Bohr's Model of the Hydrogen Atom

Bohr proposed to modify Rutherford's planetary model of the atom by making the following assumptions:

Electrons can exist only in certain special orbits about the central nucleus. The orbits are called stationary orbits or stationary states. The possible orbits are those with angular momenta that are multiples of Plank's constant.
The dynamic equilibrium of the system in the stationary states is governed by Newtonian mechanics.
Transitions between different stationary states are accompanied by the emission or absorption of radiation the frequency of which is given by the Planck formula, h$\nu$ = $\Delta$E,
where $\nu$ is the frequency (called "f" up until now) and $\Delta$E is the difference in the energy between the two stationary states.

The first assumption was made to eliminate the problem of the electrons radiating due to the acceleration in their circular orbits. This assumption was made ad hoc with no justification other than the resulting model reproduced the results of experiments. The second assumption stated that the kinematical features of the model could be calculated with classical mechanics. The third assumption was the most controversial because little was known about how these transitions might occur.