Chapter Four 



The Atom 



THE old view that the atoms, the units of the 

 elements, are ultimate and indivisible units 

 necessarily was abandoned with the discovery of 

 X-rays (W. C. Roentgen, 1895) and the discovery 

 of the spontaneous rupture of the atom in radio- 

 activity (Henri Becquerel, 1896). The heaviest 

 known atom, uranium (atomic number 92, atomic 

 weight 238.2) breaks up into a whole series of other 

 elements, of which radium (Mme. and M. Curie, 

 1898) is one. There came the knowledge that the 

 atoms are built up of positive and negative electrons, 

 and that the atom is a system that consists of a 

 positively charged nucleus which is surrounded by 

 the negative electrons (Sir Ernest Rutherford, 

 1911). 



This view of the atom having been established, 

 numerous other now well known facts about the atom 

 came to light: 



Note. — For an exhaustive treatise on the atoms and mathematical treat- 

 ment of atomic structure, see A. Sommerfeld, Atomic Structure and Spectral 

 Lines, preferably the fourth (German) edition. 



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