All elements are usually represented in chemistry in the form of a periodic system: rows (periods and rows) and columns (correspond to groups) of the table are arranged taking into account the increase in their atomic masses. The discovery of the periodic law dates back to 1869 and undoubtedly belongs to the Russian scientist chemist - Dmitry Ivanovich Mendeleev. Although many foreign sources mention the name of Juliuas Lothar Meyer, who, according to them, a year later (but independently) developed a similar system next to his name. The key to the success of the long-term efforts of the Russian chemist was the realization that previous attempts by other scientists had failed, since many of the chemical elements had not yet been discovered, so he left free space for them in his table.
The periodic law, displayed as a periodic table, is horizontally divided into seven periods. The designations of the first, second and third periods coincide with the same Roman numerals of the series: I, II, III. The fourth, fifth and sixth periods are divided into even and odd rows, identified by Roman numerals: IV, V, VI, VII, VIII and IX. And the seventh period coincides with the X next. Vertically in eighteen columns or columns, all elements are distributed in eight groups. Each group, from the first to the seventh, is divided into two columns, which are the main and secondary subgroups. The eighth group consists of four subgroups. In addition, two cells with the chemical elements of the third group - lanthanum and actinium - hide the rows, respectively called lanthanides (from 58 to 71 number) and actinides (from 90 to 103 number).
In the first period there are only two representatives: hydrogen and helium. The second and third include eight chemical elements. The periods of the fourth, fifth and sixth are long, since each contains eighteen visible elements, they are distributed in this way: in even rows contains ten, and in odd rows only eight. But considering lanthanides, the sixth period contains thirty-two chemical elements, including fourteen hidden. The seventh period is also long, it has eighteen, four of them are visible, and fourteen (actinides) are hidden. Elements of the odd rows of the fourth, fifth and sixth periods belong to the side subgroups (b), and the even rows belong to the main subgroups (a), along with those that belong to the first, second, third and seventh periods.
The periodic law establishes that all elements within the same group differ in significant similarities with each other and significantly differ from those that are part of other groups. For example, group Ia, with the exception of hydrogen, includes metals with a chemical valency plus 1, while in group VIIa, with the exception of astatine, all elements are non-metals, which in compounds usually have a valency of minus 1. Today, the periodic law is not only represented a table. It does not have a mathematical expression, but exists in the form of a statement that the properties of any chemical element, as well as the properties of all simple substances and complex compounds in which it is a part, have a periodic dependence on the magnitude of the atomic nucleus charge .
The term periodicity was first proposed by D. I. Mendeleev, despite the fact that earlier there were attempts by scientists from different countries to somehow classify known chemicals. But it was he who noticed that when they are arranged in order of increasing atomic masses, the properties of every eighth element resemble the properties of the first. In 1869, the first version of the table (at that time only 60 elements were known) was still very different from the modern form, which clearly displays the periodic law. Over time, he underwent certain changes, which consisted in the addition of new, later discovered chemical elements. But this not only did not destroy the idea of the periodicity of the properties of chemical atoms that guided the great Russian chemist, but each of them confirmed the law formulated by our scientist.
The periodic law discovered by Russian scientists and the periodic system created on its basis became a reliable foundation for modern chemistry. Due to this, Mendeleev corrected their masses in some atoms and predicted the existence of three not yet discovered elements in nature, which later found experimental confirmation, and gallium, scandium and germanium were discovered. All this led to the universal recognition of the periodic system. The value of the periodic law cannot be overestimated, since this discovery was of great importance in the development of chemistry.