The year 2019 was proclaimed by UNESCO as the “International Year of the Periodicity of the Elements” because it marks the 150th anniversary of the publication, in Russian, of the memory of Dmitry I. Mendeleev (Tobolsk, 1834 – St. Petersburg, 1907) entitled “Correlation of the properties with the atomic weight of the elements”, which lay the foundations of today’s periodic table of the elements. Chemistry “is all we are, we eat, what I see and I don’t see” and the whole universe is made up of a large number of elementary bricks, which are the elements. The periodic table of the elements (or simply periodic table) is the scheme with which the elements present in the atomic orbital s, p, d, f are treated.
During the history of chemistry, as more and more elements were discovered, it became necessary to introduce a symbolic notation that would allow them to be written in a universal, concise and unique way. Thus, different lists were created by scholars indicating the symbols of the chemical elements. With the increase in the number of known elements, it was clear the need to group them following an unequivocal logical criterion. The first grouping attempts were carried out by Johann Wolfgang Döbereiner who grouped the elements with similar chemical characteristics into three by three. The work continued in that direction until the first real periodic table, conceived by Mendeleev, which ordered the elements according to their atomic weight and exploited the periodicity of the chemical properties to bring together in the same groups elements with similar chemical properties. It was only in 1913 that, to overcome the inconsistencies found in some elements due to the presence in nature of these as a mixture of several isotopes in different percentages, the chemist Moseley proposed a grouping according to the atomic number, linearly increasing and independent of the quantity neutron.
The periodic table of Mendeleev, except for some slight modifications, is the periodic table that we still use today. Over the years, new elements are synthesized and added to the other elements of the periodic table, which however retains the periodicity characteristics observed by Mendeleev.
The story of how dozens of elements have been enclosed in a periodic table goes beyond a person and a point in time. Scientists classified and predicted elements before and after Dmitri Mendeleev’s painting of 1869. And many others worked to find and explain these new substances. Noble gases, radioactivity, isotopes, subatomic particles and quantum mechanics were all unknown in the mid-nineteenth century.
Marie Curie is the most celebrated, for her double research on Nobel prizes on radioactivity and for discovering polonium and radium. Demonstrating the discovery of a new element is difficult. The first step is to find unusual activities, chemical behaviors or physical properties that cannot be attributed to known elements, such as unexplained radioactive emissions or spectroscopic lines. Therefore the element, or its compound, must be isolated in sufficient quantity so that it can be weighed, tested and compared.
“Ci sono soltanto due possibili conclusioni: se il risultato conferma le ipotesi, allora hai appena fatto una misura; se il risultato è contrario alle ipotesi, allora hai fatto una scoperta.”
Enrico Fermi
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