State any two postulates of Bohr’s theory of hydrogen atom.

Answer (any two):

1. Stationary orbits: The electron revolves around the nucleus only in certain discrete circular orbits, in which it does not radiate energy. The centripetal force is provided by the electrostatic attraction of the nucleus: \(\dfrac{mv^2}{r}=\dfrac{1}{4\pi\varepsilon_0}\dfrac{e^2}{r^2}\).
2. Quantisation of angular momentum: The allowed orbits are those for which the angular momentum of the electron is an integral multiple of \(\dfrac{h}{2\pi}\): \(mvr=\dfrac{nh}{2\pi}\), \(n=1,2,3,\dots\)
3. Frequency condition: When the electron jumps from a higher orbit of energy \(E_m\) to a lower orbit of energy \(E_n\), it emits a photon of frequency \(\nu\) given by \(h\nu=E_m-E_n\).