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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