The element eka–tantalum predicted by Mendeleev is, perhaps, the only one of the radioactive elements that had been discovered earlier than it is generally recognized. We are taking about the element number 91 situated between thorium and uranium. Its long–lived isotope has a considerable half–life (34 300 years) and, therefore, it should be accumulated in the uranium ores; moreover, it emits alpha rays. If we look at the accepted date of its discovery (1918) it would be reasonable to ask why it was discovered so late. We shall answer this question later.
Now let us discuss the family of uranium–238 (see Table 1 and Diagram 1). The notorious element UX discovered by Crookes, which in fact started the hunt for radioelements, is designated as uranium-X1 in Table 1. This name was given to it much later, after the discovery of the radioelement designated as uranium-X2.
In February 1913 Soddy suggested that an unknown radioelement should exist between the element UX of Crookes and the element U-II discovered in 1911 in the uranium family. The properties of the new element, according to Soddy, should be those of eka–tantalum. This hypothetical radioelement seemed to have its rightful place in the fifth group of the periodic system which did not contain any radioelements by a strange whim of nature. Strictly speaking, it was not really strange. Uranium–238 (or U-I), the originator of this family, and U–II, a member of the family, are uranium isotopes; both of them have very long half–lives in comparison with other radio elements. It was not so easy to identify uranium–II against the background of uranium–I. It was just as not easy to detect the precursor of uranium–II, that is, the hypothetical eka–tantalum UK2.
This was done in mid–March 1913 by K. Fajans and his young assistant O. Göring who detected a new beta–emitting radioelement with half–life of 1.17 min and chemical properties similar to those of tantanium. In October of the same year they clearly stated that UX2 was a new radioactive element located between thorium and uranium and suggested to name it brevium (from the Greek for “short–lived”).
The symbol UX2 took its place in the uranium family but the symbol Bv could hardly he put into box No. 91 of the periodic system though the new element was intensely studied in many laboratories and its discovery was verified by British and German scientists.
At any rate, the statement that element No. 91 was discovered in 1913 does not seem controversial. But why then does not its history start with this date?
If the world war I had started brevium would, perhaps, have a better fate. But the war put a stop to radiochemical studies and sharply curtailed exchanges of information. Eka–tantalum had to be discovered for the second time.
For a long time the actinium family was the most difficult to understand among the three radioactive families. Which element is its originator? The answer was not clear. If it was actinium then its half–life had to be of the same order as the half–lives of thorium and uranium. This seemed to be unlikely though the half–life defied evaluation. At any rate, it was negligible in comparison with the Earth’s age.
Since actinium was regarded as the originator of the family the question of its precursors was meaningless and this attitude contributed to the delay of the discovery of eka–tantalum. Another suggestion was that the actinium family was not independent but just a branch of the uranium family. This suggestion was discussed by radiochemists back in 1913–1914 by which time brevium had already been discovered. But the discussion yielded no meaningful results and actinium continued to be the head of its family though under false pretenses (as almost everybody agreed).
A decisive role in further development was played by the radioelement UY, a thorium isotope of discovered in 1911 by the Russian radiochemist G. Antonov who worked in Rutherford’s laboratory. The radioelement UX1 (also a thorium isotope) in the uranium family emits beta particles and gives rise to brevium (UX2).
The French scientist A. Picard in 1917 suggested that a similar situation had to prevail at the origin of the family which was still known as the actinium family. His idea, which was confirmed only much later, was that the originator of this family was a third, still unknown uranium isotope (in addition to U–I and U–II). Picard named it actinouranium. When it emits alpha particles it converts into UY which, in its turn, convers into actinium. An intermediate product of the process should be a radioelement belonging to the fifth group of the periodic system. This sequence of transformations can be written as
This suggestion simultaneously answered the question about UY whose position in the radioactive family was unclear. This constructive, though fairly bold, suggestion was worth verifying.
In England the next stage in the search for eka–tantalum was carried out by Soddy and his assistant A. Cranston. They were lucky and in December 1917 they wrote a paper on their discovery of eka–tantalum as product of beta–decay of uranium–Y. But their data on eka–tantalum were rather poor in comparison with the report by the German chemists O. Hahn and L. Meitner.
The paper by the Germans was published earlier though it was submitted to the journal later than the paper by the British scientists. But the important thing is not the publication data. Hahn and Meitner not only extracted the new radioelement; they conducted all possible studies of its properties, evaluated its half–life and measured the mean free path of alpha particles. The German and British scientists are said to be co–discoverers of element No. 91 though the contribution made by the Germans is, undoubtedly, more significant. The tale of the discovery may be ended with the noble gesture of Fajans who did not claim the discovery of eka–tantalum (though he had every right to do so) but just suggested changing the name brevium to protactinium (from the Greek for “preceding actinium”) since the latter radioelement was a much longer–lived isotope.
Thus, the symbol Pa appeared in the periodic system. Its isotope with the longest half–life has a mass number of 231. A few milligrams of pure Pa2O5 were extracted in 1927.