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appear at the same position as the bright lines in the emission spectrum
of the substance.
 For Example- Emission spectrum of sodium has two bright lines in the
yellow region of 589 nm and 589.6 nm where as its absorption spectrum
has two dark lines corresponding to the same wavelengths.
 Drawbacks of Rutherford's atomic model
 It could not explain the stability of atom.
 It was not able to explain the line spectrum of the element.
 BOHR'S ATOMIC MODEL :
POSTULATES :
 An atom consists of a small, heavy positively charged nucleus in the
centre and the electrons revolve around it in circular orbits.
 The electrons revolve only in those orbits which have a fixed value of
energy and radius.
1312
1 
The energy is given by E n   Z 2 kJmol .
n 2
. 0 529n 2
The radius is given by r  A
0
n
Z
 As long as an electron revolves in a particular orbit it neither emit nor absorb energy.
 Since the electrons revolve only in those orbit which have fixed value of energy,so
electrons in an atom can have only discrete values of energy. Therefore we can say that
the energy of an electron is quantized.
 Those orbits are permitted in which the angular momentum of an electron is an
nh
integral multiple of h/2ᴫ.i.e. mvr 
 2
 Energy is emitted or absorbed only when the electron jumps from one orbit to the
other.
 The frequency of radiation absorbed or emitted when transition occurs between two
stationary states that differ in energy by ∆E, is given by :
 E E  E
   2 1
h h

Where E1 and E2 are the energies of the lower and higher allowed

energy states respectively.