NIST logo


Atomic Weights & Isotopic Compositions banner

Column Descriptions

Isotope: Atomic number, Symbol, and Mass number

Relative Atomic Mass (of the isotope): Ar(X), where X is an isotope
[formerly called atomic weight (see Standard Atomic Weight)]

These values are scaled to Ar(12C) = 12, where 12C is a neutral atom in its nuclear and electronic ground state. Thus, the relative atomic mass of entity X is given by: Ar(X) = m(X) / [m(12C) / 12] . If # is present, the value and error were derived not from purely experimental data, but at least partly from systematic trends. The 2003 Atomic Mass Evaluation does not extend beyond an atomic mass of 293.

Representative Isotopic Composition: Mole fraction of the various isotopes

In the opinion of the Subcommittee for Isotopic Abundance Measurements (SIAM), these values represent the isotopic composition of the chemicals and/or materials most commonly encountered in the laboratory. They may not, therefore, correspond to the most abundant natural material. The uncertainties listed in parenthesis cover the range of probable variations of the materials as well as experimental errors. These values are consistent with the values published in Atomic Weights of the Elements, 2001.

Standard Atomic Weight (common usage): Ar(X), where X is an element
[more appropriately called relative atomic mass of the element]

The relative atomic mass of an element is derived by averaging the relative atomic masses of the isotopes of that element. These values are scaled to Ar(12C) = 12, where 12C is a neutral atom in its nuclear and electronic ground state. These values are dependent on the origin and treatment of the material. The uncertainties are listed in parenthesis. Brackets [ ] indicate the mass number of the most stable isotope.

Notes: Notes for Representative Isotopic Composition and Standard Atomic Weight
a Air reference material used for the best measurement.
b Tank hydrogen has reported 2H mole fractions as low as 0.000032.
c Materials depleted in 6Li and 235U are commercial sources of laboratory shelf reagents. In the case of Li, such samples are known to have 6Li mole fractions in the range of 0.02007 - 0.07672, with natural materials at the higher end of this range. In the case of U, the 235U mole fractions are reported to range from 0.0021 to 0.007207, far removed from the natural value.
d The Commission on Atomic Weights and Isotopic Abundances recommends that the value of 272 be employed for 14N/15N of N2 in air for the calculation of 15N mole fractions from measured δ15N values.
e The original data for Sn were adjusted to account for possible errors due to 115In contamination, and an error in the 114Sn abundance.
f The abundance of this radiogenic isotope may vary substantially.
g Geological specimens are known in which the element has an isotopic composition outside the limits for normal material. The difference between the atomic weight of the element in such specimens and that given in the table may exceed the stated uncertainty.
h An electron multiplier was used for the Te measurements and the measured abundances were adjusted using a "square root of the masses" correction factor.
i Commercially available Li materials have atomic weights that range between 6.939 and 6.996; if a more accurate value is required, it must be determined for the specific material.
Note: The range given in Atomic Weights of the Elements 1995 was 6.94 to 6.99.
m Modified isotopic compositions may be found in commercially available material because it has been subject to an undisclosed or inadvertent isotopic fractionation. Substantial deviations in the atomic weight and isotopic composition of the element from that given in the table can occur.
r Range in isotopic composition of normal terrestrial material prevents a more precise value being given; the tabulated value should be applicable to any normal material.
w Fresh water reference material used for the best measurement.

Atomic Weights & Isotopic Compositions main page