NIST Physics Laboratory Atomic Weights and Isotopic Compositions of the Elements

[skip navigation] National Institute of Standards and Technology NIST Physics Laboratory 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 1995 Update To The Atomic Mass Evaluation does not extend beyond an atomic mass of 273.

 

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 to be published in Atomic Weights of the Elements 1997.

 

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 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.
c The range of 2H in tank hydrogen is 0.0032 - 0.0184 atom %.
d 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 abundances in the range 2.007 - 7.672 atom %, with natural materials at the higher end of this range.
e 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 atom % 15N from measured delta15N values.
f Evaluated isotopic composition is for most but not all commercial samples. Some isotopes are altered because of the contribution of a daughter product from the radioactive decay of another element. These isotopes are: 40Ca, 87Sr, 138Ce, 143Nd, 176Hf, 187Os, 206Pb, 207Pb, 208Pb, and 235U. Because the abundances must add up to 100 percent the abundances of all isotopes of an element are affected.
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 these measurements and the measured abundances were adjusted using a square root of the masses.
m Modified isotopic compositions may be found in commercially available material because it has been subjected 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 limits the precision of isotopic composition and atomic weight. The tabulated values should be applicable to any normal material.
w Fresh water reference material used for the best measurement.

Atomic Weights and Isotopic Compositions Main Page