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Section: 1 | Standard Atomic Weights |
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John R. Rumble, ed., CRC Handbook of Chemistry and Physics, 103rd Edition (Internet Version 2022), CRC Press/Taylor & Francis, Boca Raton, FL.
If a specific table is cited, use the format: "Physical Constants of Organic Compounds," in CRC Handbook of Chemistry and Physics, 103rd Edition (Internet Version 2022), John R. Rumble, ed., CRC Press/Taylor & Francis, Boca Raton, FL.

STANDARD ATOMIC WEIGHTS

This table of atomic weights includes the changes made through 2020 by the International Union of Pure and Applied Chemistry (IUPAC) Commission on Isotopic Abundances and Atomic Weights (CIAAW) (Refs. 1, 5-9).

IUPAC made a significant policy change in 2009 (Refs. 2-4). Each atomic weight had previously been given as a single value with an uncertainty that took into account both the measurement uncertainty and the variation in isotopic abundance in samples of the element from different terrestrial sources. For a variety of reasons (Ref. 3), this fails to give complete information on the natural variability in isotopic abundance of several elements. Therefore, beginning in 2009 the recommendations expressed the atomic weights of several elements as intervals rather than single numbers plus uncertainties. The symbol for these intervals is [a, b], where a is the lower bound of values found in normal materials and b the upper bound. For the other elements in the table, a single recommended atomic weight value is given; the number in parentheses following the value gives the uncertainty in the last digit. CIAAW frequently makes adjustments based on emerging measurement results on samples from various geographical locations. For example, atomic weights for hafnium (2019) and lead (2020) have recently been revised.

Table 1 gives the atomic weights of the elements listed in alphabetical order by name. Table 2 gives conventional reference atomic weights for 14 elements whose entries in Table 1 are intervals rather than single numbers. These conventional reference values are suggested for use on samples of unspecified origin and for calculation of molecular weights in tables intended to be broadly applicable. They have been selected such that most or all natural terrestrial atomic-weight variations are covered in an interval of plus or minus one in the last digit. It should be emphasized that the conventional values are not simply midpoints of the intervals, but rather represent the best judgment of the data evaluators.

References

  1. Wieser, M. E., et al., Pure Appl. Chem. 85, 1047, 2013. [https://doi.org/10.1351/PAC-REP-13-03-02]
  2. Wieser, M. E., and Coplen, T. D., Pure Appl. Chem. 83, 359, 2011. [https://doi.org/10.1351/PAC-REP-10-09-14]
  3. Coplen, T. B., and Holden, N. E., Chemistry International, 33, 2, p. 10, 2011.
  4. Berglund, M., and Wieser, M. E., Pure Appl. Chem. 83, 397, 2011. [https://doi.org/10.1351/PAC-REP-10-06-02]
  5. Chemistry International, 35, 6, p. 17, 2013, <www.ciaaw.org>.
  6. Chemistry International, 37, 5-6, p. 26, 2015.
  7. Meija, J., et al., Pure Appl. Chem. 88, 265, 2016. [https://doi.org/10.1515/pac-2015-0305]
  8. Possolo, A., van der Veen, A. M. H., Meija, J., and Hibbert, B., Interpreting and propagating the uncertainty of the standard atomic weights (IUPAC Technical Report), Pure Appl. Chem. 90, 395–424, 2018. [https://doi.org/10.1515/pac-2016-0402]
  9. See also Atomic Weights of the Elements 2019, <www.ciaaw.org>.
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TABLE 1. Standard Atomic Weights 2020



ElementSymbolCAS Reg. No.Atomic numberAtomic weight
Continued on next page...
ActiniumuAc7440-34-889
AluminumAl7429-90-51326.9815384(3)
AmericiumuAm7440-35-995
AntimonygSb7440-36-051121.760(1)
Argong,rAr7440-37-118[39.792, 39.963]
ArsenicAs7440-38-23374.921595(6)
AstatineuAt7440-68-885
BariumBa7440-39-356137.327(7)
BerkeliumuBk7440-40-697
BerylliumBe7440-41-749.0121831(5)
BismuthuBi7440-69-983208.98040(1)
BohriumuBh54037-14-8107
Boronm,*B7440-42-85[10.806, 10.821]
Bromine*Br7726-95-635[79.901, 79.907]
CadmiumgCd7440-43-948112.414(4)
CalciumgCa7440-70-22040.078(4)
CaliforniumuCf7440-71-398
Carbon*C7440-44-06[12.0096, 12.0116]
CeriumgCe7440-45-158140.116(1)

  • gGeological specimens are known in which the element has an isotopic composition outside the limits for the 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.
  • mModified isotopic compositions may be found in commercially available material because the material has been subjected to an undisclosed or inadvertent isotopic fractionation. Substantial deviations in atomic weight of the element from that given in the table can occur.
  • rRange in isotopic composition of normal terrestrial material prevents a more precise atomic weight being given; the tabulated value and uncertainty should be applicable to any normal material.
  • uElement has no stable isotopes. See “Table of the Isotopes” in Section 11 for individual isotopic masses. However, four such elements (Bi, Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these elements standard atomic weights are tabulated.
  • *See Table 2.


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