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Atomic physics emerged at the turn of XIX and XX centuries, based on studies of optical spectra of gases, discovery of the electron and radioactivity. In the first stage of its development (the first quarter of XX century.,)

Atomic physics dealt mainly with the identification of atomic structure and study its properties. Experiments by Rutherford scattering of a particle with a thin metal foil (1908-1911) led to the planetary model of the atom, using this model, Bohr (1913) and Somerfield (1915) developed the first quantitative theory of the atom.

Subsequent studies of the properties of the electron and atom resulted in the creation of the mid-20s. quantum mechanics - the physical theory describing the laws of the microcosm and quantifying consider phenomena in which microscopic particles are involved. Quantum mechanics is the theoretical foundation of atomic physics.

At the same time, nuclear physics plays a role of a "testing ground" for quantum mechanics. Presentation and the findings of quantum mechanics, often inconsistent with our everyday experience, was pilot-tested in atomic physics. A striking example is the famous experiments Franck - Hertz (1913) and Stern - Gerlach (1922). By the beginning of the XX century. was a wealth of material on the optical spectra of atoms. It was found that each chemical element has its own line spectrum, characterized by regular, ordered arrangement of spectral lines. Quantum mechanics relates the observed regularities in the spectrum of energy levels of the atom. In 1913, German physicists James Franck and G. Hertz fulfilled experience, which gave a direct experimental evidence that the internal energy of the atom is quantized and can therefore vary only discretely, i.e., certain portions. They measured the energy of free electrons expended on the excitation of mercury atoms. The main element of the set - glass evacuated cylinder with three soldered electrodes: cathode, anode, grid (prototype of the modern vacuum triode). The container located under the mercury vapor pressure of 1 mm Hg. The electrons left the cathode and accelerated in the field between the cathode and the grid (the accelerating voltage U), and then stopped in a field between the grid and anode (retarding voltage Ux). On the way from the cathode to the anode, electrons collide with mercury atoms
The experiment measured the strength of the anode current as a function of accelerating voltage U. The experimental curve has a number of distinct peaks separated by 4.9 V. If U <4, 9 in the collisions of electrons with mercury atoms are elastic (atomic excitation takes place), so the current increases gradually with increasing U. Upon reaching the value of U = 4, 9 In the beginning inelastic collisions due to the excitation of mercury atoms, as a result of the current strength decreases sharply. With further increase in U increases to the current again until the voltage reaches 9.8 V, when the electron is able to bring two atoms. At a voltage of 4.9V, the electron acquires the energy of 4.9 eV. Thus, the shape of the curve shows that for the excitation of mercury atom requires energy equal to 4.9 eV. This is, obviously, the same amount of energy, which the mercury atom is exchanged with an electron.

Observation of the emission gas shows at the same time the emergence of the full spectrum of mercury atoms.