332 Dr. A. W. Stewart on Atomic Structure 



At first sight the main difference between the two phe- 

 nomena appears to lie in the fact that the /3-ray change is 

 spontaneous, whilst the chemical change of valency is a 

 controllable process ; but even the spontaneity of the /3-ray 

 change finds its parallel among certain of the stable elements. 

 Thus when the chloride of monovalent indium is dissolved in 

 water, it is spontaneously converted into metallic indium and 

 the chloride of trivalent indium. Reduced to its essentials, 

 this change corresponds to the loss of two negative electrons 

 from two of the monovalent indium atoms ; and no external 

 forces are required .to bring about the phenomenon*. The 

 case of indium is not an isolated one, as this type of reaction 

 appears to be the most general which is exhibited by inorganic 

 compounds. 



Another parallelism between the /3-ray change and the 

 conversion of an ion into a new one of higher valency may 

 be adduced. In several cases, elements are found which exist 

 in monovalent and trivalent forms, or in the divalent and 

 quadrivalent condition only, instead of yielding a complete 

 series of mono-, di-, tri-, and quadrivalent varieties. Thus 

 thallium forms the chlorides T1C1 and T1C1 3 , but does not 

 give rise to the intermediate T1C1 2 . It may be asked why 

 these intermediate forms are not isolated when electrical 

 charges are removed step by step from substances of lower 

 valency. 



The state of affairs among the radio-elements throws some 

 light upon this point. The conversion of T1C1 into T1C1 3 is 

 paralleled by two consecutive /3-ray changes in the radio- 

 elements; and in the following table the results of such 

 successive changes are given. These examples have been 

 selected in which no disturbing factor in the form of an 

 alternative a-ray change occurs. The figuresf give the 

 average life of the element. 



Gi '°»p n -ssH%r+ ^^5^ Group (N+2) - 



Uranium-X, Uranium X 2 Uranium-2 



35'5 days 1*65 minutes 3 x 10 6 years 



Mesothorium-1 Mesothorium-2 Radiothorium 



7*9 years 8*9 hours 2*01 years 



Radium-D Radium-E Radium-F 



24 years 7'20 days 196 days 



* Even when solvent action is assumed, the spontaneity of the change 

 retains its importance from the present point of view, 

 t Soddy, 'The Chemistry of the Radio-elements.' 



