The periodic table of the elements, or simply periodic table, is a scheme which allows to classify and organize elements chemical based on their properties and characteristics.
The German chemist Julius Lothar Meyer (1830-1895) and the Russian mathematician Dmitri Mendeleev (1834-1907) were the first experts to have applied the properties of elements from the periodic functions of their atomic mass.
The history of the periodic table is related to the discovery of the chemical elements and the need to organize them somehow. After several attempts, it is Mendeleev which has succeeded in creating a periodic system on the basis of the atomic mass.
The Russian placed items with common properties in the same column. Moreover, he left gaps in his table because he found that there might be other items remaining to be discovered.
It is important, however, to recognize the role of John Dalton (1766-1844) in the development of the concept of atomism chemical, which was given to the possible combinations of atoms of substances. Dalton chose the mass of an atom of hydrogen as a reference unit and created a system of relative Atomic masses.
The first periodic table of Mendeleev, who was not very well received by the scientific community, included 63 items, sorted by ascending order by their atomic weights and classified into groups based on common properties such as the Valencia.
Over time, Mendeleev added the General formulas of hydrides and oxides of each group. Towards the end of the 19th century, the periodic table began to include group 0 (with noble gases, expressed as rare), so called because of the chemical inactivity (of Valencia 0).
Nowadays, the periodic table of the elements is an important resource for the study of the chemistry from secondary education.