On the recommendation of the United Nations General Assembly and the specific resolve of the United Nations Educational, Scientific and Cultural Organisation (UNESCO), the year 2019 was declared as the International Year of the Periodic Table of Chemical Elements (IYPT 2019). This decision was based on a strong appreciation of international character of scientific cooperation pursued in several frontier areas of basic and applied sciences related to the Periodic Table. It should commemorate the 150th anniversary of the creation of that Table. As expected, the decision was hailed with great enthusiasm by a large number of national, regional and international chemical societies and other scientific organisations. The IYPT was officially launched on 29 January 2019 at the UNESCO headquarters in Paris, France. Since then different types of functions, e.g. conferences, symposia, popular lectures, exhibitions and social gatherings, etc. have been going on around the world, with the aim to increase the public awareness of science in general, and chemistry in particular. Furthermore, a large number of journals are publishing editorials, commentaries or special issues, and a few publishers are bringing out special pamphlets to mark the occasion. Even a few newspapers and magazines have printed relevant articles. Since radiochemistry has contributed substantially to the extension of the Periodic Table as well as to the development of various applications related to it, this special issue of Radiochimica Acta is being published as a part of international celebrations. In this Editorial we shortly describe the origin and development of the Periodic Table and give a brief overview of its present status, discussing some related areas of particular significance to radiochemistry.
2 Origin and development of the Periodic Table
A large number of chemical elements were discovered in the second half of the 18th century and the first half of the 19th century. Several chemists were attempting to discern some distinct patterns in their physico-chemical properties. In 1860 the first ever international chemistry conference was held in Karlsruhe, Germany, under the chair of August Kekulé, with many pioneering chemists participating, among them Robert Bunsen and Michael Faraday. The main aim of the conference was to establish standardisation methods in chemistry. During this conference the famous Italian chemist Stanislav Cannizzaro delivered a masterly lecture, as a result of which the chemistry community adopted a unified system of atomic masses. This turned out to be a crucial step towards establishing the periodic system. At that conference a young Russian chemist named Dmitri Mendeleev was also present, having come to Germany in 1859 on a 2-year state stipend to work under guidance of Robert Bunsen and Gustav Kirchhoff at the University of Heidelberg. After his return to St. Petersburg in Russia, Mendeleev conjectured an arrangement of the then known 63 elements and presented a draft table, based on atomic masses, in March 1869 to the Russian Chemical Society. A year earlier, i.e. in 1868, the German chemist Lothar Meyer had written down a very similar periodic system of elements. In the following years the two tables had to undergo severe tests. For a long time the priority dispute between Meyer and Mendeleev remained. In recognition of their works, they were jointly awarded the Davy Medal in England in 1882. It was the most prestigious award at that time. In later years, the contribution of Mendeleev received more attention. He had predicted a few missing elements, many of which, but not all, were found later to be correct.
The Periodic Table of Elements was originally constructed empirically to take account of the periodicity in their chemical properties. Some of the discoveries starting in 1890s related to the structure and divisibility or even breakup of the atom (e.g. electrons, radioactivity, etc.) were not accepted by Mendeleev till his death in 1907. He saw in them a threat to his own discovery of elements as individual units. Ironically, through subsequent development of the atomic theory and, above all, the quantum mechanics, combined with the experimental evidence that the atomic number and not the atomic mass is the deciding factor in the characterisation of an element, the Periodic Table was embedded in a theoretical framework.
3 Present status and areas of interest
The present status of the Periodic Table of Elements and some important related areas of interest are presented in this special issue in a coherent way. It is divided into three parts, namely, actinides and transactinides, nuclear energy, and medical radionuclides. In each area comprehensive reviews written by authors in the forefronts of their respective fields are presented. As Editors we greatly appreciate their efforts. A brief overview of each area is given below.
3.1 Actinides and transactinides
The Mendeleev’s Periodic Table of Elements ended with uranium (Z=92). From 1940 onwards, Glenn T. Seaborg and his co-workers at the University of California, Berkeley, USA, synthesised through nuclear reactions the elements neptunium (Z=93) to lawrencium (Z=103) and, similar to the lanthanides, a new series of elements, the actinides, was established. It was based on a strong collaboration between chemists and physicists over a period of about 20 years. Efforts to make new elements beyond lawrencium, i.e. transactinides, then slowed down. But a new pick-up came in the mid 1990s at the Heavy Ion Research Facility in Darmstadt, Germany, with many national and international collaborations, followed by other intensified efforts at the Joint Institute of Nuclear Research in Dubna, Russia, and later at the RIKEN laboratory in Japan. The Periodic Table of Elements is now extended up to oganesson (Z=118) where the seventh period of the Periodic Table finishes.
In Part A of this special issue the extension of the Periodic Table beyond uranium is discussed in detail. The physico-chemical experiments leading to the discoveries of various new elements are briefly described and the chemical properties deduced from very challenging studies on a few atoms of each element produced are elucidated, particularly with regard to their placement in the extended Periodic Table. A review of theoretical attempts to understand the very strong relativistic effects in the heaviest elements, which might destroy the periodicity of the chemical behaviour, is also presented.
3.2 Nuclear energy
The tremendous amount of energy released in the splitting of the nucleus 235U when hit by a neutron (i.e. the fission process) has been harnessed to produce electricity. It is a mature technology having been in use for more than 60 years. Worldwide about 450 nuclear power reactors are in operation and 16% of the total electricity production is of nuclear origin; in some countries, especially in France, it is more than 75%. It is a very clean form of energy because in its generation almost no CO2 is emitted. The major problem, however, is the formation of long-lived radioactive waste. Most of the present development efforts are therefore related to management and disposal of the waste. Extensive fundamental studies on speciation, immobilisation, partitioning and transmutation of the waste, consisting of mainly actinides and long-lived fission products, are underway. Here the group character of the Periodic Table of Elements plays an important role. The actinides and lanthanides behave quite differently as compared to fission products which are mostly transition metals.
In Part B of this special issue some newer ideas in fuel reprocessing and partitioning of radioactive products, under development in China, India and the European Union, are described in detail. The chemistry of some special elements, like protactinium, which pose difficulty in fuel reprocessing, is elucidated. Furthermore, an analysis of the hitherto somewhat unknown effect of the actinides on proteins is described for the first time. Finally, a timely report on the contamination of the Fukushima Nuclear Power Plant Station with actinides is presented.
Today, the heavy mass region of the Periodic Table of Elements is closely connected with the energy production via fission. Furthermore, some of the actinides are also used as energy sources in space research. It should, however, be pointed out that the light mass region of the Periodic Table of Elements is also of great potential interest for energy production via the fusion process. With the increasing progress in the international test reactor facility (ITER) at Cadarache, France, the role of the chemistry of light elements will certainly enhance.
3.3 Medical radionuclides
Besides the extension of the Periodic Table beyond the heavy element uranium, two lower mass elements not existent in nature were also artificially synthesised. They were named as technetium (Z=43) and promethium (Z=61). The former is a transition metal and the latter belongs to the group of rare earths. Because of its suitable nuclear properties and versatile chemistry a radionuclide of technetium, namely 99mTc (T½=6.0 h) has become the most important radionuclide in diagnostic nuclear medicine involving Single Photon Emission Computed Tomography (SPECT). About 40 million patients per year are investigated worldwide using this radionuclide.
Some characteristics of the Periodic Table of Elements influence the various medical uses of radionuclides. The property of α-particle emission from many heavy mass nuclei makes them attractive for α-targeted therapy. In the region of the lightest mass elements, on the other hand, the radionuclides decay predominantly by positron emission. Since those elements are major constitutes of all living systems, organ function studies can be advantageously performed using Positron Emission Tomography (PET). As regards medium mass elements, a mixture of three decay modes, viz. β− emission, β+ emission and electron capture (EC), occurs and the radionuclides are finding versatile applications both in diagnosis and internal radiotherapy. Since most of those elements have metallic character, the significance of organo-metallic complexes is rapidly enhancing.
The periodicity of the Periodic Table of Elements is reflected to some extent in the uptake of radionuclides and radiopharmaceuticals by living systems. Alkali and alkali-like metal radionuclides (e.g. 38K, 82Rb, 201Tl1+) find application in cardiac blood flow studies. Other examples are the specific attachment of radionuclides of trivalent metals (e.g. 86Y, 177gLu) to tumour seeking agents, or halogens to biomolecules (via an analogue approach) for study of metabolic processes.
Some of the above mentioned aspects of medical radionuclide development and application are treated in detail in Part C of this special issue.
Over the last 150 years the Periodic Table of Elements has been extended to about double of its original size (in this period of time, 7 new natural radioelements and 28 artificial radioelements were discovered), and the Periodic Table has received a very strong scientific base, thanks to sophisticated physical and mathematical approaches attached to it. Its scope and applications have been extensively broadened, starting from fundamental aspects of relativistic influence on chemical bonding, covering fission and fusion energy systems and extending to organ function studies via molecular imaging as well as to internal radionuclide targeted therapy. Many more applications are expected to emerge in the future.