Bohr proposed a model of the hydrogen atom in 1911. He proposed two postulates:
1st postulates: An electron cannot revolve around the nucleus in all possible orbits as suggested by the classical theory.The electron can revolve round the nucleus only in those allowed or permissible orbits for which the angular momentum of the electron is an integral multiple of (h/2π). Here h is plank's constant having value 6.64×10-34 Js
Then, Angular momentum, mvr=nh/2π
Here n is principal quantum number, m is mass of electron and v is velocity in an orbit of radius r.
2nd postulates: When electron revolves in permitted orbits they do not radiate energy. An atom radiates energy only when an electron jumps from a higher energy state to the lower energy state and the energy is absorbed when it jumps from lower to higher energy orbit.
If E1 and E2 are energy associated with first and second orbits respectively, then the frequency f of the radiation emitted is given by
hf=E2 - E1
This is called bohr's frequency condition.