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In 1911, after carrying out experiments on the scattering of alpha particles of atoms Rutherford put forward and substantiated the planetary model of atomic structure based on the analysis of experimental results. According to this model, the atom consists of a heavy positively charged nucleus of very small dimensions, around which electrons are moving in some orbits. The name reflects the obvious analogy with the atom to the solar system, in which planets move according to some definite orbits around the massive center of attraction – the Sun.

However, in contrast to the planetary model of the solar system, the planetary model of the atom is self-contradictory from the point of view of classical physics. This is primarily due to the presence of the charge of an electron.

According to the laws of classical electrodynamics, an electron orbiting the nucleus, like any rapidly moving charged particle radiates electromagnetic waves. The spectrum of this radiation should be continuous, that is, contain electromagnetic waves of any wavelength.

In addition, continuous emission decreases the electron energy. Therefore, due to the radiation the radius of the orbit of the moving electron is required to be reduced, and, in the end, the electron must fall into the nucleus. In other words, the planetary model of the atom in classical physics is unstable.

In 1913, Bohr showed that it was possible to save the planetary model of the atom, introducing the quantization ideas into the theory of the atom while highlighting some of the orbits, allowed for the electron motion. It is evident that quantum Planck’s constant should be in the quantization rules. And since the quantum of action has dimension of angular momentum, Bohr added the condition of quantization of angular momentum of an electron moving around the nucleus to the theory.

The simplest atom is the hydrogen atom, containing a single electron moving in a closed orbit in the Coulomb field of the nucleus. In the first approximation, the atomic nucleus can be regarded as fixed, and the electron orbits – circular orbits.

Bohr formulated the basic tenets of the theory of the hydrogen atom in the form of three postulates:

- An electron in an atom can move only on certain fixed orbits. This movement corresponds to a stationary state of the atom with the same total energy. This means that electron moving in a closed stationary orbit does not radiate energy.
- Permission stationary orbits are those for which the angular momentum of an electron is equal to an integer multiple of the value of Planck’s constant.
- Radiation or photon absorption occurs in the transition of an atom from one stationary state to another.

In 1911, after carrying out experiments on the scattering of alpha particles of atoms Rutherford put forward and substantiated the planetary model of atomic structure based on the analysis of experimental results. According to this model, the atom consists of a heavy positively charged nucleus of very small dimensions, around which electrons are moving in some orbits.

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