For example, boron from Turkey has a lower relative atomic mass than boron from California, because of its different isotopic composition. The definition deliberately specifies " An atomic weight…", as an element will have different relative atomic masses depending on the source. The IUPAC definition of relative atomic mass is:Īn atomic weight (relative atomic mass) of an element 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. Here the "unified atomic mass unit" refers to 1⁄ 12 of the mass of an atom of 12C in its ground state. Relative atomic mass (atomic weight) - The ratio of the average mass of the atom to the unified atomic mass unit. The prevailing IUPAC definitions (as taken from the " Gold Book") are:Ītomic weight - See: relative atomic mass Relative atomic mass is therefore a more general term that can more broadly refer to samples taken from non-terrestrial environments or highly specific terrestrial environments which may differ substantially from Earth-average or reflect different degrees of certainty (e.g., in number of significant figures) than those reflected in standard atomic weights. However, relative atomic mass (atomic weight) is still technically distinct from standard atomic weight because of its application only to the atoms obtained from a single sample it is also not restricted to terrestrial samples, whereas standard atomic weight averages multiple samples but only from terrestrial sources. Relative atomic mass is also frequently used as a synonym for standard atomic weight and these quantities may have overlapping values if the relative atomic mass used is that for an element from Earth under defined conditions. It is a synonym for atomic weight, though it is not to be confused with relative isotopic mass. This quotient also explains the word relative: the sample mass value is considered relative to that of carbon-12. This comparison is the quotient of the two weights, which makes the value dimensionless (having no unit). Relative atomic mass is determined by the average atomic mass, or the weighted mean of the atomic masses of all the atoms of a particular chemical element found in a particular sample, which is then compared to the atomic mass of carbon-12. The term "relative atomic mass" now seems to be replacing "atomic weight" as the preferred term, although the term " standard atomic weight" (as opposed to the more correct " standard relative atomic mass") continues to be used. Still, both terms are officially sanctioned by the IUPAC. Standard atomic weight is nevertheless the most widely published variant of relative atomic mass.Īdditionally, the continued use of the term "atomic weight" (for any element) as opposed to "relative atomic mass" has attracted considerable controversy since at least the 1960s, mainly due to the technical difference between weight and mass in physics. "Atomic weight" is often loosely and incorrectly used as a synonym for standard atomic weight (incorrectly because standard atomic weights are not from a single sample). It is sometimes interpreted as the expected range of the relative atomic mass values for the atoms of a given element from all terrestrial sources, with the various sources being taken from Earth. The more common, and more specific quantity known as standard atomic weight ( A r,standard) is an application of the relative atomic mass values obtained from many different samples. ![]() For example, due to a different mixture of stable carbon-12 and carbon-13 isotopes, a sample of elemental carbon from volcanic methane will have a different relative atomic mass than one collected from plant or animal tissues. This quantity can vary significantly between samples because the sample's origin (and therefore its radioactive history or diffusion history) may have produced combinations of isotopic abundances in varying ratios. įor a single given sample, the relative atomic mass of a given element is the weighted arithmetic mean of the masses of the individual atoms (including all its isotopes) that are present in the sample. These definitions remain valid : 134 even after the 2019 redefinition of the SI base units. ![]() Since both quantities in the ratio are masses, the resulting value is dimensionless. The atomic mass constant (symbol: m u) is defined as being 1 / 12 of the mass of a carbon-12 atom. Relative atomic mass (symbol: A r sometimes abbreviated RAM or r.a.m.), also known by the deprecated synonym atomic weight, is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a given sample to the atomic mass constant.
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