Module 1 : Atomic Structure
Lecture 1 : Structural Chemistry
  1.6

Foundations of Quantum Theory

Bohr attempted to explain atomic structure by certain postulates, which deviated in several ways from the classical or Newtonian mechanics. In Newtonian mechanics, the position and the momentum (or the velocity) of the particle (or a collection of particles) can be simultaneously determined at each instance of time. By an analogy to the orbits of planets in planetary motion, Bohr postulated that an electron of an atom exists in “stationary” states in these orbits, which he assumed to be circular.
 
Since this postulate alone was not sufficient to explain the spectral lines obtained by Balmer, Lyman, Paschen and others, he further postulated that the angular momentum of the electron in these orbits ( given by mvr, where m is the electron mass, v, its velocity and r, its radius in the circular orbit) was quantized and was equal to n where with h = Planck's constant = 6.626 x 10-34 Js, and n can take on only integer values. Whenever the atom absorbed an energy E which was equal to the difference of energy between any two levels of the atom, there was a change of state of the atom from one stationary state to another stationary state. Using these postulates, the following formula for the energy difference between the two levels can be easily derived.

 
E2 – E1 = - RH ( 1/n22 – 1/n12) (1.1)

where RH = 109737 cm-1 is called the Rydberg constant and E2 and E1 are the energy levels of the second and the first stationary state, respectively.

Using this formula, Bohr could obtain all the lines of the spectrum of H-atom which is shown in the Figure 1.6.
 
 
While this theory explained the structure of the hydrogen atom well, it could not explain the structures of other atoms and molecules.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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