Which atomic masses are used in calculating standard atomic weights?
During the 19th century and until the early 1940s, atomic weights were determined using chemical methods, the most common of which is precise gravimetric determination using chemical stoichiometry. Nowadays, the atomic weight of an element is determined from a knowledge of the relative amounts of isotopes of an elements and the atomic masses of individual isotopes, which typically are known to 1 × 10-8. The atomic weights of monoisotopic elements are known most precisely and the uncertainly is directly related to the uncertainty in the atomic mass. For polyisotopic elements, the uncertainty in atomic weights primarily depends upon the uncertainty in the determination of the relative amounts of the isotopes, which typically is performed with a mass spectrometer. The atomic masses used in CIAAW calculations for several decades have been generated by A. H. Wapstra, G. Audi, and their coworkers. Atomic masses used in CIAAW calculations [download xls 29 KB ] {Atomic Masses.xls} Atomic masse
