Is Isotopy Possible in Hydrogen and Oxygen?

(O. Stern and M. Volmer. Are the deviations of atomic weights from integrality explainable by isotopy? Ann. d. Physik 59, p. 225—1919.)

The deviation of atomic weights from whole numbers can be explained either by the motion of electrons, which thereby change their mass, or by the fact that the element under investigation is a mixture of isotopes. Such an explanation is given by Fajans [Chemiker Kalender 1919]. Isotopy can be explained according to Bohr’s theory if one imagines the atomic nucleus as composed of positive and nega-

additional charges in such a way that their total charge is in both cases the same, while their masses differ. The electron shells, located from the nucleus at distances exceeding its dimensions by a thousandfold, must be identical for the same nuclear charge; and since the outer shells of the electrons determine the chemical properties of an element, it is clear that the chemical properties of two elements with the same nuclear charge but different mass will be identical and, consequently, in the two cases indicated we shall be dealing with isotopes.

Let us imagine, as Lenz first did (Sitzber. Bayer. Akad. M. Phys. Klasse 1918), that the hydrogen nucleus consists in one case of a single positive charge associated with mass 1, and in the second case of two hydrogen nuclei with unit charges and each with mass 1, bound by an electron. The mass of such a second nucleus is 2 and the charge 1.

The equations of motion of the positive charges in the nucleus show that its dimensions must be \(3.85 \cdot 10^{-12}\) cm, i.e., of the same order as ordinary atomic nuclei. By mixing hydrogen \(H\) with its isotope \(H_1\) (atomic weight 2) in suitable quantities, we can obtain, by adding 0.8% of the latter substance to hydrogen \(H\), a substance with a mean atomic weight corresponding to real hydrogen, \(H = 1.008\).

If it is admitted that hydrogen with atomic weight 2 is possible, then the possibility of obtaining in hydrogen particles with weight 3 becomes understandable, as Thomson found in canal rays (\(X_3\)). In fact, \(X_3\) is a molecule in which one hydrogen atom has atomic weight 1, and the other 2, and its formula will be \(HH_1\). However, as Stern and Volmer showed, against the possibility of the existence of an isotope in hydrogen there were considerations taken from spectral analysis. In fact, if hydrogen \(H = 1\) were accompanied in the amount of 0.8% by hydrogen \(H_1 = 2\), then, in addition to the ordinary spectral series of hydrogen belonging to \(H\), we ought to observe a second series lying at a distance

\[ \Delta\lambda = 2.7 \cdot 10^{-4}\lambda \]

from the lines of the first series (\(\lambda\) is the wavelength). These lines, however, are not observed, and this argues against the possibility of isotopy.

All the above considerations compelled Stern and Volmer to undertake in Nernst’s laboratory the decomposition of \(H_2\) and \(O_2\), obtained in the form of chemically pure substances. The decomposition was carried out by repeated diffusion of \(H_2\) (or \(O_2\)) through porous clay, through which gases with lower atomic weight pass more rapidly. After repeated diffusion, by combustion with one and the same oxygen of the corresponding \(H_2\) and of the isotope obtained from it, water was formed, the density of which was determined by means of Nernst’s very precise method using microbalances.

The same was done with oxygen, from which an isotope was separated by repeated diffusion.

The densities of the water obtained from different samples of \(H_2\) and \(O_2\) differed by \(0.6 \cdot 10^{-4}\) percent, whereas in the case of isotopy this difference should have been \(4.2 \cdot 10^{-2}\) percent. Thus Stern’s and Volmer’s experiments have shown that hydrogen and oxygen are not mixtures of isotopes, and that the deviations of their atomic weights from whole numbers may depend on the motions of electrons.

P. Lazarev.