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Nestor De La Vergne
The biennial review of atomic-weight determinations and other cognate data has resulted in changes for the standard atomic weights of 19 elements. The standard atomic weights of four elements have been revised based on recent determinations of isotopic abundances in natural terrestrial materials:cadmium to 112.414(4) from 112.411(8),
The Commission on Isotopic Abundances and Atomic Weights (ciaaw.org) also revised the standard atomic weights of fifteen elements based on the 2012 Atomic Mass Evaluation:aluminium (aluminum) to 26.981 5385(7) from 26.981 5386(8),
The Commission met in Gebze, Turkey, under the chairmanship of Dr. Willi A. Brand from 7 to 8 August 2013, prior to the 47th IUPAC General Assembly in Istanbul. At this meeting, the Commission reviewed recommendations of its Subcommittee on Isotopic Abundance Measurements (SIAM), which suggested changes in the standard atomic weights of some elements based on its review of published data.
The atomic mass, ma, of an unbound neutral atom of carbon-12, ma(12C), in its nuclear and electronic ground states is 12 Da exactly, where Da is the symbol for unified atomic mass unit, and alternative symbol is u. The atomic weight (also called the relative atomic mass) of isotope iE of element E, symbol Ar(iE), in material P is
Thus, the atomic mass of 12C is 12 Da, and the atomic weight of 12C is 12 exactly. All other atomic weight values are ratios to the 12C standard value and thus are dimensionless numbers. The atomic weight of element E, Ar(E), in a material P is determined from the relation
where x(iE)P is the amount fraction of isotope iE in material P (also called the isotopic abundance). The summation is over all stable isotopes of the element plus selected radioactive isotopes (having relatively long half-lives and characteristic terrestrial isotopic compositions) of the element. The atomic weight, Ar(E), of element E in a material can be determined from knowledge of the atomic masses of the isotopes of that element and the corresponding amount fractions of the isotopes of that element in the material. In contrast to the atomic weight of an element in any given material, the standard atomic weight is a quantity that represents the atomic weights of an element in normal terrestrial materials and, therefore, must be given with larger uncertainty for some elements than the measured atomic weight in any given material. Isotopes contributing to the determination of the atomic weight of an element include (1) all stable isotopes (not known to be radioactive), of which there are 252, and (2) selected radioactive isotopes that have relatively long half-lives and characteristic terrestrial isotopic compositions, of which there are 37. A radioactive isotope of an element is said to have a characteristic terrestrial isotopic composition [13] if it contributes significantly and reproducibly to the determination of the standard atomic weight of the element in normal materials.
The Commission has determined that new, calibrated isotopic-composition measurements could improve substantially the standard atomic-weight values of a number of elements that have relatively large uncertainties. Such elements include Gd, Hf, Pd, and Sm.
All known elements can be categorized according to the following constraints on their standard atomic weights (see Section 9 for details):Elements with no stable isotope and with no radioactive isotope having a characteristic terrestrial isotopic composition in normal materials, e.g. radon. No standard atomic weight can be determined and no value is provided in the Table of Standard Atomic weights for these elements. These elements have a white background for each element cell on the IUPAC Periodic Table of the Isotopes [14].
Elements whose standard atomic weight is determined by more than one isotope are shown on the IUPAC Periodic Table of the Isotopes with a yellow background [14]. They are subdivided into three groups:Elements have no documented evidence of variation in atomic weight for normal materials, or elements that have not been evaluated for variation in isotopic composition by an IUPAC project, e.g. tungsten. Elements in this subcategory may enter category 3b as more accurate isotopic-abundance measurements are published.
Elements with two or more isotopes having known variations in atomic weights in normal materials that exceed the uncertainty of the atomic weight derived from a best measurement of isotopic abundances and having upper and lower atomic-weight bounds determined by the Commission from evaluated, peer-reviewed, published data, e.g. hydrogen (Fig. 1). These elements have a pink background for each element cell on the IUPAC Periodic Table of the Isotopes [14].
where N(iE)P and N(jE)P are the numbers of each isotope, and iE denotes the higher (superscript i) and jE the lower (superscript j) atomic mass number of chemical element E in substance P. The isotope-delta value (symbol δ), also called the relative isotope-ratio difference, is a differential measurement obtained from isotope-number ratios of substance P and a reference material Ref.
The Table of Standard Atomic Weights 2013 is given in the order of atomic number (Table 1) and it replaces the Table of Standard Atomic Weights 2011 [43]. With minor exceptions covered by footnotes, the Table of Standard Atomic Weights is intended to apply to all normal terrestrial materials as well as materials in commerce, samples found in laboratories involved in chemical investigations, and samples in technological applications. The Table of Standard Atomic Weights does not apply to extraterrestrial materials nor to materials with deliberately altered isotopic compositions.
*Element has no stable isotopes. One or more representative isotopes are given in Table 4 with the appropriate relative atomic mass and half-life. However, four such elements (Bi, Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these elements, standard atomic weights are tabulated.
To indicate that standard atomic weights of elements with two or more stable isotopes are not constants of nature, the Table of Standard Atomic Weights 2013 lists atomic-weight intervals for the standard atomic weights of 12 such elements (B, Br, C, Cl, H, Li, Mg, N, O, S, Si, and Tl). For each of these elements, a graphical plot of natural variations of isotopic abundances and atomic weights is provided in this report, and figure numbers are provided in Table 1 for the interested reader.[1]
The Commission has changed the recommended value for the standard atomic weight of selenium, Ar(Se), to 78.971(8) from 78.96(3) based on the work of Wang et al. [49] using MC-ICP-MS. This work represents the first mass spectrometric measurement of selenium accepted by the Commission for the basis of the standard atomic weight. Historical values of Ar(Se) include [44]: 1902, 79.1; 1903, 79.2; 1934, 78.96; and 1969, 78.96(3).
In normal materials, there are 19 elements whose standard atomic weight is determined by only one isotope, which is also stable (non-radioactive). Thus, the standard atomic weight for these elements is invariant. These elements are: Be, F, Na, Al, P, Sc, Mn, Co, As, Y, Nb, Rh, I, Cs, Pr, Tb, Ho, Tm, and Au. In addition, two elements, Bi and Pa, have only one isotope that contributes to the standard atomic weight, but that isotope is radioactive. The standard atomic weights of these 21 elements are derived directly from their atomic masses.
The 2012 Atomic Mass Evaluation report (AME-2012) contains many advances in the measurement science of atomic masses [15]. The most notable increase in the reported precision of the nuclide masses, which has impact on the atomic-weight values is the 280-fold reduction in the uncertainty of the atomic mass of phosphorus-31. As for the consistency between the values of AME-2003 and AME-2012, the atomic mass of only one stable nuclide, lithium-7, is inconsistent at the 6s level of precision, which is the precision used by the Commission to calculate all standard atomic weights of the elements [54]. The AME-2012 report also provides, for the first time, covariances between the atomic-mass estimates. This allows for proper uncertainty evaluation of nuclide mass ratios with denominator other than carbon-12. Revised standard atomic weights are provided for 15 elements for which there have been improvements in the measurement precision of the atomic-mass values since the previous evaluation.
The number of significant digits reported in the full Table of Standard Atomic Weights (Table 1) exceeds the needs and the interests of many users. In the past, tables abridged to four and five significant digits have been published with the expectation that subsequent changes to the abridged values will be minimal. Noting that the truncation errors introduced in the four-significant digit table are unacceptable, the Commission resolved that five-digit table is a more appropriate form of abridged representation. Standard atomic weights abridged to five significant digits are presented in Table 2. Users seeking an atomic-weight value that is not an interval, such as for trade and commerce, can refer to a conventional atomic-weight value in Section 7.
*Element has no stable isotopes. One or more representative isotopes are given in Table 4 with the appropriate relative atomic mass and half-life. However, four such elements (Bi, Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these elements, standard atomic-weight values are tabulated.
The Commission recognizes that some users of atomic-weight data only need single values with disregard to their uncertainties. Therefore, for those elements with standard atomic weights given as intervals, the Commission provides conventional atomic-weight values (Table 3). These conventional quantity values have been selected so that most or all atomic-weight variation in normal materials is covered in an interval of plus or minus one in the last digit.
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