Foreword for PAC special issue 4^th International Conference on the Periodic Table (Mendeleev 150)

The year 2019 is the 150^th Anniversary of the publication of Dmitri I. Mendeleev’s Periodic Table of Chemical Elements, and the United Nations proclaimed 2019 the International Year of the Periodic Table, which greatly inspired a global enthusiasm and interests in the Periodic Table and chemistry in general. Its history and the people, the importance and application of the periodic law, its reasoning, the limits of the existence of nuclei and that how far the table can go, all these are interesting subjects and a lot of related discussions, seminars, workshops and celebrations have been carried out. IUPAC is honored to have taken the lead in the global celebrations, and the journal Pure and Applied Chemistry is responsible to publish some of the reports as special issues. It is my great pleasure and honor to introduce to the readers the special Issue dedicated to Mendeleev 150, which was held in Saint Petersburg, Russian Federation, July 26–28, 2019 as the 4^th International Conference on the Periodic Table.

Since the mid-nineteenth there are many scientists who devoted themselves to the discovery and development of Periodic Law of chemical elements. Dmitri I. Mendeleev has been considered the father of the Law, and Julius L. Meyer is among the most recognized. However, we will see from Mikhail Kurushkin’s paper that there have been some others who contributed greatly but almost forgotten. Viatscheslaw Romanoff is an unknown genius of the periodic system. Romanoff predicted essentially the discovery of elements #106, #111 and #118. His version of the periodic table is an unparalleled precursor to the contemporary periodic table, and is an example of extraordinary anticipation of the discovery of new chemical elements.

Mathematical description of the Periodic Law helps reasoning and understanding of the system and has been of great interests. In his paper Naum S. Imyanitov introduced a new formulation and equation description of the Periodic law, where the total number of i-electrons in an atom belonging to i-block is used as the new basis for the description of the periodicity. This made possible to propose a new formulation of the Periodic law and to describe the Periodic law by an equation. The equation provides opportunities for large-scale prediction of the properties of elements and their compounds. Theoretical and applied aspects of the application of the new formulation and equation are outlined. The suitability of the equation is exemplified by the description of the properties of atoms and elements, such as ionization energy, electron affinity, proton affinity, electronegativity, covalent atomic radii and the enthalpy of element formations in the gas phase.

Searching for new elements has been a continuous endeavor. Following the completion of the first 7 periods of the table with 118 elements, IUPAC working together with IUPAP has had ready the Criteria and guidelines for establishing priority of discovery of potential new elements as well as possible experimental developments for production and identification of new elements beyond the presently known ones. How far the periodic table can go and what is the highest proton number Z? Anatoli Afanasjev and coworkers at Mississippi State University demonstrated possible existence of three islands of stability of spherical hyper-heavy nuclei centered around (Z~138, N~230), (Z~156, N~310) and (Z~174, N~410), which are expected to be reasonably stable with respect of spontaneous fission, cluster and α-decays. In the paper “An Essay on Periodic Tables”, Pekka Pyykkö reviewed the history of the PT, from Döbereiner’s triads to the theoretical predictions up to element 172, and discussed a number of particular issues: Why may Z=172 be a limit for stable electron shells? What are the expected stability limits of the nuclear isotopes? When are formally empty atomic orbitals used in molecular electronic structures? What is ’Secondary Periodicity’? When do the elements (Ir, Pt, Au), at the end of a bond, simulate (N, O, I), respectively?

In Viktor Vyatkin’s paper “Spin-Orbital Exclusion Principle and the Periodic System”, it is shown that in the real configuration of the atom there is a spin-orbit periodicity, which develops in cycles consisting of two periods. An explanation of the reason for the appearance of the subshell 4s earlier than 3d is given, and the nature of the empirical rule “n+l” is revealed. It was concluded that the evolution of the electronic structure of an atom in the process of increasing the nuclear charge does not follow the path of sequential formation of shells consisting of electrons with the same principal quantum number, but along the path of periodically increasing the number of electrons in radial groups. A spin-orbital periodic table has been compiled, the prototype of which is Janet’s left-step table. Charles Janet’s left-step table was considered by some earlier studies one of the best representation for the Periodic System.

Peter Atkins discussed the symmetry beneath the table which inspiringly explores the foundations of the table in a different way from the conventional approach. On the other hand, W. H. Eugen Schwarz discussed the physical origin of the periodicities in the system of chemical elements. He argues that only if the internal structure of the Periodic Table is connected to scientific facts of the chemical world, the structure of this Periodic Table can be rationalized quantum physically, by elucidating the details of the energies and radii of the atomic core and valence orbitals in the leading electron configurations of chemically bonded atoms. He emphasizes that Periodic Tables designed after some creative concept for the overall appearance are of interest in non-chemical fields of wisdom and the arts.

In her paper, “Levi’s Periodic System vs. Mendeleev’s Periodic System”, Elena Ghibaudi made a thought-provoking comparison between Primo Levi and Dmitri Mendeleev, based on their peculiar ways of conceiving their professional role of chemist, their life experiences, their achievements and their thought. She pointed out that the Weltanschauung of these two figures, despite their having lived in distinct historical periods and their belonging to distinct cultures, was deeply influenced by the fact of being chemists: chemistry was – for both of them – a tool for interpreting the world around them and acting effectively in it. The chemistry Levi talks about in his writings is not just a narrative pretext: it is part of his vision of the world and a means of survival in the hellish context of the extermination camp. Similarly, Mendeleev’s idea of chemistry was always related to the life context and the human condition: this explains his pedagogical concerns and the attention payed to social, economic and cultural issues typical of his time. Both Levi and Mendeleev were chemists for whom chemistry was a means of civil engagement. Their writings show that chemistry was a source of inspiration for their ethics.

In this special issue there are also a few papers contributed to the inspiring history of the discoveries and the people. David Seaborg tells us the life and contributions to the Periodic Table of Glenn T. Seaborg who is the first person to have an element named after him while he was still alive. The second person to have this honor is Yuri Ts. Oganessian the “Mr. 118”. Mendeleev’s Limitations is discussed by Philip J. Stewart. The history on nihonium is given by Hideto En’yo.

The discovery of the periodic system of chemical elements and the principle of periodicity made it possible not only to predict the properties of still unknown elements, but also influenced other areas of natural science such as physics, chemistry of subatomic and supra-atomic objects, biology, and areas there is a need for natural classification. In his paper, Eugene Babaev made a review of the most striking attempts to generalize the principle of periodic system to objects other than chemical elements in some areas of natural science.

In conclusion, the conference Mendeleev 150 offered an excellent opportunity for us (chemists, physicists, mathematicians, historian, philosophers, etc.) to get together to think back, review the current, and looking forward to the future of the periodic law. The past is enlightening, the present inspiring, and the future yearning.