In the Bohr model, electrons travel around the nucleus in circular orbits (stationary states) that exist only at specific, fixed distances from the nucleus. The energy of each orbit is also fixed, or quantized. Bohr further proposed that no radiation was emitted by an electron orbiting the nucleus in a stationary state. It was only when an electron jumped, or made a transition, from one stationary state to another that radiation was emitted or absorbed.
Work Step by Step
The electron is never observed between states. The emission spectrum of an atom consists of discrete lines because the stationary states exist only at specific, fixed energies.