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Discovery of Neptunium elements

Neptunium

Of course, Fermi never presented the Queen of Italy with a test–tube containing a salt of the first transuranium element. It is no more than a typical newspapermen’s copy. But it is true that Fermi had in his hands element 93 though it could not be proved at the time. In his experiments the uranium target consisted of two isotopes, namely, uranium–238 and uranium–235. The latter underwent fission under the effect of slow neutrons giving rise to fragments which were the nuclei of the elements belonging to the central part of the periodic system. They greatly complicated the chemical situation but this was understood only when fission was discovered.

But uranium–238 absorbed neutrons converting into uranium–239, a new isotope of uranium. This beta–active isotope gave rise to an isotope of the first transuranium element with an atomic number of 93. This was just what Fermi and his group thought. But the future neptunium was hard to distinguish among the multitude of fragments. This is why the experiments in mid–thirties yielded no results. The discovery of Hahn and strassmann decisively stimulated actual synthesis of transuranium elements. To start, a reliable technique was needed for detection of the atoms of element 93 in a mass of fission fragments. As the masses of these fragments were comparatively small they had to travel longer distances (had longer paths) than the atoms of element 93 with a large mass.

Thus went the argument of E. McMillan, an American physicist from the University of California. Back in the spring of 1939 he started to analyse the distribution of uranium fission fragments along their paths. He managed to obtain a sample of fragments whose path was very short and in this sample he found traces of a radioactive substance with a half–life of 2.3 days and a high radiation intensity. Other parts of the fission fragments did not exhibit such activity. McMillan demonstrated that this unknown substance was a fission product of a uranium isotope which was also found in the short–path fragments. Thus, the reaction sequence first suggested by Fermi was written as

\[_{92}^{238}U+n\,\,\to \,_{92}^{239}U{{\xrightarrow{{{\beta }^{-}}}}^{239}}93\

Now the search was no longer conducted in darkness. Chemical analysis had then to be the final step in verification of the new element. On summer vacations McMillan invited his friend, the chemist P. Abelson, and this visit played a crucial part in the discovery of element 93. Together they established the chemical nature of the new element with a half–life of 2.3 days. The element could be chemically separated from thorium and uranium though in some aspects it was similar to them. But the new element was in no way similar to rhenium. This finally refuted the hypothesis that element 93 had to be eka–rhenium.

At the beginning of 1940 the Physical Review journal reported the real discovery of element 93. It was named neptunium after the planet that is beyond Uranus in the solar system (there is some analogy to the periodic system where neptunium follows uranium).

Synthesis of neptunium exhibited a significant feature which was to prove typical for syntheses of all transuranium elements (and other synthesized elements, too). First, one isotope with a certain mass number was synthesized. For neptunium this was neptunium–239. From that time it became a rule to data a discovery of a new transuranium element by the time of reliable synthesis of its first isotope. But sometimes this isotope proved to be so short–lived that it was difficult to subject it to physical and chemical analyses let alone find a useful application for it. A study of a new element would best be conducted with its longest–lived isotope. In the case of neptunium this was neptunium–237 synthesized in 1942 in the following reaction:

\[_{92}^{238}U\,\,(n,\,\,2n)\,\,_{92}^{237}U\xrightarrow{{{\beta }^{-}}}_{93}^{237}Np\

This isotope has a half–life of 2.2 × 106 years. However, its synthesis involves great technical difficulties. Therefore, all the initial studies of the properties of neptunium were performed with its third isotope, neptunium–238, synthesized in the nuclear reaction (d, 2n) Np. Therefore, the history of transuranium elements notes also the date of synthesis of the isotope that is most convenient for analysis but which is by no means always the longest–lived one. Starting from neptunium the American scientists for a long time played a leading part in discoveries of transuranium elements. This can easily be explained by the fact that the USA hardly experienced the hardships of the World War II. It should be noted, however, that in 1942 element 93 was independently synthesized by the German physicist K. Starke.

In 1944 a weighable amount (a few micrograms) of neptunium was synthesized. Now it is produced in tens of kilograms in nuclear reactors. Thirteen neptunium isotopes are currently known. One of them (neptunium–237) was found in 1952 in nature. This is another example when a previously synthesized element was found in nature and for which two discovery dates can be given (as for technetium, promethium, astatine, and francium).

Discovery of Transuranium elements

Transuranium elements

 Transuranium elements are all the elements whose numbers are higher than 92, that is, the elements that directly follow uranium. Now 15 such elements are known. How many more transuranium elements can be found? The answer is still unknown. This is one of the fascinating mysteries of science.

Though the first transuranium element, neptunium, (No. 93) was born not so long ago, in 1940, the question about possible existence of such elements was raised much earlier. Mendeleev did not ignore it either. He believed that even if the transuranium elements would be found on Earth their number will be limited. This was his opinion in 1870. For more than 25 years the problem remained open. Every year saw several erroneous reports on discoveries of new elements but not once the element in question had an atomic mass greater than that of uranium. It seemed axiomatic that uranium was the last element in the periodic system though nobody could say why.

But when radioactivity was discovered thorium and uranium, that is the heaviest elements in the Mendeleev table, were found to possess this property. It would logically seem that the transuranium elements had existed in nature in past but, being highly unstable, had decayed to other, known elements. This simple explanation had a hidden trap, namely, the possible half–lives of even the nearest right–hand neighbours of uranium were quite unknown. Nobody could state with certainty that these hypothetical elements were less stable than uranium and thorium. Thus, it would be reasonable to look for natural transuranium elements.

Years passed and occasionally allegedly successful discoveries of the first transuranium element were reported in scientific journals. As theoretical physics developed it repeatedly attempted to explain the break–off of the periodic system at uranium. Many of these explanations were fascinating but none convincing. In other words, in the twenties of this century the question of transuranium elements looked as vague as in the last quarter of the 19th century.

One amazing hypothesis appeared, however, against this dismal background though at first scientists treated it with suspicion. Only 40 years later the hypothesis found a new meaning. It was put forward in 1925 by the German scientist R. Swinne who looked for the transuranium elements in a peculiar material–a dust of space origin collected on the icefields of Greenland. A sample of the dark powder was given to the Stockholm museum by the well–known polar explorer E. Nordenskjöld in the eighties of the last century. Swinne hoped to find in this powder traces of transuranium elements with the numbers 106–110 and in one of his reports he even mentioned that he had recorded an X–ray spectrum containing lines that, in his opinion, corresponded to element 108. But nobody believed him and he himself discontinued his work.

Swinne made a theoretical study of the variation of various properties of radioelements and, in particular, half–lives. He came to the conclusion that the elements directly following uranium had to have short half–lives. But the elements with the numbers in the ranges between 98 and 102 and between 108 and 110 could be expected to have sufficiently long half–lives. Where to look for them? Swinne suggested that the best bet would be not terrestrial minerals but space objects. This is why he studied the dust of space origin collected in Greenland. All this was quite fascinating but not substantiated and therefore looked like doomed for oblivion.

Now we come to the point in time when the words transuranium elements started to be linked with the word synthesis.

Paradoxical as it seems the attempts to synthesize new elements (namely, trasuranium elements) had started a few years before technetium was produced. The stimulus for this work was the discovery of neutron. The scientists regarded this chargeless elementary particle as possessing infinite penetrating capacity and being capable of producing a wide variety of transformations of all kinds of elements. Thus, all laboratories that had neutrons sources started to bombard with neutrons targets made of various materials including uranium. Especially active in this work was the Italian physicist E. Fermi who was the leader of a group of young enthusiasts at the University of Rome.

They detected some new activity in irradiated uranium. As they irradiated uranium–238 it absorbed neutrons converting into an unknown uranium isotope with a mass number of 239. Since the isotope had an excess of neutrons it exhibited a definite tendency to beta decay. If the left–hand side of the reaction equation is 239U– then the right–hand side is necessarily 23993.

Fermi and his young coworkers argued in approximately this way (though not very clearly as many concepts of nuclear physics at the time were not sufficiently developed). Now chemical verification was needed to prove the synthesis of the first transuranium element. It had to be demonstrated that the activity induced by neutrons in uranium did not belong to any of the preceding elements. This was established within the limits of the capacity of radiochemistry. Thus, Fermi and his group had in their hands a new element, a transuranium one, and one that was the first to be discovered owing to the nuclear synthesis (all this happened in 1934). Fermi and his group, however, were not completely sure of their results. Meanwhile the news about the new element leaked to the press and the discovery was embellished with non–existent details such as Fermi presenting the Queen of Italy with a test–tube containing a dissolved salt of element 93. A lot of such false sensations were published in press while the group continued to assess the results obtained after irradiation of uranium with neutrons.

They extracted several beta–active substances from the uranium target. Two of them were chemically peculiar as they could be precipitated with manganese (IV) oxide easier than the elements preceding uranium. This observation suggested that element 93 was eka–rhenium–a manganese analogue. It was named auzonium (Ao). Being beta–active it could convert into the next element with Z = 94 known as hesperium (Hs). Fermi described this series of nuclear transformations in the following way:

\[_{92}^{238}U+n\xrightarrow{{}}_{92}^{239}U\xrightarrow{{{\beta }^{-}}}_{93}^{239}Ao\xrightarrow{{{\beta }^{-}}}_{94}^{239}Hs\

This series was continued further by the German scientists O. Hahn, L. Meitner, and F. Strassmann, highly experienced radiochemists, particularly O. Hahn who had made his name having had discovered several radioelements. As a result of careful studies the number of new transuranium elements increased by three (including element 97):

\[_{94}Hs{{\xrightarrow{{{\beta }^{-}}}}_{\,\,95}}EkaIr\xrightarrow{{{\beta }^{-}}}\,{{\,}_{96}}EkaPt\xrightarrow{{{\beta }^{-}}}\,{{\,}_{97}}EkaAu...\                       

The prefix Eka means that the respective transuranium elements were considered to be analogues of iridium, platinum, and gold from the sixth period of the periodic system. But it was precisely here that a serious mistake was made, which took quite a time to be found. The properties of the nearest transuranium elements were, in fact, quite different.

The history of science knows of many marvelous insights which seemed at first quite unsubstantiated. One of them was the idea put forward by I. Noddack back in 1934 that when uranium was bombarded with neutrons the uranium nuclei did not convert into new elements at all, rather, they were split into fragments which were the nuclei of lighter, known elements. Her colleagues made light of Noddack’s idea and Hahn’s comments were especially ironic. But his irony turned to be the irony of fate.

Meanwhile other scientists tried to ascertain what happened to uranium under neutron bombardment. I. Joliot–Curie and her coworker, the Serbian physicist P. Savich, particularly carefully analysed and irradiated uranium target. Among the resulting activities they detected traces of a chemical element whose properties were very similar to those of actinium, that is, an element preceding uranium, rather than following it in the periodic table. Soon it was found to have more in common with lanthanum than with actinium. Thus, one of the products obtained after bombardment of uranium with slow neutrons was similar to lanthanum.

If I. Joliot–Curie and Savich had not drawn a line at cautiously stating that the unknown element was similar to lanthanum but had definitely proven that it was lanthanum they would have become the authors (or, at least, coauthors) of one of the greatest discoveries of the 20th century. (it would be in order here to recall that lanthanum has the number 57 and uranium the number 92 and to recall the idea of I. Noddack, too). This seemed more than improbable. But facts remained facts. The results of I. Joliot–Curie and Savich looked so convincing that O. Hahn took it upon himself to verify them, the very same O. Hahn who was an ardent opponent of these results. This meant that he started to question his former opinions.

Hahn, together with his coworker Strassmann, reproduced the experiments of the French scientists whom he so recently had regarded as his opponents. Almost all the results were confirmed. The uranium target contained isotopes of lanthanum and its preceding neighbour in the periodic system, barium. As a chemist, Hahn could not doubt this. As a physicist, he was baffled by the fact.

The fact was that under neutron bombardment uranium nuclei seemed to split into two fragments and these fragments were the nuclei of the isotopes of elements belonging to the centre of the periodic system. Nuclear physics never encountered such a phenomenon. But facts had to be faced and the German scientists concluded that uranium nuclei were capable of breaking down under neutron bombardment.

This happened on December 23, 1938. The scientists immediately reported their discovery. Later Hahn reminisced that after posting the report it all had seemed so improbable to him that he had wished that he could take the letter back from the post box. The improbable proved to be right. A few days later a letter from Hahn was received by L. Meitner who had worked with him for many years. She, together with her nephew, the physicist O. Frisch, attempted theoretical treatment of this phenomenon.

To a certain extent, nuclei can be likened to drops of liquid and scientists have repeatedly tried to draw an analogy between the properties of a nucleus and those of a drop of liquid. If we transfer a sufficient energy to a drop and make it move it can break down to smaller drops. If a nucleus is excited (by a neutron, say) then it can also split into smaller fragments. Gradually, a uranium nucleus is deformed, it elongates, narrowing appears in it, and, finally, it splits into two parts. This is how Meitner and Frisch described the process of splitting of the uranium nuclei. They wrote that the process was remarkably similar to division of bacterial cells by which they propagate and suggested naming the effect “nuclei fission”.

A uranium nucleus splits into two fragments liberating an enormous amount of energy in the process. Other products of fission were free neutrons. They could hit other uranium nuclei leading to their fission and so on. Under favourable conditions a chain fission reaction could occur in a uranium lump producing a nuclear explosion of immense power. As early as 1940 the Soviet scientists Ya. Zel’ dovich and Yu. Khariton developed a rigorous theory of the chain fission reaction. Man mastered a process which, apparently, was unknown in nature. This was the most comprehensive process of transformation of elements man had ever encountered. The fragments of uranium fission were found to contain isotopes of 34 elements, from zinc (number 30) to gadolinium (number 64). The fission reaction proved to be a veritable factory of radioactive isotopes.

Uranium fission caused by neutrons was forced or artificial. Not each uranium nucleus could be split and not each neutron could produce fission. When scientists had studied the fission mechanism in more detail they understood that the intensity of fission was higher under the effect of slow neutrons and if the uranium isotope with a mass number of 235 was used. The other uranium isotope, uranium–238, experienced fission only when bombarded by fast neutrons. Can there be a natural process similar to artificial uranium fission? N. Bohr thought about that and put forward a hypothesis about possible spontaneous uranium fission (without external energy being transferred to the nuclei).

The Soviet scientists G. Flerov and K. Petrzhak attempted an experimental verification of this hypothesis. But how to establish that fission of the uranium nuclei was really spontaneous? Random neutrons of cosmic rays getting into the laboratory could distort the results of experiments. This is why one autumn midnight of 1940 Flerov and Petrzhak went down to one of the deepest stations of Moscow underground railway. There, tens of metres under the surface of earth, the harmful effect of cosmic rays could be escaped. The same night they obtained the final proof of the existence of a new type of radioactive transformations, namely, spontaneous fission of nuclei (they worked only with uranium–238). Later many isotopes of heavy elements (thorium and, particularly, transuranium elements) were found to exhibit this mechanism of radioactive decay. At present science knows of about a hundred nuclei of various elements capable of spontaneous fission. The mechanism of spontaneous fission is similar to that of fission under neutron bombardment.

We now know enough to embark of the tale of the discoveries of individual transuranium elements since it is just in this range of elements that spontaneous fission plays a very significant part. The history of transuranium elements covers forty years and during this, by modern standards, fairly long period scientists managed to take fifteen steps beyond uranium up to element 107. If we take a frame of reference and plot the numbers of elements from 1 to 92 along the horizontal axis and the years of their discovery along the vertical axis the resulting plot will look like a seismogram of a catastrophic earthquake. A similar plot for the transuranium elements is a comparatively smoothly rising line exhibiting distinct peaks. Each new synthesis of a transuranium element meant an increase in the atomic number by one (with a single exception).

The history of syntheses saw its periods of breakthroughs and slack periods. The first breakthrough period was from 1940 to 1945 when four transuranium elements were synthesized, namely, neptunium (Z = 93), plutonium (Z = 94), americium (Z = 95), and curium (Z = 96). The period till 1949 was a slack time and no new elements were discovered. In the next breakthrough period from 1949 to 1952 four more transuranium elements were added to the periodic system, namely berkelium (Z = 97), californium (Z = 98), einsteinium (Z = 99), and fermium (Z = 100). In 1955, fifteen years after the synthesis of the first tranuranium element, one more element, mendelevium (Z = 101), was synthesized. The next 25 years saw much less syntheses and only six new elements appeared in the periodic system. Here scientists encountered an entirely new situation and many former criteria for evaluating discoveries of elements proved inapplicable.

This changing pattern is by no means random, all the breakthroughs and failures had their quite objective causes. They will be apparent when we discuss syntheses of transuranium elements one by one starting from the first one, neptunium.

Discovery of Francium element

Discovery of element : Francium

The element No. 87 has a place of its own in the history of radioactive elements. Though its natural abundance is extremely small it was found originally in nature. But we shall tell its story in detail in the part of the book dealing with artificial elements. This will be better for many reasons. Here the first part of the book comes to an end.

Part two

Synthesized Elements

The idea of transmutation (transformation) of elements was born in distant times. The idea was upheld by alchemists for their specific aims. But all attempts to achieve transmutation proved futile. As chemistry was developing into an independent full–fledged science and accumulating knowledge of the structure and properties of matter the very feasibility of transformation of elements was questioned. By the end of the 19th century serious scientists ignored this problem though did not dare to refute it definitely.

But at the very end of the century an event happened which suggested the paradoxical idea that continuous transmutation of elements takes place in nature. This event was the discovery of radioactivity. But only a relatively small part of elements at very end of the periodic system are subjected to natural transmutation.

Radioactive transformations are independent of human will. All attempts to affect the course of natural radioactive processes failed. When the nuclear model of atomic structure was formulated it became clear that radioactivity is a nuclear phenomenon. The structural features of nuclei determine the capacity for radioactive decay.

The nuclear charge Z is the primary parameter of a chemical element. When a nucleus emits alpha or beta particles its charge changes so that the nature of the chemical element alters. One element is transformed into another. If we are dealing with a stable chemical element its nuclear charge Z will never change by itself. It will change if we can restructure its nucleus in some way, decrease or increase the number of protons in the nucleus. Only then will the nuclear charge change and artificial transmutation of a chemical element will take place.

Rutherford was the first to carry out artificial transmutation of elements. In 1919 he bombarded nitrogen with alpha particles and obtained oxygen atoms. This first in history artificial nuclear reaction can be described by the following equation: \[_{7}^{14}N\text{ }+_{2}^{4}He\text{ }\to _{8}^{17}O\text{ }+_{1}^{1}H\  or, in a shorter form, \[_{7}^{14}N(\alpha ,\,\,p)\,\,_{8}^{17}O\ 

Alpha particles for a long time remained the only available means for conducting nuclear reactions. The energy of naturally produced alpha particles in not high; therefore, they could penetrate the nuclei of only a relatively small number of elements and such events were extremely rare. This limited the scope of artificial transmutation of elements. The situation changed significantly as a result of two discoveries made in the thirties. In 1932 the British scientist J. Chadwick discovered a neutral elementary particle proved to be a universal instrument for performing nuclear transformations since it was not repulsed by positively charged nuclei. Two years later the French physicists Irene and Frederic Joliot – Curie discovered artificial radioactivity and detected a new type of radioactive transformation, namely, positron decay, that is, emission of positrons. It became clear that radioactive isotopes could be produced artificially by means of nuclear reactions for many stable elements.

One can ask what made possible the production of artificial radioactive isotopes in large numbers? The answer is that it was the work of experimental physicists who designed fine instruments for conducting measurements, developed special techniques for performing and studying nuclear reactions and, together with chemists, found methods for isolating traces of radioactive substance. Moreover, the range of particles available for bombardment of nuclei was extended considerably when alpha particles, protons, and neutrons were joined by deuterons (nuclei of a heavy hydrogen isotope deuterium), and later by multiply charged ions of such elements as boron, carbon, nitrogen, oxygen, neon, etc. Finally, physicists have built powerful accelerators capable of accelerating charged particles to very high velocities. All these advances paved the way for artificial synthesis of new elements.

Discovery of : Protactinium Element

Protactinium

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

    \[AcU\xrightarrow{\alpha }UY\xrightarrow{\beta }~EKaTa~\xrightarrow{\alpha }Ac\]

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.

Radio elements and their Families

Radio elements and their Families

Before the discovery of polonium and radium there were seven empty slots in the periodic system between bismuth and uranium. While the number of newly found radioactive elements was small there were no problems with their location in the periodic system. But emanations were a baffling problem. They had identical properties and therefore could not be assigned to different boxes of the periodic system, for instance, to the two empty boxes corresponding to the unknown heavy analogues of iodine and cesium. This would be an unnatural thing to do.

But even if we leave the enigmatically radon family alone the situation still remains unclear. In 1900 W. Crookes observed a strange phenomenon. After fractional crystallization of a uranium compound he obtained a filtrate and a precipitate. Uranium remained in the solution but it exhibited no activity. On the contrary, the precipitate did not contain uranium but exhibited a high–intensity radioactivity. On the strength of his observations Crookes made a paradoxical conclusion that uranium was not radioactive by itself, and its radioactivity was due to some admixture which he managed to separate from uranium. As if he had ill premonitions, Crookes refrained from giving the admixture any definite name and referred to it as uranium–x (UX). Later it was found that uranium restores its activity after separation of UX which was just a much more active substance. Thus, UX could be regarded as a new radioactive element.

Two years later E. Rutherford and F. Soddy discovered similar temporary disappearance of activity in thorium. The respective admixture was named (by analogy) thorium–x (ThX). Rutherford and Soddy attempted to find an answer to the fundamental question: what happens with a radioactive element in the process of emission of radiation? Does the chemical nature of the element remain unchanged or does it change? They made a valuable observation that the emanation of thorium was produced by ThX rather than by thorium itself. In other words, they identified the first step of radioactive transformations:

Th → ThX → EmTh

This was the event that played the decisive part in developing the theory of radioactive decay. According to Rutherford and Soddy, the mechanism of radioactive decay consists in transformation of chemical elements and in their natural transmutation. This was particularly clear in the case of radium, which converted into radon after emission of alpha radiation. Somewhat later, the alpha particle was found to be a doubly ionized helium atom. The decay of radium gave rise to two new elements, namely, radon and helium:

Ra → Rn + He

This suggestion was soon verified in the experiments of Ramsay and Soddy.

Rutherford and Soddy argued further that all the known radioactive elements were not absolutely independent but were generically linked to each other (converted successively one into another). These elements can be said to make up three radioactive families–the uranium, thorium, and radium families named after the originating element of the respective family. Many questions still remained unanswered. How many radioactive elements make up a family? What elements end the families? And finally, what kind of a “material entity” is a radioactive element and what is its real nature?

The last question was not just an abstract one since starting from the early years of the 20th century the number of radioactive substances started snowballing and the problem of their arrangement in the periodic system became very acute.

New radioactive substances became known under a variety of names such as radioactive bodies, activities, and radioactive elements. Scientists were aware that they encountered new, unknown material entities. Most of them manifested their existence only by their radioactive properties, namely, the radiation intensity, the decay type, and the half–life. But nothing or almost nothing could be said about their chemical nature. The old classical chemistry of elements always dealt with weighed quantities of substances so that a new element (or its compound) could be extracted in a material form, its reactions could be studied and its spectrum could be recorded. For most newly discovered radioactive elements all this was unfeasible. Hence, it was not unreasonable to ask whether they were elements in the proper chemical sense of the word.

The first researchers of radioactivity disagreed on this account. The Curies and Debierne assumed that all new radioactive substances were elementary in nature, and, hence, were new chemical elements. The discoveries of polonium, radium, and actinium, apparently, supported this viewpoint and these scientists stubbornly adhered to it even when numerous reports on discoveries of new radioactive substance started to pour in. But this stubbornness only fuelled the controversy.

Rutherford and Soddy held another viewpoint. In their opinion, radioactive substances could have different natures. Proceeding from their concept of radioactive families they argued that there exist relatively stable radioactive elements, that is, uranium, thorium, and radium, which give rise to the families or series of radioactive substances. Their chemical nature is well known and, thus, they can be classified as ordinary elements with only the property of radioactivity distinguishing them from other elements. The elements which close the radioactive families are normal stable elements (it was already vaguely surmised that lead had to close the radioactive families). According to Rutherford and Soddy, between these two types of elements there exist intermediate substances whose main feature in instability and which cannot be described in chemical terms. They are not elements in the conventional sense of the word, they are just something like atomic fragments. It was suggested to name them “metabolons” (from the Greek for transforming bodies). This approach did away with the problem of location of these substances in the periodic system.

But the name “metabolon” was not widely accepted. Soddy himself soon came to regarding metabolons as chemically individual substances, just like normal radioactive elements. In 1902 the British physicist G. Martin introduced the term radioelement which will be explained below. Here we shall just emphasize that the terms radioelement and radioactive element are by no means identical though they are sometimes confused in literature.

The entire history of radiochemistry in the first two decades of the 20th century is essentially the search for new radioelements and their genetic links to the earlier discovered ones. The compositions of radioactive families became increasingly clear and the families were acquiring features of systems of radioelements just as the periodic system classified the stable elements. The former radium family proved to be a part of the uranium family but there emerged the new actinium family whose originator could not be identified for a long time (this was definitely done only in 1935). Most radioelements were short–lived products whose half–lives were measured in seconds or, at best, in minutes. It was extremely difficult to determine the chemical natures and the places of radioelements in their radioactive families; even the cumbersome and monotonous work on separation of the rare–earth elements could not be compared to this task, which would need an entire book to describe it. Therefore, we have just to present here the chronological data on the discoveries of radioelements (see Table 1-3). The current composition of the three radioactive families is shown in Diagram 1.

Each radioactive family contains two characteristic groups of elements. The radioelements preceding the emanations are comparatively long–lived; on the contrary, the elements following the emanations have very short half–lives. Special notation was worked out to identify them using

Uranium–238 family

Radioelement Date of discovery Discoverers
Uranium–I

Uranium–X1

Uranium–X2

Uranium–II

Ionium

Radium

Emanation of radium

Radium–A

Radium–B

Radium–C

Radium–C′

Radium–C′′

Radium–D (radiolead)

Radium–E

Radium–F (polonium)

1896*

1900

1913

1911

1907

1898

1900

1903

1904

1903

1903

1909

1912

1900

1904

1905

1898

A. Becquerel

W. Crookes

K. Fajans, O. Göhring

H. Geiger, J. Nattal

B. Boltwood

The Curies, G. Bemont

E. Dorn

E. Rutherford, H. Barnes

P. Curie, J. Danne

P. Curie, J. Danne

P. Curie, J. Danne

O. Hahn, L. Meitner

K. Fajans

K. Hofmann, E. Strauss

K. Hofmann, L. Gonder,

W. Wölf

E. Rutherford

The Curies

* The date of discovery of uranium radioactivity

letters A, B, and C, alongside the symbols of respective elements (Ra, Th and Ac). The groups of these short–lived elements were known as active sediments; they were the most difficult elements to analyse and served as a source of much confusion and numerous errors. But it was their study that made a significant contribution to the development of the new science of radiochemistry.

Uranium–235 family

Radioelement Date of discovery Discoverers
Uranium–235 (AcU)

Uranium–U

Protactinium

Actinium

Radioactinium

Actinium–K

Actinium–X

Emanation of actinium

Actinium–A

Actinium–B

Actinium–C

Actinium–C′

Actinium–C′′

1935

1911

1918

1918

1899

1902

1906

1939

1900

1904

1905

1902

1911

1904

1904

1908

1913

1914

A. Dempster

G. Antonov

O. Hahn, L. Meitner

F. Soddy, J. Cranston

A. Debierne

F. Giesel

O. Hahn

M. Pereil

A. Debierne

F. Giesel

T. Godlewski

F. Giesel

H. Geiger

A. Debierne

H. Brooks

O. Hahn, L. Meitner

E. Marsden, R. Meitner

E. Marsden, P. Perkins

Thorium–232 family

Radioelement Date of discovery Discoverers
Thorium

Mesothorium–I

Mesothorium–II

Radiothorium

Thorium–X

Emanation of thorium

Thorium–A

Thorium–B

Thorium–C

Thorium–C′

Thorium–C′′

1898*

1907

1908

1905

1902

1899

1910

1899

1903

1909

1906

H. Schmidt, M. Curie

O. Hahn

O. Hahn

O. Hahn

E. Rutherford, F. Soddy

E. Rutherford

H. Geiger, E. Marsden

E. Rutherford

E. Rutherford

O. Hahn, L. Meitner

O. Hahn

* The date of discovery of thorium radioactivity.

As the composition of radioactive families approached the one we know the need for reasonable placement of radioelements in the periodic system became increasingly evident. After all, each of radioelements manifested chemical similarity to one or another conventional element occupying a certain box in the system.

But the number of radioelements was too large. Ramsay described the prevailing situation by the French saying embarrass de richesses (confusing abundance). By the beginning of the second decade of this century about 40 radioelements had been discovered. Some groups of elements were so similar in their chemical properties that they could not be separated with any of the available methods. (For instance, all three emanations, then thorium, ionium and radiothorium, and finally radium and thorium–X).

But the atomic masses of radioelements in each of such groups differed considerably, sometimes by a few units. The situation was indeed confusing. Some scientists suggested leaving many radioelements outside the periodic table, but more creative people were not satisfied with such a solution. In 1909 the Swedish scientists D. Strömholm and T. Svedberg suggested placing several radioelements into one box of the table (soon it was clear that they were right). The British radiochemist A. Cameron supported the idea of the Swedes in 1910.

Though back in 1903 radioactivity was proved to be accompanied with transformation of elements scientists for a long time could not give a definite answer to the question what exactly happens with a radioelement when it emits the alpha or beta particle. An answer to this question would allow to understand where in the periodic system a given radioelement is shifted owing to radioactive decay. The structure of an atom was still unknown and any changes in the nature of a radioelement could be identified by comparing its chemical properties to the properties of its product. But this was often extremely difficult to do since radiochemists had to work with exceedingly small amounts of substances. In many instances the chemical “portrait” of a radioelement had to be drawn from the secondary features.

Tenacious work of scientists and accumulation of experimental data made it possible to formulate the law of radioactive displacement. Though many scientists took part in this work the main contributions were made by F. Soddy and the Polish chemist K. Fajans and therefore this law is known as the Soddy–Fajans law. According to it, alpha decay gives rise to a radioelement displaced two boxes to the left from the starting position in the periodic table while beta decay displaces the product one box to the right. When it was shown that the charge of an atomic nucleus equals the number of the respective element in the periodic system the above empirical law was explained in the following way: an alpha particle removes from a nucleus a position charge of two and therefore the number of the starting element (the charge of its nucleus) is decreased by two while emission of a beta particle increases the positive charge of the nucleus by one.

The displacement law provided for harmonious relationship between radioactive families and the periodic system of elements. After several successive alpha and beta decays the originators of the families converted into stable lead giving rise in the process to the natural radioactive elements found between uranium and bismuth in the periodic table. But then each box in the system had to accommodate several radioelements. They had identical nuclear charges but different masses, that is, they looked as varieties of a given element with identical chemical properties but different masses and radioactive characteristics. In December 1913 Soddy suggested the name isotopes for such varieties of elements (from the Greek for the “common place”) because they occupy the same box in the periodic system.

Now it is clear that radioelements are just isotopes of natural radioactive elements. The three emanations are the isotopes of the radioactive element radon, the number 86 in the periodic system. The radioactive families consist of the isotopes of uranium, thorium, polonium, and actinium. Later many stable elements were found to have isotopes. An interesting observation may be made here. When a stable element was discovered this meant simultaneous discovery of all its isotopes. But in the cases of natural radioactive elements individual isotopes were discovered first. The discovery of radioelements was the discovery of isotopes. This was a significant difference between stable and radioactive elements in connection with the search for them in nature. No wonder that the periodic system was badly strained when accommodation had to be found for the multitude of radioelements, –it was a classification of elements, after all, not isotopes. The discovery of the displacement law and isotopy greatly clarified the situation and paved the way for future advances.

Zinc

 

Zinc

Zinc is also one of the elements whose compounds have been known to mankind from time immemorial. Its best known mineral was calamine (zinc carbonate). Upon calcination it yielded zinc oxide, which was widely used, for instance, for treating eye diseases.

Although zinc oxide is comparatively easily reduced to free metal, it was obtained in a metal state much later than copper, iron, tin, and lead. The explanation is that reduction of zinc oxide with coal requires high temperature (about 1100oC). The boiling point of the metal is 906oC; therefore, highly volatile zinc vapour escapes from the reaction zone. Before metallic zinc was isolated, its ores were used for making brass, an alloy of zinc and copper. Brass was known in Greece, Rome, India, and China. It is an established fact that Romans produced brass for the first time during the reign of Augustus (B.C.20-A.D.14). Interestingly, the Roman method of preparing brass was still used up to the 19th century.

It is impossible to establish when metallic zinc was obtained. In ancient Dacian ruins an idol was found containing 27.5 percent of zinc. Zinc was possibly obtained during brass production as a side product.

In the 10-11th centuries the secret of zinc production was lost in Europe and zinc had to be imported from India and China. It is believed that China was the first country to produce zinc on a large scale. The production process was extremely simple. Earthenware filled with calamine were tightly closed and piled into a pyramid. The gaps between the posts were filled with coal and the posts were heated to red heat. After cooling the pots, where zinc vapours condensed, were broken and metal ingots were extracted.

Europeans rediscovered the secret of zinc production in the 16th century when zinc had already been recognized as an independent metal. During the next two centuries many chemists and metallurgists worked on with methods of zinc extraction. A great deal of credit should go to a. Marggraf who published in 17

46 a large treatise, Methods of Extraction of Zinc from Its Native Mineral Calamine. He also found that lead ores from Rammelsberg (Germany) contained zinc and that zinc could be obtained from sphalerite, natural zinc sulphide.

The name “zinc” originates from the Latin word denoting leucoma or white deposit. Some scholars relate “zinc” to the German word zink, which means lead.