The postulates of Bohr's atomic model (for the hydrogen atom):
1. The electrons revolve around the nucleus in circular orbits. This is the same assumption as in Rutherford’s model and the centripetal force necessary for the circular motion is provided by the electrostatic force of attraction between the electron and the nucleus.
2. The radius of the orbit of an electron can only take certain fixed values such that the angular momentum of the electron in these orbits is an integral multiple of \(\frac{h}{2 \pi}, h\) being Planck’s constant. Such orbits are called stable orbits or stable states of the electrons and electrons in these orbits do not emit radiation as is demanded by classical physics. Thus, different orbits have different and definite values of angular momentum and therefore, different values of energy.
3. An electron can make a transition from one of its orbits to another orbit having lower energy. In doing so, it emits a photon of energy equal to the difference in its energies in the two orbits.