# 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.

$E_{2}&space;-&space;E_{1}&space;=&space;h\nu$

where $E_{1}$ $=$ energy of first orbit

$E_{2}&space;=$ energy of second orbit

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

$\nu&space;=$ 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, $mvr&space;=&space;\frac{nh}{2\pi}$

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/&space;2\pi$$2h/2\pi$$3h/2\pi$$4h/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 $He^{+},&space;Li^{2+}$ 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.