A provincial chemistry teacher, John Dalton , discovered the Law of Multiple Relations in 1803. This theory says that if a specific chemical element can form compounds with other elements, then each part of the mass will account for a part of the mass of another substance, and the relations between them will be the same as between small integers. This was the first attempt to explain the complex structure of matter. In 1808, the same scientist, trying to explain the law he discovered, suggested that atoms can have different masses in different elements.
The first atom model was created in 1904. The electronic structure of the atom in this model, scientists called "pudding with raisins." It was believed that an atom is a body with a positive charge in which its components are uniformly mixed. Such a theory could not answer the question of whether the components of the atom are in motion or at rest. Therefore, almost simultaneously with the theory of "pudding," the Japanese Nagaoka proposed a theory in which the structure of the electron shell of an atom was likened to the solar system. However, referring to the fact that when rotating around an atom, its components must lose energy, and this does not comply with the laws of electrodynamics, Vin rejected the planetary theory.
However, after the discovery of the electron, it became clear that the structure of the atom is more complex than it was imagined. Questions arose: what is an electron? How is it arranged? Are there other subatomic particles?
By the beginning of the twentieth century, planetary theory was finally accepted. It became clear that each electron, moving in the orbit of the nucleus like a planet around the Sun, has its own trajectory.
But further experiments and studies have disproved such an opinion. It turned out that the electrons do not have their own trajectory, however, it is possible to predict the region in which this particle most often appears. Orbiting around the nucleus, the electrons form an orbital, which is called the electron shell. Now it was necessary to study the structure of the electron shells of atoms. Physicists were interested in questions: how exactly do electrons move? Is there orderliness in this movement? Maybe the movement is chaotic?
The ancestor of atomic physics N. Bohr and a number of such major scientists have proved: electrons rotate by shell-layers, and their motion corresponds to certain laws. It was necessary to closely and thoroughly study the structure of the electron shells of atoms.
It is especially important to know this structure for chemistry, because the properties of a substance, it was already clear, depend on the structure and behavior of electrons. From this point of view, the behavior of the electron orbital is the most important characteristic of this particle. It was found that the closer the electrons are located to the atom’s nucleus, the more efforts must be made to break the electron-nucleus bond. The electrons located next to the nucleus have a maximum bond with it, but a minimum energy reserve. For external electrons, on the contrary, the bond with the nucleus weakens, and the energy reserve increases. Thus, electron layers are formed around the atom. The structure of the electron shells of atoms has become clearer. It turned out that energy levels (layers) form particles with close energy reserves.
Today it is known that the energy level depends on n (this is a quantum number) and corresponds to integers from 1 to 7. The structure of the electron shells of atoms and the largest number of electrons at each level is determined by the formula N = 2n2.
A capital letter in this formula denotes the largest number of electrons in each level, and a small letter indicates the serial number of this level.
The structure of the electron shell of atoms establishes that in the first shell there can be no more than two atoms, and in the fourth - no more than 32. The external, complete level contains no more than 8 electrons. Layers where there are fewer electrons are considered incomplete.