ARTICLES 25T 



docs not exceed a certain limit. The number of atoms or 

 groups which another atom can hold with a maximum force is 

 therefore of necessity restricted, and whilst, by the resultant 

 attractive force of this close aggregate, other atoms or molecules 

 may be held, the union will be feeble and liable to be dis- 

 turbed by the competing attraction of other molecules among 

 which they may find themselves, e.g. if molecules are so at- 

 tached they may be broken off when brought into solution, 

 or atoms under similar circumstances may be ionised. 



The maximum number of atoms or molecules which a 

 grouping atom can hold in stable combination with itself is 

 termed by Werner the " co-ordination number." This is found 

 to be to a considerable extent independent of the nature of 

 the central atom, and for many elements is six. 



These six atoms, or groups, are so firmly held in a more or 

 less complete enveloping sphere or shell enclosing the central 

 nuclear atom, that they are incapable of separate ionisation. 

 Other atoms or groups held by the residual affinity of the 

 central group are in an outer or ionisable zone. 



These theories have been chiefly developed in connection 

 with the complicated addition compounds which certain 

 metallic salts, notably those of trivalent cobalt and of chromium, 

 form with other molecules, as for example : 



CoCl 3 -6NH 3 , CoCl 3 2NH 3 -2H 2 0, Co(N0 2 ) 3 - 3 NH 3 , 

 Co(N0 2 ) 3 -2NH 3 -KN0 3 



According to the views of Werner, which have brought such 

 apparently differently constituted compounds into an ordered 

 system, they all contain a radicle which may be basic, neutral 

 or acidic, consisting of a trivalent metallic atom such as cobalt 

 or chromium associated with six other molecules, atoms or 

 groups which may be ammonia or some similar basic substance 

 (as ethylene diamine or pyridine), or water or halogen atoms, 

 or groups such as N0 9 , or partly one and partly others ; e.g. the 

 radicles in the compounds cited are 



[Co(NH 3 ) 9 ] +++ , [Co(NH 3 ) 2 (H a O) 8 Cl 2 r, [Co(NH 3 ) 3 (N0 8 ) 3 ]°, 



[Co(NH 3 ) 8 (NO) 3 ) 4 ]- 



The valency and chemical character of these radicles are found 

 to depend not only upon the grouping metallic atom, but also 

 upon the nature of the six molecules, atoms, or groups with 



