"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).
The table below shows the IUPAC 2007 atomic weights (refs 3-5). Values for elements 110-118 are in references 6-15.
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 periodically by IUPAC.
[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.
P. de Bievre and H.S. Peiser for IUPAC in Pure & Appl. Chem., 1992, 64, 1535.