CHEMICAL AND RADIOACTIVE PROPERTIES OF HEAVY ELEMENTS*

Glenn T. Seaborg

The present article is devoted to the chemical and radioactive properties of the heavy elements, i.e., elements with atomic numbers greater than 88 and, in particular, to the properties of the newly discovered transuranium elements. Apart from the obvious importance of these elements from the standpoint of atomic energy, they are also of great interest from a purely scientific point of view. A general study of the chemical properties of these elements, and especially of those among them that belong to the transuranium series, has led to a considerable increase in our knowledge of the atomic structure of the elements in this region of the periodic system—a subject about which, naturally, we had little information only a few years ago. Similarly, the study of the radioactive properties of new isotopes in this region has considerably increased our knowledge of the properties of radioactive isotopes, as well as of the nature and regularities in these properties, which in turn has greatly broadened our information about nuclear structure. The new data obtained in recent years concerning other nuclear properties of these elements, such as, for example, their relation to fission and cross sections for neutron absorption, lie beyond the scope of this article.

Before 1940 there were no experimental data on the chemical and radioactive properties of the transuranium elements. Nor was much known about the chemical properties of the elements immediately preceding uranium in the periodic system, that is, about the elements actinium, thorium, and protactinium with atomic numbers 89, 90, and 91, respectively. Uranium itself at that time belonged among the less well-known elements. It was known that uranium possesses two oxidation states, VI and IV, and that of these the VI state is more stable under ordinary conditions in aqueous solutions. It was

* The article is a report delivered on November 16, 1945, at a conference on nuclear chemistry at the Technological Institute of Northwestern University, Evanston, Illinois, USA. The article was published in Chemical and Engineering News, December 10, 1945.

It is also known that uranium possesses another oxidation state, III, which was obtained and preserved only with great difficulty in aqueous solutions. Although a sufficient amount of information existed on the chemical compounds of the VI and IV oxidation states, knowledge of such elementary matters as the properties of metallic uranium was almost entirely lacking. In the case of thorium it had been established that there exists only one stable oxidation state in aqueous solutions—with oxidation number IV—but little was known about its compounds and practically nothing about metallic thorium. Very little work had been devoted to the study of the chemistry of pure protactinium; several compounds had been obtained and the existence of oxidation state V was suspected. Only a few tracer-type experiments had been carried out with actinium; they led to the conclusion that this element exhibits oxidation state III and in its chemical behavior is very similar to the trivalent rare earths.

TRANSITION GROUP

The sum of this information was insufficient to make it possible to conclude whether this transition group is associated with the filling of the \(d\) electron shell or the \(5f\) shell, and in any case the question of with which element the transition group begins remained unanswered. However, the discovery of the transuranium elements and the considerable accumulation of information on the chemical properties of other elements of this group, which has occurred in recent years, made it possible to draw much more definite conclusions about the atomic structure in this region of the periodic system.

NEPTUNIUM

The first transuranium element was discovered by MacMillan and Abelson of the University of California in May 1940. Using neutrons from Lawrence’s cyclotron, they were able to show by chemical means that radioactivity with a period of 2.3 days, formed upon irradiation of uranium with neutrons, belongs to the isotope \(93^{239}\), which is the decay product of 23-minute \(U^{239}\), arising from \(U^{238}\) by radiative neutron capture. Their experiments, carried out by means of the tracer method, showed that element 93 has at least two oxidation states—the higher state (or states) and the lower state (or states), analogous to the VI and IV forms of uranium. They found that for the oxidation of \(93^{239}\) from the lower state to the higher one, a greater oxidizing power is required than in the case of the corresponding states of uranium. MacMillan named element 93 neptunium, after Neptune—the planet next beyond Uranus in the solar system.

Another isotope, Np$^{237}$, was discovered at the beginning of 1942 by Wahl and Seaborg at the University of California. This isotope is the decay product of the previously discovered 7-day $\beta$-emitter U$^{237}$, which is formed as a result of the $(n,2n)$ reaction from U$^{238}$. The isotope Np$^{237}$ deserves special mention here, since it is an $\alpha$-emitter with a very long period ($2.25 \cdot 10^6$ years) and is therefore suitable as a material for studying the chemical properties of neptunium by using weighable quantities and ordinary concentrations, provided only that a method exists for obtaining it in sufficient quantities. Fortunately, the large uranium piles at Clinton and Hanford solved this problem, and a certain number of milligrams became available for chemical study. Using this material, Hindman, Magnusson, and La Chapelle (T. I. La Chapell) in the Metallurgical Laboratory of the University of Chicago*) were able to carry out an intensive study of the chemical properties of neptunium, which led to the establishment of its oxidation states and to the preparation of a number of its compounds. This work showed that neptunium has oxidation states VI, V, IV, and III, with a general shift of stability toward the lower oxidation states in comparison with uranium.

PLUTONIUM

After the discovery of neptunium, the next transuranium element to be discovered was element 94. This element was discovered by Seaborg, McMillan, Wahl, and Kennedy at Berkeley, California, at the end of 1940. The isotope discovered had mass 238 and arose when uranium was bombarded with deuterons by means of Lawrence’s cyclotron. The investigators mentioned showed that bombardment of uranium with deuterons leads to a new isotope of neptunium—to the 2-day $\beta$-emitter Np$^{238}$, formed by the $(n,2n)$ reaction from U$^{239}$. They found that this isotope of element 93 decays, transforming into an $\alpha$-emitting element 94—namely $94^{238}$, and that this $\alpha$-emitter has a period of about 50 years. Their early experiments with negligible quantities of this material showed that element 94 also has at least two oxidation states: a higher state (or states) and a lower state (or states). The experiments indicated that even stronger oxidizing agents are required than in the case of neptunium in order to oxidize element 94 from the lower form to the higher one. Wahl’s chemical investigations during this early period were especially remarkable. Element 94 was given the name plutonium, following the manner chosen in naming neptunium.

The most important isotope of plutonium is, of course, the isotope with mass 239. The isotope Pu$^{239}$, which is the daughter substance

) The Metallurgical Laboratory is the conventional name of the nuclear laboratory which, during the war, conducted secret work on atomic energy. See H. D. Smyth, Atomic Energy. (Ed.*)

2.3-day \(Np^{239}\), is an \(\alpha\)-emitter with a half-life of approximately 24,000 years. Its exceptionally important significance is due to its capacity to undergo fission under the action of slow neutrons—a property which makes this isotope comparable in importance with \(U^{235}\). Since \(Pu^{239}\) served as the material used in at least one of the bombs exploded in New Mexico, Hiroshima, and Nagasaki, there is no need to dwell on the importance of this isotope. Its importance for the problem of interest to us here is due to the fact that it is obtainable in weighable quantities, which makes it possible to study the properties of plutonium by ordinary chemical methods.

The first pure chemical compound of plutonium, free from carrier material and from any other foreign substance, was prepared by Cunningham and Werner in the Metallurgical Laboratory in Chicago on August 18, 1942. This memorable day will go down in the history of science as the day of the first isolation of weighable quantities of an artificially prepared element. The work was carried out with amounts of substance measured in micrograms, the substance having been prepared by bombarding uranium with neutrons from a cyclotron.

As is now well known, plutonium is now available in large quantities as a result of the work of the Clinton and Hanford plants, which carry out the chain reaction. Using plutonium from this source, several groups have intensively investigated the chemical properties of plutonium. The group at Los Alamos under the direction of Kennedy and Smith (C. S. Smith), which also included Wahl, Garner, and Jones, was concerned chiefly with the chemistry of plutonium from the standpoint of its purification. Latimer and Eastman at Berkeley, together with Connick, Hofmann, and others, made a significant contribution to the study of plutonium chemistry by investigating oxidation states, potentials, and reaction kinetics. Spedding, together with Sullivan, Voigt (A. F. Voigt), and Newton at Iowa State College, investigated the chemistry of complex-ion formation and other questions of plutonium chemistry. A group of chemists of the Metallurgical Laboratory of the University of Chicago was engaged in the investigation of basic solutions and dry chemistry, as well as in separation and purification processes. Somewhat before the Clinton plant began operating, a considerable part of the staff of the Metallurgical Laboratory, headed by Perlman and English, together with Brown, Cooper, Stoughton, and others, moved to the plant, where their chief occupation consisted of the further intensive development of plant processes for the separation and isolation of plutonium freed from radioactive fission products. In Chicago, several groups remained working on the fundamental problems of chemistry and purification, under the direction of Manning, Orlemann, Davidson, and Cunningham, as well as groups of the plutonium division headed by Albaugh (F. W. Albaugh), Willard, Tomlinson, and Wotton. The latter three subsequently moved to the Hanford plant in order to continue the program of investigations on separation. Much of the work of primary importance for the separation program, such as, for example, the study of fission products

and radiation chemistry, as well as chemical analyses, were carried out by another group in the Metallurgical Laboratory, but their results are beyond the scope of this article.

It was established that plutonium has oxidation states VI, V, IV, and III, and that the lower oxidation state is more stable than in the case of neptunium. A large number of plutonium compounds were also obtained and their properties determined, and it can quite rightly be said that the chemistry of plutonium is now known to us as well as, or even better than, the chemistry of most elements of the periodic system.

As a general conclusion from this work, it may be said that neptunium and plutonium are similar in chemical properties to uranium, with increasing stability of the lower oxidation states on passing to plutonium.

GROUP OF ACTINIDES

The elements from 90 to 94 are situated in the corresponding positions just below the sixth period of the transition elements from Hf to Os (atomic numbers 72 to 76), in which the electron shell \(5d\) is being filled. The transition elements Hf—Os are similar in their chemical properties to the corresponding transition \(4d\)-elements (from Zr to Ru, atomic numbers from 40 to 44). Although the first members (\(_{90}\mathrm{Th}\), \(_{91}\mathrm{Pa}\)) of the group 90—94 show a great similarity in chemical properties to the first members (\(_{72}\mathrm{Hf}\), \(_{73}\mathrm{Ta}\)) of the transition \(5d\) group and to the first members (\(_{40}\mathrm{Zr}\), \(_{41}\mathrm{Nb}\)) of the transition \(4d\) group, the last members of group 90—94, i.e. \(_{93}\mathrm{Np}\), \(_{94}\mathrm{Pu}\), show practically no similarity to \(_{75}\mathrm{Re}\) and \(_{76}\mathrm{Os}\), nor to \(_{43}\mathrm{Ru}\) and \(_{44}\mathrm{Rh}\). This indicates that filling is taking place in the \(5f\) electron shell, although from these chemical facts alone one cannot yet conclude whether uranium is the first member of this series. Although the presentation of all the evidence lies beyond the scope of this article, we shall put forward the attractive hypothesis that this series, similar to the rare earths, begins with actinium in the same sense in which the “lanthanide” series begins with lanthanum. On this basis it may be called the “actinide” series, and the first \(5f\) electron may appear in thorium. Therefore the characteristic oxidation state, i.e. the oxidation state found in members containing seven \(5f\) electrons and fourteen \(5f\) electrons, for this transition series is III.

ELEMENTS 95 AND 96

Thus, the oxidation state IV found for thorium is analogous to the oxidation state IV of cerium. From the behavior of uranium, neptunium, and plutonium it follows that three of the permissible \(5f\)-electrons are readily given up, which explains the inability of thorium to exhibit the oxidation state III. On the basis of this hypo-

teses, elements 95 and 96 should exhibit very stable states III. Indeed, element 96 should exhibit the state III almost exclusively, since with its seven \(5f\)-electrons it should have an electronic structure analogous to gadolinium with its seven \(5f\)-electrons.

The experiments of Seaborg, James, Morgan, and Ghiorso at the Metallurgical Laboratory have recently led to the identification of isotopes of elements 95 and 96 and have made it possible to study the chemical properties of these isotopes by means of tracer-method techniques. These investigators studied the products obtained as a result of bombardment of \(U^{238}\) and \(Pu^{239}\) with helium ions of very high energies (\(40\ \mathrm{MeV}\)) in the cyclotron of the University of Berkeley. The work was made possible thanks to the active participation and assistance of J. Hamilton and his group at the University of California; these investigators recently rebuilt the 60-inch cyclotron to obtain high-energy particles and carried out the irradiation.

Of course, in the case of some elements of this series the question of whether to place the electrons in the \(6d\) or \(5f\) shell may at times be of only academic interest, since the energy required for transition from the \(5f\) shell to the \(6d\) shell may lie in the region of chemical-bond energies. The electronic configuration may differ from compound to compound, or even change with a change in the physical state of a given compound. This displacement of the electronic configuration should probably be most sharply expressed among the middle members of the first half of the series, i.e., uranium, neptunium, and plutonium. Since the difference in energies between the \(5f\) and \(6d\) shells is small, while resonance effects are large, the latter may predominate in determining the very lowest level.

It may be useful here to summarize what has been said in the following propositions. At the present time the periodic system consists of 96 known identified elements, i.e., at least one isotope—stable or radioactive—is definitely known for each of the elements with atomic numbers from 1 to 96. The facts indicate that the atomic structure of the heaviest elements, i.e., of elements with atomic numbers greater than 88, corresponds to a transition series in which the \(5f\) electron shell is being filled. This series differs from the rare-earth series (14 elements with atomic numbers from 58 to 71 inclusive, following lanthanum), in which the \(4f\) shell is filled, in that the first members of this series of heavy elements are much more readily oxidized to oxidation state III. With increasing atomic number, the lower oxidation states and, in particular, state III become more stable. The first \(5f\) electron probably appears in thorium, and the stable configuration consisting of seven \(5f\)-electrons is probably completed in element 96.

There was much speculation about the possibility of the existence in nature of isotopes of the transuranium elements. Searches for such isotopes were also made. Seaborg and Perlman, as early as 1942, carried out a chemical separation of neptunium and plutonium from a certain quantity of uranium pitchblende and were able to show the presence of weak α-activity in the transuranium fraction; they attributed this activity to plutonium, namely to the isotope Pu²³⁹. The amount of plutonium in the pitchblende corresponds approximately to 1 part in 10¹⁴. This isotope of plutonium is present in similar uranium-containing ores, despite its relatively short period, probably because it is continuously formed from U²³⁸ by the absorption of neutrons, which are always present. A source of neutrons sufficient to explain the presence of the indicated amount of plutonium may, in particular, be the spontaneous fission of U²³⁸.

Plutonium is the most interesting element in this new series, and therefore it is appropriate to conclude this article with a further description of the achievements that were made in the study of the chemistry of this element. From its inception, the Metallurgical Laboratory made its principal task the development of a method for obtaining, in the free state, appreciable quantities of Pu²³⁹. This problem consisted of two main parts, which were to a considerable degree independent. There were as many different programs as there were groups of researchers. The first main part of the program consisted in developing a structure carrying out a chain reaction with the aim of obtaining Pu²³⁹ in the uranium lattice. The second part consisted in developing a method for separating plutonium from uranium and from the highly radioactive fission products formed simultaneously with the plutonium.

PRODUCTION OF PLUTONIUM

The problem of developing a process for separating plutonium is unprecedented from almost every point of view. No one had yet seen plutonium at the time when the plant design was being discussed. The chemical properties attributed to this element at that time were based on data that may be called secondary (experiments by the radioactive-tracer method).

The novelty of the problem was emphasized by the fact that Pu²³⁹ was not only the first artificially produced isotope, but also an element lying outside the limits of the classical periodic system. These curious circumstances in themselves would not have presented serious obstacles if they had not been connected with other aspects of the problem, unusual for enterprises of industrial scale.

Both the plutonium itself and the fission products from which it had to be freed were to be present in uranium in insig-

CHEMICAL AND RADIOACTIVE PROPERTIES OF HEAVY ELEMENTS

at extremely low concentrations. Their separation required specialized techniques. An enormous complication for the enterprise was the fact that these negligible quantities of fission products, in turn, had to be separated from the plutonium to such an extent that the amount remaining of each of them would be on the order of one part per million. An additional complication was that the separation process had to be conducted entirely by remote control, owing to the sharply changing γ-activity of the fission products. As a result, it seemed necessary that the process be adapted to simple equipment requiring a minimal number of operations, and that the control limits should not be too strict.

Although four methods of chemical separation were tested—namely: volatility, adsorption, solvent extraction, and precipitation—the process ultimately chosen was a precipitation process. Thompson was chiefly responsible for the conception and early development of the process actually used. The process is associated with the coprecipitation of plutonium together with a “carrier”—a procedure common in radiochemistry.

One of the most interesting and alarming aspects in the development of this process lay in the necessity of testing the process at a time when the available quantities of plutonium, produced with the cyclotron, were measured in micrograms. It was necessary to test the process at concentrations corresponding to the full level of the Hanford plant operations, and therefore the experiments had to be carried out on an ultramicrochemical scale and to use volumes on the order of a microliter. This means that the ratio between the scale of these experiments and the final scale of Hanford production was equal to \(1 : 10^{10}\). Of course, a larger scale ratio had never yet been achieved. Despite these difficulties, the chemical separation process at Hanford was successful from the very beginning, and its results surpassed all expectations. High yields and high decontamination factors (removal of the activity of fission products) were achieved from the very beginning and continued to improve with time.

The precipitation process that was applied is associated with the alternation of oxidation states IV and VI, as indicated in Smyth’s report*). The process consists in precipitating plutonium (IV) with a chemical compound serving as a carrier, then dissolving the precipitate, oxidizing the plutonium to the VI state, and precipitating the chemical compound—the carrier—while the plutonium (VI) remains in solution. Successive oxidation–reduction cycles are continued until the desired decontamination has been achieved.

) The author has in mind the well-known book by Smyth (H. D. Smyth), Atomic Energy for Military Purposes*.

This description of the Hanford separation process is, of course, a considerable simplification of the actual process. In all, approximately thirty chemical reactions are carried out, involving hundreds of operations, before the plutonium emerges from the process. Nor is the plant itself described, with its massive structures and their complex labyrinth of equipment, its piping system, and its remote control. The initial development of the design for this production facility was undertaken at a time when the world’s production of plutonium was invisible to the naked eye. This remarkable research program, using microscopic and submicroscopic quantities, marks one of the many astonishing and hitherto unprecedented achievements of the chemists who developed the chemical separation processes used in the manufacture of the atomic bomb.