Standard atomic weights of the elements 2021 (IUPAC Technical Report)
Authors: Prohaska Thomas , Irrgeher Johanna , Benefield Jacqueline , Böhlke John K. , Chesson Lesley A. , Coplen Tyler B. , Ding Tiping , Dunn Philip J. H. , Gröning Manfred , Holden Norman E. , Meijer Harro A. J. , Moossen Heiko , Possolo Antonio , Takahashi Yoshio , Vogl Jochen , Walczyk Thomas , Wang Jun , Wieser Michael E. , Yoneda Shigekazu , Zhu Xiang-Kun , Meija Juris 
Published: May 2022

Following the reviews of atomic-weight determinations and other cognate data in 2015, 2017, 2019 and 2021, the IUPAC (International Union of Pure and Applied Chemistry) Commission on Isotopic Abundances and Atomic Weights (CIAAW) reports changes of standard atomic weights. The symbol A
r°(E) was selected for standard atomic weight of an element to distinguish it from the atomic weight of an element E in a specific substance P, designated A
r(E, P). The CIAAW has changed the values of the standard atomic weights of five elements based on recent determinations of terrestrial isotopic abundances:

Ar (argon):
from 39.948 ± 0.001
to
[39.792, 39.963]

Hf (hafnium):
from 178.49 ± 0.02
to
178.486 ± 0.006

Ir (iridium):
from 192.217 ± 0.003
to
192.217 ± 0.002

Pb (lead):
from 207.2 ± 0.1
to
[206.14, 207.94]

Yb (ytterbium):
from 173.054 ± 0.005
to
173.045 ± 0.010

The standard atomic weight of argon and lead have changed to an interval to reflect that the natural variation in isotopic composition exceeds the measurement uncertainty of A
r(Ar) and A
r(Pb) in a specific substance. The standard atomic weights and/or the uncertainties of fourteen elements have been changed based on the Atomic Mass Evaluations 2016 and 2020 accomplished under the auspices of the International Union of Pure and Applied Physics (IUPAP). A
r° of Ho, Tb, Tm and Y were changed in 2017 and again updated in 2021:

Al (aluminium), 2017:
from 26.981 5385 ± 0.000 0007
to
26.981 5384 ± 0.000 0003

Au (gold), 2017:
from 196.966 569 ± 0.000 005
to
196.966 570 ± 0.000 004

Co (cobalt), 2017:
from 58.933 194 ± 0.000 004
to
58.933 194 ± 0.000 003

F (fluorine), 2021:
from 18.998 403 163 ± 0.000 000 006
to
18.998 403 162 ± 0.000 000 005

(Ho (holmium), 2017:
from 164.930 33 ± 0.000 02
to
164.930 328 ± 0.000 007)

Ho (holmium), 2021:
from 164.930 328 ± 0.000 007
to
164.930 329 ± 0.000 005

Mn (manganese), 2017:
from 54.938 044 ± 0.000 003
to
54.938 043 ± 0.000 002

Nb (niobium), 2017:
from 92.906 37 ± 0.000 02
to
92.906 37 ± 0.000 01

Pa (protactinium), 2017:
from 231.035 88 ± 0.000 02
to
231.035 88 ± 0.000 01

Pr (praseodymium), 2017:
from 140.907 66 ± 0.000 02
to
140.907 66 ± 0.000 01

Rh (rhodium), 2017:
from 102.905 50 ± 0.000 02
to
102.905 49 ± 0.000 02

Sc (scandium), 2021:
from 44.955 908 ± 0.000 005
to
44.955 907 ± 0.000 004

(Tb (terbium), 2017:
from 158.925 35 ± 0.000 02
to
158.925 354 ± 0.000 008)

Tb (terbium), 2021:
from 158.925 354 ± 0.000 008
to
158.925 354 ± 0.000 007

(Tm (thulium), 2017:
from 168.934 22 ± 0.000 02
to
168.934 218 ± 0.000 006)

Tm (thulium), 2021:
from 168.934 218 ± 0.000 006
to
168.934 219 ± 0.000 005

(Y (yttrium), 2017:
from 88.905 84 ± 0.000 02
to
88.905 84 ± 0.000 01)

Y (yttrium), 2021:
from 88.905 84 ± 0.000 01
to
88.905 838 ± 0.000 002