Pure Appl. Chem., Vol. 68, No. 12, pp. 2339-2359, 1996.
AND APPLIED CHEMISTRY
INORGANIC CHEMISTRY DIVISION
COMMISSION ON ATOMIC WEIGHTS AND ISOTOPIC ABUNDANCES*
ATOMIC WEIGHTS OF THE ELEMENTS
1995
Prepared for publication by
T. B. COPLEN
*Membership of the Commission for the period 1994-1995
was as follows:
K. G. Heumann (FRG, Chairman); T. B. Coplen (USA, Secretary); J.K. Böhlke (USA, Associate); H. J. Dietze (FRG, Associate); M. Ebihara (Japan, Associate); J. W. Gramlich (USA, Titular); H. R. Krouse (Canada, Titular); R. D. Loss (Australia, Associate); G. I. Ramendik (Russia, Titular); D. E. Richardson (USA, Associate); K. J. R. Rosman (Australia, Titular); L. Schultz (FRG, Titular); P. D. P. Taylor (Belgium, Associate); L. Turpin (France, Associate); R. D. Vocke, Jr. (USA, Associate); P. De Bièvre (Belgium, National Representative); Y. Xiao (China, National Representative); M. Shima (Japan, National Representative); A. Pires de Matos (Portugal, National Representative); N. N. Greenwood (UK, National Representative).
Abstract - The biennial review of atomic weight, Ar(E), determinations and other cognate data has resulted in changes for the standard atomic weight of the following elements:
| from | to | |
| boron | 10.811±0.005 | 10.811±0.007 |
| carbon | 12.011±0.001 | 12.0107±0.0008 |
| fluorine | 18.9984032±0.0000009 | 18.9984032±0.0000005 |
| sodium | 22.989768±0.0000006 | 22.989770±0.000002 |
| aluminium | 26.981539±0.000005 | 26.981538±0.000002 |
| phosphorus | 30.973762±0.000004 | 30.973761±0.000002 |
| scandium | 44.955910±0.000009 | 44.955910±0.000008 |
| manganese | 54.93805±0.00001 | 54.938049±0.000009 |
| cobalt | 58.93320±0.00001 | 58.933200±0.000009 |
| arsenic | 74.92159±0.00002 | 74.92160±0.00002 |
| rhodium | 102.90550±0.00003 | 102.90550±0.00002 |
| caesium | 132.90543±0.00005 | 132.90545±0.00002 |
| cerium | 140.115±0.004 | 140.116±0._001 |
| praseodymium | 140.90765±0.00003 | 140.90765±0.00002 |
| europium | 151.965±0.009 | 151.964±0.001 |
| terbium | 158.92534±0.00003 | 158.92534±0.00002 |
| holmium | 164.93032±0.00003 | 164.93032±0.00002 |
| thulium | 168.93421±0.00003 | 168.93421±0.00002 |
| platinum | 195.08±0.03 | 195.078±0.002 |
| gold | 196.96654±0.00003 | 196.96655±0.00002 |
| bismuth | 208.98037±0.00003 | 208.98038±0.00002 |
An annotation for potassium has been changed in the Table of
Standard Atomic Weights. To eliminate possible confusion in the
reporting of relative lithium isotope-ratio data, the Commission
recommends that such data be expressed using 7Li/6Li
ratios and that reporting using 6Li/7Li
ratios be discontinued. Because relative isotope-ratio data for
sulfur are commonly being expressed on non-corresponding scales,
the Commission recommends that such isotopic data be expressed
relative to VCDT (Vienna Cañon Diablo Troilite) on a scale
such that 34S/32S of IAEAS1
silver sulfide is 0.9997 times that of VCDT. Many elements have
a different isotopic composition in some nonterrestrial
materials. Some recent data on of oxygen are included in this
report for the information of the interested scientific community.
The Commission on Atomic Weights and Isotopic Abundances met under
the chairmanship of Professor K. G. Heumann from 4th-6th
August 1995, during the 38th IUPAC General Assembly
in Guildford, United Kingdom. The Commission decided to publish
the report "Atomic Weights of the Elements 1995" as
presented here and the report "Isotopic Compositions of the
Elements 1995" (ref. ).
The Commission has reviewed the literature over the previous two
years since the last report on atomic weights (ref.
) and evaluated the published data on atomic weights and isotopic
compositions on an element-by-element basis. The atomic weight,
Ar(E), of element E can be determined from a
knowledge of the isotopic abundances and corresponding atomic
masses of the nuclides of that element. The last compilations
of the isotopic abundances and atomic masses with all relevant
data were published in 1991 (ref. ) and
1993 (ref. ), respectively. The Commission
periodically reviews the history of the atomic weight of each
element emphasizing the relevant published scientific evidence
on which decisions have been made (ref.
).
For all elements for which a change in the Ar(E)
value or its uncertainty, U[Ar(E)] (in
parentheses, following the last significant figure to which it
is attributed), is recommended, the Commission by custom makes
a statement on the reason for the change and in this and recent
reports also includes a list of past recommended values over a
period in excess of the last 100 years. It should be understood
that before 1961 the values quoted were intended to be consistent
with Ar(O) = 16, whereas since that time the
values are consistent with Ar(O) = 15.9994.
An attempt is not generally made to follow the data to a time
before 1882. Between 1882 and 1903 G. P. Baxter's data are quoted
usually from his report to the International Critical Tables
(ICT). Between 1903 and 1923, Baxter's ICT values show a few
small differences from those recommended by the International
Committee on Atomic Weights. It is the latter values that are
here quoted. Uncertainties were given systematically only from
1969, from which time they should be considered to be expanded
uncertainties, but the factor determining the excess over single
standard uncertainties has deliberately not been stated.
Boron
The Commission has changed the recommended value for the standard
atomic weight of boron to Ar(B) = 10.811(7)
based on an evaluation of the variation in isotopic abundance
of naturally occurring boron-bearing substances. The Commission
decided that the uncertainty of Ar(B) should
be increased to include boron in sea water (ref.
). However, footnote "g" remains in Tables 1 and
2 to warn users that highly unusual naturally occurring boron-bearing
substances can be found with an atomic weight that falls outside
the implied range. The previous value, Ar(B) = 10.811(5),
was adopted by the Commission in 1983 (ref. ) and was based on
calibrated mass spectrometric measurements by Finley et al.
(ref. ), De Bièvre and DeBus (ref. ), and Catanzaro et
al. (ref. ), and on a mineral survey by Agyei et al.
(ref. ). Historical values of Ar(B) include:
1882, 10.97; 1894, 11; 1896, 10.95; 1900, 11.0; 1919, 10.9; 1925,
10.82; 1961, 10.811(3); 1969, 10.81(1); and 1983, 10.811(5).
Carbon
The Commission has changed the recommended value for the standard
atomic weight of carbon to Ar(C) = 12.0107(8)
based on an evaluation of the variation in isotopic abundance
of naturally occurring carbon-bearing substances. The Commission
decided that it could reduce the uncertainty in Ar(C)
and changed the value of Ar(C) from 12.011(1)
to 12.0107(8). The footnote "g" remains to warn users
of the existence of highly unusual naturally occurring carbon-bearing
substances with an atomic weight that falls outside the implied
range. The previous value, Ar(C) = 12.011(1),
was adopted by the Commission in 1969 (ref. ) with the intent
that this value covered all known terrestrial sources of carbon.
The Commission recognized the calibrated mass spectrometric measurement
by Chang et al. (ref. ). Historical values of Ar(C) include:
1885, 12.00; 1894, 12; 1896, 12.01; 1898, 12.00; 1916, 12.005;
1925, 12.000; 1938, 12.010; 1953, 12.011; 1961, 12.01115(5); and
1969, 12.011(1).
Fluorine
The Commission has changed the recommended value for the standard
atomic weight of fluorine to Ar(F) = 18.9984032(5),
based on new atomic mass data (ref. 4). The previous value, Ar(F)
= 18.9984032(9), was based on the atomic mass determination of
Wapstra and Audi (ref. ). Historical values
of Ar(F) include: 1882, 19.03; 1894, 19;
1896, 19.03; 1897, 19.06; 1900, 19.05; 1903, 19.0; 1925, 19.00;
1961, 18.9984; 1969, 18.9984(1); 1971, 18.99840(1); 1975, 18.998403(1);
and 1985, 18.9984032(9).
Sodium
The Commission has changed the recommended value for the standard
atomic weight of sodium to Ar(Na) = 22.989770(2),
based on new atomic mass data (ref. 4). The previous value, Ar(Na)
= 22.989768(6), was based on the atomic mass determination of
Wapstra and Audi (ref. 14). Historical values of Ar(Na) include:
1882, 23.05; 1909, 23.00; 1925, 22.997; 1953, 22.991; 1961, 22.9898;
1969, 22.9898(1), 1971, 22.98977(1); and 1985, 22.989768(6).
Aluminium
The Commission has changed the recommended value for the standard
atomic weight of aluminium to Ar(Al) = 26.981538(2),
based on new atomic mass data (ref. 4). The previous value, Ar(Al)
= 26.981539(5), was based on the atomic mass determination of
Wapstra and Audi (ref. 14). Historical values of Ar(Al) include:
1888, 27.08; 1894, 27; 1896, 27.11; 1900,27.1; 1922, 27.0; 1925,
26.97; 1951, 26.98; 1961, 26.9815; 1969, 26.9815(1); 1971, 26.98154(1);
and 1985, 26.981539(5).
Phosphorus
The Commission has changed the recommended value for the standard
atomic weight of phosphorus to Ar(P) = 30.973761(2),
based on new atomic mass data (ref. 4). The previous value, Ar(P)
= 30.973762(4), was based on the atomic mass determination of
Wapstra and Audi (ref. 14). Historical values of Ar(P) include:
1882, 31.03; 1894, 31; 1896, 31.02; 1900, 31.0; 1911, 31.04;
1925, 31.027; 1947, 30.98; 1951, 30.975; 1961, 30.9738; 1969,
30.9738(1); 1971, 30.97376(1); and 1985, 30.973762(4).
Scandium
The Commission has changed the recommended value for the standard
atomic weight of scandium to Ar(Sc) = 44.955910(8),
based on new atomic mass data (ref. 4). The previous value, Ar(Sc)
= 44.955910(9), was based on the atomic mass determination of
Wapstra and Audi (ref. 14). Historical values of Ar(Sc) include:
1882, 44.08; 1894, 44.0; 1897, 44.12; 1900, 44.1; 1921, 45.10;
1951, 44.96; 1961, 44.956; 1969, 44.9559(1); 1983, 44.95591(1);
and 1985, 44.955910(9).
Manganese
The Commission has changed the recommended value for the standard
atomic weight of manganese to Ar(Mn) = 54.938049(9),
based on new atomic mass data (ref. 4). The previous value, Ar(Mn)
= 54.93805(1), was based on the atomic mass determination of Wapstra
and Audi (ref. 14). Historical values of Ar(Mn) include:
1882, 54.03; 1894, 55; 1896, 54.99; 1900, 55.0; 1909, 54.93;
1953, 54.94; 1961, 54.9380; 1969, 54.9380(1); and 1985, 54.93805(1).
Cobalt
The Commission has changed the recommended value for the standard
atomic weight of cobalt to Ar(Co) = 58.933200(9),
based on new atomic mass data (ref. 4). The previous value, Ar(Co)
= 58.93320(1), was based on the atomic mass determination of Wapstra
and Audi (ref. 14). Historical values of Ar(Co) include:
1882, 59.02; 1894, 59; 1895, 59.5; 1896, 58.95; 1897, 58.93;
1898, 58.99; 1900, 59.0; 1909, 58.97; 1925, 58.94; 1961, 58.9332;
1969, 58.9332(1); and 58.93320(1).
Arsenic
The Commission has changed the recommended value for the standard
atomic weight of arsenic to Ar(As) = 74.92160(2),
based on new atomic mass data (ref. 4). The previous value, Ar(As)
= 74.92159(2), was based on the atomic mass determination of Wapstra
and Audi (ref. 14). Historical values of Ar(As) include:
1882, 75.09; 1894, 75.0; 1896, 75.09; 1897, 75.01; 1900, 75.0;
1910, 74.96; 1934, 74.91; 1961, 74.9216; 1969, 74.9216(1); and
1985, 74.92159(2).
Rhodium
The Commission has changed the recommended value for the standard
atomic weight of rhodium to Ar(Rh) = 102.90550(2),
based on new atomic mass data (ref. 4). The previous value, Ar(Rh)
= 102.90550(3), was based on the atomic mass determination of
Wapstra and Audi (ref. 14). Historical values of Ar(Rh) include:
1882, 104.29; 1894, 103; 1896, 103.01; 1900, 103.0; 1909, 102.9;
1925, 102.91; 1961, 102.905; 1969, 102.9055(1); and 1985, 102.90550(3).
Caesium
The Commission has changed the recommended value for the standard
atomic weight of caesium to Ar(Cs) = 132.90545(2),
based on new atomic mass data (ref. 4). The previous value, Ar(Cs)
= 132.90543(5), was based on the atomic mass determination of
Wapstra and Audi (ref. 14). Historical values of Ar(Cs) include:
1882, 132.92; 1894, 132.9; 1896, 132.89; 1900, 132.9; 1903, 133.0;
1904, 132.9; 1909, 132.81; 1934, 132.91; 1961, 132.905; 1969,
132.9055(1); 1971, 132.9054(1); and 1985, 132.90543(5).
Cerium
The Commission has changed the recommended value for the standard
atomic weight of cerium to Ar(Ce) = 140.116(1)
based on the calibrated thermal ionization mass-spectrometric
determination by Chang et al. (ref. ). The previous value
of Ar(Ce) = 140.115(4) was adopted in 1985 following
the application of a new set of guidelines for assessing data
(ref. ). The new measurement was also accompanied
by a limited mineral survey which showed no measurable variation
in the isotopic composition of terrestrial materials. Other high
precision measurements of isotopic abundances have been made on
cerium since 1985 (e.g. refs. & ), but these were
not calibrated measurements. Historical values of Ar(Ce) include:
1882, 140.75; 1894, 140.25; 1898, 139.35; 1900, 139; 1903, 140;
1904, 140.25; 1929, 140.13; 1961, 140.12; 1969, 140.12(1); and
1985, 140.115(4).
Praseodymium
The Commission has changed the recommended value for the standard
atomic weight of praseodymium to Ar(Pr) = 140.90765(2),
based on new atomic mass data (ref. 4). The previous value, Ar(Pr)
= 140.90765(3), was based on the atomic mass determination of
Wapstra and Audi (ref. 14). Historical values of Ar(Pr) include:
1894, 143.5; 1897, 143.60; 1900, 140.5; 1909, 140.6; 1916, 140.9;
1925, 140.92; 1961, 140.907; 1969, 140.9077(1); and 1985, 140.90765(3).
Europium
The Commission has changed the recommended value for the standard atomic weight of europium to Ar(Eu) = 151.964(1), based on the calibrated thermal ionization mass-spectrometric determination by Chang et al. (ref. ). The previous value of Ar(Eu) = 151.965(9) was adopted in 1985 following the application of a new set of guidelines for assessing data (ref. 16). The new measurement was also accompanied by a survey of 12 minerals and chemicals, but no variations in isotopic abundance were detected. This new value of Ar(Eu) has a significantly improved uncertainty. Historical values of Ar(Eu) include: 1901, discovered; 1907, 152.0; 1961, 151.96; 1969, 151.96(1); and 1985, 151.965(9).
Terbium
The Commission has changed the recommended value for the standard
atomic weight of terbium to Ar(Tb) = 158.92534(2),
based on new atomic mass data (ref. 4). The previous value, Ar(Tb)
= 158.92534(3), was based on the atomic mass determination of
Wapstra and Audi (ref. 14). Historical values of Ar(Tb) include:
1894, 160; 1907, 159.2; 1953, 158.93; 1961, 158.924; 1969, 158.9254(1);
and 1985, 158.92534(3).
Holmium
The Commission has changed the recommended value for the standard
atomic weight of holmium to Ar(Ho) = 164.93032(2),
based on new atomic mass data (ref. 4). The previous value, Ar(Ho)
= 164.93032(3), was based on the atomic mass determination of
Wapstra and Audi (ref. 14). Historical values of Ar(Ho) include:
1913, 163.5; 1925, 163.4; 1941, 164.94; 1961, 164.930; 1969,
164.9303(1); 1971, 164.9304(1); and 1985, 164.93032(3).
Thulium
The Commission has changed the recommended value for the standard
atomic weight of thulium to Ar(Tm) = 168.93421(2),
based on new atomic mass data (ref. 4). The previous value, Ar(Tm)
= 168.93421(3), was based on the atomic mass determination of
Wapstra and Audi (ref. 14). Historical values of Ar(Tm) include:
1894, 170.7; 1903, 171; 1909, 168.5; 1922, 169.9; 1925, 169.4;
1953, 168.94; 1961, 168.934; 1969, 168.9342(1); and 1985, 168.93421(3).
Platinum
The Commission has changed the recommended value for the standard
atomic weight of platinum to Ar(Pt) = 195.078(2),
based on electron impact ionization of gaseous Pt(PF3)4
and measurement of Pt+ ions in a mass spectrometer
by Taylor et al. (ref. ). The previous value of Ar(Pt)
= 195.08(3), adopted in 1979, was based on a thermal ionization
mass-spectrometric determination by White et al. (ref.
) in 1955. The uncertainty on the new atomic weight is calculated
using new guidelines approved by the Commission at its meeting
in 1993. This new measurement of Ar(Pt), although
it is not a calibrated measurement, has led to a substantial improvement
in the uncertainty of Ar(Pt). Historical values
of Ar(Pt) include: 1882, 194.87; 1894,
195; 1896, 194.89; 1900, 194.9; 1903, 194.8; 1909, 195.0; 1911,
195.2; 1925, 195.23; 1955, 195.09; 1969, 195.09(3); and 1979,
195.08(3).
Gold
The Commission has changed the recommended value for the standard
atomic weight of gold to Ar(Au) = 196.96655(2),
based on new atomic mass data (ref. 4). The previous value, Ar(Au)
= 196.96654(3), was based on the atomic mass determination of
Wapstra and Audi (ref. 14). Historical values of Ar(Au) include:
1882, 196.61; 1894, 197.3; 1896, 197.24; 1897, 197.23; 1900,
197.2; 1953, 197.0; 1961, 196.967; 1969, 196.9665(1); and 1985,
196.96654(3).
Bismuth
The Commission has changed the recommended value for the standard
atomic weight of bismuth to Ar(Bi) = 208.98038(2),
based on new atomic mass data (ref. 4). The previous value, Ar(Bi)
= 208.98037(3), was based on the atomic mass determination of
Wapstra and Audi (ref. 14). Historical values of Ar(Bi) include:
1882, 208.00; 1894, 208.9; 1895, 208; 1896, 208.11; 1900, 208.1;
1903, 208.5; 1907, 208.0; 1922, 209.0; 1925, 209.00; 1961, 208.980;
1969, 208.9806(1); 1971, 208.9804(1); and 1985, 208.98037(3).
Potassium (footnote "g")
The Commission has removed the footnote "g" from potassium.
In 1991 the Commission added the footnote "g" to potassium
in Tables 1 and 2 because Hinton et al. (ref. ) reported
isotopic abundances in naturally occurring materials outside Ar(K).
The work of Humayun and Clayton (ref. ) does not support the
results of Hinton et al., but instead indicates that isotopic
fractionation in terrestrial potassium-bearing materials must
be less than 0.5‰.
Following past practice, the Table of Standard Atomic Weights
1995 is presented both in alphabetical order by names in English
of the elements (Table 1) and in the order of atomic numbers (Table
2).
The atomic weights reported in Tables 1 and 2 are for atoms in
their electronic and nuclear ground states. The unified atomic
mass unit (u) is equal to 1/12 of the rest mass of the neutral
atom of 12C in its nuclear and electronic ground state.
The Commission wishes to emphasize the need for new precise calibrated
isotopic composition measurements in order to improve the accuracy
of the atomic weights of a number of elements which are still
not known to a satisfactory level of accuracy.
The names and symbols for those elements with atomic numbers 104
to 111 referred to in the following tables are systematic and
based on the atomic numbers of the elements recommended for temporary
use by the IUPAC Commission of the Nomenclature of Inorganic Chemistry
(ref. ). The names are composed of the following roots representing
digits of the atomic number:
1 un, 2 bi, 3 tri, 4 quad, 5 pent,
6 hex, 7 sept, 8 oct, 9 enn, 0 nil.
The ending "ium" is then added to these three roots.
The three-letter symbols are derived from the first letter of
the corresponding roots.
For atomic-weight values the uncertainties are routinely called
"expanded uncertainties" with the symbol U in
italic font. The symbol U[Ar(E)] is
an acceptable alternative to U. In past reports the Commission
has referred to relative uncertainty, which is the magnitude of
the uncertainty divided by Ar(E). Therefore,
relative expanded uncertainty is indicated alternatively as U/Ar(E)
or U[Ar(E)]/Ar(E).
Figure 1 shows the changes in the relative uncertainties, U[Ar(E)]/Ar(E),
of the recommended standard atomic weights of the elements from
1969 to 1995. Arrowheads mark the 1995 relative uncertainties.
The base of each arrow marks the position of the relative uncertainty
estimate in 1969. The change factor is represented by the length
of an arrow and equals the ratio of the relative uncertainty at
the base to that at the tip of the arrow. The symbol
¡³
indicates no change since 1969, corresponding to an "improvement
factor" of 1-this applies to fourteen elements. Only one
element (Xe) has an "improvement factor" less than 1,
indicating a loss in the estimate of relative uncertainty.
Relative lithium isotope-ratio data in geochemical and environmental studies are commonly reported as d7Li values in terms of 7Li/6Li abundance ratios. It was brought to the attention of the Commission that some laboratories are reporting d6Li values by using 6Li/7Li abundance ratios. This is confusing because (i) d7Li values are opposite in sign to d6Li values, (ii) the absolute values of d7Li and d6Li are not equal,