Astatine and Francium
In July 1925 the British scientist W. Friend went to Palestine but not as a pilgrim. Moreover, he was neither an archeologist nor a tourist visiting exotic lands. He was just a chemist and his luggage contained mostly ordinary empty bottles which he hoped to fill with samples of water from the Dead sea. Which has the highest concentration of dissolved salts on Earth. Fish cannot live in it and a man can swim in it without any danger of drowning–so high is the density of water in it.
The somber Biblical landscapes failed to dampen Friend’s hopes for success. His goal was to find in the water of the Dead Sea eka–iodine and eka–cesium which chemists had sought in vain. Sea water contains many dissolved salts of alkali metals and halogens and their concentration in the Dead Sea water must be exceptionally high. The higher the probability that they hide among them the unknown elements, namely the heaviest halogen and the heaviest alkali metal, even if in trace amounts.
Of course, Friend was not entirely original in choosing the direction of his search. As early as the end of the 19th century a chemist would not hesitate to answer the question where to look for eka–iodine and eka–cesium on Earth. The obvious answer was where natural compounds of alkali metals, in sea and ocean water, in various minerals, in deep well water, in some sea algae, and soon. In other words, the field of search was quite wide.
But all the attempts to find eka–iodine and eka–cesium failed and efforts of Friend were no exception. Now let us turn back to the last decades of the 19th century. When Mendeleev developed the periodic system of elements it contained many empty slots corresponding to unknown elements between bismuth and uranium. These empty slots were rapidly filled after the discovery of radioactivity. Polonium, radium, radon, actinium and finally protactinium took their places between uranium and thorium. Only eka–iodine and eka–cesium were late. This fact, however, did not particularly trouble scientists. These unknown elements had to be radioactive since there was not even a hint of doubt that radioactivity was the common feature of elements heavier than bismuth. Therefore, sooner or later radiometric methods would demonstrate the existence of elements 85 and 87.
The natural isotopes of uranium and thorium in long series of successive radioactive transformations give rise to secondary chemical elements. In the first decade of the 20th century scientists had in their disposal about forty radioactive isotopes of the elements at the end of the periodic system, that is, from bismuth to uranium. These radioelements comprised three radioactive families headed by thorium–232, uranium–235, and uranium–238. Each radioactive element sent its representatives to these families with the only exception of eka–iodine and eka–cesium. None of the three series had links that would correspond to the isotopes of element 85 or 87. This suggested an unexpected idea that eka–iodine and eka–cesium were not radioactive.But why? Nobody dared to answer this question. Under this assumption it was meaningless to look for these element in the ores of urnium and thorium which contained all the radioactive elements without exception.
The assumption about stability of eka–iodine and eka–cesium was not confirmed. But all efforts to find isotopes of these elements in the radioactive families met with failure. But there remained one path of investigation which seemed promising. Does a given radioactive isotope have only one or two decay mechanisms? For instance, it emits both alpha and beta particles. If so the products of decay of this isotope are isotopes of two different elements and the series of radioactive transformations at the place of this isotope experiences branching. This problem was discussed for a long time and for some isotopes this effect seemed to take place.
In 1913 the British scientist A. Cranston worked with the radioelement MsTh–II (an isotope of actinium–228). This isotope emits beta particles and converts into thorium–228. But Cranston thought that he detected a very weak alpha decay, too. If that was true the product of the decay had to be the long–expected eka–cesium. Indeed, the process is described by
But Cranston just reported his observation and did not follow the lead.
Just a year later three radiochemists from Vienna–S. Meyer, G. Hess, and F. Paneth–studied actinium–227, an isotope belonging to the family of uranium–235. They repeated their experiments and at last their sensitive instruments detected alpha particles of unknown origin. Alpha particles emitted by various isotopes have specific mean paths in air (of the order of a few centimetres). The mean path of the alpha particles in the experiments of the Austrian scientists was 3.5 cm. No known alpha–active isotope had such mean path of alpha particles. The scientists from the Vienna Radium Institute concluded that these particles were the product of alpha decay of the typically beta–active actinium–227. A product of this decay had to be an isotope of element 87.
The discovery had to be confirmed in new experiments. The Austrians were ready for this but soon the World War I started. They indeed observed alpha radiation of actinium–227 and this meant that atoms of element 87 were produced in their presence. But this fact had to be proved. It was easier to refute their conclusions. Sceptics said that the observed alpha activity was too weak and the results were probably erroneous. Others said that an isotope of the neighbouring element, protactinium, also emitted alpha particles with mean path close to 3.5 cm. Perhaps, an error was caused by an admixture of protactinium.
Elements 85 and 87 were discovered several times and given such names as dacinum and moldavium, alcalinium and helvetium, or leptinum and anglohelvetium. But all of them were mistakes. The fine–sounding names covered emptiness.
The mass numbers of all isotopes in the family of thorium–232 are divided by four. Therefore, the thorium family is sometimes referred to as the 4n family. After division by four of the mass numbers of the isotopes in the two uranium families we get a remainder of two or three. Re–spectively, the uranium–238 family is known as the (4n + 2) family and the uranium–235 family as the (4n + 3) family.
But where is the (4n + 1) family? Perhaps it is precisely in this unknown fourth series of radioactive transformations that the isotopes of eka–iodine and eka–cesium can be found. The idea was not unreasonable but not a single known radioactive isotope could fit into this hypothetical family by its mass number.
Sceptics declared, not without reason, that indeed there had been the fourth radioactive series at the early stages of Earth’s existence. But all the isotopes that comprised it including the originator of the series had too short half–lives and hence disappeared from the face of Earth long ago. The fourth radioactive tree had withered away long before mankind appeared.
In the twenties theorists attempted to reconstruct this family, to visualize its composition if it had existed. This imaginary structure had positions for the isotopes of elements 85 and 87 (but not for the radon isotopes). But this direction of search did not bring results, too. Perhaps the elusive elements did not exist at all?
But the goal was not that far. But before we start the tale about the realization of the scientists’ dreams let us turn back to the first synthesized element, namely, technetium.
Why was technetium the first? Primarily, because the choice of the target and the bombarding particles was obvious. The target was molybdenum, which could be made sufficiently pure at the time. The bombarding particles were neutrons and deutrons and accelerators were available for accelerating deutrons. This is why the discovery of technetium manifested the dawn of the age of synthesized elements. The work on promethium proved more complicated because in belonged to the rare–earth family and the main difficulties were met in determining its chemical nature.
But the task for elements 85 and 87 looked much more formidable. In their attempts to produce eka–iodine the scientists could only have one material for the target, namely, bismuth, element 83. The bombarding particles were a case of Hobson’s choice, too–only alpha particles could be used. Polonium, which precedes eka–iodine, could not be used as the material for the target. The elements with lower numbers than bismuth could not be used as targets because the scientists lacked appropriate bombarding particles to reach number 85.
Eka–cesium looked totally inaccessible for artificial synthesis. No suitable targets and bombarding particles existed in the thirties. But such is the irony of history that it was precisely element 87 that became the second after technetium reliably discovered element out of the four missing elements within the old boundaries of the periodic system. At this point in history the line of eka–iodine and eka–cesium, which had travelled parallel for such a long time, started to diverge and therefore we shall consider their discoveries separately.
Element 85 was synthesized by D. Corson, C. Mackenzie, and E. Segre who worked at Berkley (USA). The Italian physicist Segre by that time had settled in the USA and was the only one in the group who had an experience in artificial synthesis of a new element (technetium). On July 16, 1940, these scientists submitted to the prestigious physical journal Physical Review a large paper entitled “Artificial radioactive element 85”. They reported how they had bombarded a bismuth target with alpha particles accelerated in a cyclotron and obtained a radioactive product of the nuclear reaction . The product, most probably, was an isotope of eka–iodine with a half–life of 7.5 hours and a mass number of 211. Segre and his coworkers performed fine chemical experiments with the new element produced in negligible amounts and found that it was similar to iodine and exhibited weakly metallic properties.
The results seemed convincing enough. But the new element remained nameless for the time being. Further work on eka–iodine had to be delayed as the war started. It was resumed only in 1947 and the same group announced synthesis of another isotope with a mass number of 210. Its half–life was somewhat longer but still only 8.3 hours. Later it was found to be the longest–lived isotope of element 85. It was produced with a similar technique as the first isotope though the energy of the bombarding alpha particles was somewhat higher. As a result the intermediate composite nucleus (209Bi + α) emitted three rather than two neutrons and hence, the mass number of the isotope was lower by 1. Only now the new element was given the name astatine from the Greek for “unstable” (the symbol At).
But in the interval between the syntheses of the isotopes 211At and 210At a remarkable event occurred. The scientists from the Vienna Radium Institute B. Karlik and T. Bernert managed to find natural astatine. This was an extremely skillful study straining to the utmost the capacity of radiometry. The work was crowned with success and element 85 was born for the second time. As in the cases of technetium and promethium, we can name two dates in the history of astatine, namely, the year of its synthesis (1940) and the year of its discovery in nature (1943).
But when the Segre and his coworkers were preparing for irradiating a bismuth target with alpha particles the scientific community had known about the discovery of eka–cesium for more than a year. Transactions of the Paris Academy of Science published a paper headed “Element 87: AcK formed from actinium” and dated January 9, 1939. Its author was M. Perey, the assistant of the eminent radiochemist Debierne who had announced his discovery of actinium forty years earlier.
Marguerite Perey did not invent any fundamentally new methods and did not indulge in any vague and complicated speculations about possible sources of natural eka–cesium. In 1938 she came upon a paper published in 1914. The paper was signed by the Austrian chemists Meyer, Hess and Peneth. Perey attempted to prove their ideas. She obtained a carefully purified specimen of actinium–227. This isotope has a high beta–activity but sometimes it emits alpha particles, too. The mean path of such particles in air is 3.5 cm. This alpha radiation is by no means due to protactinium as the actinium specimen was sufficiently purified. Since alpha particles are emitted the eka–cesium isotope with a mass number of 223 must continuously be accumulated in the specimen. A series of experiments definitely demonstrated that, indeed, some substance with a half–life of 21 min is accumulated in the actinium specimen. Now it is the turn of chemical analysis to prove that this substance is a new element. Its properties proved to be similar to those of cesium. Perey named the new element francium in honour of her country. Only for a short period it was called actinium K (AcK) in accordance with the old nomenclature of radioelements.
The first description given by Perey to the newborn element was extremely brief: the element is formed with alpha decay of actinium –227 in the reaction
and it is alpha–active with a half–life of 21 min. Then she spent several months studying its chemical properties and demonstrated convincingly that francium is similar to cesium in all its characteristics.
None of the natural radioactive elements had such a short half–life, even the artificially synthesized element 85 had a half–life measured in hours. There were hopes to find other natural isotopes of francium with longer half–lives. But in fact francium–223 proved to be the only francium isotope found on Earth.
The only remaining path to success was synthesis but it proved very difficult. More than ten years passed after the discovery of Perey when francium isotopes were artificially synthesized. The nuclear reaction giving rise to the francium isotope with a mass number of 212 can be written in short as
This reaction is the fission of uranium nucleus by protons accelerated to very high energies. When such a fast proton hits uranium nucleus it produces something like an explosion with ejection of a multitude of particles, namely, six protons and 21 neutrons. Of course, the reaction is not due to a blind chance but is based on careful theoretical predictions. Uranium may be replaced with thorium. The reaction product, francium–212, for some time was considered to be the longest–lived isotope (a half–life of 23 min) but later the half–life was found to be only 19 min.
Artificial synthesis of francium is much more difficult and less reliable method than extraction of francium as a product of decay of natural actinium. But natural actinium is rare. What to do? A current method is to irradiate the main isotope of radium with a mass number of 226 (its half–life is 1622 years) with fast neutrons. Radium–226 absorbs a neutron and converts into radium–227 with a half–life of about 40 min. Its decay gives rise to pure actinium–227 whose alpha decay in its turn produces francium–223. The symbols At and Fr were permanently installed in boxes 85 and 87 of the periodic table and their properties proved to be exactly the same as predicted from the table. But in comparison with their unstable mates born by nuclear physics, technetium and promethium, their position is clearly unfavourable.
According to estimates, the 20-km thickness of the Earth crust contains approximately 520 g of francium and 30 g of astatine (this is an overestimation in some respects). These amounts are of the same order as the terrestrial “resources” (quotation marks are more than suitable here) of technetium and promethium. We are probably making a mistake when we talk condescendingly about astatine and francium? Not at all. Technetium and promethium are produced in large amounts, kilograms and kilograms of them. The fact is that technetium and promethium have much longer half–lives and can therefore be accumulated in larger amounts. But accumulation of astatine and francium is just unfeasible. In fact, each time their properties have to be studied they have to be produced a new.
In the radioactive families the isotopes of astatine and francium are placed not on the principle pathways of radioactive transformations but at the side branches. Here is the branch on which natural francium is born:
The isotope Ac in 99 cases out of 100 emits beta particles and only in one case it undergoes alpha decay.
The situation is even less easy in the case of the branches responsible for the formation of astatine:
What may be said about these branches? The producers of natural astatine (the polonium isotopes) are by themselves extremely rare. For them alpha decay is not just predominant but practically the only radioactivity mechanism. Beta decays for them seem something like a mishap as can be clearly seen from the following data.
There is only one beta decay event per 5 000 alpha decays of polonium–218. Things are even sadder for polonium–216 (1 per 7 000) and polonium–215 (1 per 200 000). The situation speaks for itself. The amount of natural francium on Earth is larger. It is produced by the longest–lived actinium isotope 227Ac (a half–life of 21 years) and its content is, of course, much higher than that of the extremely rare polonium isotopes capable of producing astatine.