CHEMISTRY OF THE EARTH 103 



appear to be triads, are really pentads which 

 have become facultative triads by self- 

 saturation of two of the active centres. If 

 such elements united to form a large group, 

 as the element carbon does, there would be 

 this additional element of instability, in that 

 every member of the group was unsaturated, 

 in addition to the union of the similar elements 

 to one another. It is thus seen that the 

 elements of which the valency is represented 

 by odd numbers are not suited for taking on 

 central roles in large groupings of atoms 

 into a single giant molecule. 



When the valency of a hexad is reached, 

 as has been claimed to be the case in the iron 

 group, the atom itself reaches nearly the limit 

 of internal stability, and is just able, in single 

 or dual atomic groupings, to unite with other 

 dissimilar atoms, but any higher interaction 

 between the similar hexad atoms than two 

 joined together is never found. It is indeed 

 a matter of doubt whether hexad elements 

 exist. The energetics of such forms of 

 atomic matter at the present state of our 

 knowledge are but little known, and the 

 valency theory gets into difficulties in explain- 

 ing them. It becomes clear, therefore, by 

 such processes of exclusion that the type of 

 element in which the electrons are so arranged 



