Uncertainty in atomic weights

Numerical values for the atomic weights of the elements are now reviewed every 2 y by the Commission on Atomic Weights and Isotopic 

1874 Foundation of the American Chemical Society (64 elements listed). 

1903 First international table of atomic weights (78 elements listed). 

1959 Fast table to be based on oxygen = 16 (83 elements listed). 

1997 Latest available lUPAC values (84 f 28 elements listed). 

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All members of I,ho group form several oxides and u surprising amount, of uncertainty exists concerning them. They probably nil form the simple oxide M20 by direct, union of the elements, but then is seme doubt in the case of rubidium. These oxides nnet with wider forming the characteristic liases MOir, whose strength increases with iiicn iise in atomic weight. 

Descriptive properties for a basic group of approximately 1400 inorganic compounds are compiled in Section 3. These follow a concise, revised introduction to inorganic nomenclature that follows the recommendations of the lUPAC published in 1990. In this section are given the exact atomic (or formula) weight of the elements accompanied, when available, by the uncertainty in the final figure given in parentheses. 

The conclusions on the occurrence of ion-molecule reaction in the radiolysis of ethylene are not seriously affected by the uncertainties in the neutralization mechanism. It must be assumed that neutralization results in the complex species which constitute the ionic polymer, — i.e., the fraction of the ethylene disappearance which cannot be accounted for by the lower molecular weight products containing up to six carbon atoms. 

The number in parentheses following the atomic weight value gives the uncertainty in the last digit. An... 

To prepare a calibration solution of 1 pmol L 1 ofCd+2 from a CRM of pure Cd metal, a mass of Cd is weighed into a flask, dissolved, and made up to the mark. The atomic weight of cadmium is 112.411 g mol 1 with uc = 0.008 g mol 1. Thus, 1.12411 g of pure cadmium metal dissolved in hydrochloric acid and made up to 1.00 L will have an amount concentration of 1.00 x 10 2 mol L 1. An aliquot of 0.100 mL of this solution made up to 1.00 L should create the desired solution of amount concentration 1.00 pmol Lr1. Until a proper estimate of the measurement uncertainty is made the significant fig-... 

Geological specimens are known in which the element has an isotopic composition outside the limits for normal material. The difference between the atomic weight of the element in such specimens and that given in the Table may exceed the stated uncertainty. 

For amount-of-substance measurements we include kilogram mass units, which are linked to the amount-of-substance unit in SI by the atomic-weight values. The latter differ greatly in uncertainty for different chemical entities, but are always available, with the best estimates by current knowledge of their uncertainties,... 

The relative molar masses of the gaseous components and the associated uncertainty in the relative molar masses are calculated from tables of atomic weights. The relative molar mass and uncertainties are combined with the gas purity and weighing uncertainty using the Gravcalc software. 

The atomic weights of many elements are not invariant but depend on the origin and treatment of the material.The standard values of Ar(E) and the uncertainties (in parentheses following the last significant figure to which they are attributed) apply to elements of natural terrestrial origin.The footnotes to this table elaborate the types of variation that may occur for individual elements and that may be larger than the listed uncertainties of values of Ar(E). Names of elements with atomic numbers 110 to 116 are provisional. 

The variations in isotopic composition of many elements in samples of different origin limit the precision to which a relative atomic mass can be given. The standard atomic weights revised biennially by the IUPAC Commission on Atomic Weights and Isotopic Abundances are meant to be applicable for normal materials. This means that to a high level of confidence the relative atomic mass of an element in any normal sample will be within the uncertainty limits of the tabulated value. By normal it is meant here that the material is a reasonably possible source of the element or its compounds in commerce for industry and science and that it has not been subject to significant modification of isotopic composition within a geologically brief period [43]. This, of course, excludes materials studied themselves for very anomalous isotopic composition. 

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