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.

Postulates of Bohr Theory of Atomic Structure:

• Atom consist of two parts-

1. The very small, centralized, and highly denser position is called a 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. = -E_n.

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.

Rutherford Gold Foil Experiment- Discovery of Nucleus

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