The upper part of the periodic system down to the sixth period (where the rare–earth elements are located) always seemed relatively quiet, particularly after the discovery of the group of noble gases which harmoniously closed the right–hand side of the system. It was quiet in the sense that one could hardly expect any sensational discoveries there. The debates concerned only a possible existence of elements that were lighter than hydrogen and elements lying between hydrogen and helium. On the whole, we can say in the parlance of mathematicians that this part of the periodic system was an ordered set of chemical elements.
Therefore, the more awkward and confusing seemed to be the mysterious blank slot No. 43 in the fifth period and seventh group.
Mandeleev named this element eka–manganese and tried to predict its main properties. A few times the element seemed to have been discovered but soon it proved to be an error. This was the case with ilmenium allegedly discovered by the Russian chemist R. Hermann, back in 1846. For some time even Mendeleev tended to believe that ilmenium was eka–manganese. Some scientists suggested placing devium between molybdenum and ruthenium. The German chemist A. Rang even put the symbol Dv into this box of periodic table. In 1896 there flashed and burned like a meteor lucium supposedly discovered by P. Barriere.
Mandeleev did not live to see the happy moment when eka–manganese was really found. A year after his death, in 1908, the Japanese scientist M. Ogawa reported that he found the long–awaited element in the rare mineral, molybdenite and named it nipponium (in honour of the ancient name of Japan). Alas, Asia once more failed to contribute a new element to the periodic system. Ogawa, most probably, dealt with hafnium (which was also discovered later).
Chemists grew accustomed to a few chemical elements being discovered every year and they were at a loss in the case of eka–manganese. They began to think that Mendeleev could make a mistake and no manganese analogues existed.
H. Moseley decisively refuted this skepticism in 1913. He clearly demonstrated that these analogues have their own place among the elements. In a paper dated September 5, 1925, W. Noddack, I. Tacke, O. Berg announced that they had discovered, together with element No. 75 (rhenium), its lighter analogue in the seventh group of the periodic system, namely, masurium whose number was 43. Two new symbols, Ma and Re, appeared in the periodic table, in chemical textbooks, and numerous scientific publications. The discoverers saw nothing odd in the fact that masurium and rhenium had not been discovered earlier. These elements were thought to be not too rare, however. The lateness of their discovery was attributed to another cause. A large group of trace elements in known to geochemistry. The trace elements are classified as those elements which have no or almost no own minerals but are spread in various amounts over minerals of other elements as if the nature has sprayed them with a giant atomizer. This is why the traces of masurium and rhenium were so hard to identify. Only the powerful eye of X–ray spectral analysis could distinguish them against the formidable background of other elements. There is an ancient saying that if two people do the same things this does not mean that the results will be identical. Two biographies started under the same conditions typically follow different paths. The same can be said about the fates of elements 43 and 75; one of them went a long way and found its proper place while the other’s way soon led it to a forest of errors, misunderstandings, and controversies. This was the path of masurium.
W. Prandtl got interested in the empty slots in the seventh group of the periodic table. He had his own outlook and put forward original ideas on the structure of the periodic system. He did not compile a new version of the table, though. He suggested placing the rare–earth elements each to a group though by that time most chemists had put down such an arrangement. But in Prandtl’s version of the table the seventh group happens to reveal as many as four empty slots below manganese corresponding to yet undiscovered elements (this was in 1924) whose numbers were 43, 61, 75, and 93. Prandtl believed this to do no chance occurrence but a result of a common cause that had prevented four elements from having been discovered. The German scientist, however, made his table structure too elaborate and artificial to be accepted. The final discovery of rhenium was the first indication of his errors, and his ideas on the first transuranium element (No. 93) were little thought of at the time. But he was intuitively right in thinking of a close common link between elements 43 and 61.
The belief in masurium’s existence gradually diminished. Only the original discoverers were firm. As late as the beginning of the thirties I. Noddack continued to say that in time element 43 would be commercially available as it happened with rhenium. But as the time passed and chemists again and again failed to find masurium in whatever minerals they analysed they came to believe that I. Noddack was right only by half, that is, only about rhenium. Rarest mineral specimens were tested for masurium. Some people even went as far as to claim that masurium minerals had yet to be found and would possess unheard of properties. Naturally, geochemists were quite sceptical. The imagination of some people went even further and masurium was suggested to be radioactive. That was too much, others said. But it was precisely this shot that did not go wild.
Let us talk about some concepts of nuclear physics. We have discussed isotopes. Now we meet another term–isobars–elements having the same atomic weight or mass numbers but different atomic numbers (from the Greek for “heavy”). Isobars, in other words, are isotopes of different chemical elements with different nuclear charges but identical mass numbers. Take, for instance, potassium–40 and argon–40 which have different nuclear charges (respectively, 19 and 20). Their mass numbers are identical because their nuclei contain different numbers of protons and neutrons but their total numbers are the same; potassium nucleus contains 19 protons 21 neutrons while argon nucleus has 20 protons and 20 neutrons.
Thus, the concept of isobars turned out to be the magic key that opened the door to the mystery of masurium.
When the majority of stable chemical elements were found to have isotopes–up to ten isotopes per element–the scientists started to study the laws of isotopism. The German theoretical physicist J. Mattauch formulated one of such laws at the beginning of the thirties (the basic premise of this law was noted back in 1924 by the Soviet chemist S. Shchukarev). The law states that if the difference between the nuclear charges of two isobars is unity one of them must be radioactive. For instance, in the 40K–40Ar isobar pair the first is naturally weakly radioactive and transforms into the second owing to the so–called process of K–capture. Then Mattauch compared with each other the mass numbers of the isotopes of the neighbours of masurium, that is, molybdenum (Z = 42) and ruthenium (Z = 44):
Mo isotopes 94 95 96 97 98 – 100 – –
Ru isotopes – – 96 – 98 99 100 101 102
What did he deduce from this comparison? The fact that the wide range of mass numbers from 94 to 102 was forbidden for the isotopes of element 43 or, in other words, that no stable masurium isotopes could exist.
If that was really so that meant a peculiar anomaly linked to the number 43 in the periodic system. All the atom species with Z = 43 had to be radioactive as if this number was a small island of instability amidst a sea of stable elements. This, of course, would be unfeasible to predict within the framework of purely chemical theory. When Mendeleev predicted his eka–manganese he could never imagine that this member of the seventh group of the periodic system could not exist on Earth. Of course, in those times (the early thirties) Mattauch’s law was no more than a hypothesis, though one that looked like quite capable of becoming a law. And it became just that. The physicist’s idea opened the eyes of chemists who lost all hope of finding element 43 and they saw the source of their errors. However, the symbol Ma remained in box 43 of the periodic system for a few more years. And not without a reason. All right, all masurium isotopes are radioactive. But we know radioactive isotopes existing of Earth–uranium–238, thorium–232, potassium–40. They are still found on Earth because their half–lives are very long. Masurium isotopes are, perhaps, long–lived, too? If so, one should not be too hasty in dismissing the chances of successful search for element 43 in nature.
The old problem remained open. Who knows which way the biography of masurium would take if not for the dawn of a new age–that of artificial synthesis of elements.
Nuclear synthesis became feasible after invention of the cyclotron and the discoveries of neutrons and artificial radioactivity. In early thirties a few artificial radioisotopes of known elements were synthesized. Syntheses of heavier–than–uranium elements were even reported. But physicists just did not dare to take the challenge of the empty boxes at the very heart of the periodic system. It was explained by a variety of reasons but the major one was enormous technical complexity of nuclear synthesis. A chance helped. At the end of 1936 the young Italian physicist E. Segre went for a post–graduate work at Berkley (USA) where one of the first cyclotrons in the world was successfully put into operation. A small component was instrumental in cyclotron operation. It directed a beam of charged accelerated particles to a target. Absorption of a part of the beam led to intense heating of the component so that it had to be made from a refractory material, for instance, molybdenum.
The charged particles absorbed by molybdenum gave rise to nuclear reactions in it and molybdenum nuclei could be transformed into nuclei of other elements. Molybdenum is a neighbour of element 43 in the periodic system. A beam of accelerated deutrons could, in principle, produce masurium nuclei from molybdenum nuclei.
That was just Segre’s thought. He was a competent radiochemist and understood that if masurium really were produced its amount would be literally negligible and its separation from molybdenum would present an enormously intricate task. Therefore, he took an irradiated molybdenum specimen with him back to the University of Palermo where he was assisted in his work by the chemist C. Perrier.
They had had to work for nearly half a year before they could present their tentative conclusions in a short letter to the London journal nature. Briefly, the letter reported the first in history artificial synthesis of a new chemical element. This was element 43 the futile search for which on Earth wasted so much efforts of scientists from many countries. Professor E. Lawrence from the University of California at Berkley gave the authors a molybdenum plate irradiated with deutrons in the Berkley cyclotron. The plate exhibited a high radioactivity level which could hardly be due to any single substance. The half–life was such that the substances could not be radioactive isotopes of zirconium, niobium, molybdenum, and ruthenium. Most probably they were isotopes of element 43.
Though the chemical properties of this element were practically unknown Segre and Perrier attempted to analyse them radiochemically. The element proved to be closely similar to rhenium and exhibited the same analytical reactions as rhenium. However, it could be separated from rhenium with technique used for separating molybdenum and rhenium. The letter was written in Palermo and dated June 13, 1937. It was by no means a sensation. The scientific community regarded it as just the authors going on record. The reported data were too patchy while what was needed to be convincing was precisely the details and clear results of radiochemical analysis.
Only later Segre and Perrier were recognized as heroes; indeed, they extracted from the irradiated molybdenum just 10–10g of the new element–an amount formerly undetectable Never before radiochemists worked with such negligible amounts of material. The discoverers suggested naming the new element technetium from the Greek for “artificial”. Thus, the name of the first synthesized element reflected its origin. The name, though, became generally accepted only ten years later.
Perrier and Segre received new specimens of irradiated molybdenum and continued their studies. Their discovery was confirmed by other scientists. By 1939 it was understood that bombardment of molybdenum with deutrons or neutrons produces at least five technetium isotopes. Half–lives of some of them were sufficiently long to make possible substantial chemical studies of the new element. It no longer sounded fantastic to speak about “the chemistry of element 43”. But all attempts to measure accurately the half–lives of the technetium isotopes failed. The available estimates were disheartening since none of them exceeded 90 days and this put a stop to all hopes of finding the element on Earth.
So what was technetium in the late thirties and early forties? Nothing more than an expensive toy for curious scientists. Any prospects of accumulating it in a noticeable amount were, apparently, non–existent. The fate of technetium (and not only of it) was reversed when nuclear physics discovered an amazing phenomenon–fission of uranium by slow neutrons.
When a slow neutron hits a nucleus of uranium–235 it in effect breaks the nucleus down into two fragments. Each of the fragments is a nucleus of an element from the central part of the periodic table, including technetium isotopes. It is not without a reason that a fission reactor (a large–scale nuclear energy producer) is known as a factory of isotopes. Cyclotron made possible the first ever synthesis of technetium and fission reactor allowed the chemists to produce kilograms of technetium. But even before the first fission reactor started operating Segre in 1940 found the technetium isotope with a mass number of 99 in uranium fission products in his laboratory. Having found its new birthplace in a fission reactor technetium started to turn into an everyday (paradoxical as it may be) element. indeed, fission of 1 g of uranium–235 gives rise to 26 mg of technetium–99.
As soon as technetium ceased to be a rare bird scientists found the answers to many questions that had puzzled them, and first of all about its half–lives. In the early fifties it became clear that three of technetium isotopes are exceptionally long–lived in comparison with not only its other isotopes but also many other natural isotopes of radioactive elements. The half–life of technetium–99 is 212 000 years, that of technetium–98 is one and a half million years, while that of technetium–97 is even more, namely, 2 600 000 years. The half–lives are long but not long enough for primary technetium to be conserved on Earth since its origin. The primary technetium would survive on Earth if its half–life were not shorter than one hundred fifty million years. This makes obvious the hopelessness of all search for technetium of Earth.
But technetium can still be produced in the course of natural nuclear reactions, for instance, when molybdenum is bombarded by neutrons. How can free neutrons appear on Earth? They can be produced in spontaneous fission of uranium. The process occurs as described above, only spontaneously, and gives rise to a few neutrons, apart from two large fragments, i.e. nuclei of lighter elements.
The search for technetium in molybdenum ores failed and scientists turned their attention to another possibility. If technetium isotopes are produced in fission reactors why cannot they be born in natural processes of spontaneous uranium fission?
Using as a basis the Earth uranium resources (taking the figure for the mean abundance of uranium in the 20–km thickness of the Earth crust) and assuming the same proportion of produced technetium as in the case of reactor fission we can calculate that there are just 1.5 kg of technetium on Earth. Such a small amount (though it is larger than for other synthesized elements) could hardly be taken seriously. Nevertheless, scientists attempted to extract natural technetium from uranium minerals. This was done in 1961 by the American chemist B. Kenna and P. Kuroda. Thus, technetium acquired another birthday–the day when it was discovered in nature. If the methods of artificial synthesis of technetium had failed to materialize, even then it would, sooner or later, be brought to light from the bowels of the Earth.
But ten years earlier, in 1951, sensational news about element 43 was heard. The American Astronomer S. Moore found characteristic lines of technetium in the solar spectrum. The spectrum of technetium had been recorded immediately when it had become feasible, that is, when a sufficient amount of the element had been synthesized. The spectral data had been compared with those reported by the Noddacks and Berg for masurium. The spectra had proved to be quite different making ultimately clear the mistake of the discoverers of masurium. The spectrum of the solar technetium was identical to that of the terrestrial technetium. An analogy with helium was apparent–both elements sent messages from the Sun before to be found on Earth. True, astronomers questioned the data on the solar technetium. But in 1952 the cosmic technetium once more sent a message when the British astrophysicist P. Merril found technetium lines in the spectra of two stars with the poetic names of R Andromedae and Mira Ceti. The intensities of these lines evidenced that the content of technetium in these stars was close to that of its neighbours in the periodic system, namely, niobium, zirconium, molybdenum, ruthenium, rhodium and palladium. But these elements are stable while technetium is radioactive. Though its half–life is relatively long it is still negligible on cosmic scale. Therefore, the existence of technetium on stars can mean only that it is still born there in various nuclear reactions. Chemical elements continue to be produced in stars on a gigantic scale. A witty astrophysicist named technetium the acid test of cosmogonic theories. Any theory of the origin of elements must elucidate the sequence of nuclear reactions in stars giving rise to technetium.