Explain Bohr's atomic model.

(N/A) Neils Bohr $(1913)$ was the first to explain quantitatively the general features of hydrogen atom structure and its spectrum.
Postulates of the model:
$(i)$ The electron in the hydrogen 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.
$(ii)$ The energy of an electron in the orbit does not change with time. However,the electron will move from a lower stationary state to a higher stationary state when the required amount of energy is absorbed,or energy is emitted when the electron moves from a higher to a lower stationary state. The energy change does not take place in a continuous manner.
$(iii)$ The frequency of radiation absorbed or emitted when a transition occurs between two stationary states that differ in energy by $\Delta E$,is given by:
$E = h \nu$ and $\Delta E = (E_{2} - E_{1})$
So,$\nu = \frac{\Delta E}{h}$ (Bohr's frequency rule)
Where $E_{1}$ and $E_{2}$ are the energies of the lower and higher allowed energy states respectively.
$(iv)$ The angular momentum of an electron in a given stationary state can be expressed as:
$m_{e} v r = n(\frac{h}{2 \pi})$ where $n = 1, 2, 3, \dots$
Thus,an electron can move only in those orbits for which its angular momentum is an integral multiple of $\frac{h}{2 \pi}$,which is why only certain fixed orbits are allowed.