Atomic weights of the elements 2009 (IUPAC Technical Report)
in changes for the standard atomic weights of 11 elements. Many atomic weights are
not constants of nature, but depend upon the physical, chemical, and nuclear history of the
material. The standard atomic weights of 10 elements having two or more stable isotopes
have been changed to reflect this variability of atomic-weight values in natural terrestrial
materials. To emphasize the fact that these standard atomic weights are not constants of
nature, each atomic-weight value is expressed as an interval. The interval is used together
with the symbol [a; b] to denote the set of atomic-weight values, Ar(E), of element E in normal
materials for which a ≤ Ar(E) ≤ b. The symbols a and b denote the bounds of the interval
[a; b]. The revised atomic weight of hydrogen, Ar(H), is [1.007 84; 1.008 11] from
1.007 94(7); lithium, Ar(Li), is [6.938; 6.997] from 6.941(2); boron, Ar(B), is [10.806;
10.821] from 10.811(7); carbon, Ar(C), is [12.0096; 12.0116] from 12.0107(8); nitrogen,
Ar(N), is [14.006 43; 14.007 28] from 14.0067(2); oxygen, Ar(O), is [15.999 03; 15.999 77]
from 15.9994(3); silicon, Ar(Si), is [28.084; 28.086] from 28.0855(3); sulfur, Ar(S), is
[32.059; 32.076] from 32.065(2); chlorine, Ar(Cl), is [35.446; 35.457] from 35.453(2); and
thallium, Ar(Tl), is [204.382; 204.385] from 204.3833(2). This fundamental change in the
presentation of the atomic weights represents an important advance in our knowledge of the
natural world and underscores the significance and contributions of chemistry to the wellbeing
of humankind in the International Year of Chemistry 2011. The standard atomic weight
of germanium, Ar(Ge), was also changed to 72.63(1) from 72.64(1).
Keywords: atomic-weight interval; atomic-weight range; conventional atomic-weight values;
boron; carbon; chlorine; germanium; half-lives; hydrogen; IUPAC Inorganic Chemistry
Division; lithium; nitrogen; oxygen; silicon; sulfur; thallium.
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