Bohr Theory of Atomic Structure

Bohr Theory of Atomic Structure:

  • Bohr’s model is valid only for mono-electronic systems like H, He+, Li2+.
  • It is basically an intuitive model.
  • It is given by Neil Bohr, 1913.
  • This model explains the line spectrum of hydrogen atom.
bohrs model of atom 1024x559 - Bohr Theory of Atomic Structure

Postulates of Bohr Theory of Atomic Structure:

  • Atom consist of two parts-
  1. Very small, centralized and highly denser position called as Nucleus.
  2. Extra-nuclear part containing electrons, moving around the nucleus in well-defined circular path called as orbits.
  • Electrons revolve around the nucleus with high velocity, in those orbits for which angular momentum is quantized.

i.e  mvr = nh/2π

where, ‘n’ = 1, 2, 3,…………. but in friction;  ‘v’ is the velocity; ‘m’ is mass of the electron; ‘r’ is the radius of the nth orbit

  • Different energy levels are numbers as 1, 2, 3, 4, 5 or K, L ,M, N, O.
  • The energy of the electron is constant in fixed orbit known as Stationary States or Energy Levels.
  • As the size of shell increases, energy increases but energy gap decreases (away from the nucleus energy increase from more negative to less negative and finally become zero).
  • An electron can jump from any lower to any higher orbit, by absorbing energy is equal to the energy difference of two orbits.
  • An electron can jump from any higher to any lower orbit, by emitting or releasing energy is equal to energy difference of two orbits.
  • During absorption or emission of energy, Planck’s quantum equation is obeyed i.e. E = hν, where ν‘ is the frequency of radiation absorbed or emitted and ‘h’ is Planck’s constant.
  • Ionization energy is equal and opposite to the energy of electron in that orbit i.e. I.E. = -En.

Limitations of Bohr’s Model of Atom:

  • Inability to explain the line spectra of multi-electron atoms.
  • It fails to explain the multiple or fine structure of spectral lines.
  • Splitting of lines under the influence of a strong magnetic field or electric field (Zeeman Effect and Stark Effect).
  • It violates de Broglie concept of dual character of matter.
  • It violates Heisenberg’s uncertainty principle.
  • Bohr theory never explains the union of atom to form molecules.
  • Inability to explain the shape of molecule.
  • It cannot explain the brightness of spectral lines.
  • Bohr gives a flat model of the atom and the concept of orbit considering the electron as a particle but nowadays it is believed that atom possesses 3D model and concept of orbitals.
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