1915^ Radio-Activity and Periodic System 31 



of positive electricity and the expulsion of these two positive charges 

 from the atom affects the valency of the product, as Fajans has pointed 

 out, just as in ordinary electro-chemical changes of valency. If the 

 atom were initially in Group IV., for example, its ion is tetravalent and 

 carries four atomic charges of positive electricity. Two such charges 

 having been expelled with the a particle, the product is in the di-valent 

 Group II., non-separable from radium. The mass in this case is four 

 units less. So with the ^ ray change. The j3 particle is a negative 

 electron and the loss of this single atomic charge of negative elec- 

 tricity increases the positive valency of the product by one. Radium B, 

 for example (in Group IV.), expels a ^ particle and becomes radium 

 C (in Group V.). Whenever two or more radio-elements fall in the 

 same place in the Periodic Table, then, independently of all consider- 

 ations as to the atomic mass, the nature of the parent element, and the 

 sequence of changes in which they result, the elements in question are 

 chemically non-separable and identical. As will later appear, this 

 identity extends also to most of the physical properties such as volatility 

 and spectrum reactions.* 



To express this "newlj revealed complexity of matter," 

 Soddv has suggested the word isotope. A group of two or 

 more elements occupying the same place in the periodic table, 

 differing in atomic weight yet chemically non-separable, is 

 isotopie. There are possibly seven such elements isotopic 

 with lead. Radium is one of four isotopes. The chemistry of 

 thirty-seven radio-elements is thus reduced to a smaller 

 number of about ten types. 



Two fundamental changes in the old views as to the sys- 

 tem are indicated here. First, the position of an element is 

 not fixed but can be changed in either of two ways— by a 

 change in valence (which, as is well known, can be brought 

 about in various ways), and again by disintegration due to 

 ray-emission. Secondly, more than one element can occupy 

 a given position in the system. This is independent of the 

 atomic weight, but such elements are chemically inseparable. 

 This involves the giving up of all idea of the properties 

 being functions of the atomic weights and necessitates the 

 formulation of the law anew. 



The place occupied by an atom is not solely determined 



Soddy, "The Radio-active Elements and the Periodic Eaw." 



