The Conceptions of Atomic and Molecular Structure

Having now considered the development of the special principles underlying the quantum mechanics of the atom, we shall indicate briefly the development of our knowledge regarding the material substratum to which they apply. 

The phenomena of electrolysis first led to the hypothesis of the atomic structure of electricity. Subsequently the carriers of negative electricity were detected in the free state as the cathode rays and the /3-rays of radioactive substances. Prom the deviation of these rays in electromagnetic fields, the ratio ejm, of charge to mass of the particles, could be determined. It was found that 

On the assumption that the same elementary quantum of electricity is concerned both here and in electrolysis (which can be verified approximately by experiment), we are led to the conclusion that the mass of these negative particles of electricity is about an 1830th part of that of a hydrogen atom. These carriers of negative electricity are called electrons and it can be shown, by optical and electrical experiments, that they exist as structural units in all matter. By making use of the fact that it is possible to produce on very small (ultra-microscopic) metal particles, and oil drops, a charge of only a few electrons, and to measure it, very accurate values have been found for the charge carried by an electron. Millikan found.  

Positive electricity has only been found associated with masses of atomic magnitude. Positive rays have been produced and studied it will suffice to mention a-rays of radioactive substances, anode rays and canal rays. The determination of ejm from deviation experiments gave the mass of the a-particles to be that of the helium atom for the particles of the anode rays the mass is that of the atom of the anode material, while for the particles of the canal rays the mass is that of an atom of the gas in the tube. We must therefore assume that each atom consists of a positive particle, at which is concentrated most of its mass, and of a number of electrons. In the neutral atom the number of the elementary 

But if, on the basis of these conceptions and the classical principles, we now attempt to develop a mechanical theory of the atom, we encounter the following fundamental difficulty a system of moving electric charges, such as is pictured in this model, would continually lose energy owing to electromagnetic radiation and must therefore gradually collapse. Further, all efforts to deduce the characteristic structure of the series spectra on the basis of the classical laws have proved fruitless. 

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