Forces between Atoms and Chemical Affinity. Ill 



of the smaller sphere than at that of the larger one. In a 

 case like this the field due to'the molecule may be unsym- 

 metrical and the molecules M may attach themselves to one 

 side of A rather than to the other ; we may express this by 

 saying that ODe atom in the molecule A can form these 

 molecular complexes while the other can not. In the 

 illustration given above we have supposed that the lack of 

 symmetry is due to the difference in size of the atoms in the 

 molecule A. Although the meaning we attach to the idea of 

 atomic volume depends to a large extent on the view we 

 take of the constitution of the atom, yet on almost any view 

 the electric force at the surface of a charged atom would be 

 larger for an atom with a small " volume " than for one with 

 a larger one. We may see an illustration of this in the case 

 of the alkali metals. These can lose a negative corpuscle 

 with exceptional ease, which may be taken as an indication 

 that the force exerted by a positively charged atom on a 

 corpuscle at the surface of the atom is exceptionally small. 

 Now the atoms of the alkali metals are distinguished for 

 their large atomic volumes, and their electropositive character 

 increases from lithium to caesium along with their atomic 

 volumes. We should thus expect that, at any rate with 

 elements of the same type, those with a small atomic volume 

 would be more likely to form molecular complexes than 

 those with larger ; that, for example, lithium would be more 

 likely to form molecular compounds than caesium. The fact 

 that of the chlorides of the alkali metals lithium chloride is 

 the most deliquescent, may be an example of this effect. 



Any want of symmetry in the B molecules may result in 

 their attaching themselves to one of the atoms in the A mole- 

 cule in preference to the other : thus, if in the electrostatic 

 doublet in the B molecule the electric force close to the 

 negative end of the doublet is greater than that close to the 

 positive end, B will tend to attach itself to the atom in A 

 which has the positive charge, when there is intra-molecular 

 ionization in A ; while if the force close to the positive end 

 of B was greater than that close to the negative, B would 

 attach itself to the other atom in A. We thus see that the 

 atoms in a molecule A to which other molecules are linked 

 will depend on the nature of B as well as on that of A. 

 And it does not follow because water of hydration is linked 

 to one atom in a molecule of A, that the molecules of 

 all the other substances which show intra-molecular ionization 

 — ammonia, alcohol, hydrochloric acid and the like — will 

 necessarily be linked to the same atom. 



The formation of these molecular compounds would 



