Postulate I
Bohr postulated that in an atom, electron/s could revolve in stable orbits without emitting radiant energy and each atom can exist in certain stable states. Also, each state has a definite total energy. These are stationary states of the atom.
Postulate II
Bohr defined these stable orbits in his second postulate. According to this postulate:
An electron revolves around the nucleus in orbits
The angular momentum of revolution is an integral multiple of h/2pi – where h is Planck’s constant [h = 6.6 x 10-34 Js].
Hence, the angular momentum (L) of the orbiting electron is: L = nh/2pi
Postulate III
According to this postulate, an electron can transition from a non-radiating orbit to another of a lower energy level. In doing so, a photon is emitted whose energy is equal to the energy difference between the two states.