State Bohr's Atomic Model

First Postulate - In an atom, the electron revolve around the nucleus in circular orbits with constant speed

Second Postulate - the radius of the orbit of an electron can only take certain fixed values such that the angular momentum oof the electron in these orbits is an integral multiple of h/2π , h being the Planck' constant.

Third Postulate - an electron can make a transition from one of its orbits to orbit to another having lower energy. In doing so, it emits a photon of energy equal to the difference in its energies in the two orbits.