The postulate of Bohr’s theory are as follows:
(1) Bohr’s First Postulate: An electron revolving around the nucleus is not permitted to move in every possible orbit. It can move only in some “special orbits”. These orbits are known as stationary orbits. An electrons in a stationary orbit cannot lose energy according to the demands of classical electromagnetic theory.
To define these special orbits Bohr further postulated that the angular momentum of an electron in a stationary orbit is always an integral multiple of h/2x where h is Plancks constant. For an electron of mass m moving with a velocity v in an orbit of radius r, we must have
mvr = n h/2π
where n is an integer having permitted values 1, 2, 3,...... n is known as the principal quantum number. Each value of n corresponds to a stationary orbit.
(2) Bohr’s second postulate is designed to explain the emission of radiations by atoms. This postulate state that whenever an electron jumps from one stationary orbit to another the energy difference between the two is either emitted or absorbed as a quantum of radiation. Expressed mathematically
En\(_2\) - En\(_2\) = hv
where En\(_1\) and En\(_2\) are the energy of electron in the n\(_1^{th}\) and n\(_2^{th}\) orbit and v is the frequency of radiation emitted when electron makes a transition from n\(_2^{th}\) orbit to n\(_1^{th}\) orbit.