The ratio of the average mass of the atom to the unified atomic mass unitĪlthough it is not stated explicitly in the Goldbook what is meant by "average mass", it is likely and plausible that the averaging is over different isotopes weighted by terrestrial isotopic abundance. Relative atomic mass (atomic weight), A r On the other hand, the official IUPAC publication, IUPAC Goldbook, defines: Thus A r(X) is the numerical value of m(X) when m(X) is expressed The relative atomic mass A r(X) of an elementary particle, atom, or more generally an entity X, is defined by A r(X) = m(X) / m u, where m(X) is the mass of X. This usage is followed by Mohr and Taylor, who state that (the atomic mass constant m u is a twelfth of the mass of 12C): ![]() Relative Atomic Mass (of the isotope): A r(X), where X is an isotope We followed NIST, see the NIST web site, where clearly and unambiguously the relative mass is defined of an isotope. The concept of "relative atomic mass" is in principle a simple one, yet there is some confusion about its definition. For instance, the HCl molecule has standard atomic weight 1.00794 + 35.453 = 36.461, which is the value used in almost all chemical calculations. By using averaged masses the chemist accounts for the fact that different isotopes occur in nature. (For historical reasons the term "weight" is still used here.) In most of practical chemistry the standard weight is used as "the" mass of an element. The atomic mass averaged over isotopic abundances is called the standard atomic weight. Of all the chlorine atoms occurring on earth 75.78 % is of the lighter kind, while 24.22 % is the heavier isotope. It has two stable isotopes: 35Cl (with a mass of 34.96885271 u) and 37Cl (with a mass of 36.96590260 u). In off-the-shelf chemicals the concentrations of different isotopologues are determined by the terrestrial natural abundances of the isotopes. In most of practical chemistry different isotopologues-always present in "off-the-shelf" chemicals-are of no concern whatsoever.Hydrogen chloride, for instance, would be seen as a mixture of the following isotopologues: H– 35Cl, D– 35Cl, H– 37Cl, and D– 37Cl. In these fields it is common to consider the samples as mixtures of of different isotopologues, in much the same way as when the sample consists of different compounds. That is, one can distinguish the spectral peaks arising from the different isotopologues, (same molecule, different isotopic composition) in the sample. ![]()
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