THE SMALLEST PARTICLES OF MATTER 15 



Emptiness in the Atom 



In connection with cathode-ray experiments conducted with 

 Heinrich Hertz of radio-wave fame, Phillip Lenard (Nobel prize, 

 1905) observed that cathode rays could pass through a "window" 

 of thin aluminum foil and still retain sufficient energy to cause 

 fluorescence and phosphorescence. Lenard concluded that the 

 atoms in the foil must have a very open structure, and also that 

 they might have localized positive and negative electric charges. 

 Sir J. J. Thomson, in developing this idea, calculated how nega- 

 tive electrons would be distributed in a sphere of positive charge, 

 a matter of importance in understanding the Periodic Table. 

 Professors William Draper Harkins (Chicago), Gilbert N. Lewis 

 (California), and Dr. Irving Langmuir (Nobel prize, 1928), also 

 contributed much to establishing how electrons are distributed in 

 atoms. 



Meanwhile, Rutherford had been investigating scattering of 

 alpha particles by atoms; but his assistant, Dr. E. Geiger, had 

 observed only small scattering (about one degree) in thin pieces 

 of heavy metal. However, Rutherford set Geiger and a young stu- 

 dent, E. Marston, the task of finding out whether any alpha 

 particles could be scattered through a large angle by impact upon 

 an atom. Within a few days, to their great astonishment, they 

 observed some alpha particles coming backward. In a lecture 

 given shortly before his death, Lord Rutherford said: 



"It was quite the most incredible event that has ever happened to 

 me in my life. It was almost as incredible as if you fired a 15-inch 

 shell at a piece of tissue paper and it came back and hit you. On 

 consideration I realized that this scattering backwards must be the 

 result of a single collision; and when I made calculations I saw that 

 it was impossible to get anything of that order of magnitude unless 

 you took a system in which the greater part of the mass of the atom 

 was concentrated in a minute nucleus. It was then that I had the 

 idea of an atom with a minute massive centre carrying a charge. I 

 worked out mathematically what laws the scattering should obey, and 

 I found that the number of particles scattered through a given angle 

 should be proportional to the thickness of the scattering foil, the 

 square of the nuclear charge, and inversely proportional to the fourth 

 power of the velocity. These deductions were later verified by Geiger 

 and Marsden in a series of beautiful experiments." 



The extent of the "wide open spaces" in atoms is shown in the 

 following comparison: If the tiny proton nucleus of a hydrogen 



