TRANSURANIUM ELEMENTS OF THE MENDELEEV PERIODIC TABLE OF ELEMENTS
Russian physicists were the first in the world to synthesize six of the transuranium elements with atomic numbers 113-118. The results make it possible to refine existing models of the nucleus and nuclear reactions, to better understand the chemical properties of long-known and widely used elements, to understand how the electronic structure of atoms and molecules determines their specific chemical properties. Further progress on the Periodic Table may lead to the formation of a completely new group of elements, superactinides (starting with element 121) with interesting properties.
Synthesis of elements 115, 117 and 118 was carried out in the Joint Institute for Nuclear Research (JINR, Dubna) employing the U-400 accelerator complex of the Flerov Laboratory of Nuclear Reactions in reactions of accelerated ions of Ca-48 with actinide targets. Further, the results obtained in Dubna were confirmed by researchers in Germany (GSI, Darmstadt) and the USA (Berkeley). Earlier, in 2011, the IUPAC recognized the priority of Russians in the discovery of elements 114 and 116.
In general, synthesis of elements 114 through 118 was the first experimental discovery of “the islands of stability” in the domain of superheavy elements that is of principal fundamental importance for nuclear physics (study of nuclear structure), for chemistry (verification of the fundamental D.I. Mendeleev law of periodicity of properties of chemical elements), for understanding of one of the key questions of the modern science that is the formation of heavy nuclei (process of nucleosynthesis), etc.
All in all, in the last 50 years, the Mendeleev Periodic Table was replenished by 17 new elements (102 – 118), 9 of which were synthesized in JINR; among the latter are the heaviest (superheavy) five elements of the Periodic Table produced in the recent decade.
Today FLNR JINR is the recognized world leader in synthesis and study of the properties of new superheavy elements of the Mendeleev Periodic Table. However, scientific competition in this field increases, and today (taking into account the results obtained in FLNR JINR) the corresponding programs are approved in Germany, Japan, China, France and the USA. Further progress of these studies here in JINR is connected with the development in the institute of the first in the world Factory of Superheavy Elements. This is to be based on the new accelerator of heavy ions, the most powerful in this energy range, with intensity 10 times higher than has been achieved by today, that will allow to set goals of synthesizing new elements with atomic number 119 and 120 and further, that are the first elements of the eighth period of the Mendeleev Periodic Table.
The discovery of new elements in the Mendeleev Periodic Table has always been of interest to the public. It’s not even so much the scientific significance of these discoveries as the fact that at school everyone studied the Periodic Law, and some even remembered the symbols for the elements. But now behind these discoveries are complex studies in nuclear physics and radiochemistry, about which many people have no idea.
At present, new elements are synthesized only with heavy ion accelerator facilities (previously, they were found in terrestrial minerals, products of nuclear reactors and nuclear explosions). For the synthesis of the most stable isotopes, such combinations of nuclei are chosen that contain as many neutrons as possible and the composite nuclei have low excitation energy. The yield of the produced heavy elements is extremely small – individual atoms or tens of atoms produced, sometimes, after months of irradiation process. Half-life takes seconds and sometimes fractions of a millisecond. It is quite difficult to isolate the nuclei of new elements from the mixture of nuclear reaction products formed and to correctly identify them. For this purpose, special facilities are created that register a chain of decays with emission of alpha particles and formation of isotopes of lighter elements, sometimes the chain ends with spontaneous nucleus fission.
In Russia, since the 1950s, work on the synthesis of new elements with the help of heavy ion accelerator facility has been carried out in Dubna under the leadership of Academician G. N. Flyorov (1913-1990), the founder of this field. Now these works are carried out under the supervision of Academician Yu.Ts. Oganessian. There are only a few accelerator facilities in the world where it is possible to produce transactinides (i.e. elements with Z nuclear charge of more than 103).
According to the regulations, the authors are to suggest names of the new elements. What scientific significance can the discovery of new elements have? What does it change in our ideas about the nuclear structure and the properties of elements in general?
From the physical point of view, these results may be important for a better understanding of the nuclear structure and nuclear interaction. Since the 1960s, there has been a heated discussion of the existence of so-called “the islands of stability” in Z nuclear charges 114 and 126 as a manifestation of the nuclear shell model. Therefore, producing the first transactinides of a much longer half-life than predicted by the old liquid drop model, was really fundamentally important. Now no one doubts the shell model. The results of studies of the new elements and new isotopes allow to refine the existing models and nuclear reactions. Although no fundamentally new phenomena are expected, new data is always useful. Obviously, the top of “the island of stability” cannot be reached by existing methods: there are simply no such combinations in nuclear reactions – there are not enough neutrons in the resulting isotopes. Earlier, there were attempts to detect super heavy elements in Nature that could be so long-existed as the Solar System. But these attempts were unsuccessful. The once stated results have found neither experimental nor theoretical confirmation.
From the chemical point of view, the situation is somewhat different. Fundamentally new phenomena can indeed be expected here. It is about the so-called “relativistic effects”. A nucleus with a large charge will cause an electron to have a high velocity, and the Schrödinger equation no longer works in this case. In particular, the dumbbell-shaped p-electrons undergo changes in the VII period, and one of them turns into a ball-shaped one. As a result, the electronic structure of atoms changes. The new elements may have a significant deviation of chemical properties from those extrapolated from the Periodic Table and the emergence of unusual chemical properties.
One way or another, the study of relativistic effects makes it possible to better understand the chemical properties of long-known and widely used elements. It also makes it possible to better understand how the electronic structure of atoms and molecules, which can be calculated, determines their specific chemical properties. This is still far from being a fully resolved issue. Further progress on the Periodic Table may lead to the formation of a completely new group of elements, superactinides (starting with element 121) with interesting properties.