Periodic Law

All elements are usually represented in chemistry in the form of a periodic system: rows (periods and rows) and columns (corresponding to groups) 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 Mendeleyev. Although many foreign sources close to his name mention the name Julius Lothar Meier, who, they claim, a year later (but independently) developed a similar system. The key to the success of the many years of efforts of the Russian chemist was the realization that previous attempts by other scientists failed, since many of the chemical elements were not yet open, so in his table he left room for them.

The periodic law, displayed as a periodic table, is divided horizontally 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 series, identified by Roman numerals: IV, V, VI, VII, VIII and IX. And the seventh period coincides with the X series. 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, representing the main and secondary subgroups. The eighth group consists of four subgroups. In addition, two cells with chemical elements of the third group - lanthanum and actinium - conceal the series, called respectively lanthanides (from 58 to 71 numbers) and actinides (from 90 to 103 numbers).

In the first period only two representatives: hydrogen and helium. The second and third include eight chemical elements. The fourth, fifth and sixth periods are lengthy, since each of the eighteen visible elements enters, they are distributed in this way: in even numbers there are ten, and in odd numbers there are only eight. But if we take into account the lanthanides, then the sixth period contains thirty-two chemical elements, including fourteen hidden ones. The seventh period is also long, it has eighteen, four of them are visible, and fourteen (actinides) are hidden. The elements of the odd series of the fourth, fifth and sixth periods belong to the sub-subgroups (b), and the even series belong to the main subgroups (a), along with those that refer to the first, second, third and seventh periods.

The periodic law establishes that all elements within one group differ in significant similarities with each other and differ markedly from those that are part of other groups. For example, group Ia, with the exception of hydrogen, includes metals with a chemical valence of plus 1, while in group VIIa, with the exception of astat, all elements are non-metals, which in compounds usually have a valence minus 1. Today, the periodic law is not only Table. He does not have a mathematical expression, but exists in the form of the assertion that the properties of any chemical element, as well as the properties of all simple substances and complex compounds in which he enters, have a periodic dependence on the magnitude of the atomic nucleus charge .

The term periodicity was proposed for the first time by DI Mendeleev, in spite of the fact that attempts by scientists from different countries to somehow classify known chemical substances have been earlier . But it was he who noticed that when they are arranged in order of increasing atomic masses, the property of each eighth element resembles 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 one, which clearly reflects 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, which guided the great Russian chemist, but each of them confirmed the law formulated by our scientist.

Open to Russian scientists, the periodic law and the periodic system created on its basis became a reliable foundation of modern chemistry. Thanks to which Mendeleev corrected some of their masses for some atoms and predicted the existence of three elements not yet discovered 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 can not be overestimated, since this discovery was of great importance in the development of chemistry.