Actinide elements discovery

Ion exchange (qv see also Chromatography) is an important procedure for the separation and chemical identification of curium and higher elements. This technique is selective and rapid and has been the key to the discovery of the transcurium elements, in that the elution order and approximate peak position for the undiscovered elements were predicted with considerable confidence (9). Thus the first experimental observation of the chemical behavior of a new actinide element has often been its ion-exchange behavior—an observation coincident with its identification. Further exploration of the chemistry of the element often depended on the production of larger amounts by this method. Solvent extraction is another useful method for separating and purifying actinide elements. 

The first actinide metals to be prepared were those of the three members of the actinide series present in nature in macro amounts, namely, thorium (Th), protactinium (Pa), and uranium (U). Until the discovery of neptunium (Np) and plutonium (Pu) and the subsequent manufacture of milligram amounts of these metals during the hectic World War II years (i.e., the early 1940s), no other actinide element was known. The demand for Pu metal for military purposes resulted in rapid development of preparative methods and considerable study of the chemical and physical properties of the other actinide metals in order to obtain basic knowledge of these unusual metallic elements. 

Usually the discovery of a new element is announced in a scientific paper or at a scientific meeting. The actinide elements curium and americium were announced to the world in a 1945 childrens radio show called Quiz Kids. The show s guest scientist on November 11 that year was a young scientist named Glenn Seaborg. One of the children on the show asked Seaborg if any new elements had been discovered lately. Seaborg happily shared the news that his lab had in fact created two new elements with atomic numbers 95 and 96. 

Uranium was the first actinide element to be identified. In 1789, M. H. Klaproth discovered the presence of a new element in a sample of pitchblende (impure, mineralized form of UO2). Klaproth named the element uranite after the recently discovered planet Uranus. Nearly 100 years later, Becquerel made the initial discovery of the radioactive behavior of uranium through experiments with uranium minerals and photographic plates. 

Actinium and the actinide elements (thorium, etc.) also are listed separately at the bottom. Each actinide element is somewhat similar to actinium and to the lanthanide element with which it is paired. The discovery of hafnium (Hf) in 1923 and rhenium (Re) in 1925 completed the Periodic Table through uranium except for four blank spaces. 

All these new discoveries, of course, verified Seaborg s theory, and the transuranium elements, along with thorium, protactinium and uranium, are now called the actinide elements. They all fit in the Periodic Table between actinium and the element eka-hafnium. Eka-hafnium is the tentative name given to the undiscovered element with the atomic number 104 which lies directly below hafnium in the Periodic Table and which is expected to have chemical properties similar to those of hafnium. 

The discovery that the actinide elements are like the rare earth elements was most useful in identifying them. 

This is the way that all of the actinide elements heavier than curium were chemically identified in their discovery experiments. 

The recognition of the similarity in chemical properties between the actinide and lanthanide elements was an important contributing factor in the synthesis and isolation of the transcurium elements. Most of the chemical identification was carried out by eluting the elements from columns of cation exchange resin. The pattern of the elution behavior from the resin bed of the lanthanide elements made it possible to predict with good accuracy the expected elution position for a new actinide element (Fig. 16.7). This technique constituted the most definitive chemical evidence in the discovery experiments for the elements from atomic numbers 97 through 101. More recently these conclusions have been confirmed by spectroscopy. 

The actinides are a row of radioactive elements from thorium to lawrencium. They were not always separated into their own row in the periodic table. Originally, the actinides were located within the d-block following actinium. In 1944, Glenn Seaborg proposed a reorganization of the periodic chart to reflect what he knew about the chemistry of the actinide elements. He placed the actinide series elements in their own row directly below the lanthanide series. Seaborg had played a major role in the discovery of plutonium in 1941. His reorganization of the periodic table made it possible for him and his coworkers to predict the properties of possible new elements and facilitated the synthesis of nine additional transuranium elements. 

The nature of bonding in classical coordination and organometallic complexes of actinide elements has attracted interest since their discovery. Several papers have emphasized, through various experimental techniques, some covalence in the metal-ligand bonding and now... 

It has been more than fifty years since the discovery of the transuranium elements. The initial activities in this field established the fundamental solution and solid-state chemistry of the first two of these elements and their compounds under the auspices of the Manhattan Project. New separation methods including solvent extraction techniques and uranium isotope separation played a leading role in these programs. Tracer techniques were widely used to determine solubilities (or solubility liinits) of transuranium compounds as well as to obtain information about the coorination chemistry in aqueous solution. A little later, special solvent extraction and ion-exchange techniques were developed to isolate pure transplutonium elements on the milligram and smaller scale. The second edition of The Chemistry of the Actinide Elements, published in 1986 (i), covers most of these topics. A detailed overview of the history of transuranium chemistry is given in Transuranium Elements A Half Century (2). 

The basis for the family relationships among the lanthanide elements themselves, and of periodic relationships that include the actinide elements, is their elemental and ionic electronic configurations. The fascinating story of the discovery and development of this truth can be found in several chapters in volume 11 of this Handbook on the Physics and Chemistry of Rare Earths (1988) and in the chapter by Seaborg (ch. 118) in this volume. The oxides provide a delicately sculpted model of the relationships between each element and other members of its series and also of the relationships between the two series. The electronic configurations for the ground state of the atoms and the relevant valence states of these elements are listed in table 1. The similarities and lack of congruence for the oxides of these two series is discussed in section 3. 

The strong similarity in the solution chemistry of the 4f and the 5f elements is most evident for the trivalent oxidation state of both families. The discovery experiments of the transplutonium actinides depended directly on this similarity as it allowed very accurate predictions of the chemical properties of the to-be-discovered elements. However, the actinide series is not an exact analog of the lanthanide elements. For example, while the stability of the trivalent oxidation state is a primary characteristic of all the lanthanide elements, trivalency is not the most stable state for the actinide elements of Z = 90-94 and 102. The greater stability of Nof q, relative to No, j,+ is not observed in the 4f analog although Yb " can be present in reducing systems. Such differences are related to the dififerences in the relative energies of the (n)f, (n -I- l)d and (n -I- 2)s orbitals when n = 4 (Ln) and 5 (An). 

Table I summarizes the discovery or synthesis of all of the actinide elements.

The actinide elements missing in 1915 were aU discovered in the period 1917-1940. Number 43 technetium was also discovered in 1940, so now only one of the 92 elements on earth was missing. It was number 61, promeihium Pm, one of the rare earth elements. Its discovery in 1945 was described in Chapter 17 Rare earths. 

In 1940 E. McMillan and P. H. Abelson at the University of California, Berkeley, irradiated U with neutrons this led to the formation of element number 93, which they called neptunium [52.18]. What they did can be described by the first formulas in Table 52.3. Their successful result was the start of intensive work that led to the discovery of all the actinide elements. In fact neptunium and also plutonium occur in nature. Minute amounts of these elements are produced in the radioactive decay of uranium. 

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