Standard atomic weights (IUPAC 1997)

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Definition

"An 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 ¹²C" in its nuclear and electronic ground state. (Refs. 1 and 2)

A sample of any element consists of one or more isotopes of that element. Each isotope is a different weight. The relative amounts of each isotope for any element represents the isotope distribution for that element. The atomic weight is the average of the isotope weights weighted for the isotope distribution and expressed on the ¹²C scale as mentioned above.

For a discussion of the use of "weight" rather than "mass" (preferred by physicists), see the discussion in ref. 1.

The "1999 values" were published in 2001 (ref. 12) and are entitled "Atomic Weights of the Elements 1999". These are available online (Refs. 13 and 14). Minor modifications (The "2001 values") to these are available online (Refs. 15 and 14).

Units

atomic mass units

Notes

The table below of atomic weights is adapted from the 1997 report of the IUPAC Atomic Weights and Isotopic Abundances Commission entitled "Atomic Weights of the Elements 1997" (ref 3). 1995 values are in ref. 4, 1993 values are in ref. 5, and 1991 values are in ref. 6. Values for elements 110-112 are in references 7, 8, and 9. Values for elements 114, 116, and 118 are taken from reference 910. For a discussion of atomic weight values from 1882-1997, see ref. 11. This table will be updated for the latest 1999 values shortly.

The standard atomic weights apply to the elements as they exist naturally on Earth, and the uncertainties take into account the isotopic variation found in most laboratory samples. Further comments on the variability are given in the footnotes.

The number in parentheses following the atomic weight value gives the uncertainty in the last digit. An entry in [square brackets] indicates the mass number of the longest-lived isotope of an element that has no stable isotopes and for which a standard atomic weight cannot be defined because of wide variability in isotopic composition (or complete absence) in nature.

The values given for elements without a stable nuclide are those for the longest lived isotope quoted in Reference 2.

Commercially available samples of lithium have atomic weights between about 6.96 and 6.99. More accurate values would require a determination for the specific sample.

Values of standard atomic weights are reviewed biennially by IUPAC.

Footnotes

• [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.
• [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 atomic weight of the element from that given the table can occur.
• [r] range in isotopic composition of normal terrestrial material prevents a more precise atomic weight being given; the tabulated atomic weight value should be applicable to any normal material.

Literature sources

1. P. de Bievre and H.S. Peiser for IUPAC in Pure & Appl. Chem., 1992, 64, 1535.
2. N.N. Greenwood and A. Earnshaw in Chemistry of the Elements, 2nd edition, Butterworth, UK, 1997.
3. Pure & Appl. Chem., 1999, 71, 1594-1607.
4. Pure & Appl. Chem., 1996, 68, 2339-2359.
5. Pure & Appl. Chem., 1994, 66, 2423.
6. Pure & Appl. Chem., 1992, 64, 1519.
7. Element 110
8. Element 111
9. Element 112
10. Element 114, 116, 118
11. T.B. Coplen and H.S. Peiser for IUPAC in Pure & Appl. Chem., 1998, 70, 237.
12. T.B. Coplen for IUPAC, "Atomic Weights of the Elements 1999", in Pure & Appl. Chem., 2001, 73, 667-683.
13. http://www.physics.curtin.edu.au/iupac/docs.html
14. http://www.chem.qmw.ac.uk/iupac/AtWt/.
15. Summary of Minutes of Commission Meeting at IUPAC General Assembly, Brisbane, Australia, 30 June-2 July 2001.