Standard Atomic Weights Research Articles

Isotopic Compositions of the Elements 1997

The Commission’s Subcommittee for the Isotopic Composition of the Elements has carried out its biennial review of isotopic compositions, as determined by mass spectrometry and other relevant methods. This involves a critical evaluation of the published literature, element by element, and forms the basis of the Table of Isotopic Compositions of the Elements as Determined by Mass Spectrometry presented here. New guidelines have been used to arrive at the uncertainties on the isotopic abundances and there are numerous changes to the table since it was last published in 1991. Atomic Weights calculated from this table are consistent with Ar(E) values listed in the Table of Standard Atomic Weights 1997.

Isotopic compositions of the elements 1997 (Technical Report)

Abstract

Atomic Weights of the Elements 1995

The biennial review of atomic weight, Ar(E), determinations and other cognate data has resulted in changes for the standard atomic weight of 21 elements. The five most significant changes are: boron from 10.811±0.005 to 10.811±0.007; carbon from 12.011±0.001 to 12.0107±0.0008; arsenic from 74.92159±0.00002 to 74.92160±0.00002; cerium from 140.115±0.004 to 140.116±0.001; and platinum 195.08±0.03 to 195.078±0.002. An annotation for potassium has been changed in the Table of Standard Atomic Weights. To eliminate possible confusion in the reporting of relative lithium isotope-ratio data, the Commission recommends that such data be expressed using Li/7Li6 ratios and that reporting using Li/6Li7 ratios be discontinued. Because relative isotope-ratio data for sulfur are commonly being expressed on noncorresponding scales, the Commission recommends that such isotopic data be expressed relative to VCDT (Vienna Cañon Diablo Troilite) on a scale such that S/34S32 of IAEA-S-1 silver sulfide is 0.9997 times that of VCDT. Many elements have a different isotopic composition in some nonterrestrial materials. Some recent data on oxygen are included in this article for the information of the interested scientific community.

Atomic Weights of the Elements 1993

The biennial review of atomic weight, Ar(E), determinations and other cognate data has resulted in changes for the standard atomic weight of titanium from 47.88±0.03 to 47.867±0.001, of iron from 55.847±0.003 to 55.845±0.002, of antimony from 121.757±0.003 to 121.760±0.001, and of iridium from 192.22±0.03 to 192.217±0.003. Recent investigations on chlorine and bromine confirmed the presently accepted values of Ar(Cl) and Ar(Br). To emphasize the fact that the atomic weight of lithium 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 isotope-ratio data for stable hydrogen, carbon, and oxygen are commonly being expressed on non-corresponding scales, the Commission recommends that such isotopic 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 information of the interested scientific community.

1993 Table of standard atomic weights abridged to five significant figures

1993 Table of standard atomic weights abridged to five significant figures

Isotopic Compositions of the Elements 1989

The Subcommittee for Isotopic Abundance Measurements (SIAM) of the IUPAC Commission on Atomic Weights and Isotopic Abundances has carried out its biennial review of isotopic compositions, as determined by mass spectrometry and other relevant methods. The Subcommittee’s critical evaluation of the published literature element by element forms the basis of the Table of Isotopic Compositions of the Elements as Determined by Mass Spectrometry 1989, which is presented in this Report. Atomic Weights calculated from the tabulated isotopic abundances are consistent with Ar(E) values listed in the Table of Standard Atomic Weights 1989.

Atomic Weights of the Elements 1989

The biennial review of atomic weight, A_r(E), determinations, and other cognate data has resulted in changes for nickel from 58.69±0.01 to 58.6934±0.0002 and for antimony from 121.75±0.03 to 121.757±0.003 due to new calibrated measurements. Because the measurement of the isotopic composition of mercury has also been improved during the last two years, the Commission was able to reduce the uncertainty of the atomic weight of this element from 200.59±0.03 to 200.59±0.02. Due to the nearly constant isotopic composition of protactinium in nature, where 231Pa is the predominant isotope, the atomic weight of this element was fixed to 231.03588±0.000 02. The Table of Isotopic Compositions of the Elements 1989 will be published as a companion paper to that on Atomic Weights of the Elements 1989. The Table of Standard Atomic Weights Abridged to Five Significant Figures and current data on isotopic compositions of nonterrestrial material are included to benefit users who are more concerned with the length of time during which a given table has full validity to the precision limit of their interest. The Table of Atomic Weights to Four Significant Figures was prepared and has been published separately.

Atomic Weights of the Elements 1987

The International Union of Pure and Applied Chemistry Commission on Atomic Weights and Isotopic Abundances has reviewed recent literature and confirmed the atomic weight values published in 1985, with one minor change. The current table of standard atomic weights is presented.

Standard atomic weights of the Elements (1986)

Standard atomic weights of the Elements (1986)

HOW Good Are the Standard Atomic Weights?

Science has not stood still since 1961. Based mostly on more searching analysis and better mass spectrometry, the 1983 IUPAC Table of Atomic Weights contains a good deal of better data than the 1961 table. It is now well recognized that atomic weights are not all constants of nature. It is thus appropriate to assign an atomic weight to a given sample. Accordingly IUPAC feels free to define the standard atomic weight of an element as referring to our best knowledge of the atomic weight of an element in normal natural terrestrial sources. Its uncertainty is implied by the precision of its tabulated value and arises from experimental uncertainty and variability of isotopic composition in such sources. The IUPAC table is now called the Table of the Standard Atomic Weights, as recommended by the IUPAC Commission now called Commission on Atomic Weights and Isotopic Abundances (CAWIA). A subcommittee of CAWIA has just completed another element by element review of their atomic weights. How good are they now. In this simplified discussion the author is chiefly addressing chemical analysts, who perhaps place too much confidence in the accuracy to these values. 5 references, 4 tables.

How good are the standard atomic weights?

How good are the standard atomic weights?

Molecular Weight Comparisons from Density and X-Ray Data. The Atomic Weights of Calcium and Fluorine

Molecular weights have been compared by combination of the x-ray and density data for the crystals, calcite, diamond, lithium fluoride, sodium chloride, and potassium chloride. By assumption of the chemical atomic weights of carbon, chlorine, potassium, lithium, and sodium, values for the atomic weights of calcium and fluorine have been calculated. The method employed consists of a series of independent calculations from the results of which final weighted average values have been taken for the atomic weights of calcium and fluorine. This has yielded 40.0849±0.0030 and 18.9967±0.0013, respectively. These values are most probably the most precise values for these constants at the present time. From the results of the present work it becomes apparent that the determination of atomic weights from comparison of molecular weights by combination of density and x-ray data is as reliable as other standard atomic weight procedures.

Density of Sodium Chloride The Atomic Weight of Fluorine by Combination of Crystal Density and X-Ray Data

The densities of carefully purified crystals of sodium chloride have been determined by the method of "crystal flotation" in pure ethylene dibromide. The results yield ${d}_{27.634\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}}=2.16165\ifmmode\pm\else\textpm\fi{}0.00002\frac{\mathrm{g}}{\mathrm{ml}},$ which reduces to ${d}_{20\ifmmode^\circ\else\textdegree\fi{}}=2.16366\ifmmode\pm\else\textpm\fi{}0.00003\frac{\mathrm{g}}{\mathrm{ml}}.$It is found that six successive precipitations of NaCl in the manner employed by Richards and Wells in their determination of the atomic weights of sodium and of chlorine are required to effect purification to constant density (\ifmmode\pm\else\textpm\fi{} about ${4\ifmmode\times\else\texttimes\fi{}10}^{\ensuremath{-}6}$ g/ml), and also that exposure to air produces surface contamination sufficient to cause erratic changes in apparent density that may amount to as much as ${5\ifmmode\times\else\texttimes\fi{}10}^{\ensuremath{-}4}$ g/ml within a few minutes. Combination of our value for ${d}_{\mathrm{NaCl}}$ with that of C. A. Hutchison and H. L. Johnston for ${d}_{\mathrm{LiF}}$ and with Straumanis, Ievins, and Karlsons' value for the lattice constant of LiF relative to the Siegbahn value for NaCl yields 0.443640\ifmmode\pm\else\textpm\fi{}0.000025 for the ratio of the molecular weights of LiF and NaCl, respectively. With the adoption of 22.997 (International Atomic Weight Committee) for the atomic weight of sodium, this ratio yields 18.994\ifmmode\pm\else\textpm\fi{}0.001 for the atomic weight of F. With the adoption of 22.994 (Birge) for sodium, the F atomic weight comes out 18.992. Either of these figures is in reasonable agreement with the value 22.995\ifmmode\pm\else\textpm\fi{}0.002, based on densities and lattice constants of fluorite and calcite, and with the gas density determinations for compounds of F, but are somewhat lower than the mass spectrograph value of 18.999\ifmmode\pm\else\textpm\fi{}0.001 for ${\mathrm{F}}^{19}$. It appears that the determination of relative molecular weights by combination of x-ray and density data are as reliable, in favorable cases, as by other standard atomic weight methods. In calcite, fluorite, and rock salt crystals, used to obtain the data underlying these computations, there is no evidence of any appreciable influence of "crystal mosaic" patterns which Zwicky thought might influence crystal densities by as much as 1 percent.

The origin of primary lead ores

Lead concentrated in ore deposits is called common lead or ore-lead. Its standard atomic weight (1936) is 207.21. The lead dispersed through in minute quantities is called rock-lead. Rock-lead consists partly of common lead, which was originally present in rock-material; and partly of radiogenic lead, which has been generated in same rock-material as a result of radioactive disintegration of uranium and thorium during geological time, duration of which is now taken to be about 2,000 million years. From available determinations of lead, uranium, and thorium in various it is shown: (a) that average atomic weight of granitic rock-lead should have progressively decreased during geological time from 207.21 at beginning to 207.14 at present day; (b) that average atomic weight of basaltic rock-lead should have similarly decreased from 207.21 to 207.10. The atomic weight of rock-lead concentrated in sublimate mineral, cotunnite, from 1906 magma of Vesuvius, is 207.05, a result which confirms inference that rock-lead has an atomic weight significantly lower than that of common lead. The atomic weight of ore-lead, however, is found to be 207.21 + or - .01, and--as far back as 1,300 million years--to be independent of geological age of ore. It follows from evidence presented that ore-lead has not been derived from granitic or basaltic rocks, or from sediments formed from such rocks, and that it has no genetic connection with acid or basic magmas. Ore-lead must therefore have ascended from depths below sialic and basaltic layers of lithosphere. Gregory9s hypothesis that the source of ores appears to lie in a zone deeper than that of ordinary igneous rocks is thus largely confirmed, so far as lead ores are concerned. The data for peridotites, however, are too few as yet to justify extension of this important generalisation to ultrabasic and magmas.