((CIAAW2013)) adds a full reference (<ref>...</ref>) for the standard atomic weight (Ar, standard) of chemical elements as defined by IUPAC (commission: CIAAW). The source is:
Primarily, the atomic weights second here are defined by CIAAW [1]:
"Standard Atomic Weights". Commission on Isotopic Abundances and Atomic Weights (CIAAW). 2015. Retrieved 2015-09-20. (68 elements)
CIAAW also published additional lists in spreadsheet format (2013, no longer available). They had additional conventional values for trade usage, and some background information on the numbers and calculations. (12 elements: H, B, Br, C, Cl, Li, Mg, N, O, S, Si, Tl).
Coplen, Tyler B.; Brand, Willi A. (2013), Atomic weights of the elements 2013 (xls), CIAAW
Some unstable elements can naturally occur, and so are listed (4 elements: Bi, Th, Pa, U)
Additional values are those of the most stable isotopes, as given in List of chemical elements (34 elements). This source is not supported by the CIAAW publications.
CIAAW site or table 1 in PAC, has footnotes g, m, r.
)CIAAW may publish changes to atomic weights (including its precision and derived values). Since 1947, any update this is done in odd years nominally; the actual date of publication may be some time later.