Question #fd2a5

Bohr took as starting point for his atomic model the theory of the quanta (by Plank) and Einstein's explanation of the photoelectric effect.

Plank had successfully explained that radiation can only exist in certain finite amounts, that he called quanta (although he never believed his own explanation was true, only a mathematic convenience).

Bohr decided to use this fact applied to atoms. If radiation can only exist in certain finite values, the gradual collapse of the electrons into the nucleus of Rutherford's model could not happen. Instead, electrons would jump down from one orbit to another lower orbit instantly, emitting one of Einstein's photons as radiation.

With this in mind, he postulated that the electrons orbits could not have just any value for their radii, they could only exist at certain finite values. Being inside of the nucleus would also be a forbidden value, as it would emit an infinitely energetic photon, which is nonphysical. While existing in this allowed orbits, electrons do not radiate.

Electrons in an allowed orbit can jump to a lower orbit, emitting a photon of radiation whose energy would be the energy difference between both orbits. Or, in the same manner, they could absorb a photon of the appropriate energy and be promoted to a higher orbit. These jumps between orbits are called "quantum jumps" (this is the origin of the idiom).

This was a very simple model, based on postulates rather than on a complete theory (Heisenberg, Schrodinger and Born would be the ones to develop it), but even this very simple model could account perfectly for the behavior of the hydrogen atom, which was a resounding success.