34 SECTIONAL ADDRESSES. 



like the nitrogen in ammonia. Where co-ordination occurs this limitation 

 is removed ; the atom can give or take as many electrons as may be 

 necessary, and in the fully co-ordinated atom it will have a fully shared 

 valency group. Its maximum co-ordination valency, or co-ordination 

 number, is therefore half the number of electrons in its maximum valency 

 group. 



In this way the conception of the co-ordinate link as being a covalency, 

 that is, a link of two shared electrons, differing from the ordinary covalency 

 only in this, that the two electrons both come from one of the linked atoms 

 instead of one from each, provides the mechanism required to explain the 

 existence and the properties of the co-ordination compounds of Werner. 

 This conclusion removes the apparent contradiction between organic and 

 inorganic compounds ; it refers the structure of molecules of both classes 

 to the same physical principles, and exhibits the original co-ordination 

 theory of Werner and the older structural theory as two aspects of the 

 same general process. It further removes two objections which might 

 have been urged against the co-ordination theory as it was originally 

 proposed. The first of these is that it seemed to assign a unique position 

 to one or two of the atoms in a molecule, which were regarded as ' co- 

 ordination centres ' in some way governing the structure of the whole. 

 This is obviously an incorrect view of the molecule, in which every atom 

 is in a sense as important as every other. We can now see that this is in 

 fact the case, and that the nitrogen in an ammine, for example, is just 

 as inuch a centre of co-ordination as the metal. The second point is that 

 the distinction which Werner made between principal and subsidiary 

 valencies, which was always unsatisfactory, now disappears. It originated 

 in a desire to retain the valencies of the structural theory, while recog- 

 nising the formation of more links than the structural theory would 

 permit. It has long been clear that there was no ground for maintaining 

 the existence of this distinction within the co-ordination complex. The 

 electronic theory shows that the difference between a normal and a 

 co-ordinate covalency is in their method of formation ; when they have 

 been formed both alike consist of two shared electrons. 



The further application of these ideas to those compounds with which 

 Werner's name is most closely connected is an inquiry of great interest, 

 but I do not propose to pursue it here. I would rather consider some 

 more general questions. We have been led, in seeking an explanation 

 of the structure of co-ordination compounds, to the conception of a third 

 form of atomic linkage in addition to the recognised forms of electro- 

 valencies and covalencies ; or, as we should rather say, we have found 

 that a covalency can arise in a second way. This new method is peculiar 

 in that it allows of the combination of apparently saturated atoms or 

 molecules with one another, and it is therefore the condition which makes 

 the association of liquids possible. Links of this type are not confined to 

 inorganic compounds, but are widely spread in organic chemistry, as 

 Werner himself showed. Co-ordination is thus of great importance 

 throughout the whole of chemistry. Now that we understand the physical 

 mechanism which underlies it, we may hope to arrive at some idea of 

 its characteristic properties, and it will be well to consider what new 

 light these throw on various problems of chemistry in general. 



