By decision of the UN General Assembly 2019 was declared an International Year of the Periodic Table of Chemical Elements. On this occasion, in Bulgaria, an anniversary postage stamp was issued. Search for a link between the properties of the chemical elements and their atomic weight was undertaken before Mendeleev by many chemists. It is characteristic of all these classifications that the observed proximity in the properties of the elements has been attributed to chance. Mendeleev, however, was the only one who perceived that beyond the periodicity of the elements’ properties there is an important natural law. With the discovery of the structure of atoms, the periodic table of chemical elements found its scientific explanation. The periodic table set an important task to chemistry – to look for the relationship between the composition and the properties of the substances, including the disclosure of the regularities which are the basis of the so-called “anomalies”.
By decision of the UN General Assembly 2019 was declared an International Year of the Periodic Table of Chemical Elements. With this act the United Nations are respecting the role of chemistry in the sustainable development and the solutions it gives to global challenges to humanity. The occasion is the 150th anniversary of the publication by Dmitry Mendeleev (Fig. 1) in 1869 of a table of the chemical elements known at that time, in an ascending order of their atomic masses. On this occasion, the International Union of Pure and Applied Chemistry (IUPAC) organized series of events and encouraged his National Adhering Organizations to mark this anniversary with celebrations, publications and, etc. , . In some of the countries anniversary postage stamps were issued .
The creation of the Periodic Table of the Chemical Elements is one of the greatest generalizations in physicochemical science after the atomic molecular theory and the laws of thermodynamics, a synthesis that links the lessons learned from the experience gained over time into a unified system and thus creates one of the foundations for further development of natural sciences.
The pursuit of human thought to seek unity in the infinite multitude gave birth to the idea of primary matter. The creator of atomism Democritus postulated: “There is only matter and void” . In this ancient age, when people believed in the existence of many gods, Democritus assumed that only matter exists.
The first scientific definition of a chemical element as a pure chemical substance which cannot be subdivided into simpler ones, was given by Robert Boyle (1627–1691) in 1661  (Fig. 2). Antoine Lavoisier (1743 –1794) made the first list of chemical elements in 1789, which contained 33 elements, including light and heat . In 1815, Jöns Jacob Berzelius (1779–1848) made a classification of elements, compounds and minerals and divided elements on the basis of their chemical and physical properties as metals and nonmetals (metalloids) . He also introduced modern chemical symbols. In 1829, Johann Wolfgang Döbereiner (1780–1849) united the elements into groups of three elements, the so-called triads, pointing out that some properties of the elements and their compounds are arithmetic means of the properties of the two end elements .
The basic concepts of chemistry, such as atom and molecule, and the determination of atomic weights, were eventually clarified only in 1860 when, at the initiative of August Kekulé (1829–1896) the first International Congress of Chemistry took place in Karlsruhe. At this congress, a paper was distributed among the participants by Stanislao Cannizzaro (1826–1910) . In this paper Cannizzaro insist on the distinction, previously hypothesized by Avogadro, between atomic and molecular weights and shows on wide experimental basis the difference between atomic weights and equivalent weights, a key step on the pathway to the discovery of the periodic table a decade later.
Mendeleyev attended this forum, and Cannizzaro’s work inspired him organize the elements in his table. The congress was also attended by Julius Lothar Meyer (1830–1895).
After the Karlsruhe congress, there was a boom in the search for a link between the properties of the chemical elements and their atomic weight:
In 1862, Alexandre-Emile Béguyer de Chancourtois (1820–1886) put the 50 elements known by that time (ordered by ascending atomic masses) on a helix drawn on a cylindrical surface and showed that elements with similar properties are located one under another . He was the first to notice the periodicity in the properties of the elements.
In 1863, John Alexander Reina Newlands (1837–1898) ordered the elements by eight in seven octaves, and found that every eighth element has properties analogous to those of the first element . Newland called this rule an “octave law”, but he was ridiculed by his contemporaries, and the Chemical Society refused to publish his findings. Nevertheless, Newlands drew a table of elements and used it to predict the existence of missing elements such as germanium. The chemical community only recognized the significance of his discovery 5 years after Mendeleyev has already been acknowledged.
In 1864, Julius Lothar Meyer published a table of 44 elements arranged according to valence. The table showed that elements with similar properties often have the same valence .
At the same time, the English chemist William Odling (1829–1921) published a table of 57 elements ordered by atomic weight, and mentioned the idea of a periodic law, but did not pass it consistently. Subsequently, he offered a classification of the elements according to their valence .
In 1867, the German chemist Gustavus Detlef Hinrichs (1836–1923) published a helical periodic system based on the atomic spectra and atomic weights and on the chemical similarities of the elements .
It is characteristic of all these classifications of chemical elements that none of their authors attempted to link the groups into a common system. The observed proximity in the properties of the elements has been attributed to chance, the elements were seen as individuals having nothing in common with each other. Evidence of this is the great distrust to the report by Newlands in 1864, in which he announced the regularity found by him in the change of the properties of chemical elements with an increase in atomic weight. Some of the audience asked Newland with a mockery of whether it would not be right if the elements were ordered by the initial letters of their names – so the thought of a common connection between the properties of all elements was alien to the minds of the chemists then. And this was only 5 years before the recognition of the periodic system.
In 1869, Dmitry Ivanovich Mendeleev (1834–1907) published an extended version of this table, also ordered by ascending atomic masses  (see Fig. 3). It contained 63 chemical elements and became the basis of the periodical table used today.
Lothar Meyer inspired by Cannizzaro, also drew his own table, much more detailed than that of Mendeleev . However, it was published only a few months later and, as one might expect, a cruel priority dispute arose between them.
Mendeleev’s priority, however, was not in these few months, but in his firm conviction that an important natural law was revealed. Using the periodic system, Mendeleev straightened the atomic weights of some of the elements so that they can take such places in the table as to fit their properties. On this occasion, Lothar Meyer wrote in “Liebig’s Annalen der Chemie und Pharmacie”: “It would be premature to change the atomic weights adopted today because of such an uncertain starting point”. Meyer was right as a man of strict scientific thought, but Mendeleev was right as all the data obtained since 1869 confirmed his atomic weights.
Mendeleev’s triumph came, however, after predicting the existence and properties of three previously unknown elements. Not a few years passed when the elements Ga, Sc and Ge were discovered. The properties of these elements were very close to those Mendeleev had predicted.
The first impetus for the recognition of Mendeleev’s priority was the discovery of gallium in 1875 by Paul-Émile Lecoq de Boisbaudran (1838–1912) . Mendeleev compared the properties of gallium with those of the predicted by him ekaalumium. He wrote a letter to Lecoq de Boisbaudran, pointing out that the density of gallium determined by him (4.7 g/cm3) did not match to its place in the periodic system, according to which it should be 5.9–6 g/cm3. Lecoq de Boisbaudran repeated his experiment with thoroughly purified gallium and, convincing himself in Mendeleev’s rightfulness, became his most zealous supporter.
Friedrich Engels (1820–1895), taking into account Mendeleev’s achievement, wrote in “Dialectics of Nature”: “Mendeleev, unconsciously applying Hegel’s law on the passage of quantity into quality, did a scientific feat that could boldly be put on the level of Le Verrier’s discovery, who calculated the orbit of the still unknown planet Neptune” . On this occasion, I would make the following comment. Undoubted is the success of the 25-year-old John Couch Adams (1819–1892) who, ensuing from the deviations in the movement of the planet Uranium assumed that there must be another unknown planet , and of the professor of celestial mechanics Urbain Jean Joseph Le Verrier (1811–1877) who repeated the calculations of Adams and made astronomers acquainted with them . But this prediction was only possible because of the perfection and accuracy of astronomical laws. There was no law in chemistry to allow predictions at this level. It is in the periodic system that Mendeleev fathomed out a law that allowed such predictions in chemistry. And this at a time when nothing was yet known about the structure of the atom.
In any significant scientific discovery, three important stages can be distinguished: the emergence of the idea, the realization of the idea and the imposition of the discovery, i.e. to make people perceive and acknowledge it. The idea of a relationship between the properties of the chemical elements and their atomic weight had lived for 40 years. Mendeleev, however, was the only one who perceived that beyond the periodicity of the elements’ properties there is an important natural law.
The formulation of the periodic law given by Mendeleev in July 1871 reads: “The properties of the elements, as well as the properties of the simple and complex bodies formed by them, are in a periodic dependence of their atomic weight”.
The periodic table solved a number of questions in chemistry. In the first place, it includes all the chemical elements in a common system – the periodic system of chemical elements. An important consequence is that between two adjacent elements, the existence of elements with intermediate atomic weights, both on the horizontal and the vertical of the table, is excluded. Hence, the number of chemical elements is limited. There can be no lighter element than hydrogen. On the other end of the table, as seen later, the elements become more and more unstable, so there must be an upper limit as well.
The periodic table set an important task to chemistry – to look for the relationship between the composition and the properties of the substances. In other words, to explain the properties of substances with the specifics of the elements forming them. And hence, the more distant task of expressing the whole set of possible chemical changes of the substances. In this sense, the periodic system is the first step in trying to derive a general formula expressing the chemical behavior of the substances.
Periodic regularity, however, does not possess the accuracy of physicochemical laws. It has the character of a scheme that generally reflects only qualitative dependencies. No physical or chemical property of the chemical elements or their compounds changes in a way to be expressed by an exact numerical relation. What’s more, there are catastrophic jumps in the periodic system. For example, the melting temperature of carbon is above 3600°C while that of its neighboring nitrogen falls to −210°C. There are also clear “offences”: PCl5 and SbCl5 are stable under normal conditions, while AsCl5 was obtained only in 1976 in liquefied chlorine and it decomposes at about −50°C.
Therefore, nowadays we should refrain from speaking of a periodic law, as no exceptions can exist in a natural law.
Mendeleev guessed that understanding the nature of the valence will clarify the nature of the periodic law. In 1911, Ernest Rutherford (1871–1937), in his studies on the radioactive decay of radon, created the concept that atoms consisted of an atomic nucleus and electrons, and offered the planetary model for the structure of the atom . Niels Henrik David Bohr (1885–1962) created the quantum model of the structure of atoms, according to which electrons move by not arbitrary but precisely defined orbits , , .
Antonius Johannes van den Broek (1870–1926) suggested the hypothesis that the atomic numbers of the elements are determined by the charge of the atomic nucleus and they are not merely serial numbers , which was experimentally confirmed by Henry Gwyn Jeffreys Moseley (1887–1915) in 1913 , . Thus it became clear that both the periodic law and the valence have a common genesis. With the discovery of the structure of atoms, the periodic table of chemical elements found its scientific explanation.
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