The postulates of Bohr's atomic model (for the hydrogen atom):
(1) The electron revolves with a constant speed in a circular orbit around the nucleus. The necessary centripetal force is the Coulombs force of attraction of the positive nuclear charge on the negatively charged electron.
(2) The electron can revolve without radiating energy only in certain orbits, called allowed or stable orbits, in which the angular momentum of the electron is equal to an integral multiple of h/2π, where h is Planck's
constant.
(3) Energy is radiated by the electron only when it jumps from one of its orbits to another orbit having lower energy. The energy of the quantum electromagnetic radiation, i.e., the photon, emitted is equal to the energy difference of the two states.
