MODERN CONCEPTS IN PHYSICS LANQMUIR 235 



classical laws. This model enabled him to derive certain mathemat- 

 ical equations from which he was al)le to calculate the frequencies 

 corresponding to the dilierent lines in the spectra of hydrogen and 

 other elements, these frequencies being obtained from fundamental 

 quantities such as the charge and mass of tlie electron and the quan- 

 tum constant A, and did not involve any quantities dependent on the 

 properties of the elements in question. The agreement between the 

 theory and experiment was practically perfect, often enabling tiie 

 frequency to be calculated with an accuracy of one part in 200,000. 



Such remarkable success made most physicists and chemists be- 

 lieve that Bohr's model, for the hydrogen atom at least, was sub- 

 stantially correct. That is, the}' believed that Bohr's work proved 

 that in a normal hydrogen atom the electron really described a 

 circular orbit around a nucleus having a diameter and a frequency 

 given by Bohr's model. Bohr himself never attached any such 

 importance to the mechanical model, realizing that the important 

 steps that he had taken consisted mainly in the introduction of new 

 concepts and more particularly in the mathematical equations by 

 which the observed frequencies in the spectral lines could be 

 calculated. 



"Within recent years, largely through the work of Bohr himself 

 and his students, and Sommerfeld, Schroedinger, and others, this 

 theory of the hydrogen atom has undergone changes. According to 

 Bohr's original model the radiation of energy corresponding to a 

 spectral line resulted from transition in which the electron passed 

 from one stationary orbit to another. No physical picture of this 

 transition seemed possible. To account for the known phenomena 

 it seemed necessary that the transition should occur so rapidly that 

 the electron would have to move from one orbit to another with a 

 velocity greater than that of liglit, and yet the train of waves in 

 the resulting radiation lasted for relatively long periods of time, 

 about 10'' seconds. Kadiant energy could be absorbed by the atom 

 only if the frequency was just that which was capable of trans- 

 ferring an electron from one orbit to another definite orbit. Thus 

 only one frequency could be absorbed at a time by an atom. It 

 was found, however, that the frequencies corresponding to many 

 lines could be scattered by a single atom. This seemed to reciuire 

 the presence within any given atom of a number of oscillators as 

 great as the number of lines in the spectrum. One of the greatest 

 arguments in favor of the original Bohr theory was that it avoided 

 just this sort of complication in the atom. 



To get rid of difliculties such as these, Heisenbrrg and Bohr real- 

 ized that it was necessary to sweep out of the theories of atomic 

 structure the many concepts which were characteristic of the me- 



