IDA NODDACK - On Element 93

About four months ago an article was published in this journal dealing with the missing elements in the periodic table.¹ At the end of that article the possibility of discovering transuranic elements (that is, elements beyond uranium in the periodic table) was discussed. A few weeks later it was reported, first in the newspapers and then also in technical journals, that two scientists, Professor Fermi in Rome and Mr. Koblic in Joachimsthal, independently had discovered the element with atomic number 93.

We wish to consider first the work of Fermi.² Fermi investigated the possibility of induced radioactivity by means of neutrons. The so-called induced radioactivity had been discovered by Curie and Joliot in the bombardment of atomic nuclei by alpha particles.

Fermi put beryllium powder and radium emanation (i.e., radon gas, element 86) together into a glass container. The radon emits alpha particles, and when these strike beryllium nuclei, neutrons are released. The neutrons penetrate the walls of the glass container and can then act on materials placed nearby. Fermi brought a large number of materials, either in element or compound form, near to the neutron source for irradiation. The irradiated materials were then placed in front of a Geiger counter. Many elements were found to emit beta particles for some time after irradiation, and so verified the induced radioactivity.³ We will not discuss here Fermi's proposed explanation⁴ of the rather complicated observations, since we are interested only in the one example dealing with the supposed production of element 93. To study induced radioactivity of uranium, Fermi took a solution of uranium nitrate, from which all radioactive daughter products had been removed, and brought it near his neutron source. With a Geiger counter he was able to show that the solution became radioactive and emitted beta particles after irradiation. The analysis of the decay curve showed that not only one but at least five different radioactive half lives were produced. Fermi emphasized that it was still uncertain whether the different radioactive decays occur in series (one after the other) or in parallel.

Fermi was able to make a chemical separation of one of the new radioelements, which had a half life of 13 minutes. He did this by adding manganese salt and concentrated nitric acid to the uranium nitrate solution, then heating to the boiling point and adding sodium chlorate. The resulting manganese dioxide precipitate was found to contain almost all of the beta activity with the 13 minute half life. Fermi next tried to show that the radioelement which is responsible for this beta activity was not an isotope of any known element near uranium. To show this he added known beta emitting isotopes of the following elements to the acid solution of uranium nitrate: protactinum (91), thorium (90), actinium (89), radium (88), bismuth (83), and lead (82). When sodium chlorate is added to precipitate the manganese dioxide, none of these beta-emitting isotopes are found in the precipitate, according to Fermi. Since the unidentified new radioelement does precipitate with manganese, and since it could not be an isotope of radon (86) or francium (87) either according to its properties, Fermi concludes that it might be the unknown element 93 (or perhaps 94 or 95).

This method of proof is not valid. Fermi compared his new beta emitter not only with the immediate neighbor of uranium, namely protactinium, but also considered several other elements down to lead. This indicates that he thought a series of consecutive decays was possible (with emission of electrons, protons, and helium nuclei), which eventually formed the radioelement with the 13 minute half life. It is not clear why he did not investigate the element polonium (84) which is also between uranium (92) and lead (82), and why he chose to stop at lead. The old view that the radioactive elements form a continuous series which ends at lead or thallium (81) is just what the previously mentioned experiments of Curie and Joliot had disproved. Fermi therefore ought to have compared his new radioelement with all known elements. It is known from analytic chemistry that numerous elements will precipitate with manganese dioxide if they are present as compounds, atoms, or colloids in a nitric acid solution.

In order to test how various elements behave with Fermi's precipitation method, we made up 100 cm³ of a 55% nitric acid solution containing a few milligrams of nearly all stable elements in dissolved or colloidal form. 200 mg manganese nitrate was added to this solution, it was heated to boiling, and then 2 g of potassium chloride (dry) was added slowly. The resulting manganese dioxide precipitate was then tested chemically and spectroscopically for the presence of other elements. The precipitate contained the following elements: Ti, Nb, Ta, W, Ir, Pt, An, and Si with almost the total quantity of each element which had been in solution; Sb, Ph, Bi, Ni, and Co with partial amounts.

As previously noted, Fermi also did not investigate if polonium (84) goes into the manganese precipitate. An experiment was carried out with polonium which showed that this element does go almost completely into the MnO₂ precipitate.⁵ Therefore, the proof that the new (13 minute) radioelement has atomic number 93 is in no sense successful, since Fermi's method of eliminating other possibilities has not been carried through to completion.

One could assume equally well that when neutrons are used to produce nuclear disintegrations, some distinctly new nuclear reactions take place which have not been observed previously with proton or alpha-particle bombardment of atomic nuclei. In the past one has found that transmutations of nuclei only take place with the emission of electrons, protons, or helium nuclei, so that the heavy elements change their mass only a small amount to produce near neighboring elements. When heavy nuclei are bombarded by neutrons, it is conceivable that the nucleus breaks up into several large fragments, which would of course be isotopes of known elements but would not be neighbors of the irradiated element.

The finding that the new radioelement comes down together with rhenium sulfide when this is precipitated from an acid solution also does not speak for element 93. In the first place, rhenium sulfide readily absorbs other materials. Secondly, the prediction of the probable properties of 93 make it appear not at all certain that this element forms a sulfide which is stable in acid.

Furthermore, if Fermi's interpretation of his experiments were correct, then an additional necessary conclusion, which was not given by him, is that the beta decay of element 93 would produce element 94. It should be relatively easy to separate this chemically from element 93.

One must await further experiments, before one could claim that element 93 has really been found. Fermi himself is careful in this respect, as has been mentioned previously, but in one article about his experiments⁶ and also in the reports found in the newspapers it is made to appear that the results are already certain.

The second statement about the discovery of element 93 comes from Odolen Koblic.⁷ He reported that he had obtained a substantial quantity of element 93 at Joachimsthal (Czechoslovakia) out of the wash water from roasted pitchblende ore. (The pitchblende was believed to contain perhaps 1% of this element.) He described chemical properties of the element and its compounds, determined its atomic weight, assumed that it was the radioactive parent substance of protactinum (91), and gave it the name "bohemium" after his native land. This report also was uncritically accepted and widely distributed in the newspapers of the whole world.

Through the intervention of Dr. M. Speter, Mr. Koblic sent to me two samples of his material, with the request to investigate it for the presence of element 93. Both the chemical analysis and X-ray spectra showed that the material did not contain any 93; it did show instead a mixture of silver, thallium vanadate and tungstate salts, with excess tungstic acid. After being told of these findings, Koblic became convinced of the presence of tungsten, and withdrew his claims to the discovery of element 93.⁸ It is unnecessary therefore to discuss here the reactions which Koblic attributed to the supposed new element, since they can all be explained as reactions from a mixture of vanadium and tungsten.⁹

A short time ago (August 11, 1934) news was received from the United States that Smith and Steinbach in New Jersey succeeded in finding an element with a higher atomic number than uranium. However there are only some not very clear newspaper reports available, so that one cannot tell what was done so far.

¹ I. Noddack, Z. Angewandte Chemie 47, 301 (1934).
² E. Fermi, Nature 133, 898 (1934).
³ Of course not all the atoms of the irradiated material become radioactive, but only an amount which is undetectable by weighing, perhaps several hundred atoms in this case.
⁴ E. Fermi, Nature 133, 757 (1934).
⁵ 1 am indebted to Dr. J. Fränz for preparing the polonium and making the radioactive measurements.
⁶ Nature 133, 863 (1934).
⁷ 0. Koblic, Chemiker-Ztg. 58, 581 (1934).
⁸ 0. Koblic, Chemiker-Ztg. 58, 683 (1934); Österr. Chemiker-Ztg. 37, 140 (1934).
⁹ In withdrawing his claim, Mr. Koblic mentioned only the tungsten, not the vanadium, although he was informed by letter concerning the approximate amounts of both elements in his samples, and the supposed "bohemium-reactions" were explained due to a mixture containing both vanadium and tungsten.

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