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According to Rutherford model of the atom, electrons seem to revolve around the nucleus. Otherwise, the electrons will be attracted to the nucleus by the electric attraction and stick together. When electrons are attached to the nucleus of an atom, it means that the atom is squeezed, that is, the material is squeezed. But in reality, this doesn't happen.
Niels Bohr presented an extraordinary prescription in 1913. The key to this prescription is the transition or quantum jump. It was not theoretically derived, but an emergency measure tailored to the stability of atoms and the phenomenon of emitted light (line spectrum).
Bohr put forward the following bold postulates to explain the stability and line spectrum of atoms that the Rutherford atomic model could not explain.
① In an atom, the electron revolves only in a fixed orbit. When revolving in such a set orbit, electrons do not emit or absorb light. An atom in this state is called a stationary state.
② When electrons transition or quantum jump from one orbit to another, atoms discontinuously absorb or emit energy in the form of monochromatic light. The frequency of light emitted or absorbed is proportional to the difference in energy between the two orbits. υ=E/h=(E1-E2)/h.
③ When an electron moves in a circular orbit, its momentum is limited to an integer multiple of Planck's constant. In other words, the angular momentum of an electron (orbital radius r × momentum) is the integer multiple of Planck's constant divided by 2π. (Angular momentum L=nh/2π, Bohr's quantum condition, quantization postulate)
④ When electrons are in a stable orbit (steady state), their behavior follows the laws of classical mechanics (correspondence principle).
Bohr's early model was further generalized by Arnold Sommerfeld of the University of Munich and reborn as the 'Bohr-Sommerfeld Model'. This Bohr-Sommerfeld model is called “The old quantum theory” meaning a theory before quantum mechanics was completed.
However, in 1923, the Bohr-Sommerfeld model faced a serious crisis. The most direct reason is that this model fails to explain the second simplest helium atom (two electrons) after hydrogen, which has one electron.
A more fundamental problem is that Bohr's postulates could not be explained physically at the time. According to classical mechanics, the motion of an object follows causal determinism. In other words, if you know the current state, you should be able to know the before and after states. Therefore, it was a serious problem that the Bohr model could not explain when and where electrons in atomic orbits transition (quantum jump).
Accordingly, the quantum jump has emerged as a hot issue in the physics world. Except for Bohr's Copenhagen School, most physicists, such as Einstein, Schrödinger, and Pauli, criticized the 'quantum jump' as a mysterious and bizarre hypothesis.
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