by Sergio Cristallo

The periodic table: yesterday and today      FORWARD

The 2019 has been declared by UNESCO "International Year of the periodic Table", to celebrate the 150th anniversary of the publication of the Periodic Table by the russian chemist Dmitrij Mendeleev. In fact, March the 6th 1869, Mendeleev formally presented to the Russian Chemical Society his classification of chemical elements, with the title: "The Dependence between the Properties of the Atomic Weights of the Elements".
Mendeleev was not the first to propose such a kind of classification for chemical elements. At that epoch, in fact, other Tables were already published, in which the elements were ordered basing on the increasing atomic number or grouping elements with similar chemical properties. One of the main features of Mendeleev' Table is the fact that he left empty slots, basing on the periodic properties he had previously identified. For instance, he hypothesized the existence of three elements, naming them eka-silicon, eka-aluminum and eka-boron, being exactly one slot below the corresponding known elements (see Figure 1). Those elements were discovered in the following years (they are germanium, gallium and scandium, respectively).


At that epoch more than 60 elements were known; today, the number has almost doubled (see Figure 2). The last 4 elements (nihonium, moscovium, tennessine e oganesson) were officially presented in 2016.
The Periodic Table includes both stable elements (i.e. with an endless lifetime) and unstable elements (i.e. they transform to another chemical species). In nature, all elements can be found, apart from the Superheavy ones, created artificially and with extremely short lifetimes.
In the Periodic Table, elements are ordered with increasing atomic numbers (symbol Z), i.e. with increasing number of “protons” within the nucleus. The proton is a sub-atomic particles, with a mass equal to 1.7x10-24 gr (about 2000 billionths of billionths of billionths of gram) and a positive charge equal to 1.6x10-19 C (16 billionths of billionths of Coulomb, which is the charge transported in 1 second by an electric flux of 1 Ampere). Atoms are globally neutral, because around nuclei (positively charged) there are electrons, with negative charge. Inside nuclei there are also neutrons, which have a mass almost identical to protons, but without charge. Therefore, an atom consists of a nucleus (with protons and neutrons), surrounded by a "cloud" of electrons. The nucleus charge provides its atomic number (Z); its mass, instead, is commonly identified by the sum of nucleons (A=protons+neutrons: the masses of electrons are negligible with respect to nuclei).
Each chemical element has its typical "isotopic composition". “Isotopes” are atoms with the same atomic number, but with a different number of neutrons. In nature, there are mono-isotopic elements (as fluorine or gold), elements with many stable isotopes (tin has 10 isotopes: the largest number) and elements without stable isotopes (as technetium, which transforms to molybdenum).
The interested reader can explore the IUPAC Periodic Table of the Elements and Isotopes .


Download Figure 2 (pdf;jpg)