330 Dr. A. W. Stewart on Atomic Structure 



retain in their orbits the " cometary " negative electrons 

 which circulate around it. 



With regard to the expulsion of charged helium atoms 

 from radioactive elements, it is assumed that the a-particle 

 consists of four positive and two negative electrons : the pair 

 of negative electrons being situated at the foci of an ellipse 

 around the circumference of which two positive charges 

 revolve. The extra pair of positive charges travel in longer, 

 " cometary" orbits; so that they are easily detachable when 

 in aphelion. It must be admitted that there is a difficulty in 

 accounting for their attraction by the atomic nucleus, which 

 in this case is electrically neutral ; but as this attraction is a 

 matter of practice and not of theory, it must be admitted as 

 possible even if no theory can be adduced to account for it. 



The formation of the a-particle is due, as has been said, 

 to the collision of two systems each containing two positive 

 and one negative electron. This does away with the necessity 

 for postulating the presence of actual helium atoms within 

 the structure of radioactive elements, an hypothesis which is 

 fraught with difficulties owing to the fact that the helium 

 atom has a volume of 26 "6, whilst the uranium atom, which 

 emits eight helium atoms, has a volume of only 12*8. The 

 collision hypothesis also accounts for the presence of the two 

 extra positive charges which invariably accompany the helium 

 atom in its ejection. 



In this model atom, as in most others, the valency of an 

 element is taken as the difference between the total positive 

 and the total negative charge of the atom ; but the variation 

 in valency caused by a- or /3-ray changes is assumed to be 

 brought about by alterations in the inner zones of the atomic 

 structure, whilst chemical changes of valency are accounted 

 for on the assumption that the number of the electrons in the 

 cometary orbits is altered. No definite conclusions can be 

 drawn with regard to the relative numbers of electrons in 

 the various zones, beyond the suggestion put forward above 

 that the number of electrons in the innermost negative core 

 of metallic atoms is less than that of the electrons in the 

 intermediate positive orbits ; though probably, as Soddy has 

 indicated, the surplus number of positive charges in the two 

 inner zones combined is equal to the atomic number of the 

 atom. 



In order to test still further this conception of the atom, it 

 is necessary to examine evidence in a different field. Among 

 the radioactive elements, two classes can be distinguished. 

 In the first place there are certain groups of elements which 

 are chemically inseparable but which differ from one another 



