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IUPAC Revises Atomic Weight Values lUPAC's new table of atomic weights based on carbon-12 ends differences between older scales

D o w C o r n i n g

The new table of atomic weights based on carbon-12, now officially released by the International Union of Pure and Applied Chemistry, has replaced the table based on the former chemist's standard, natural oxygen. IUPAC adopted carbon-12 as the standard in August (C&EN, Aug. 21, page 22) , following similar action by the Inter­national Union of Pure and Applied Physics in replacing its former scale based on oxygen-16. The new table for both physicists and chemists in­corporates changes in scale and latest information on atomic weights.

The change in uranium is a case in point. Partly because of the change in scale and partly because of a re­examination of data, the atomic weight of uranium goes from 238.07 to 238.03.

The listing for uranium also shows the method of dealing with fairly stable radioactive elements that have variable distributions of isotopes. The atomic weight given is that for the material normally handled by chemists. It is the value for the naturally occurring distribution of isotopes. Lead is an­other example of handling the problem of isotopic distribution. Lead ob­tained from minerals of radiogenic origin has variable isotopic composi­tion, but the major sources of lead are of sufficiently constant composition to justify a listing for the element.

Most radioactive elements, whether of natural or artificial origin, do not have a fixed isotopic composition. Their composition depends on the history of the specific sample. Thus no atomic weights are given for such elements in the official table. A table listing selected isotopes of the radio­active elements is to be included in the full report of the Commission on Atomic Weights. That report will be published in the Comptes Rendus of the Montreal IUPAC conference. This table will include the half lives and mode of disintegration of isotopes that either have the longest half-life or that are the best known isotopes.

Three elements whose atomic weights have been changed signifi­cantly are silver, from 107.880 to 107.870; chlorine, 35.457 to 35.453; and bromine, 79.916 to 79.909.

The atomic weights for several ele­ments which have only one isotope (or are close to being mononuclidic) are given to more significant figures than previously. Aluminum, for instance, has an atomic weight of 26.9815 in the new table instead of 26.98. Ar­senic, carbon, cobalt, fluorine, iodine, and sodium are some of the elements for which atomic weights are given to a greater number of significant figures.

The range of the atomic weights of six elements (as a result of natural variations in their isotopic composi­tion ) has now been determined. These are boron, carbon, hydrogen, oxygen, silicon, and sulfur. In older tables, only the range for sulfur was large enough to be noted. The discovery of the isotopic composition of oxygen was the original reason physicists stopped using natural oxygen and started a scale of atomic weights based on the single isotope oxygen-16. Nat­ural oxygen is made up of 01 ( i with small proportions of O17 and O18. The proportions of O17 and 0 1 S are not constant. They depend upon whether the sample comes from the atmos­phere, the ocean, or the earth's crust.

Normally, the rule for precision of an atomic weight value is that the last digit is good to ±1/i» digit. Thus for strontium, the atomic weight of 87.62 is 87.62 ± 0.005, or within the range 87.615 to 87.625. For some elements, the atomic weights are known more accurately than to ±VL> of the final digit. For chromium, the atomic weight is 51.996 ± 0.001. If this value were rounded off to the second decimal, it would appear as 52.00 with an assumed experimental uncertainty of ±0.005. Such additional data on experimental uncertainties for a few other elements are given in appropri­ate footnotes to the table.

Adoption of the carbon-12 scale does not mean that the element carbon has the exact atomic weight of 12. The exact number 12 is taken as the relative nuclidic (atomic) mass of the isotope C12. Natural carbon is made up of C12, about VU of C1·'5, and some­times a trace of C14. Thus the atomic weight of carbon is 12,01115 ± 0.00005, slightly varied by differences in isotopic distribution.

42 C & E N NOV. 2 0, 1961

TABLE OF ATOMIC WEIGHTS 1961

(Based on Carbon-12)

Symbol Atomic

No. Atomic Weight Symbol

Atomic No.

Atomic Weight

Actinium Ac 89 Mercury Hg 80 200.59 Aluminum AI 13 26.9815 Molybdenum Mo 42 95.94 Americium Am 95 Neodymium Nd 60 144.24 Antimony Sb 51 121.75 Neon Ne 10 20.183 Argon Ar 18 39.948 Neptunium Np 93 Arsenic As 33 74.9216 Nickel Ni 28 58.71 Astatine At 85 Niobium Nb 41 92.906 Barium Ba 56 137.34 Nitrogen Ν 7 14.0067 Berkelium Bk 97 Nobelium No 102 Beryllium Be 4 9.0122 Osmium Os 76 190.2 Bismuth Bi 83 208.980 Oxygen 0 8 15.9994' Boron Β 5 10.811- Palladium Pd 46 105.4 Bromine Br 35 79.909b Phosphorus Ρ 15 30.9738 Cadmium Cd 48 112.40 Platinum Pt 78 195.09 Calcium Ca 20 40.08 Plutonium Pu 94 Californium Cf 98 Polonium Po 84 Carbon C 6 12.01115» Potassium Κ 19 39.102 Cerium Ce 58 140.12 Praseodymium Pr 59 140.907 Cesium Cs 55 132.905 Promethium Pm 61 Chlorine CI 17 35.453b Protactinium Pa 91 Chromium Cr 24 51.996b Radium Ra 88 Cobalt Co 27 58.9332 Radon Rn 86 Copper Cu 29 63.54 Rhenium Re 75 186.2 Curium Cm 96 Rhodium Rh 45 102.905 Dysprosium Dy 66 162.50 Rubidium Rb 37 85.47 Einsteinium Es 99 Ruthenium Ru 44 101.07 Erbium Er 68 167.26 Samarium Sm 62 150.35 Europium Eu 63 151.96 Scandium Sc 21 44.956 Fermium Fm 100 Selenium Se 34 78.96 Fluorine F 9 18.9984 Silicon Si 14 28.086-Francium Fr 87 Silver Ag 47 107.870b

Gadolinium Gd 64 157.25 Sodium Na 11 22.9898 Gallium Ga 31 69.72 Strontium Sr 38 87.62 Germanium Ge 32 72.59 Sulfur S 16 32.064-Gold Au 79 196.967 Tantalum Ta 73 180.948 Hafnium Hf 72 178.49 Technetium Te 43 Helium He 2 4.0026 Tellurium Te 52 127.60 Holmium Ho 67 164.930 Terbium Tb 65 158.924 Hydrogen H 1 1.00797- Thallium ΤΙ 81 204.37 Indium In 49 114.82 Thorium Th 90 232.038 Iodine 1 53 126.9044 Thulium Tm 69 168.934 Iridium Ir 77 192.2 Tin Sn 50 118.69 Iron Fe 26 55.847b Titanium Ti 22 47.90 Krypton Kr 36 83.80 Tungsten W 74 183.85 Lanthanum La 57 138.91 Uranium U 92 238.03 Lead Pb 82 207.19 Vanadium V 23 50.942 Lithium Li 3 6.939 Xenon Xe 54 131.30 Lutetium Lu 71 174.97 Ytterbium Yb 70 173.04 Magnesium Mg 12 24.312 Yttrium Y 39 88.905 Manganese Mn 25 54.9380 Zinc Zn 30 65.37 Mendelevium Md 101 Zirconium Zr 40 91.22

a The atomic weight varies because of natural variations in the iso- h The atomic we'ght is believed to have an experimental uncertainty topic composition of the element. The observed ranges are boron, of the following magnitude: bromine, ±0.002; chlorine, ±0.001; ±0.003; carbon, ±0.00005; hydrogen, ±0.00001; oxygen, chromium. ±0.001- iron, ±^003; silver, ±0.003. For other ±0.0001; silicon, ±0.001; sulfur ±0.003. elements the ast digit given is believed to be reliable to ±0.5.

Courtesy of the International Union of Pure and Applied Chemistry and Butterworth Scientific Publications

NOV. 20, 1961 C & E N 43