Bohr’s Theory
This model was based on quantum theory of radiation and Classical law of physics.
Bohr model is applicable only for single electron species like H, He+, Li2+etc.Bohr model is based on particle nature of electron.
The important postulates on which Bohr’s Model is based are the following:
1. Atom has a nucleus where all protons and neutrons are present. The size of nucleus is very small and it is present at the centre of the atom.
2. Negatively charged electron revolves around the nucleus in the same way as the planets revolve around the sun.The path of electron is circular.
3. The attraction force (Columbic or electrostatic force) between nucleus and electron is equal to the centrifugal force on electron.i.e. Attraction force towards nucleus = centrifugal force away from nucleus.
4. Electrons can revolve only in those orbits in which angular momentum ( ) of electron is integral multiple of
where : n = +ve Integer number (n=1,2,3,4,…………)
h = Planck’s constant
π= constant
5. The orbits in which electron can revolve are known as stationary orbits because in these orbits energy of electron is always constant.
6. Each stationary orbit is associated with definite amount of energy therefore these orbits are also called as energy levels and are numbered as 1,2,3,4,5, ……… or K,L,M,N,O, ……….. from the nucleus outwards.
7. The emission or absorption of energy in the form of photon can only occur when electron jumps from one stationary state to another & it is
Energy is absorbed when electron Jumps from inner to outer orbit and is emitted when electron moves from outer to inner orbit.
Drawback’s of Bohr Model:
1. Bohr model is applicable for single electron species & fails to explain the spectrum of multielectron species.
2. Bohr could not explain the basis why angular momentum is integral multiple of h/2 .
3. Bohr model does not explain splitting of spectral lines in magnetic field(Zeeman effect).
4. Bohr model does not explain splitting of spectral lines in electric field(Stark effect).
5. According to Bohr theory the spectrum of isotopes of hydrogen was expected to be same. However the spectrum of deuterium & tritium was found to be slightly different from that of hydrogen.