Neils Bohr proposed a model of an atom based on atomic spectra of hydrogen.
He retained the basic concepts of Rutherford’s model i.e., the atom has a positively charged nucleus at the center and the electrons revolve around the nucleus. He applied Planck’s Quantum theory for revolving electron.
The main postulates are:
Electrons revolve around the nucleus with high velocity in circular paths called orbits or shells.
As long as the electron is in a particular orbit, its energy is constant. Therefore, these orbits are called stationary orbits.
Each stationary orbit is associated with a definite energy and is known as energy level. These energy levels are named as K, L, M, N, ….. etc. or numbered as 1,2,3,4… etc.
When an electron jumps from a lower energy level to a higher energy level, the difference in energy is emitted as radiation in quanta.
When an electron jumps from a lower energy level to a higher energy level, the difference in energy is absorbed as radiation in quanta.
where energy of first orbit
energy of second orbit
h = Planck’s constant
frequency of radiation
The angular momentum of the electron revolving in a stationary orbit is equal to integral multiples of .
n = integer (1,2,3,4…)
h = Planck’s constant
m = mass of the electron
v = velocity of electron
r = radius of the circular orbit
Therefore, the angular momentum of the electron in various orbits is equal to , , , etc.
Thus, the momentum is said to be quantized.
Merits of Bohr’s atomic model:
- This theory explains the various spectral series of hydrogen atom in excellent manner.
- This theory also explains the spectral series of ions like etc.
- This theory is highly successful in the aspect of calculating the radius and the energy of each orbit of hydrogen atom.
Limitations of Bohr’s atomic model :
- Bohr’s atomic theory could not explain the spectra of atoms having more than one electron. Examples: He, Li, Be, B, C.
- This theory doesn’t explain splitting of spectral lines into a group of lines under the influence of magnetic field and electric field.
- This theory could not justify the Quantization of angular momentum.
- This theory could not account for the formation of chemical bonds.
- This theory assumes electron as a particle. This is against the wave nature of electron proposed by de Broglie.
- This theory gives the position and velocity of electron accurately and simultaneously. This is against the uncertainty principle proposed by Heisenberg.