The hydrogen atom can only exist at certain quantized energy levels called stationary states. An atom does not emit energy when in one of these stationary states. An atom can change between stationary states by absorbing or emitting a photon with energy equal to the difference between the two states.

4. The hydrogen atom has only certain allowable energy levels which he called as stationary states.

5. The atom does not radiate energy while in one of its stationary states.

6. The atom changes to another stationary state only by absorbing or emitting a photon whose energy equals the difference in energy between the two states. It cannot lie in between two steps.