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Atomic Weight of Radium, History

The Approximate Atomic Weight of Radium

Radium belongs to the alkaline earth group of metals, and, therefore, it should be divalent. This has been verified by electro- endosmotic measurements, and by diffusion experiments.

From a study of the spectrum Runge and Precht concluded that the atomic weight should be about 258, but by a different formula Watts deduced 226.56 from the same data. Measurements of the electrical conductivity of radium bromide solutions indicated that the former was probably too high. From consideration of the numerical relationships of the Periodic Table, Wilde concluded that the atomic weight should be approximately 184.

The Exact Atomic Weight of Radium

The first chemical determination of the atomic weight was undertaken by Mme. Curie. Samples of radioactive barium chloride of gradually increasing activity were precipitated with silver chloride. With the weakest of these, values for the atomic weight were obtained which were not materially different from the atomic weight of barium. As the activity increased, however, the atomic weight also began to increase. At the same time the product was examined spectroscopically. When the radium and barium spectra were of almost equal intensity the atomic weight was 173.8. Ultimately a product was obtained which contained only a trace of barium and had an atomic weight of 225.

In 1902 a sample of radium chloride, which still showed the three principal barium lines with considerable intensity, gave values lying between 220 and 224. For all these determinations the same sample of radium chloride was used, the radium being recovered from the solution and retransformed into chloride.

Analyses of much larger quantities of radium chloride were made by Honigschmid. He used the utmost care in preparing pure specimens by recrystallising from hydrochloric acid and precipitating by alcohol until the product appeared to be unchanged by fifty further crystallisations from hydrochloric acid and eleven precipitations by alcohol. The water of crystallisation was driven off, first in nitrogen and then in a current of hydrochloric acid gas.

The variations in the atomic weight were not more than ±0.03. At the end of the work the material was again crystallised twenty times from hydrochloric acid solution, but no change in atomic weight was observed.

Ramsay and Gray, using the quartz microbalance, and working on quantities of less than 3 mgm., found the change in weight on transforming the anhydrous bromide into the chloride by heating in a current of hydrochloric acid. Three experiments gave 226.53, 226.22, and 226.33, mean 226.36.

Honigschmid then revised his atomic weight determinations. He transformed the chloride into bromide and recrystallised thirty times. By spectroscopic analysis the salt appeared to be free from barium. From a series of six analyses he found that 5.06791 grm. of radium bromide precipitated 4.93389 grm. of silver bromide, corresponding with an atomic weight of 225.96, and from two analyses that the bromine in 1.28825 grm. of radium bromide was precipitated by 0.72045 grm. of silver, giving 225.97.

He also carried out two determinations by Ramsay and Gray's method, converting nearly 1 grm. of bromide into chloride. This gave 225.94.

In all, seventeen determinations were made in which the extreme values were 225.99 and 225.93, giving a mean value of 225.97±0.012.

The International Commission on Atomic Weights for 1925 have adopted the value

Ra = 225.95.

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