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Today, we students, and those who undertake the fascinating study trip into science can often use essential supports and equipments for learning, but we aren’t well aware of their routine use, of how much effort was behind their realization.
In chemistry the periodic table is an example: comfortable, easy to use, fundamental instrument to the problems but…do people know who invented it or how the periodic table started?
If you share our purpose, come along with us on this trip to the origin of the periodic table but…, before starting, we have to explain to those who don’t like the subject very much or have only a superficial knowledge, what this famous table is.
The periodic table of the elements or periodic System is a diagram that contains all the chemical elements, arranged in the order of their ascending atomic number and the periodicityof their characteristics. But before describing the table, we have to wonder: what we mean with chemical elements
Chemical elements refer to substances that are constituted by the same atoms, all containing the same number of protons. Nowadays over one hundred elements are known, some of which, the so called trans-uranium elements, are very radioactive. These, in fact, do not exist in nature and man produces them by bombarding the atomic nucleuses of other elements with electrically loaded particles.
A first classification subdivides the chemical elements in metals and non metals; the metals are electropositive or rather they tend to give up electrons, on the contrary the non-metals are electronegativeor rather they tend to gain electrons. It is possible to include a third group of so called semimetals that possess intermediate properties. If the elements are catalogued according to their atomic number, proportional to the positive charge of the nucleus, we notice a (unique) periodicity of the physical and chemical characteristics, and this suggests a further division into families, like for example the family of the alkaline-earth metals, of the lanthanides, of the noble gases and so on.
Closing this digression, the periodic table is subdivided in to seven horizontal lines, called periods, and into eighteen vertical columns, called groups. The first three periods are defined as short periods. The others, called long periods, contain from eighteen to thirty-two elements.
According to the method introduced by IUPAC (International Union for Pure and Applied Chemistry), the groups are counted from one to eighteen proceeding from left to right.
All the elementsbelonging to a definite group present very similar chemical properties and at the same time different from other groups.
We talked of periodicityin the chemical-physical characteristics of the elements, but what does periodicity mean?
In chemistry periodicity means a law that states that the chemical and physical properties of the elements are systematically, repeated according to the increasing atomic numbers.
This definition sends us back to its history, exactly when, in the XIX century, with the development of electrochemistry, the British scientists Humphry Davy and Michael Faraday discovered new elements and started a careful study on their chemical properties. Consequently, we must admit that the passionate 1800 isn’t only Romanticism or pure patriotism as critics say, on the contrary this century presents, in the area of research, very surprising events and discoveries … (the fact remains that the discoveries in other centuries were very important too!). Then we can state that in the field of science too, and in particular of chemistry, the 19th century was important for the levels of knowledge reached by chemistry.
In the first decade of 1800 fourteen elementswere added to the short list, that it was already known.
We remember the chemists who took part in this discovery: Davy, Gay-Lussac and Thenard, Wolloston, Berzelius, Tennant, HatchettandEkeberg. Davy, through electrolysis, focused on six new types; Gay-Lussac and Thenard isolated boron; Wolloston discovered palladiumandrhodium while Berzelius cerium. Tennant recognized osmium and iridium; followed by Hatchet with his columbium (officially niobium) and Ekeberg with tantalum.