There was one major objection to the Rutherford model of atom and Bohr was the one to point it out. So while giving his Theory of hydrogen atom, he took the essential features from the Rutherford model of atom but made a change in order to account for the stability of the atom. He made the following postulates: –
a) An atom consists of a centrally located nucleus having a positive charge and responsible for most of the mass of the atom.
b) Electrons revolve around with nucleus at centre and in certain fixed circular orbits with definite radius.
c) The permitted or fixed orbits are such that the angular momentum of an electron is integral multiple of h/2π, h being the Planck’s constant.
The angular momentum L=mvr=n (h/2π), m is the mass of the electron; v is the velocity of the electron in the orbit, n is any integer starting from1.
n is the principal quantum number. The above equation is known as Bohr quantization postulate.
d) Electrons do not radiate energy while moving in their permitted orbit. These orbits are called non-radiating or stationary orbits. This is the manner in which Bohr accounted for the stability of the atom.
e) When an electron jumps from one orbit to another, it absorb/radiate energy. An electron absorbs energy when it jumps from a lower to higher orbit and radiate while jumping from a higher to lower orbit.
Let us assume an electron jumps from orbit ni to nf (nf>ni), then the energy absorbed by the electron is hγ=ΔE=Ef-Ei, Ef and Ei being the energy of fth and ith orbit respectively.
The above equation is known as Bohr frequency condition
