In this article it says:
The electrons can only orbit stably, without radiating, in certain orbits (called by Bohr the "stationary orbits") at a certain discrete set of distances from the nucleus. These orbits are associated with definite energies and are also called energy shells or energy levels. In these orbits, the electron's acceleration does not result in radiation and energy loss as required by classical electromagnetics. The Bohr model of an atom was based upon Planck's quantum theory of radiation.
My question is: how can this be if it is actually impossible? Any accelerating point charge should emit radiation, as explained here, for example. It is not like I can posit the existence of an orbit where the electron is stable, just out of mid air. What am I missing here?
Answer
The following passage has been extracted from Bohr's Nobel lecture:
While in contradiction to the classical electromagnetic theory no radiation takes place from the atom in the stationary states themselves, a process of transition between stationary states can be accompanied by the emission of electromagnetic radiation, which will have the same properties as that which would be sent out according to the classical theory from an electrified particle executing an harmonic vibration with constant frequency.
Bohr proposed this as an postulate (premise or starting point of reasoning). So, in a sense, he said this as a definition.
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