(i) The electron in the atom can move around the nucleus in a circular path of fixed radius and energy. These paths are called orbits, stationary states or allowed energy states. These orbits are arranged concentrically around the nucleus. Radiation can occur only when the electron jump from one orbit to another. The atom will be completely stable in the state with the smallest orbit, since there is not orbit of lower energy into which the electron can jump.
(ii) According to Bohr's model, radiation (energy) is absorbed if the electron moves from the orbit of smaller Principal quantum number to the orbit of higher Principal quantum number, whereas the radiation (energy) is emitted if the electron moves from higher orbit to lower orbit. It explains the origin of spectral lines in H-atom.
