Forces between Atoms and Chemical Affinity. 781 



If we take the view that the forces exerted by the atoms 

 are due to charges of positive and negative electricity, the 

 negative electricity being on the corpuscles, then the force 

 exerted by an atom in the region around it will depend 

 mainly upon the mobility of its corpuscles. 



For if an atom A has some corpuscles which can move 

 freely about in the atom, then, if an electrical system of any 

 kind is brought into the neighbourhood of A, these corpuscles 

 will move into such a position that there will be attraction 

 between A and the system. For example, if A contained a 

 single free corpuscle and the system B were an electrical 

 doublet, the corpuscle would move into such a position as to 

 attract the doublet in whatever position that might be 

 placed, and as the corpuscle is extremely mobile this shift 

 can take place with great facility. 



If, however, the corpuscles were fixed in the atom, then if 

 it were not in such a position as to attract B originally (and 

 it is just as likely to be in a position to repel as to attract), 

 in order to attract it, the whole atom A, whose mass is 

 very much greater than that of a corpuscle, would have 

 to wheel round, and this on account of the much greater 

 inertia of the atom would be a matter of much greater 

 difficulty than the motion of the corpuscle in the former 

 case. 



Thus we may regard an atom all of whose corpuscles are 

 fixed as incapable of exerting any great attraction on other 

 atoms around it unless these are in particular positions, we 

 shall suppose that an atom in this state is what the chemists 

 call saturated. So that our criterion of saturation will be 

 that all the corpuscles in the atom are held fast by the forces 

 acting upon them. 



We regard the negatively electrified corpuscles in an atom 

 as arranged in a series of consecutive layers ; those in the 

 inner layers we suppose are so firmly fixed that they do not 

 adjust themselves so as to cause the atom to attract other 

 atoms in its neighbourhood. There may, however, be a ring 

 of corpuscles near the surface of the atom which are mobile 

 and which have to be fixed if the atom is to be saturated. 

 We suppose, moreover, that the number of corpuscles of this 

 kind may be anything from to 8, but that when the number 

 reaches 8 the ring is so stable that the corpuscles are no 

 longer mobile and the atom is so to speak self-saturated. 



The number of these mobile corpuscles in an atom of an 

 element is equal to the number of the group in which the 

 element is placed on Mendeleef's arrangement. 



Thus helium and neon have no free corpuscles ; hydrogen, 



