The concept of the atom (from the Greek word meaning indivisible) introduced by Democritus (ca. 460 B.C. - ca. 370 B.C.) is perhapsone of the most brilliant scienti c ideas in the history of mankind.Despite the fact that the atom is quite an ancient concept and that chemistry has a central place within the pure sciences, its development is very recent (the last 250 years). As a modern science, we can associate its birth with the publication of the Lavoisier treatise in 1785. The notion of an elementary substance (which cannot be divided into others with lower mass) would open a new, quantitative era in chemistry. In Portugal, the development of chemistry was promoted by the Pombaline reforms to the university system, which introduced the teaching of chemistry and led to the creation of the Chymico Laboratory (inaugurated in 1775/76) at the University of Coimbra.
The association of Lavoisier's concept of an elementary substance with Democritus' atomism was made by Dalton in his book "NewSystem of Chemical Philosophy" (1808). While "atoms" di eredonly in shape and size in Democritus' atomism, it was only with Dalton that each elementary substance was attributed to a di erenttype of atom.
As new atoms were discovered and information about their properties grew, "families" of atoms with similar chemical behavioursbegan to be identi ed. Döbereiner's triads (1817) (e.g. lithium,sodium and potassium or chlorine, bromine and iodine), the Telluric helix of Chancourtois (1862) or Newlands' law of octaves (1863) are examples of this. However, the father of the periodic table,in the sense of the systematic classi cation of all known elements(63 as of 1869), was the Russian Dmitri Ivanovich Mendeleev, whorst published it in the Journal of the Russian Chemical Societyand whose summary was republished in German in the Zeitschrift für Chemie (12, 405-406 1869). Mendeleev's classi cation wasextraordinary, not only because of its systematic nature, but also because of its predictive character: on the basis of the gapsin his periodic classi cation (based on atomic mass) he predictedthe existence of ten new elements, of which seven have been discovered. To better understand Mendeleev's visionary character we must remember that in 1869 the electron (Thomson, 1897), the proton (Moseley, 1913) and the neutron (Chadwick, 1932) had still not been discovered.
In 2019, 150 years have passed since this extraordinary creation of human genius and the United Nations General Assembly has proclaimed 2019 the International Year of the Periodic Table. As an icon of chemistry, the Periodic Table represents the central character of this science and its interfaces with other areasof scienti c knowledge and technologies. The celebration decreedby the United Nations is an opportunity to demonstrate the centrality of chemistry and its role in the resolution of humanity's greatestchallenges in the elds of the environment, energy, food, health andsustainable development. The celebrations should also serve to raise society's awareness about the role of chemistry as a science that promotes development and well-being, and to motivate new generations to pursue careers in science and technology.
Adelino Galvão /Secretary General of the Portuguese Society of Chemistry
