498 On the Structure of the Atom. 



It should be borne in mind that there is no inherent 



impossibility on the nucleus theory that atoms may differ 



considerably in atomic weight and yet have the same nucleus 



charge. This is most simply illustrated by radioactive 



evidence. In the following table the atomic weight and 



nucleus charge are given for a few of the successive elements 



arising from the transformation of uranium. The actual 



.... 

 nucleus charge of uranium is unknown, but for simplicity it 



is assumed to be 100. 



uccessive Elements . . Ur^ TJr X^ UrX -> Ur -> Io^ Ra 



Atomic weights 238*5 234-5 234-5 234-5 230'5 226-5 



Charge on nucleus 100 98 99 100 98 90 



Following the recent theories, it is supposed that the- 

 emission of an a particle lowers the nucleus charge by two- 

 units, while the emission of a /3 particle raises it by one unit.. 

 It is seen that Ur! and Ur 2 have the same nucleus charge 

 although they differ in atomic weight by four units. 



If the nucleus is supposed to be composed of a mixture of 

 hydrogen nuclei with one charge nnd of helium nuclei with 

 two charges, it is a priori conceivable that a number of atoms 

 may exist with the same nucleus charge but of different 

 atomic masses. The radioactive evidence certainly supports 

 such a view, but probably only a few of such possible atoms 

 would be stable enough to survive for a measurable time. 



Bohr * has drawn attention to the difficulties of construct- 

 ing atoms on the " nucleus " theory, and has shown that the 

 stable positions of the external electrons cannot be deduced 

 from the classical mechanics. By the introduction of a con- 

 ception connected with Planck' s quantum, he has shown 

 that on certain assumptions it is possible to construct simple 

 atoms and molecules out of positive and negative nuclei, e. g~ 

 the hydrogen atom and molecule and the helium atom, which 

 behave in many respects like the actual atoms or molecules.. 

 While there may be much difference of opinion as to the 

 validity and of the underlying physical meaning of the 

 assumptions made by Bohr, there "can be no doubt that the 

 theories of Bohr are of great interest and importance to all 

 physicists as the first definite attempt to construct simple- 

 atoms and molecules and to explain their spectra. 



University of Manchester. 

 February 1914. 



* Bohr, Phil. Mag. xxvi. pp. 476, 857 (1913). 



