atomic structure

Bohr Model of Atom – Class 11 Chemistry Chapter 2 Notes

Bohr Model of atom:

            Neil Bohr in 1913 put forward a theory to develop atomic model and to overcome shortcomings of Rutherford model.

Bohr theory has following postulates.

  1. Electrons revolve around the nucleus in fixed orbits (i.e. orbits of particular radii). Each orbit or shell is associated with a definite amount of energy.
  2. Electrons while remaining in any of the allowed orbits do not radiate energy.
  3. Energy is evolved or absorbed when electron jumps from one orbit to another orbit.
  4. When electron jumps from lower to higher orbit, energy is absorbed and is released when electron jumps from higher orbit to lower orbit.
  5. The energy released or absorbed during electron jump from one orbit to another is equal to the difference of energies of those two orbits. This energy is in the form of quantum.

ΔE = E2 – E1 = hﬠ                   Where ΔE = Change in energy

i.e.    Energy released or absorbed         

                                                            E1 = Energy of lower orbit

                                                            E2 = Energy of higher orbit

                                                            hﬠ = Quantum of energy

                                                            h = Plank’s constant

                                                            ﬠ = frequency of radiations

  • Electron while revolving in orbits has angular momentum which is some integral multiple of h/2л.

We can derive various formulae on the basis of Bohr’s theory.

Defect’s of Bohr’s Model:

Bohr’s theory is good enough to explain the spectrum of hydrogen atom and hydrogen like cations (He+, Li+2, Be+3) but:

  • It fails to explain the spectrum of atoms and ions having more than one electrons.
  • It fails to explain the fine lines in the spectrum obtained in the high resolving power spectrometer.
  • It also cannot explain Zeeman effect and Stark effect.
  • Bohr model of atom goes against the Heisenberg’s uncertainty principle.

“Appearance of additional lines in the spectrum when obtained in magnetic field”, is called Zeeman effect.

“Appearance of additional lines in the spectrum when obtained in electric field”, is called

Stark effect.                             

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