Atomic Weights of the Elements 1993

Cite as: Journal of Physical and Chemical Reference Data 24, 1561 (1995); https:// doi.org/10.1063/1.555967 Submitted: 28 February 1995 . Published Online: 15 October 2009

IUPAC Commission on Atomic Weights and Isotopic Abundances


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Journal of Physical and Chemical Reference Data 24, 1561 (1995); https://doi.org/10.1063/1.555967 24, 1561

© 1995 American Institute of Physics for the National Institute of Standards and Technology. Atomic Weights of the Elements 1993

IUPAC Commission on Atomic Weights and Isotopic Abundances 431 National Cenrer, Reston, VA 22092

Received February 28, ] 995

The biennial review of atomic weight, Ar(E), determinations and other cognate data has resulted in changes for the of from 47.88 ±O.03 to 47.867±O.OOl, of from 55.847±O.003 to 55.845±O.002, of from 121.757±O.003 to 121.760±O.OOl. and of from 192.22±O.03 to 192.217±O.OO3. Recent investigations on and confinned the presently accepted values of Ar(Cl) and Ar(Br). To emphasize the fact that the atomic weight of commonly available in laboratory reagents can vary significantly, the value of lithium, Ar(Li), was enclosed in brackets and a footnote was added. As a result of several changes, the Table of Standard Atomic Weights Abridged to Five Significant Figures has been updated. Because relative -ratio data for stable , , and are commonly behlg expressed on non-corresponding scales, the Commission recommends that' such isotoi.jc data be expressed only relative to the references VSMOW and VPDB. Because many elements have a different isotopic composition in some non-terrestrial materials, recent data on non-terrestrial materials are included in this report for the infonnation of the interested scientific community. ©1995 American Institute of Physics and American Chemical Society.

Key wordS: atOmic weight; critiCal evaluation; elements; isotopiC compOSitions.


1. Introduction ...... 1561 3. Relative atomic masses and half-lives of selected 2. Comments on Some Atomic Weights and Annota- ...... 1571 tions...... 1562 4. Standard atomic weights abridged to five significant 2.1. Titanium ...... 1562 figures ...... ' 1572 2.2. Iron ...... 1562 5. Examples of observed maximum isotopic variation 2.3. Antimony...... 1562 and corresponding atomic weights in non-terrestrial 2.4. Iridium...... 1562 materials due to different processes...... 1574 2.5. Lithium...... 1562 6. Examples of isotopic compositions and correspond- 3. The Table of Standard Atomic Weights 1993 . . .. 1563 ing atomic weights in different extraterrestrial 4. Relative Atomic Masses and Half-lives of Selected sources...... 1575 Radionuclides ...... 1563 5. 1993 Table of Standard Atomic Weights Abridged List of Figures to Five Significant Figures...... 1563 SJ. Introduction ...... 1563 1. The isotopic composition of and carbon in interstellar SiC grains (average size 5Jl.m) separated 6. Reporting Relative Abundance Data for Stable Hy­ from the carbonaceous chondrite Murchison. . . .. 1564 drogen, Carbon, and Oxygen ...... 1563 7 . Non-Terrestrial Data...... 1564 1. Introduction 7.1. Processes...... 1565 7.2. Sources ...... 1565 The Commission on Atomic Weights and Isotopic 8. Other Projects of the Commission ...... 1566 Abundances met under the chairmanship of Professor 9. References...... 1576 K. G. Heumann from 6th_8 th August 1993, during the 37th IUPAC General Assembly in Lisbon, Portugal. The Commis­ List of Tables sion decided to publish the report "Atomic Weights of the

2 Elements 1993" as presented here. 1. Standard atomic weights 1993 [Scaled to AreC) = The Commission has reviewed the literature over the previ­ 12] ...... 1566 ous two since the last report I and evaluated the pub­ 2 2. Standard atomic weights 1993 [ScaJed to Are C) lished data on atomic weights and isotopic compositions on 12] ...... ]569 an element-by-element basis. The atomic weight of an ele­ ment (Tables 1 and 2) can be detennined from a knowledge of the isotopic abundances and corresponding atomic masses ©1994 by the International Union of Pure and Applied (JUPAC). of the of that element. The latest compilation of the Reprints available from ACS; see Reprints List at back of issue. isotopic abundances and atomic masses with an relevant data nnd.7 -26Rg/gS/2414l11561/16/$10.00 1561 J. Phys. Chern. Ref. Data, Vol. 24, No.4, 1995 1562 IUPAC COMMISSION ON ATOMIC WEIGHTS AND ISOTOPIC ABUNDANCES was published in 1991,2 and 1985,3 respectively. The Com­ high purity by Taylor et aZ. 20 The magnitude of the uncer­ mission periodically reviews the history of the atomic weight tainty on this value is mainly due to the variations of iron of each element emphasizing the relevant published scientific isotopic composition found by Dixon et al.21 in geological evidence on which decisions have been made.4 and biological samples. The previous value of Ar(Fe) = Membership of the Commission for the 1991-1993 55.847(3) was assigned in 1961,22 based on -spectromet­ was as follows: K. G. Heumann (FRG, Chairman); T. B. ric measurements. Historical values of Ar(Fe) [values before Coplen (USA, Secretary); H. J. Dietze (FRG, Associate); M. 1961 are based on Ar(O) = 16 exactly; more recent values are Ebihara (, Associate); J. W. Gramlich (USA, Titular); H. concordant with Ar(O) = 15.9994] include: 1894,56.04; 1896, S. Hertz (USA, Associate); H. R. Krouse (, Titular); R. 56.02; 1900, 56.0; 1901, 55.9; 1909, 55.85; 1912, 55.84; D. Loss (, Associate);G. Ramendik (, Associ­ 1940, 55.85; and 1961, 55.847(3). ate); K. J. R. Rosman (Australia, Titular); L. Schultz (FRG, Associate); M. Shima (Japan, Titular); P. Taylor (Belgium, 2.3. Antimony Associate); L. Turpin (France, Associate); K. Wade (UK, Titular); P. De Bievre (Belgium, National Representative); E. The Commission has changed the recommended value for the Giesbrecht (, National Representative); N. N. Green­ standard atomic weight of antimony to Ar(Sb) = 121.760(1), (UK, National Rcprcscntativc); D. Richardson, (USA, based on the calibrated positive thermal ionization mass-spcc­ National Representative); Y. Xiao (, National Repre­ trometric determination by Chang et al.23 The previous value sentative ). of Ar(Sb) = 121.757(3), which was adopted by the Commis­ sion in 19892 was based on the mass-spectrometric measure­ 2. Comments on Some Atomic Weights ment by de Laeter and Hosie,24 which was supported by other and Annotations high quality measurements by Chang et al.,25 and by Wachs­ mann and Heumann.26 Historical values of Ar(Sb) [values 2.1. Titanium before 1961 are based on Ar(O) = 16 exactly; more recent values are concordant with Ar(O) = 15.9994] include: 1894, The Commission has changed the recommended value for 120.23; 1896, 120.43; 1900, 120.4; 1924, 120.2; 1925, the standard atomic weight of titanium to Ar(Ti) = 47.867(1) 121.77; 1961, 121.75; 1969, 121.75(3); and 1989, 121.757(3). based on the calibrated positive thermal ionization mass-spec­ A survey of five stibnite and five laboratory trometric determination by Shima and Torigoye.5 The previ­ reagents was also carried out by Chang et al.23 No evidence ous value, Ar(Ti) = 47.88(3), was based on recalculation of the of isotopic fractionation for any of the terrestrial materials chemica16.7 and the available mass-spectrometric determina­ was found. tions,8.9.,Il.12,13 but was weighted towards the calibrated mass- spectrometric measurements of Belsheim. 13 Although highly 2.4. Iridium precise mass-spectrometric determinations by Heydegger et al., 14 Niederer et al.15 and Niemeyer and Lugmair16 have The Commission has changed the recommended value for been published since 1979, none of those were calibrated the standard atomic weight of iridium to Ar(Ir) = 192.217(3) measurements. Therefore, the Commission retained the previ­ based on recent high precision measurements using both pos­ ous values of ArCTi) and its uncertainty. Historical values of itive and negative thermal ionization . The Ar(Ti) [values before 1961 are based on AlO) = 16 exactly; previous value, AlIr) = 192.22(3), was based on two mass­ more recent values are concordant with Ar(O) = 15.9994] spectrometric determinations by Sampson and Bleakney27 and 28 include: 1894, 48; 1896, 48.15; 1903, 48.1; 1927, 47.90; Baldock. A more recent ArCIr) value reported by Creaser 1969,47.90(3); and 1979,47.88(3). et al.29 was not considered by the Commission due to the fact It should he noted that the two calihrateCl measnrements hy that only a single measurement was made. Recent work by 30 31 Shima and Torigoye5 and Belsheim13 show excellent agree­ Walczyk and Heumann and Chang and Xia0 were evalu­ ment. Titanium is an abundant, widely distributed element, ated by the Commission. The work of Walczyk and Heumann yet the previous value of Ar(Ti) carried the relative uncertainty was chosen by the Commission as the best measurement due of 6.3 XI 0-4. This uncertainty was the result of instrumental to its better precision; however, both measurements are in difficulties with titanium and not because of natural iso­ agreement with their stated uncertainties. The new value rep­ topic variability. Hoggll and Belsheim13 searched for, but resents a significant improvement in the precision of the found no measurable variability in the isotopic composition atomic weight and is in agreement with the previous measure­ of terrestrial titanium. Furthermore, recent investigations ments. Historical values of Ar(Ir) [values before 1961 are have not detected any variations in isotopic abundance in based on Ar(O) = 16 exactly; more recent values are concor­ dant with Ar(O) = 15.9994] include: 1894, 193.1; 1896, '193.12; 1900,193.1; 1903, 193.0; 1909, 193.1; 1953,192.2; 2.2. Iron 1969, 192.22(3).

The Commission has changed the recommended value for 2.5. lithium the standard atomic weight of iron to ArCFe) = 55.845(2) based on recent calibrated positive thermal ionization mass-spectro­ The Commission has noted with concern the commercial metric measurements carried out on a metallic iron sample of dissemination of significant quantities of laboratory reagents ATOMIC WE~GHTS OF THE ELEMENTS 199~ 563

that have been artificially depleted in 6Li, resulting in labels J\udi and Wapstra.3 The half-lives listed are those provided by 1m containers of reagents with incorrect atomic-weight val­ Holden.33,34,35.36 lICS, which actually may range from 6.94 to 6.99. To make t'hemists aware of this problem, the Commission has enclosed 1993 Table of Standard Atomic Weights 11 r(Li) in brackets in Tables 1 and 2. To emphasize that 6Li Abridged to Five Significant Figures depleted materials arc commonly available (especially in the U.S. and Europe), the listed value is therefore marked with a 5.1. Introduction dagger, and the footnote for the dagger reads: Commercially available Li materials have atomic The detail and the number of significant figures in the weights that range between 6.94 and 6.99; if a IUPAC Table of Standard Atomic Weights, as found in the more accurate value is required, it must be deter­ biennial reports of the Commission on Atomic Weights and mined for the specific material. Isotopic Abundances, published in Pure and Applied Chem­ An article discussing the variation in Ar(Li) win be pre­ istry, exceed the needs and interests of most users, who are pared during 1994. more concerned with 'the length of time during which a given table has validity to the precision limit of their interests. 3. The Table of Standard Atomic Vtleights Accordingly, the Commission on Atomic Weights and Iso­ 1993 . topic Abundances in 1987 decided to prepare for publication a revised and updated version of the 1981 Table of Atomic Following past practice. the Table of Standard Atomic Weights Abridged to Five Significant Figures. or fewer where Weights 1993 is presented both in alphabetical order by uncertainties do not warrant even five-figure accuracy (this names in English of the elements (Table 1) and in the order currently applies to eight elements). When an atomic weight of atomic numbers (Table 2). is known to more than five significant figures, it is abridged The Commission wishes to emphasize the need tor new in this Table to the five-figure value closest to the unabridged precise calibrated isotopic composition measurements in or­ best value. When the sixth digit of the unabridged value is 5 der to improve the accuracy of the atomic weights of a num­ exactly, it is rounded up or down to make the fifth digit in this ber of elements which are still not known to a satisfactoPj abridged Table even. The single-digit uncertainty in the tabu­ level of accuracy. lated atomic weight is held to be symmetric - that is, it is The names and symbols for those elements with atomic applicable with either a positive .or a negative sign. numbers 104 to 109 referred to in the following tables are The abridged Table (Table 4) is here given with the reason­ systematic and based on the atomic numbers of the e1ements able hope that not even one of the quoted values will need to recommended for temporary use by the HJPAC Commission be changed because of every biennial revision of the of the Nomenclature of .32 The names are unabridged Table, although the quoted uncertainties may composed ot the following representing digits of the have altered. Moreover, any change in an abridged value will : probably be by only one unit in the last significant figure or by adding a fifth significant figure where only four can be 1 un~ 2 bi, 3 tri, 4 quad, 5 pent, given now. Such constancy in these values is desirable for 6 hex, 7 sept, 8 oct, 9 enn, o nil. textbooks and numerical tables derived from atomic-weight data. However, it should be remembered that the best atomic­ The ending "ium" is then added to these three roots. The weight values of 27 elements are sti]] uncertain by more than three-letter symbols are derived from the first letter of the one unit in the fifth significant figure. The annotated warnings corresponding roots. of anomalous geological occurrences, isotopically altered ma­ terials, and variability of radioactive elements are relevant even in the abridged Table. The footnote concerning lithium 4. Relative Atomic i\~asses HaU .. Uves is particularly important. Selected RadlonucUdes

The Commjssion on Atomic Weights and Isotopic Abun­ Re~ative Abundance Data dances as in previous years publishes a table of relative rogen~ Carbon~ and atomic masses and haif-lives of selected radionuc1ides for Oxygen ;s©t()Jpes' elements without a stable (see Table 3). Since the Commission has no prime responsibility for the dissemination It has come to the attention of the Commission that relative of such values, it has not attempted either to record the isotopic ratio data for carbon~ and oxygen are com- precision possible or to make its tabulation comprehensive. expressed by different authors on non-corresponding There is no general agreement on which of~hc isotopes of lhe scales, To ehrninate possible confusion in the reporting of radioactive elements is, or is likely to be judged, such isotopic abundances, the Commission recommends that: tant." Various criteria such as "longest half-lire", "produc 2H/IH relative ratios of an substances be expressed tion in quantity", "used commerciallyn, etc., have IK'Cll ;\p­ relative to VSMOW (Vienna Standard IVlean Ocean ) plied in the Commission's choice. The relative aiolni,,' 11):1.'),')(':--; 1)11