Atomic weights relative uncertainty

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 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. 

Note The number in parentheses indicates the uncertainty in the last digit of the atomic weight. Monoisotopic mass (relative atomic mass) refers here to the mass of a specific nuclide (isotope). Atomic weight from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12C. 

Davy proposed the name ioron (borax + carbon) because the element comes from the mineral borax and is similar in appearance to carbon. Borax, found in deserts and formerly volcanic areas, remains the principal source of boron compounds. One of the largest open-pit borate mines in the world is located in a California mining town appropriately called Boron. Boron is one of the few elements whose percent isotopic abundance varies significantly from one region to another. For example, the borates from California are somewhat low in °B, whereas those from Turkey are high in this isotope. In fact, there is more relative uncertainty in the atomic weight of boron than in that of any other element. 

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