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Bohr atom model : Neils Bohr in 1913, modified Rutherford’s atom model in order to explain the stability of the atom and the emission of sharp spectral lines.
Bohr proposed the following postulates :
(i) An electron cannot revolve round the nucleus in all possible orbits. The electrons 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(where h is Planck’s constant =6.626 × 10-34Js). These orbits are called stationary orbits or non radiating orbits and an electron revolving in these orbits does not radiate any energy.
If m and v are the mass and velocity of the electron in a permitted orbit of radius r then angular momentum of electron = mvr = nh/2π, where n is called principal quantum number and has the integral values 1,2,3…
This is called Bohr’s quantization condition.
(ii) An atom radiates energy, only when an electron jumps from a stationary orbit of higher energy to an orbit of lower energy. If the electron jumps from an orbit of energy E2 to an orbit of energy E1, a photon of energy hν= E2-E1 is emitted. This condition is called Bohr’s frequency condition.