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      Atomic Weight, History
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Atomic Weights of Lead, History

In his list of atomic weights Dalton attributed to lead the value 95; this is really the equivalent referred to O = 7. Berzelius, in 1811, took into account the three oxides of lead: litharge, red lead, and the peroxide; but since at this time no independent guiding principle was known whereby to fix the magnitude of an atomic weight, no just conclusion could be drawn from the analysis of these compounds.

Berzelius was then of the opinion that the atomic weight of lead is 414, attributing to litharge the formula PbO2. In 1826, however, on account of the generalisations of Dulong and Petit and Mitscherlich, he halved this value, and wrote Pb = 207; nevertheless Gmelin, in the same year, because of the confusion in which fundamental principles were involved at that period, reduced the atomic weight to an equivalent, and adopted the round figure Pb = 104.00.

That the atomic weight of lead is approximately 207 is shown by the following facts:

  1. The specific heat of lead between 18° C. and 100° C. is 0.031 (Magnus). Assuming, according to Dulong and Petit's law, a mean atomic heat of 6.4, the atomic weight is about 207.
  2. Mitscherlich's law of isomorphism provides additional confirmation of this magnitude; for lead salts are isomorphous with corresponding salts of the alkaline earth metals, and the complex fluorides K3HPbF8 and K3HSnF8, as well as the complex chlorides M2PbCl6 and M2SnCl6, are isomorphous.
  3. Lastly, lead occupies an appropriate position in the Periodic Table, in which it has been placed on account of its atomic weight being 207.

Analytical determinations of the atomic weight of lead were made by Berzelius, Turner, Anderson and Svanberg, Marignac, Dumas, Stas, Betts and Kern, Baxter and Wilson, Baxter and Thorvaldson, and Baxter and Grover.

The experiments of Marignac were made upon lead chloride with the purpose of checking the atomic weight of chlorine. Two sets of gravimetric estimations were carried out in 1846 in which (a) lead was heated in an atmosphere of chlorine and was thus converted into the chloride, the ratio found being

Pb:PbCl2 = 100:134.201

whence Pb = 207.34.

(b) Lead chloride was dissolved in water and the chlorine precipitated by silver nitrate. The ratio found was

PbCl2: 2AgCl = 100:103.21

whence Pb = 206.84.

In 1859 Dumas obtained the ratio

PbCl2:2Ag = 128.750:100

whence Pb = 206.88.

The determination of the atomic weight of lead was included in the classical researches of Stas. The work, which was published in 1860, comprised the conversion of pure lead into nitrate and sulphate respectively, the following results being obtained:

(a) The lead nitrate was ignited in a current of dry air at 140° C. to 160° C.

Pb:2NO3 = 100:59.9704

whence Pb = 206.79.

(b) The lead nitrate was ignited in vacuo at 155° C.

Pb:2N03 = 100:59.9645

whence Pb = 206.81.

(c) With lead sulphate the following ratio was determined:

Pb:S04 = 100:46.4275

whence Pb = 206.91.

Betts and Kern determined, by an electrochemical method, the ratio

2Ag:Pb = 100:95.814

whence Pb = 206.73.

The above results are usually considered to be slightly too low for ordinary lead and have been discarded in favour of the more accurate work of Baxter and Wilson. These investigators adopted a method similar to that of Marignac; the amount of silver, in the form of nitrate, required to precipitate completely a known weight of lead chloride was determined, as well as the weight of silver chloride produced. Special care was taken in the preparation of the lead chloride, which was made in different ways and recrystallised several times from hydrochloric acid solution in platinum vessels; and the specimens employed were required not to darken when heated in a stream of hydrogen chloride, and to form a clear solution in water after ignition. The ratios PbCl2:2Ag and PbCl2:2AgCl were determined, the former as the mean of nine, the latter as the mean of six experiments. The end point of the titration in the former experiments was determined by the use of the nephelometer. The following results were obtained

(a) PbCl2:2Ag = 128.8478:100

whence Pb = 207.089.

(b) PbCl2:2AgCl = 96.9783:100

whence Pb = 207.097.

The mean value of the two sets of experiments is Pb = 207.093. Subsequently Baxter and Thorvaldson carried out similar experiments with lead bromide prepared by precipitating lead nitrate solution with hydrobromic acid, and recrystallising the salt by dissolving it in hot concentrated hydrobromic acid, precipitating with water and drying it in a current of nitrogen and hydrogen bromide.

The two ratios PbBr2:2Ag and PbBr2:2AgBr were determined, and in each case the average was

Pb = 207.19.

In view of the discrepancy between the value Pb = 207.09 obtained from lead chloride by Baxter and Wilson and the value Pb = 207.19 obtained as above from the bromide, Baxter and Grover collected normal lead from various geographical and mineralogical sources, purified it, and determined its atomic weight both by the bromide and the chloride method with the following results:

PbBr2:2AgPb = 207.20
PbBr2:2AgBrPb = 207.18
PbCl2:2AgPb = 207.21
PbCl2:2AgClPb = 207.22
The mean value is Pb = 207.20; where Ag. = 107.880, Br = 79.916, Cl = 35.457.

In consequence of these results the value Pb = 207.20 has been adopted by the International Committee on Atomic Weights since 1916.

The Atomic Weight of Lead from Radioactive Sources

Until the development of the science of radioactivity the idea that the atomic weight of an "element " might vary according to its source had not entered the minds of chemists. If, however, the same " element " should be the final disintegration product of two other elements of different atomic weights, which have passed through different stages of disintegration with loss of α- and β-particles, the final "element" might exist in two isotopic forms, chemically indistinguishable, but differing from each other by several units of atomic weight.

Now Soddy and Hyman have found that Ceylon thorite contains 0.39 per cent, of lead monoxide, whose lead is believed to be derived from the thorium in the mineral by radioactive change. Moreover, it has been calculated that the lead isotope derived from thorium should have an atomic weight of 208.4, since the thorium atom, of atomic weight 232.4, loses six helium atoms, of atomic weight 4, in the course of radioactive change. This is distinctly greater than the atomic weight of ordinary lead, Pb = 207.2.

Soddy and Hyman determined the atomic weight of thorite lead by the method of Baxter and Wilson, and found the values 208.5 and 208.3, with a mean value 208.4, which agrees with that indicated by the facts of radioactivity.

Equally striking were the results obtained by Richards and Lembert, who, after confirming the accuracy of the value Pb = 207.1, as recognised by the International Atomic Weights Committee (1915), determined the atomic weights of lead obtained from a variety of radioactive minerals, with the following results:

Lead from North Carolina uraninite – 206.40
Lead from Joachimsthal pitchblende – 206.57
Lead from Colorado carnotite – 206.59
Lead from Ceylonese thorianite – 206.82
Lead from English pitchblende – 206.86

Maurice Curie has also obtained a somewhat analogous series of results with lead obtained from a variety of radioactive minerals, but unfortunately the sources of these are not specified.

Lead from carnotite – 206.36
Lead from yttro-tantalite – 206.54
Lead from pitchblende – 206.64
Lead from monazite – 207.08
Lead from galena – 207.01

Honigschmid and Mile. St. Horovitz found the value 206.736 as the mean of nine determinations of the atomic weight of purified lead that had been obtained from pitchblende; and, subsequently, by employing 20 kilos of selected purest Joachimsthal pitchblende, and also pure reagents, found the atomic weight of uranium-lead to be 206.405. Still more remarkable is the value 206.046 ± 0.014 obtained by Marckwald from a uranium ore occurring in an old primary geological formation in Morogoro, East Africa; and with this must be placed the value 206.063 ± 0.008 determined by Honigschmid and Mile. Horovitz in a sample of broggerite from Moos, Norway, whilst the atomic weight of common lead determined in the same way was found to be 207.180 ± 0.006. The arc and spark spectra of the two leads wrere, however, absolutely identical.

It has been pointed out by Soddy that whilst the atomic weight of lead from thorium minerals should be greater than that of ordinary lead - viz. 232.4 - 6×4 = 208.4 - the atomic weight of lead from uranium minerals should be less - viz. 238.2 - 8×4 = 206.2; that the extreme values found for the atomic weight of lead from these sources are 207.77 and 206.08 respectively; and that the densities of these two products are 11.376 and 11.273 respectively, the atomic volume of lead being the same whatever its source.

These are the first occasions in which the atomic weight of an element has been found to vary with its geographical source, and further researches along these lines will be awaited with interest.

The results clearly confirm the views of Russell, Fajans, and Soddy on the significance of radioactive change in reference to the Periodic System.
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