Bohr’s atomic model

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.

E_{2} - E_{1} = h\nu

where E_{1} = energy of first orbit

E_{2} = energy of second orbit

h = Planck’s constant (6.625 \times10^{-34} J. sec)

\nu = frequency of radiation

The angular momentum of the electron revolving in a stationary orbit is equal to integral multiples of \frac{h}{2\pi}.

Angular momentum, mvr = \frac{nh}{2\pi}

where

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 1h/ 2\pi2h/2\pi3h/2\pi4h/2\pi 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 He^{+}, Li^{2+} 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.