Chemical properties of the heaviest elements

The centrifugal separator of the AKUEVE system is also used for phase separation in the SISAK technique [84]. SISAK is a multistage solvent extraction system that is used for studies of properties of short-lived radionuclides, e.g., the chemical properties of the heaviest elements, and solvent extraction behavior of compounds with exotic chemical states. In a typical SISAK experiment, Fig. 4.34, radionuclides are continuously transported from a production... 

In the discussion of the rather unexpected chemical results [42], it was suggested that the chemical properties of the heaviest elements cannot reliably be predicted by simple extrapolations of trends within a group of elements , and that relativistic, quantum chemical calculations for compounds of Nb, Ta, Pa, and Db are needed to understand in detail the differences in the halide complexing of the group-5 elements . 

One century after the beginning of most dramatic changes in physics and chemistry, after the advent of quantum theory and in the year of the 100th anniversary of Paul A.M. Dirac, modern relativistic atomic and molecular calculations clearly show the very strong influence of direct and indirect relativistic effects not only on electronic configurations but also on chemical properties of the heaviest elements. The actual state of the theoretical chemistry of the heaviest elements is comprehensively covered in Chapter 2. It does not only discuss most recent theoretical developments and results, where especially up to date molecular calculations dramatically increased our insights over the last decade, but it also relates these results to experimental observations. 

Investigations of chemical properties of the heaviest elements belong to the most fundamental and important areas of chemical science. They seek to probe the uppermost reaches of the Periodic Table of the elements where the nuclei become extremely unstable and relativistic effects on electronic shells are very strong. This makes both theoretical and experimental research in this area extremely exciting and challenging. 

In the discussion of these rather unexpected chemical results [75], it was suggested that the chemical properties of the heaviest elements cannot reliably be... 

Nuclear chemistry consists of a four-pronged endeavor made up of (a) studies of the chemical and physical properties of the heaviest elements where detection of radioactive decay is an essential part of the work, (b) studies of nuclear properties such as structure, reactions, and radioactive decay by people trained as chemists, (c) studies of macroscopic phenomena (such as geochronology or astrophysics) where nuclear processes are intimately involved, and (d) the application of measurement techniques based upon nuclear phenomena (such as nuclear medicine, activation analysis or radiotracers) to study scientific problems in a variety of fields. The principal activity or mainstream of nuclear chemistry involves those activities listed under part (b). 

Advances in relativistic quantum theory and computational methods made it possible to predict properties of the heaviest element compounds by performing accurate calculations of their electronic structures. Relativistic atomic and molecular calculations in combination with various models were useful in helping to design sophisticated and expensive chemical experiments. Experimental results, in turn, were helpful in defining the scope of the theoretical problems and provided an important input. The synergism between the theoretical and experimental research in the last decade led to better understanding the chemistry of these exotic species. 

Extensive DFT and PP calculations have permitted the establishment of important trends in chemical bonding, stabilities of oxidation states, crystal-field and SO effects, complexing ability and other properties of the heaviest elements, as well as the role and magnitude of relativistic effects. It was shown that relativistic effects play a dominant role in the electronic structures of the elements of the 7 row and heavier, so that relativistic calculations in the region of the heaviest elements are indispensable. Straight-forward extrapolations of properties from lighter congeners may result in erroneous predictions. The molecular DFT calculations in combination with some physico-chemical models were successful in the application to systems and processes studied experimentally such as adsorption and extraction. For theoretical studies of adsorption processes on the quantum-mechanical level, embedded cluster calculations are under way. RECP were mostly applied to open-shell compounds at the end of the 6d series and the 7p series. Very accurate fully relativistic DFB ab initio methods were used for calculations of the electronic structures of model systems to study relativistic and correlation effects. These methods still need further development, as well as powerful supercomputers to be applied to heavy element systems in a routine manner. Presently, the RECP and DFT methods and their combination are the best way to study the theoretical chemistry of the heaviest elements. 

The study of the chemical properties of the heaviest known elements in the Periodic Table is an extremely challenging task and requires the development of unique experimental methods, but also the persistence to continuously improve all the techniques and components involved. The difficulties are numerous. First, elements at the upper end of the Periodic Table can only be artificially synthesized "one-atom-at-a-time" at heavy ion accelerators, requiring highest possible sensitivity. Second, due to the relatively short half-lives of all known transactinide nuclides, very rapid and at the same time selective and efficient separation procedures have to be developed. Finally, sophisticated detection systems are needed which allow the efficient detection of the nuclear decay of the separated species and therefore offer unequivocal proof that the observed decay signature originated indeed form a single atom of a transactinide element. 

Achievements in the area of the theoretical chemistry of the heaviest elements are overviewed. The influence of relativistic effects on properties of the heaviest elements is elucidated. An emphasis is put on the predictive power of theoretical investigations with respect to the outcome of "one-atom-at-a-time" chemical experiments. 

Recent advances in relativistic quantum theory and computational methods have raised the research in the theoretical chemistry of the heaviest elements to a qualitatively new level. It became possible to predict properties of the heaviest elements, their gaseous compounds and complexes in solutions with a sufficiently high accuracy. On the basis of those calculations, the behaviour of the heaviest element species in specific chemical experiments was reliably predicted and confirmed by specially designed experiments. 

For frontier research on both the chemical and nuclear properties of the heaviest elements, large quantities of heavy actinides with relatively long half-Kves, such as Es and are... 

Early reviews on predictions of transactinide element properties based on relativistic atomic calculations and extrapolations are those of [1-5]. More recent reviews on the theoretical chemistry of the heaviest elements covering also investigations of molecular, complex, and solid-state properties are those of [6-14]. Chemical and physical properties of the heaviest elements including theoretical aspects are also discussed in [15]. 

Studies of the chemical properties of the heaviest actinides (Z > 101) and aU of the transactinides (Z > 103), including superheavy elements (SHEs), depend on the use of atom-at-a-time chemistry. They cannot be produced by simple neutron... 

The last decade was marked with the discovery of five new members of the Periodic Table The heaviest elements of the last transition element series 110 through 112 were identified in the Gesellschaft fiir Schwerionenforschung (GSI), Darmstadt [1-3] and some decay chains and fission products associated with production of even more heavy elements 116 and 114 were recently reported by the Joint Institute for Nuclear Research (JINR), Dubna [4], This period of time was also very fruitful with studying chemical properties of the very heavy elements [5-9],... 

Though substantial progress has been reached in understanding the chemistry of the heaviest elements, there is still a number of open questions needed to be answered from both the experimental and theoretical points of view. For the superactinides, investigations of the chemical properties will be even more exciting, since the resemblance of their properties to those of their lighter homologues will be much less pronounced. 

DU has 40% less radioactivity than natural uranium, but may contain trace levels of plutonium, neptunium, americium, technetiiun, and U, which increase the radioactivity by 1% but are insignificant with respect to chemical and radiological toxicity (Force Health Protection Readiness Policy Programs, 2008 Sztajnkrycer and Often, 2004 WHO, 2001). Because of the decreased radioactivity of DU, it is believed that DU is a safer form than natural uranium, while maintaining the same chemical properties. As the heaviest occurring element, uranium is extremely dense and both uranium and DU are often used in applications which require such dense metals. 

Single atom chemistry is of particular importance if only single atoms are available for chemical studies, as in the case of the heaviest elements. The short-lived isotopes of these elements can only be produced at a rate of one atom at a time, and the investigation of their chemical properties requires special considerations. 

Because of the uniqueness of divalency within a series of elements that are commonly trivalent, most of the chemical research concerning the heaviest actinides has been concentrated on studies of lower oxidation states. The chemical properties of the trivalent ions of the lathanides and actinides are virtually the same throughout both series and, for this reason, there has been little incentive to specifically study this oxidation state in Md, No, and Lr. This close relationship between the scientific significance and the research completed up to now is strongly correlated with the extraordinary effort required to produce experimental information concerning these elements. In a scientific sense, the principle of cost-effectiveness has governed the selection of research topics. Beside the scientific effort required, there are additional restrictions to obtaining extensive experimental data. 

In this chapter, results of recent theoretical investigations in the chemistry of the heaviest elements are reviewed. Chemical properties, trends and an analysis of the role of relativistic effects are discussed. The results obtained by various calculational methods are critically compared. Special attention is paid to the predictions of properties of superheavy elements studied by experiment. 

Except for few properties, like volatility or complex formation, many others cannot be directly measured for the heaviest element compormds. They can only be evaluated. For example, the chemical composition of superheavy element compounds is not known and can only be assumed on the basis of analogy in the experimental behaviour with that of the lighter congeners in chemical groups. Ionisation potentials (IP), electron affinities (EA), dissociation energies or geometrical structures can not presently be measured at all, and can only be determined via quantum-chemical calculations. Thus, in the area of the heaviest elements theory starts to become extremely important and is often the only source of useful chemical information. 

This was the first time that predictions of extraction behaviour of the heaviest elements based on relativistic quantum-chemical calculations were made, and also confirmed by specially designed experiments. Only by considering all possible equilibria in the aqueous phase including hydrolysis could this imexpected behaviour be predicted. Simple extrapolations of properties within the group would have shown the straightforward and, consequently, wrong trend. 

Although a richness of information has been obtained, a number of open questions still remain. For elements which were chemically identified, like Rf or Sg, a more detailed study, both theoretical and experimental, should follow. Elements 109, 110 and 111 are still to be studied experimentally the prerequisites for their successful experimental studies should be similar to those of the lighter transactinides. These include the existence of isotopes long enough for chemical studies, knowledge of their nuclear decay properties, so that they can be positively identified, synthesis reactions with the highest possible cross sections and suitable techniques for their separation. For those elements, predictions of the chemical behaviour are a matter of future research. Especially difficult will be the accurate prediction of adsorption of the heaviest elements on various surfaces, or their precipitation from aqueous solutions by determining electrode potentials. For that, further developments in accurate calculational schemes are needed. More sophisticated methods are needed to treat weak interactions, which are important for physisorption processes. 

One element Fermi bombarded with slow neutrons was uranium, the heaviest of the known elements. Scientists disagreed over what Fermi had produced in this transmutation. Some thought that the resulting substances were new transuranic elements, while others noted that the chemical properties of the substances resembled those of lighter elements. Fermi was himself uncertain. For the next several years, attempts to identify these substances dominated the research agenda in the international scientific community, with the answer coming out of Nazi Germany just before Christmas 1938. 

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