Postulates of Bohr Atomic Theory

Niels Bohr's 1913 model for the hydrogen atom rests on four postulates. First, the electron can move around the nucleus only in one of several fixed circular paths of definite radius and definite energy; these permitted paths are called orbits, stationary states, or allowed energy states, and they are arranged concentrically around the nucleus in order of increasing energy. Second, the energy of an electron sitting in one of these orbits does not change with time — the electron only moves from a lower stationary state to a higher one if it absorbs exactly the right amount of energy, and only moves from a higher state to a lower one by emitting energy; this energy change always happens in a discontinuous jump, never gradually. Third — Bohr's frequency rule — when a transition occurs between two stationary states differing in energy by $\Delta E$, the frequency of the radiation absorbed or emitted is $\nu = \frac{E_2-E_1}{h}$, where $E_1$ and $E_2$ are the energies of the lower and higher states respectively. Fourth, the electron's angular momentum in a given stationary state is restricted (quantised) to integral multiples of $\frac{h}{2\pi}$: $mvr = n\times\frac{h}{2\pi}$ for $n = 1,2,3,\ldots$ — recalling that angular momentum for a particle moving on a circle of radius r at speed v works out to $mvr$ (moment of inertia $I=mr^2$ tim …