60 BELL SYSTEM TECHNICAL JOURNAL 



differences between the band-spectra of compounds involving such 

 atoms, which are well explained by attributing to the several atoms 

 identical nucleus-charges and electron-systems, but nucleus-masses 

 standing to one another in the same ratios as the observed masses of the 

 atoms do.^ But I must not give all the evidence for the nuclear atom- 

 model, or this article will be swamped. 



Among the reasons for ascribing radioactivity to the nucleus, the 

 primary one has already been introduced — radioactivity, like mass, is 

 unalterable; and another has already been stated, though without 

 mentioning its relevance to this question. Certain radioactive ele- 

 ments emit charged atoms of helium; and since outside of the nucleus 

 nothing except electrons is provided in the atom-model, these charged 

 atoms must be supposed to proceed out of the nuclei. This argument 

 could not be used upon the radioelements which emit electrons; but 

 even for these there are reasons for suspecting that some of the electrons 

 which issue from them do not come out of the family of electrons 

 surrounding the nucleus, but from some other place. For instance, 

 it is possible and usual to pry electrons out of various locations in the 

 circumnuclear family; but when this is done, the resulting "ionized" 

 atoms promptly take in one electron or as many more as they have 

 lost, and revert to their original state and nature. This does not 

 happen with the radioactive atoms which emit beta-rays; the depar- 

 ture of the electron effects an irreversible change, the atom is altered 

 for good and all. It does not however acquire a permanent positive 

 charge; it takes on an electron and makes good its loss of charge. 

 This is best explained by supposing that the original atom lost an 

 electron originally located in the nucleus, and added one to the cir- 

 cumnuclear family, keeping its net charge equal to zero but undergoing 

 a rearrangement of its charges. 



By accepting the idea that certain of the charged particles emerging 

 from a radioactive element issue from the nuclei of its atoms, it is pos- 

 sible to express and explain very simply a celebrated law of radio- 

 activity which was discovered by Fajans and Soddy in the early days 

 of the nuclear atom-model and helped greatly to establish it. 



When an atom of a radioelement of atomic number Z emits an 

 alpha-particle with its charge + 2e, its nuclear charge diminishes by 

 that amount. It becomes an atom with nuclear charge (Z — 2)e and Z 

 electrons. The diminished nuclear charge cannot hold the entire 

 electron-family; two of its members depart, and the atom becomes an 

 atom of nuclear charge (Z — 2)e and (Z — 2) circumnuclear or orbital 

 electrons. The radioelement changes into an element two steps farther 

 down in the procession of the elements. 



^ Introduction, p. 400. 



