Quantum Conditions in the Atom

the electron was first known as a negatively charged particle. The work of Rutherford shows that it is localized in the outer parts of the atom and is 1841 times lighter than the hydrogen atom nucleus. The latter carries the positive charge and most of the atomic weight. 

The spectrum of a particular atom is unique. At the beginning of the last century, the atom was frequently thought to consist of a number of oscillators, one for each spectral line. Ritz found that sometimes two frequencies in a spectrum for a given atom sum up to another frequency of the same atom. It was also considered as very remarkable that the frequencies of the hydrogen atom seemed to have a strange integer relationship, known as Bahner s rule. 

FIGURE 1.3 Apparatus used by Thomson in his historic experiment. A, anode with a slit B, slit to sharpen the cathode ray further C, cathode D and E, two insnlated plates connected with the opposite poles of a high voltage battery F, recording eqnipment. The magnetic field is perpendicular to the electric field. 

A rough understanding of the spectrum and structure of atoms was first achieved by Niels Bohr in Copenhagen. He realized that a classical treatment of the hydrogen atom, similar to planet motion, would not give rise to a discrete spectrum, not even a stable atom. Therefore, he introduced stationary states for the first time. This was a hint of what was to come in the form of quantum mechanics about 10 years later. His principal assumptions are worth citing, since the main differences between classical theory and what is reqnired by a novel theory are clearly stated  

That the dynamical equilibrium of the systems in the stationary states can be discussed by help of the ordinary mechanics, while the passing of the systems between different stationary states cannot be treated on that basis. 

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