JACQUES LOEB 685 



IV. Theoretical Considerations. 



KosseP has suggested tjhat for chemical reactions of atoms we may 

 replace the atoms by simplified models consisting only of the charges 

 of the positive nuclei in the center and the ring of valency electrons. 

 The distance between the positive nucleus and the outermost ring 

 or shell of electrons we call the radius of the atom or of the ion. While 

 a monatomic, monovalent cation has one excess positive charge in 

 its nucleus a monatomic, monovalent anion has one excess negative 

 charge in its periphery. Although these positive and negative charges 

 are alike, the relative electrostatic action of ions with the same charge 

 must vary with the radius. Cations act through the excess positive 

 charge on the nucleus, and the electrostatic action of the nucleus 

 must become the greater the smaller its radius; i.e., the nearer the 

 positive nucleus can get to the body on which it is supposed to act. 

 This explains why the accelerating as well as the depressing efficiency 

 of a cation on the rate of diffusion of water through the membrane 

 increases inversely with the radius of the cation; namely, in the order 

 Rb < K < Na < Li, where Rb has the minimal effect. 



When the nucleus has more than one excess charge the accelerating 

 as well as the depressmg efficiency increases with the valency, since 

 ceteris paribus the electrostatic action of a cation must increase with 

 the number of excess charges of its nucleus. 



We understand also why the accelerating as well as the depressing 

 efficiency of an anion increases directly with its radius, since the 

 electrostatic effect of a monovalent anion on an outside body is 

 determined by its excess electron and hence will be the greater the 

 greater the distance between the valency electron and its positive 

 nucleus. 



It is also obvious why the efficiency of an anion should increase 

 with its valency; i.e., with the number of valency electrons it con- 

 tains in excess of its nuclear charges. 



We can only speak of ionic radius in the case of monatomic ions 

 like K or CI; when, however, an ion consists of more than one atom, 

 e.g. NO3, CNS, we have more than one positive nucleus and each 

 nucleus has ' its own rings or shells of electrons. The term ionic 



' Kossel, W., Ann. Physik, 1916, xlix, 229. 



