Bohr's postulates, the two fundamental assumptions introduced in 1913 by the Danish physicist N. Bohr to explain (within the framework of the Rutherford model of the atom) the stability of the atom and the regularities of spectra: the existence of stationary states of the atom, corresponding to a discrete series of permitted values Εi(i=1, 2,3,...) of its energy, a change in which is associated with a quantum (abrupt) transition from one stationary state to another (the first postulate); the condition on the frequencies v of the electromagnetic radiation emitted in a radiative quantum transition of the atom from the state with energy Εi to the state with energy Εk:Εi-Εk = hv (the second postulate). Bohr's postulates became the basis of the Bohr theory of the atom; they received a theoretical justification in quantum mechanics.