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The atomic weight Ar(E) of element E is defined by the International Union of Pure and Applied Chemistry (IUPAC) as the relative molar mass 12M(E)/M(12C), where M(E) is the molar mass of the element and M(12C) is the molar mass of pure carbon-12 (IUPAC, 1994). The molar mass is defined as the mass per unit amount of substance of the specified entity; thus M(E) = Ar(E) × 10−3 kgmol−1.

Whereas the atomic weights of the nuclides are invariant, those of the elements depend, through variations in the isotopic composition, on the origin and treatment of the material. Values of Ar(E) recommended by IUPAC are given in Table 1 for the first 103 elements in order of increasing atomic number Z. The values given generally relate to the elements as they exist naturally on earth and the uncertainty in the last digit is then given as the first note; other notes elaborate the kinds of variation to be expected for each element. In the case of unstable elements, the atomic weight of the longest-lived known isotope is given.

Table 1. Atomic weights Ar(E) of the elements E [IUPAC (1994)]

g     Geologically exceptional specimens are known in which the element has an isotopic composition outside the limits for normal material. The difference between values of M in such specimens and that given might considerably exceed stated uncertainty.
m    Modified isotopic compositions can be found in commercially available material because it has been submitted to an undisclosed or inadvertent isotopic separation. Substantial deviation from the given Ar can occur.
r     Range in isotopic composition of normal terrestrial material prevents a more precise value of Ar being given; the tabulated value of Ar should be applicable to any normal material.
A    Radioactive element, having no stable nuclide, that lacks a characteristic terrestrial isotopic composition. The atomic weight of the longest-lived nuclide is given. See IUPAC (1994) for the atomic weights of other known nuclides.
X    An element, having no stable nuclide, that exhibits a range of characteristic terrestrial compositions of long-lived radionuclide(s) such that a meaningful value of Ar can be given.


IUPAC: Pure and Applied Chemistry (1994) 66, 2433-2444.


  1. IUPAC: Pure and Applied Chemistry (1994) 66, 2433-2444. DOI: 10.1351/pac199466122423
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