To explain the line spectra emitted by atoms, as well as the stability
of atoms, Niel’s Bohr proposed a model for hydrogenic (single elctron)
atoms He introduced three postulates and laid the foundations of
quantum mechanics:
(a) In a hydrogen atom, an electron revolves in certain stable orbits
(called stationary orbits) without the emission of radiant energy (b) The stationary orbits are those for which the angular momentum
is some integral multiple of h/2p (Bohr’s quantisation condition