Level: introductory+
Reference: William Ramsay & Robert Whytlaw Gray, "Density of the emanation of radium," Comptes Rendus 151, 126-8 (1910).
Notes: William Ramsay (1852-1916) was awarded the Nobel Prize in chemistry in 1904 for the discovery of the noble gases. Ramsay had a part in the discovery of argon, the identification of helium on earth[1], the proposal that argon and helium belonged in a new column of the periodic table, and the discovery of krypton, neon, and xenon to fill that column. This exercise, however, is based on work Ramsay did with Robert Whytlaw Gray several years later on the densest of the noble gases.
In this exercise, students are asked to carry out essentially the same calculations as Ramsay & Gray, to determine the atomic weight from vapor density measurements. Ramsay & Gray did not spell out these calculations, but they do report results for each sample and a mean. For sample III, Ramsay & Gray reported an atomic weight of 227, which was enough to raise the mean atomic weight to 220. I suspect that the discrepancy is due to a transcription error in the volume of sample III: that the volume of sample II was repeated by mistake rather than the measured volume of sample III. The error is small enough, however, to be immaterial to the exercise.
Ramsay and Gray proposed a new name for the gas, noting that "radium emanation" was clumsy. They proposed niton, a name that did not catch on. Within a few years of this paper, this emanation of radium and similar emanations produced in the radioactive decay of thorium and actinium would be recognized as isotopes of the element we now know as radon. In fact the other emanations were also produced by α decay of other isotopes of radium that are formed in the decay sequences of thorium and actinium.
Pedagogical note: I rated this exercise as introductory+ because it requires some preliminary steps. It could easily be made less involved by providing a formula for the molar mass of a gas sample based on mass, volume, temperature. Attention could also be focused on the vapor density problem by having the student carry out a calculation for just one sample (i.e., not bringing in the issue of precision).
Solutions: To download solutions, go to:
http://web.lemoyne.edu/giunta/classicalcs/ramsay.doc
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