Forces between Atoms and Chemical Affinity. 775 



the work required to separate the constituents will be equal 

 to 2MM'/d 3 , where d is the distance between the centres of 

 the doublets. Now we may take d as comparable with 

 10" 8 cm. and put d~xXl6~ s cm., and from the values 

 given on page 764 M and M' are comparable with 10~ 18 , and 

 are say y x 10 ~ 18 , z x 10 ~ 18 respectively. The values found 

 for M and M' show that y and z will be small compared 

 with unity except for molecules in which there is intra- 

 molecular ionization, and even then they will not be large, 

 as the largest value we found was that for water, where 

 J\l = 2*l x 10~ 18 . Expressed in terms of x, y, z, the work 



2 vz 

 2MM'/d 3 required to separate A and B is equal to -~- X 10" 12 



i X 



ero-. The work required to separate the atoms in a molecule 

 of iodine is about 2 x 10" 12 erg, and this molecule is dissoci- 

 ated to a very considerable extent at temperatures of 300° 

 or 100° G. It follows from this that if yz/x 2 were as small, 

 say, as 1/10, the compound AB would be almost wholly 

 dissociated at ordinary temperatures ; for this compound to 

 exist then yz/x 2 must not be a small fraction. With moments 

 each as large as that in the ammonia molecule, this condition 

 is fulfilled, but it is evident from the numbers that there is 

 not any very great margin ; and if one of the moments fell 

 to as small a value as we might expect when there is no 

 molecular ionization, it would only be in cases where x is 

 exceptionally small that we could expect the compound to 

 exist at ordinary temperatures. The study of the magnitudes 

 involved thus leads us to the conclusion that though this 

 molecular combination may occur not infrequently, it will be 

 by no means universal and will probably be limited to special 

 types of molecules. Let us suppose, however, that this con- 

 dition is fulfilled, and that a compound AB can exist : what 

 are the prospects of A being able to hold a second molecule 

 of B in combination ? It is clear that the first molecule will 

 have taken up the strongest part of the electric field round A ; 

 if a second molecule B moves up to A, it has to take up 

 a position where the attraction which binds it to A is less 

 intense than that which bound the first molecule ; and, more- 

 over, since the doublets in the two molecules of B will repel 

 each other, the one first in the field will be driven from its 

 specially favourable position, so that when the two molecules 

 of B are present in the compound AB 2 , neither is attracted 

 to A quite so strongly as the single molecule B in the 

 compound AB. Similar remarks will apply when a third 

 molecule B is added to form the compound AB 3 ; in this case 

 none of the B molecules will be held quite as firmly as in 



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