Dear Colleagues,

From the moment the concept of a "chemical element" appeared in chemical science, the problem of determining their assortment and their systematization has been one of the key problems of general chemistry. Attempts at such a systematization were repeatedly made at different times and in different countries, in particular by the English chemists J. Newlands and W. Odling, by the French chemist Alexandre-É. de Chancourtois, and by the German chemist L. Meyer. Without detracting from their achievements in solving this problem, one cannot fail to note the following important circumstance: all these attempts to systematize chemical elements by these and other scientists, until the end of the 1960s of the 19th century, were actually nothing more than particularistic (sometimes purely random) observations and comparisons.

Accordingly, they led only to one or another particular generalization, devoid of those basic features that should be inherent in the law of nature. A real breakthrough in this direction was made in 1869 thanks to the discovery by the Russian chemist Dmitriy I. Mendeleev of a fundamental regularity according to which the properties of chemical elements are in a periodic dependence on their atomic masses (in a more accurate modern interpretation, on the charge of the nuclide), and which is currently known as "DI Mendeleev's Periodic Law”. Mendeleev proposed a tabular version of the system of chemical elements, which became the prototype for one of the most common interpretations of this law, namely, the so-called “short-periodic” variant. This option is still the most common in Mendeleev's homeland—the Russian Federation; in the world as a whole, it became the most popular the so-called “long-periodic” variant proposed by another great scientist, the Swiss chemist A. Werner.

At present, there are three main problems associated with the systematization of chemical elements. First, how many chemical elements exist and what are the regularities of their mutual arrangement in the periodic system. Second, if is there any quantitative (i.e., expressed by a strict mathematical formula) regularity between at least some property of a chemical element and the charge of its nuclide. Third, the search for new and improvement of existing variants for the interpretation of the Periodic Law in order to identify the most optimal among them. Therefore, the first two of these problems, despite the simplicity of their formulations, still do not have an unambiguous answer; as for the third problem, the total number of interpretations of D.I. Mendeleev's Periodic Law is already more than 1000 and continues to grow from year to year, and the question of which of them should be considered the most closely corresponding to the formulation of the Periodic Law remains open.

Taking this into account, this Special Issue is intended to include primarily original full papers and brief communications on any research that in one way or another aims at solving any of the three above problems. We also welcome articles that will present results that contribute to the decision of any particular issues not directly related to the above three problems, but related to the general problem of systematization of chemical elements (for example, connected with historical events and/or facts related to it). Review articles may also be submitted for publication in this Special Issue (including author's reviews focusing primarily on their authors' own publications).

Prof. Dr. Oleg V. Mikhailov
Guest Editor

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• I. Mendeleev’s periodic law
• Periodic System of chemical elements
• “half-long-periodic” variant
• “long-periodic” variant
• “short-periodic” variant