BOHR'S MODEL

In 1913, Niels bohr, a great name in physics, suggested that the puzzle of hydrogen spectra may be solved if we make the following assumptions.

Bohr's postulates

1. The electron revolves round the nucleus in circular orbits.
   
2. The orbit of the electron around the nucleus can take only some special values of radius. In these orbits of special radii, the electron does not radiate energy as expected from Maxwell's law. These orbits are called stationary orbits. 
   
3. The energy of the atom has definite value in a given stationary orbit. The electron can jump from one stationary orbit to other. If it jumps from an orbit of higher energy E2 to an orbit of lower energy E1,  it emits a photon of radiation. The energy of the photon is E2 - E1. The wavelenght of the emitted radiation is given by the Einstein - Planck equation. E2 - E1= hν= hc/λ. The electron can also absorb energy for some sources and jump from a lower energy orbit to higher energy orbit.
4. In stationary orbits, the angular momentum l  of the electron about the nucleus is an integral multiple of the Planck constant h divided by 2π. This last assumption is known as Bohr's quantization rule and the assumptions 1 to 4 are known a Bohr's postulates 

LIMITATION OF BOHR'S MODEL

Bohr's model was great success at a time when the physicist were struggling hard to understand the discrete wavelengths in hydrogen spectra. Even today the model is very popular among beginners and nonphysicists , who can visualise the inside of the atom as electrons going in the circle around the nucleus. However, the model did not go too far. It could not be extended for atoms or ions having more than one electron. Even helium spectrum was beyond the scope of Bohr's model. As technology improved and the wavelenghts measured with more accuracy, deviations were observed in the case of hydrogen spectral lines. Thus, at least seven components having slightly different dpectral lines are revealed in what was previously known as 656.3 nm line. On the theoretical side also, the model is not quite consistent with the physics in totality. Bohr's model more look like a patch on Maxwell's electromagnetism. Maxwell's theory is not replaced or refuted but it is arbitrarily assumed that in certain orbits, electrons get the licence to disobey the laws of electromagnetism and are allowed not to radiate energy.