The Atomic Weights of Radioactive Substances

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Abstract
(1) The isotopic weights of Pb206, Pb207, and Pb208, on the chemical scale O = 16.0000, are obtained by three distinct methods, using in each case the assumption that they differ by an amount very close to unity. The results for Pb206 are these: (1) from packing fraction of lead, 205.965; (2) from study of the packing fraction curve, 205.992; from relative abundance of isotopes in leads of known atomic weight, 206.003. Mean, 205.987±0.03. By taking into account a recent atomic weight determination on lead from Bedford cyrtolite, reasonable rounded values for the three isotopes are 205.98, 206.98 and 207.98±0.03. (2) From these values the isotopic weights of the radioactive substances are obtained by adding the mass (and the mass‐equivalent of the energy) lost in disintegration. The calculated values for radium and thorium, 226.02±0.03 and 232.03±0.03, do not agree with the International values 225.97±0.01 and 232.12±0.01. The calculated weights for U238 and U234 are 238.04±0.03 and 234.04±0.03, respectively, while the heretofore accepted atomic weight of U is 238.14±0.01. In view of Aston's conclusion that U does not contain more than 3 percent of U239 or any higher isotope, it appears probable that the accepted value for uranium is somewhat too high. (3) W — N is plotted as a function of N, for the three radioactive series, W being the isotopic weight and N the nearest integer. The graphs obtained are almost linear, and their slopes are nearly identical. In an average way they conform to the equation W — N = — 0.02+0.00214 (N — 206).

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