Determination Of Atomic Weight Research Articles

TSAW—a lifelong challenge or simply an unsolved mystery?

Abstract The table of standard atomic weights (TSAW) provides of the numbers included in all reprints of the periodic tables of the elements, in all textbooks in schools, high schools and universities, found on the internet, and numerous databases [1]. Whenever one needs to relate masses of an element to molar quantities, standard atomic weights come into the play. They are of fundamental importance in science, education, technology, industries, trade and commerce. It is therefore not surprising that the determination of atomic weights has a central role in chemistry. Also, it is not surprising that it needs a whole commission of experts to set-up such table with highest confidence even though at first sight it seems to be a simple compilation of numbers. That commission of experts is the Commission on Isotopic Abundances and Atomic Weights (referred to as the Commission in the text). (For more information about the Commission visit ciaaw.org).

Standard atomic weights of the elements 2021 (IUPAC Technical Report)

Abstract Following the reviews of atomic-weight determinations and other cognate data in 2015, 2017, 2019 and 2021, the IUPAC (International Union of Pure and Applied Chemistry) Commission on Isotopic Abundances and Atomic Weights (CIAAW) reports changes of standard atomic weights. The symbol A r°(E) was selected for standard atomic weight of an element to distinguish it from the atomic weight of an element E in a specific substance P, designated A r(E, P). The CIAAW has changed the values of the standard atomic weights of five elements based on recent determinations of terrestrial isotopic abundances: Ar (argon): from 39.948 ± 0.001 to [39.792, 39.963] Hf (hafnium): from 178.49 ± 0.02 to 178.486 ± 0.006 Ir (iridium): from 192.217 ± 0.003 to 192.217 ± 0.002 Pb (lead): from 207.2 ± 0.1 to [206.14, 207.94] Yb (ytterbium): from 173.054 ± 0.005 to 173.045 ± 0.010 The standard atomic weight of argon and lead have changed to an interval to reflect that the natural variation in isotopic composition exceeds the measurement uncertainty of A r(Ar) and A r(Pb) in a specific substance. The standard atomic weights and/or the uncertainties of fourteen elements have been changed based on the Atomic Mass Evaluations 2016 and 2020 accomplished under the auspices of the International Union of Pure and Applied Physics (IUPAP). A r° of Ho, Tb, Tm and Y were changed in 2017 and again updated in 2021: Al (aluminium), 2017: from 26.981 5385 ± 0.000 0007 to 26.981 5384 ± 0.000 0003 Au (gold), 2017: from 196.966 569 ± 0.000 005 to 196.966 570 ± 0.000 004 Co (cobalt), 2017: from 58.933 194 ± 0.000 004 to 58.933 194 ± 0.000 003 F (fluorine), 2021: from 18.998 403 163 ± 0.000 000 006 to 18.998 403 162 ± 0.000 000 005 (Ho (holmium), 2017: from 164.930 33 ± 0.000 02 to 164.930 328 ± 0.000 007) Ho (holmium), 2021: from 164.930 328 ± 0.000 007 to 164.930 329 ± 0.000 005 Mn (manganese), 2017: from 54.938 044 ± 0.000 003 to 54.938 043 ± 0.000 002 Nb (niobium), 2017: from 92.906 37 ± 0.000 02 to 92.906 37 ± 0.000 01 Pa (protactinium), 2017: from 231.035 88 ± 0.000 02 to 231.035 88 ± 0.000 01 Pr (praseodymium), 2017: from 140.907 66 ± 0.000 02 to 140.907 66 ± 0.000 01 Rh (rhodium), 2017: from 102.905 50 ± 0.000 02 to 102.905 49 ± 0.000 02 Sc (scandium), 2021: from 44.955 908 ± 0.000 005 to 44.955 907 ± 0.000 004 (Tb (terbium), 2017: from 158.925 35 ± 0.000 02 to 158.925 354 ± 0.000 008) Tb (terbium), 2021: from 158.925 354 ± 0.000 008 to 158.925 354 ± 0.000 007 (Tm (thulium), 2017: from 168.934 22 ± 0.000 02 to 168.934 218 ± 0.000 006) Tm (thulium), 2021: from 168.934 218 ± 0.000 006 to 168.934 219 ± 0.000 005 (Y (yttrium), 2017: from 88.905 84 ± 0.000 02 to 88.905 84 ± 0.000 01) Y (yttrium), 2021: from 88.905 84 ± 0.000 01 to 88.905 838 ± 0.000 002

Whom should we credit for the discovery of isotopes?

Whom should we credit for the discovery of isotopes? The first suggestion of an idea, the first experimental proof, or the development of a new method that clearly reveals the isotopes? Stromholm and Svedberg, Fajans and Soddy interpreted patterns of radioactive decay, which became confirmed theory on the solid basis of the very accurate atomic weight determinations by Richards and his coworkers. The mass spectrograph measurements by Aston provided major extension of the concept of isotopes to much of the rest of the periodic table. Thus, Stromholm and Svedberg, Fajans and Soddy (and others) provided an idea, which Richards proved, and Aston made generally useful. Richards should be listed along with Fajans, Soddy, and Aston for fundamental contributions to establishing isotopes as part of the modern language of science. As much credit should be given to the experimental proof as to the suggestion or naming of the concept.

Accurate Determination of the Absolute Isotopic Composition and Atomic Weight of Molybdenum by Multiple Collector Inductively Coupled Plasma Mass Spectrometry with a Fully Calibrated Strategy.

A fully calibrated strategy has been investigated for the first time for the accurate determination of absolute isotopic composition and atomic weight of molybdenum using multiple-collector inductively coupled plasma mass spectrometry. The correction for instrumental mass bias was performed using synthetic isotope mixtures, which were gravimetrically prepared with all of the seven high-purity and isotopically enriched molybdenum isotope materials together. Six natural molybdenum materials, including molybdenum standard solution NIST SRM 3134, were accurately measured and yielded the absolute isotopic composition (in atom %, k = 1) of 92Mo-14.690(18), 94Mo-9.173(6), 95Mo-15.865(5), 96Mo-16.666(3), 97Mo-9.588(4), 98Mo-24.307(16), and 100Mo-9.711(13). These isotopic data enable an atomic weight Ar(Mo) of 95.9466(34) (k = 2) to be calculated, which is slightly lower than the current standard atomic weight 95.95(1) and with a much improved uncertainty. The associated uncertainties were evaluated according to the Guide to Expression of Uncertainty in Measurement of ISO/BIPM and Monte Carlo simulation to ensure that all sources of uncertainty were fully accounted for. A particular characteristic of the proposed new approach is that mass bias correction factor K for each isotope ratio of molybdenum can be achieved via fully experimental determination without using the traditional semiempirical correction mathematical models. In addition, the relationship between mass of isotope and bias per mass unit β was investigated based on the thorough measurement data.

Atomic weights of the elements 2013 (IUPAC Technical Report)

Abstract 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), molybdenum to 95.95(1) from 95.96(2), selenium to 78.971(8) from 78.96(3), and thorium to 232.0377(4) from 232.038 06(2). 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), arsenic to 74.921 595(6) from 74.921 60(2), beryllium to 9.012 1831(5) from 9.012 182(3), caesium (cesium) to 132.905 451 96(6) from 132.905 4519(2), cobalt to 58.933 194(4) from 58.933 195(5), fluorine to 18.998 403 163(6) from 18.998 4032(5), gold to 196.966 569(5) from 196.966 569(4), holmium to 164.930 33(2) from 164.930 32(2), manganese to 54.938 044(3) from 54.938 045(5), niobium to 92.906 37(2) from 92.906 38(2), phosphorus to 30.973 761 998(5) from 30.973 762(2), praseodymium to 140.907 66(2) from 140.907 65(2), scandium to 44.955 908(5) from 44.955 912(6), thulium to 168.934 22(2) from 168.934 21(2), and yttrium to 88.905 84(2) from 88.905 85(2). The Commission also recommends the standard value for the natural terrestrial uranium isotope ratio, N(238U)/N(235U)=137.8(1).

Characterization of a series of absolute isotope reference materials for magnesium: ab initio calibration of the mass spectrometers, and determination of isotopic compositions and relative atomic weights

Three different MC-ICPMS instruments were calibrated by means of synthetic isotope mixtures to enable absolute Mg isotope ratios with expanded uncertainties of ≤0.15‰.

Atomic weights of the elements 2011 (IUPAC Technical Report)

The biennial review of atomic-weight determinations and other cognate data has resulted in changes for the standard atomic weights of five elements. The atomic weight of bromine has changed from 79.904(1) to the interval [79.901, 79.907], germanium from 72.63(1) to 72.630(8), indium from 114.818(3) to 114.818(1), magnesium from 24.3050(6) to the interval [24.304, 24.307], and mercury from 200.59(2) to 200.592(3). For bromine and magnesium, assignment of intervals for the new standard atomic weights reflects the common occurrence of variations in the atomic weights of those elements in normal terrestrial materials.

Experimental determination of the Avogadro constant

Since the time Cannizzaro expounded a system of atomic weight determination in the Karlsruhe Congress in 1860 [10], and firmly established Avogadro hypothesis, there have been dozens of experimental methods for determining the Avogadro number. Many of these methods seek to determine a fundamental physical constant such as charge of an electron or the Boltzmann constant whose value for one mole of particles is precisely known. Ingenious methods that have been devised for the measurement of these fundamental constants are discussed in this article.

Atomic weights of the elements 2009 (IUPAC Technical Report)

The biennial review of atomic-weight determinations and other cognate data has resulted in changes for the standard atomic weights of 11 elements. Many atomic weights are not constants of nature, but depend upon the physical, chemical, and nuclear history of the material. The standard atomic weights of 10 elements having two or more stable isotopes have been changed to reflect this variability of atomic-weight values in natural terrestrial materials. To emphasize the fact that these standard atomic weights are not constants of nature, each atomic-weight value is expressed as an interval. The interval is used together with the symbol [a; b] to denote the set of atomic-weight values, A r(E), of element E in normal materials for which a ≤ A r(E) ≤ b. The symbols a and b denote the bounds of the interval [a; b]. The revised atomic weight of hydrogen, A r(H), is [1.007 84; 1.008 11] from 1.007 94(7); lithium, A r(Li), is [6.938; 6.997] from 6.941(2); boron, A r(B), is [10.806; 10.821] from 10.811(7); carbon, A r(C), is [12.0096; 12.0116] from 12.0107(8); nitrogen, A r(N), is [14.006 43; 14.007 28] from 14.0067(2); oxygen, A r(O), is [15.999 03; 15.999 77] from 15.9994(3); silicon, A r(Si), is [28.084; 28.086] from 28.0855(3); sulfur, A r(S), is [32.059; 32.076] from 32.065(2); chlorine, A r(Cl), is [35.446; 35.457] from 35.453(2); and thallium, A r(Tl), is [204.382; 204.385] from 204.3833(2). This fundamental change in the presentation of the atomic weights represents an important advance in our knowledge of the natural world and underscores the significance and contributions of chemistry to the well-being of humankind in the International Year of Chemistry 2011. The standard atomic weight of germanium, A r(Ge), was also changed to 72.63(1) from 72.64(1).

Nuclear applications of inorganic mass spectrometry

There are several basic characteristics of mass spectrometry that are not always fully appreciated by the science community. These characteristics include the distinction between relative and absolute isotope abundances, and the influence of isotope fractionation on the accuracy of isotopic measurements. These characteristics can be illustrated in the field of nuclear physics with reference to the measurement of nuclear parameters, which involve the use of enriched isotopes, and to test models of s-, r-, and p-process nucleosynthesis. The power of isotope-dilution mass spectrometry (IDMS) to measure trace elements in primitive meteorites to produce accurate Solar System abundances has been essential to the development of nuclear astrophysics. The variety of mass spectrometric instrumentation used to measure the isotopic composition of elements has sometimes been accompanied by a lack of implementation of basic mass spectrometric protocols which are applicable to all instruments. These metrological protocols are especially important in atomic weight determinations, but must also be carefully observed in cases where the anomalies might be very small, such as in studies of the daughter products of extinct radionuclides to decipher events in the early history of the Solar System. There are occasions in which misleading conclusions have been drawn from isotopic data derived from mass spectrometers where such protocols have been ignored. It is important to choose the mass spectrometer instrument most appropriate to the proposed experiment. The importance of the integrative nature of mass spectrometric measurements has been demonstrated by experiments in which long, double beta decay and geochronological decay half-lives have been measured as an alternative to costly radioactive-counting experiments. This characteristic is also illustrated in the measurement of spontaneous fission yields, which have accumulated over long periods of time. Mass spectrometry is also a valuable tool in the determination of neutron capture cross-section measurements and the application of such determinations in Planetary Science.

Atomic weights of the elements 2007 (IUPAC Technical Report)

The latest evaluation of atomic weight determinations and other cognate data has warranted five changes for the standard atomic weights of the elements, A r(E), from those published previously in the Table of Atomic Weights 2005. The revised standard atomic weight of nickel, A r(Ni), is 58.6934(4); zinc, A r(Zn), is 65.38(2); molybdenum, A r(Mo), is 95.96(2); ytterbium, A r(Yb), is 173.054(5); and lutetium, A r(Lu), is 174.9668(1). Standard atomic weight tables abridged to four and five significant figures were also evaluated. The Commission-recommended value for the isotope-amount ratio of n(40Ar)/n(36Ar), which is of importance in geochronology and geochemistry, has been changed to 298.56(31) from 296.03(53) based on new measurements. Atmospheric O2 is recognized as an international measurement standard, along with Vienna Standard Mean Ocean Water (VSMOW) and Vienna Pee Dee Belemnite (VPDB) carbonate for measurement and reporting of differences in relative oxygen isotope-amount ratios δ(17O) and δ(18O).

The role of mass spectrometry in atomic weight determinations

The 1914 Nobel Prize for Chemistry was awarded to Theodore Richards, whose work provided an insight into the history of the birth and evolution of matter as embedded in the atomic weights. However, the secret to unlocking the hieroglyphics contained in the atomic weights is revealed by a study of the relative abundances of the isotopes. A consistent set of internationally accepted atomic weights has been a goal of the scientific community for over a century. Atomic weights were originally determined by chemical stoichiometry--the so-called "Harvard Method," but this methodology has now been superseded by the "physical method," in which the isotopic composition and atomic masses of the isotopes comprising an element are used to calculate the atomic weight with far greater accuracy than before. The role of mass spectrometry in atomic weight determinations was initiated by the discovery of isotopes by Thomson, and established by the pioneering work of Aston, Dempster, and Nier using sophisticated mass spectrographs. The advent of the sector field mass spectrometer in 1947, revolutionized the application of mass spectrometry for both solids and gases to other fields of science including atomic weights. Subsequently, technological advances in mass spectrometry have enabled atomic masses to be determined with an accuracy better than one part in 10(7), whilst the absolute isotopic composition of many elements has been determined to produce accurate values of their atomic weights. Conversely, those same technological developments have revealed significant variations in the isotope abundances of many elements caused by a variety of physiochemical mechanisms in natural materials. Although these variations were initially seen as an impediment to the accuracy with which atomic weights could be determined, it was quickly realized that nature had provided a new tool to investigate physiochemical and biogeochemical mechanisms in nature, which could be exploited by precise and accurate isotopic measurements. Atomic weights can no longer be regarded as constants of nature, except for the monoisotopic elements whose atomic weights are determined solely by the relative atomic mass of that nuclide. Stable isotope geochemists developed mass spectrometric protocols by the adoption of internationally accepted reference materials for the light elements, to which measurements from various laboratories could be compared. Subsequently, a number of heavy elements such as iron, molybdenum and cadmium have been shown to exhibit isotope fractionation. The magnitude of such isotope fractionation in nature is less than for the light elements, but technological developments, such as multiple collector-inductively coupled plasma-mass spectrometry, have enabled such fractionation effects to be determined. Measurements of the atomic weights of certain elements affect the determination of important fundamental constants such as the Avogadro Constant, the Faraday Constant and the Universal Gas Constant. Heroic efforts have been made to refine the accuracy of the atomic weight of silicon, with the objective of replacing the SI standard of mass--the kilogram--with the Avogadro Constant. Improvements in these fundamental constants in turn affect the set of self-consistent values of other basic constants through a least-squares adjustment methodology. Absolute isotope abundances also enable the Solar System abundances of the s-, r-, and p-process of nucleosynthesis to be accurately determined, thus placing constraints on theories of heavy element nucleosynthesis. Future developments in the science of atomic weight determinations are also examined.

Atomic Weights of the Elements 2005

The latest evaluation of atomic weight determinations and other cognate data has warranted 16 changes for the standard atomic weights of the elements, Ar(E), from those published previously in the Table of Atomic Weights 2001. The revised standard atomic weight are as follows: Ar(Al)=26.9815386(8), Ar(Bi)=208.98040(1), Ar(Cs)=132.9054519(2), Ar(Co)=58.933195(5), Ar(Au)=196.966569(4), Ar(La)=138.90547(7), Ar(Mn)=54.938045(5), Ar(Nd)=144.242(3), Ar(P)=30.973762(2), Ar(Pt)=195.084(9), Ar(Sm)=150.36(2), Ar(Sc)=44.955912(6), Ar(Na)=22.98976928(2), Ar(Ta)=180.94788(2), Ar(Tb)=158.92535(2), Ar(Th)=232.03806(2). A recommendation is made that δ13C values of all carbon-bearing materials be measured and expressed relative to Vienna-Pee Dee Belemnite on a scale normalized by assigning consensus values of −46.6‰ to L-SVEC lithium carbonate and +1.95‰ to NBS 19 calcium carbonate.

Atomic weights of the elements 2005 (IUPAC Technical Report)

The latest evaluation of atomic weight determinations and other cognate data has warranted 16 changes for the standard atomic weights of the elements, Ar(E), from those published previously in the 2001 Table of Atomic Weights. The revised standard atomic weights are as follows: Ar(Al) = 26.981 5386(8), Ar(Bi) = 208.980 40(1), Ar(Cs) = 132.905 4519(2), Ar(Co) = 58.933 195(5), Ar(Au) = 196.966 569(4), Ar(La) = 138.905 47(7), Ar(Mn) = 54.938 045(5), Ar(Nd) = 144.242(3), Ar(P) = 30.973 762(2), Ar(Pt) = 195.084(9), Ar(Sm) = 150.36(2), Ar(Sc) = 44.955 912(6), Ar(Na) = 22.989 769 28(2), Ar(Ta) = 180.947 88(2), Ar(Tb) = 158.925 35(2), Ar(Th) = 232.038 06(2). A recommendation is made that δ13C values of all carbon-bearing materials be measured and expressed relative to Vienna Pee Dee Belemnite (VPDB) on a scale normalized by assigning consensus values of -46.6 ‰ to L-SVEC lithium carbonate and +1.95 ‰ to NBS 19 calcium carbonate.

Ellen Gleditsch: Duty and responsibility in a research and teaching career, 1916-1946

ABSTRACT Ellen Gleditsch (1879-1968) became Norway's first authority of radioactivity and the country's second female professor. After several years in international centers of radiochemistry, Gleditsch returned to Norway, becoming associate professor and later full professor of chemistry. Between 1916 and 1946 Gleditsch tried to establish a laboratory of radiochemistry at the University of Oslo, a career which included network building, grant applications, travels abroad, committee work, research, teaching, supervision, popularization, and war resistance work. Establishing a new field was demanding; only under her student, Alexis Pappas, was her field institutionalized at Oslo. This paper presents Gleditsch's everyday life at the Chemistry Department, with emphasis on her formation of a research and teaching laboratory of radiochemistry. Her main scientific work during this period is presented and discussed, including atomic weight determination of chlorine, age calculations in minerals, the hunt for actinium's ancestor and investigations on 40K.

Edward Williams Morley and the Atomic Weight of Oxygen: the Death of Prout's Hypothesis Revisited

Prout's hypothesis was influential in—if not necessary for—the establishment of the atomic weight of oxygen, a figure conclusively demonstrated in 1895. Ironically, the successful determination of oxygen's weight also led to a final refutation of the hypothesis (at least the hypothesis in the classical sense). But more than this, the end of Prout's hypothesis via the determination of oxygen's atomic weight was due to three fundamental changes that characterized the way chemistry was practised and communicated in the late nineteenth century. First, encyclopaedia‐like presentations of past atomic‐weight investigations became the focus in numerous and influential studies. Second, there was a dramatic change in the way professional publications presented investigations and experiments, characterized by experimental detail and apparatus design at the expense of theoretical discussion. Finally, the production of hydrogen became the focus of research, as it was one of the principal components of any investigation into atomic weights. Here I present Prout's hypothesis in its historical context by focusing on these three developments and their influence on the research of Edward Williams Morley. While doing so I also illustrate the way marginalized scientists were able to take advantage of their otherwise dubious position and participate in an active and important role in atomic‐weight investigations.

Atomic weights of the elements. Review 2000 (IUPAC Technical Report)

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 ethical, legal, and international standards. The standard atomic weights of the elements 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 invariably 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 carbon isotope of mass number 12. Technological advances in mass spectrometry and nuclear-reaction energies have enabled atomic masses to be determined with a relative 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 precision 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 variations increasingly place a constraint on the uncertainties with which some standard atomic weights can be stated, they provide numerous opportunities for investigating a range of important phenomena in physical, chemical, cosmological, biological, and industrial processes. This review reflects the current and increasing 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 constants 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.

Catalyst design and mechanistic aspects of the alternating copolymerisation of ethene and carbon monoxide by diphosphine-modified palladium catalysisThe illustration of John Dalton (reproduced courtesy of the Library and Information Centre, Royal Society of Chemistry) marks the 200th anniversary of his investigations which led to the determination of atomic weights for hydrogen, nitrogen, carbon, oxygen, phosphorus and sulfur.

This article addresses fundamental mechanistic aspects related to the perfectly alternating copolymerisation of CO and ethene in MeOH through the analysis of both actual reactions catalysed by palladium(II)–diphosphine precursors and stoichiometric reactions with model compounds. Most recent in situ studies of CO/ethene copolymerisation, applying high-pressure IR and NMR techniques, have been reviewed and commented.

Inorganic nanotubesThe illustration of John Dalton (reproduced courtesy of the Library and Information Centre, Royal Society of Chemistry) marks the 200th anniversary of his investigations which led to the determination of atomic weights for hydrogen, nitrogen, carbon, oxygen, phosphorus and sulfur.

Inorganic nanotubesThe illustration of John Dalton (reproduced courtesy of the Library and Information Centre, Royal Society of Chemistry) marks the 200th anniversary of his investigations which led to the determination of atomic weights for hydrogen, nitrogen, carbon, oxygen, phosphorus and sulfur.

Atomic Weights of the Elements 1997

The biennial review of atomic weight determinations, Ar(E), and other cognate data has warranted no changes for the standard atomic weights of the elements from that previously published in the Table of Atomic Weights 1995 with the exception of a new footnote for lithium. The names for the transfermium elements with atomic numbers 101 to 109 have been officially adopted, and element 112 has been synthesized. Many elements in non-terrestrial matter have isotopic compositions different from those in similar terrestrial substances that may affect their atomic weight. Some recent isotopic data on xenon and oxygen in non-terrestrial materials are included in this report together with a discussion of the possible processes causing these changes.