Question

What are the main postulates of Bohr's theory of an atom ?

Answer 1

1) An electron can revolve around the nucleus in certain fixed orbits of definite energy without emission of any radiant energy. Such orbits are called stationary orbits.

2) An electron can make a transition from a stationary state of higher energy $E_2$ to a state of lower energy $E_1$ and in doing so, it emits a single photon of frequency,.

3) Only those orbits are allowed for which the angular momentum of the electron is an integral multiple of $\frac{h}{2\pi }$