Relative atomic mass

Relative atomic mass (symbol: Ar; sometimes abbreviated RAM or r.a.m.), also known by the <a href="/facts/Deprecation/1aQL7rnP">deprecated</a> synonym atomic weight, is a <a href="/facts/Dimensionless_quantity/TINrpQIq">dimensionless</a> <a href="/facts/Physical_quantity/S8pWBEpK">physical quantity</a> defined as the ratio of the average <a href="/facts/Mass/HHdc8XmF">mass</a> of <a href="/facts/Atom/69syB8dH">atoms</a> of a <a href="/facts/Chemical_element/0jducgWD">chemical element</a> in a given sample to the <a href="/facts/Atomic_mass_constant/IMATU3vf">atomic mass constant</a>. The atomic mass constant (symbol: mu) is defined as being ⁠1/12⁠ of the mass of a <a href="/facts/Carbon-12/nJYECzEr">carbon-12</a> atom. Since both quantities in the ratio are masses, the resulting value is dimensionless. These definitions remain valid: 134  even after the <a href="/facts/2019_revision_of_the_SI/g0h07jCX">2019 revision of the SI</a>.
For a single given sample, the relative atomic mass of a given element is the <a href="/facts/Weighted_arithmetic_mean/VHyJfPxb">weighted arithmetic mean</a> of the masses of the individual atoms (including all its <a href="/facts/Isotope/KD9B1y6o">isotopes</a>) that are present in the sample. This quantity can vary significantly between samples because the sample's origin (and therefore its <a href="/facts/Radioactivity/Ya6FVjt4">radioactive</a> history or diffusion history) may have produced combinations of isotopic <a href="/facts/Natural_abundance/dGjaAwc9">abundances</a> in varying ratios. For example, due to a different mixture of stable carbon-12 and <a href="/facts/Carbon-13/Iho4LUtW">carbon-13</a> isotopes, a sample of elemental carbon from volcanic <a href="/facts/Methane/kBHneYnh">methane</a> will have a different relative atomic mass than one collected from plant or animal tissues.
The more common, and more specific quantity known as <a href="/facts/Standard_atomic_weight/8arqbQsq">standard atomic weight</a> (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 <a href="/facts/Isotope_geochemistry/Pq2IXawb">taken from Earth</a>. "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 <a href="/facts/Weight/9NMHCsQd">weight</a> and mass in physics. Still, both terms are officially sanctioned by the <a href="/facts/IUPAC/2AV6myCK">IUPAC</a>. 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.

Relative atomic mass open-in-new

Relative atomic mass