FAQs

1. Why don't some elements have a standard atomic weight?

More than 25 elements have no stable nuclides and do not have a characteristic terrestrial isotopic composition, including Tc, Pm, Po, At, Rn, Fr, Ra, Ac, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, and Rg.  However, three such elements (Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these elements an atomic weight is tabulated in the Commission's Table of Standard Atomic Weights.

2. Aren’t the atomic weights of the elements constants of nature?

Some of the atomic weight can be considered in the same category as constants of nature.  For example, the standard atomic weight of fluorine in the 2005 Commission report is 18.998 4032(5).  The uncertainty is 2.6 × 10^–8. Compare this to uncertainties of 5 × 10–8 for the Avogadro constant, 1.7 × 10^–6 for the Boltzmann constant, and 5 × 10^–8 for the Planck constant (See http://physics.nist.gov/cuu/Constants/index.html). The atomic weight of an element is equal to the sum of the products of the fraction of each isotope of the element and the atomic mass of the respective isotope, which typically is known with an uncertainty of 1 x 10–8.  In the case of fluorine and other monoisotopic elements, which comprise only a single isotope, the atomic weights of these elements are known with low uncertainties because the respective atomic masses are known with high accuracy.

However, the majority of chemical elements are polyisotopic and their atomic weights are limited by the uncertainty with which the fraction of each isotope of the element can be determined.  These fractions commonly are determined by mass spectrometry. A close look at the Table of Standard Atomic Weights indicates that the uncertainty in atomic weight of some elements can be relatively large.  For example, the figure below for boron is figure 3 of Isotope-abundance variations of selected elements, T.B. Coplen, J.K. Böhlke, P. De Bièvre, T. Ding, N.E. Holden, J.A. Hopple, H.R. Krouse, A. Lamberty, H.S. Peiser, K. Revesz, S.E. Rieder, K.J.R. Rosman, E. Roth, P.D.P. Taylor, R.D. Vocke, Jr., and Y.K. Xiao, Pure Appl. Chem. 74, 1987-2002 (2002).  [full text - pdf 506 KB] {SNIF.pdf}

This figure shows that not only is the atomic weight of boron not a constant of nature, but the atomic weight of boron varies by as much as 1 part in 1000 in various terrestrial materials because of the variability in fraction of the two stable isotopes, ¹⁰B and ¹¹B, in terrestrial materials.  To know the precise atomic weight of a boron sample, the isotope-amount ratio must be determined.

3. Which atomic masses are used in calculating standard atomic weights?

During the 19th century and until the early 1940s, atomic weights were determined using chemical methods, the most common of which is precise gravimetric determination using chemical stoichiometry.  Nowadays, the atomic weight of an element is determined from a knowledge of the relative amounts of isotopes of an elements and the atomic masses of individual isotopes, which typically are known to 1 × 10^-8.  The atomic weights of monoisotopic elements are known most precisely and the uncertainly is directly related to the uncertainty in the atomic mass.  For polyisotopic elements, the uncertainty in atomic weights primarily depends upon the uncertainty in the determination of the relative amounts of the isotopes, which typically is performed with a mass spectrometer.  The atomic masses used in CIAAW calculations for several decades have been generated by A. H. Wapstra, G. Audi, and their coworkers.

Atomic masses used in CIAAW calculations [download xls 29 KB  ] {Atomic Masses.xls}

Atomic masses currently used in CIAAW calculations are derived from: G. Audi, A. H. Wapstra, and C. Thibault. Nucl. Phys. A 729, 337 (2003).

4.  What is a monoisotopic element?

An element is considered to be monoisotopic by CIAAW if it has one and only one isotope that is either stable or has a half-life greater than 1 x 10¹⁰ a.  At various times, the term “mononuclidic” has been used synonymously with “monoisotopic”; similarly, radioactive isotopes have been referred to as “radionuclides” or  “radioisotopes”.  Thus, CIAAW considers Th to be monoisotopic because ²³²Th has a half-life of 1.4 x 10¹⁰ a and the half-lives of the other isotopes of Th are all less than 1 x 10¹⁰ a.  In contrast, Pa is not classified as a monoisotopic element because the half-life of the most abundant isotope ²³¹Pa is only 3.25(1) x 10⁴a.  All isotopes of elements of atomic number greater than 83 have half-lives less than 10¹⁰ a, except for ²³²Th.  The following 21 elements are considered to be monoisotopic in the evaluation of the atomic weights: Be, F, Na, Al, P, Sc, Mn, Co, As, Y, Nb, Rh, I, Cs, Pr, Tb, Ho, Tm, Au, Bi and Th.

5. How often are the standard atomic weights of the chemical elements evaluated and updated?

Over the past three decades the Table of Standard Atomic Weights has been updated biennially except for 2003.