Relative atomic mass

Not to be confused with atomic mass.
See also: standard atomic weight

Relative atomic mass (symbol: Ar; 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. The atomic mass constant (symbol: mu) is defined as being ⁠1/12⁠ of the mass of a carbon-12 atom.[1][2] Since both quantities in the ratio are masses, the resulting value is dimensionless. These definitions remain valid[3]: 134  even after the 2019 revision of the SI.[a][b]

For 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. 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. 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.

The more common, and more specific quantity known as standard atomic weight (Ar,standard) is an application of the relative atomic mass values obtained from many different samples. 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.[8] "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). Standard atomic weight is nevertheless the most widely published variant of relative atomic mass.

Additionally, 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.[9] Still, both terms are officially sanctioned by the IUPAC. 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.

  1. ^ International Union of Pure and Applied Chemistry (1980). "Atomic Weights of the Elements 1979" (PDF). Pure Appl. Chem. 52 (10): 2349–84. doi:10.1351/pac198052102349.
  2. ^ International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry, 2nd edition, Oxford: Blackwell Science. ISBN 0-632-03583-8. p. 41. Electronic version.
  3. ^ a b International Bureau of Weights and Measures (20 May 2019), The International System of Units (SI) (PDF) (9th ed.), ISBN 978-92-822-2272-0, archived from the original on 18 October 2021
  4. ^ "2022 CODATA Value: molar mass of carbon-12". The NIST Reference on Constants, Units, and Uncertainty. NIST. May 2024. Retrieved 2024-05-18.
  5. ^ Tiesinga, Eite; Mohr, Peter J.; Newell, David B.; Taylor, Barry N. (30 June 2021). "CODATA recommended values of the fundamental physical constants: 2018". Reviews of Modern Physics. 93 (2). doi:10.1103/RevModPhys.93.025010. PMC 9890581.
  6. ^ "Standard Uncertainty and Relative Standard Uncertainty". CODATA reference. NIST. Archived from the original on 24 July 2023. Retrieved 30 August 2023.
  7. ^ a b Mohr, Peter J; Newell, David B; Taylor, Barry N; Tiesinga, Eite (1 February 2018). "Data and analysis for the CODATA 2017 special fundamental constants adjustment". Metrologia. 55 (1): 125–146. doi:10.1088/1681-7575/aa99bc.
  8. ^ Definition of element sample
  9. ^ de Bièvre, Paul; Peiser, H. Steffen (1992). "'Atomic Weight' — The Name, Its History, Definition, and Units" (PDF). Pure and Applied Chemistry. 64 (10): 1535–43. doi:10.1351/pac199264101535.


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