A) Postulates of Bohr.s theory :
1) Electrons revolve around the nucleus in certain, definite circular paths, called orbits. They are also. known as energy levels or shells.
2) Each orbit is associated with a definite amount of energy. Hence these orbits are called energy orbits or energy levels or energy shells.
3) As the value of n increases, the size and energy of the orbit increases.
4) As long as the electrons revolve in the circular orbits, they neither emit nor absorb energy. Hence, these orbits are also called stationary orbits.
5) More than one stationary orbit is possible for any electron and the angular momentum of the revolving electron is quantized. That is, the electrons revolve in those circular orbits in which the angular momentum of the revolving electron is an integral multiple of `h//2pi` .
This can be expressed as `mvr=nxx(h)/(2pi)`
where m = mass of the electron,
v = velocity of the electron
r= radius of the orbit ,
h= Planck’s constant,
n = Principal quantum number.
6) The energy of electron changes when the electron jumps from one orbit to another orbit. When the electron jumps from a lower orbit to a higher orbit, energy is absorbed. Similarly. wheń the electron jumps from a higher orbit to a lower orbit, energy is emitted. The energy emitted or absorbed is given by the equation, `DeltaE=E_(2)-E_(1)=hv`
where
`E_(1)` = Energy of lower.orbits,
`E_(2)` = Energy of higher orbits,
`DeltaE` = Difference in energy,
h= Planck’s constant,
v= Frequency .
B) Limitations :
Bohr.s model of atom could explain the i) stability of atom
ii) the line spectrum of hydrogen atom and ions with a single electron `(He^(+), Li^(2+), Be^(3+) ......)`
But this model could not explain the following observations :
i) Bohr.s model cannot explain the spectra of atoms or ions having more than one electron .
ii) When the spectrum of hydrogen was taken with a spectrascope of high resolving power,it was observed that-each line is actually .a group of line’. Bohr.s-model cannot explain this fine structure in the atomic spectra.
iii) It cannot explain Zeeman effect (splitting of spectral lines in the- presence of magnetic field) and stark effect (splitting of spectral lines in an electric field).
iv) It also cannot explain the ability of atoms to form molecules by chemical bonds.
v.) It failed to explain the dual nature of electrons and also the geometry and shapes of molecules.
