Bohr’s atomic model

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## Bohr’s atomic model

To overcome the objections of Rutherford’s model, Bohr proposed atomic model.The important postulates of Bohr’s atomic model are,

1)  Electrons revolve around the nucleus in circular path without radiating energy. These are called orbits.

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2) Each orbit is associated with definite energy . Thus orbits are also called energy levels.When value of  ‘n’ (orbit number) increases then radius of orbit (r) & energy of electron (E) increases.When electron moves in a given orbit , it neither absorbs nor emits energy.

3) When electron goes from an orbit to another, then energy is absorbed or emitted in the form of a radiation . The wavelength ‘ λ ‘ of the radiation absorbed or emitted is given by,

### E2 – E1 =ΔE = hv  = hc / λ

h= Planck’s constant

E= energy absorbed or emitted

When electron jumps from higher energy level (E2) to lower energy level (E1), then energy is emitted.When electron jumps from lower energy level (E1) to higher energy level (E2) ,then energy is absorbed.

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4) An electron can move only in those orbits in which the angular momentum of the electron is h/2π or integral multiple of h/2π .

### Angular momentum(mvr) = nh/2π

n =  Orbit number or principal quantum number (n =  1,2,3….)

h= Planck’s constant

m= mass of electron

v = velocity of electron

r= radius of orbit in which electron is revolving

## Facts explained by  Bohr’s atomic model-

1)  It explains the stability of atom because electrons revolve around the nucleus in circular orbits without losing energy.

2) It explains the line spectrum of  the  atoms. When an electron jumps from lower to higher or higher to lower energy level, then lines of definite frequency or wavelength are obtained in the atomic spectra.

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3) It is possible to calculate the radius & energy of electron in various orbits.

## Defects of Bohr’s atomic model-

1) This model could not explain the spectra of atoms or ions having more than one electron like He, Li, Na etc.

It only explains the spectra of those atoms or ions which contains only one electron like H- atom , He+, Li++ etc.

2) It does not explain the Zeeman  &  Stark effect. Zeeman effect is splitting of spectral lines under the influence of magnetic field . Stark effect is splitting of spectral lines under the influence of electric field.

3) Bohr’s atomic model does not explain the fine spectrum of hydrogen atom. (When spectrum of hydrogen atom is seen by high resolving spectroscope , then it is found that single line is made up of so many fine lines.)