There may be variations of the statements and number of postulates. the meaning is important.
Niels Bohr postulated (like assumptions) the following :
1. The Electron revolves around the nucleus in circular orbits.
2. The orbit of electrons can take only special values of radius. In these orbits
electrons do not radiate energy. These are called stationary orbits.
3. The energy of the atom has a definite value in a stationary orbit. When an
electron jumps from one orbit (energy level E2) to another orbit of lower energy
E1, it emits a photon of radiation, with energy E2 - E1.
E2 - E1 = h n
4. In stationary orbits, the angular momentum L of the electron about the nucleus is an integral multiple of h/2π.