 747 20Ca Calcium 748 © 2003 IUPAC, Pure and Applied Chemistry 75, 683–800 686 J. R. de LAETER et al. 21Sc Scandium 749 22Ti Titanium 750 23V Vanadium 750 24Cr Chromium 751 25Mn Manganese 752 26Fe Iron (Ferrum) 752 27Co Cobalt 753 28Ni Nickel 753 29Cu Copper (Cuprum) 754 30Zn Zinc 755 31Ga Gallium 755 32Ge Germanium 756 33As Arsenic 757 34Se Selenium 758 35Br Bromine 759 36Kr Krypton 759 37Rb Rubidium 760 38Sr Strontium 760 39Y Yttrium 761 40Zr Zirconium 761 41Nb Niobium 762 42Mo Molybdenum 762 43Tc Technetium 763 44Ru Ruthenium 763 45Rh Rhodium 764 46Pd Palladium 764 47Ag Silver (Argentum) 765 48Cd Cadmium 766 49In Indium 767 50Sn Tin (Stannum) 767 51Sb Antimony (Stibium) 768 52Te Tellurium 769 53I Iodine 770 54Xe Xenon 770 55Cs Caesium (Cesium) 771 56Ba Barium 771 57La Lanthanum 772 58Ce Cerium 773 59Pr Praseodymium 773 60Nd Neodymium 774 62Sm Samarium 774 63Eu Europium 775 64Gd Gadolinium 776 65Tb Terbium 776 66Dy Dysprosium 777 67Ho Holmium 777 68Er Erbium 778 69Tm Thulium 778 70Yb Ytterbium 779 © 2003 IUPAC, Pure and Applied Chemistry 75, 683–800 Atomic weights of the elements: Review 2000 687 71Lu Lutetium 779 72Hf Hafnium 780 73Ta Tantalum 780 74W Tungsten (Wolfram) 781 75Re Rhenium 781 76Os Osmium 782 77Ir Iridium 782 78Pt Platinum 783 79Au Gold (Aurum) 783 80Hg Mercury (Hydrargyrum) 784 81Tl Thallium 784 82Pb Lead (Plumbum) 785 83Bi Bismuth 785 90Th Thorium 786 91Pa Protactinium 786 92U Uranium 787 REFERENCES 788 APPENDIX A: SOURCES OF REFERENCE MATERIALS 800 PREFACE Purpose of the review This review describes the gradual evolution of knowledge, understanding, and detailed information on the atomic weights of the chemical elements and their isotopic compositions in normal materials, as evaluated regularly by IUPAC. Atomic weights at the start of the 20th century were a well-recognized part of chemistry, but are now interdisciplinary, both in their measurements and their applications. Under these circumstances, such a review has clear purposes and aims at: • tracing the history of concepts, such as that of “atomic weights” once believed to be constants of nature; • describing the methods of atomic-weight determinations; • giving current knowledge of the best values of the atomic weights based on the elements’ isotopic compositions; • indicating the estimated uncertainties of all these data in accord with methods of measurement science, as established during the century; • adopting scales of measurements in accord with the modern system of units as also developed during the century; • warning of known limitations of the above data for exceptional materials; and • exploring applications for the differential measurement of isotopic composition to all branches of materials science. This review is not primarily concerned with definitions and technical terms, as this matter is the responsibility of IUPAC’s Interdivisional Commission on Nomenclature and Symbols (IDCNS), re- cently renamed Interdivisional Committee on Terminology, Nomenclature and Symbols (ICTNS). Questions of semantics generally, when derived by logic or based on historic use, or in accord with con- ventions in related fields, are vigorously debated. Even CAWIA, whose very name includes the much- disputed term “atomic weight,” has not been able to avoid involvement. When Tomas Batuecas, President of the Atomic Weights Committee, persuaded the authorities in the IUPAC Bureau in 1963 to change the term to “atomic mass”,
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