VALENCE 213 



S0 4 os a whole. It is evidently bivalent, H 2 I (S0 4 ) n , Zn n (S0 4 ) n . 

 Similarly, in K^NOs) 1 , and in H I (N0 3 ) 1 , the N0 3 is clearly uni- 

 valent. H 3 I (PO 4 ) 111 shows P0 4 to be trivalent. 



Valence also by Displacement. In the foregoing instances, 

 we have learned the valence of an element or radical by studying 

 its combinations. But, clearly, if an element is displaced from 

 combination, atoms of equal total valence must take its place. 

 Thus the action : 



Zn + 2HC1 -* ZnCl 2 + H 2 



shows Zn displacing 2H 1 , and the valence of Zn must therefore be 

 two. We see that this is the case for, on displacing the 2H, it 

 combines with 2C1 1 . 



Summary. We may now sum up all these facts by saying: 

 The valence of an element is a number representing the capacity 

 of one atomic weight of the element to combine with, or displace, 

 atomic weights of other elements, the unit of such capacity being 

 that of one atomic weight of hydrogen or chlorine. To make a 

 corresponding statement for the valence of a radical, we sub- 

 stitute, in the foregoing sentence, the word radical for element, 

 and the word formula-weight for atomic weight. 



Application in Making Formula? and Equations. We 



can see at once that the rule of valence will be of great assistance 

 to us in making formulae and equations. Suppose, for example, 

 that we burn a piece of aluminium foil in chlorine, and get the 

 white aluminium chloride. What is its formula? Up to this 

 point, we should simply have looked for it in a book. And if, 

 subsequently, we had required the formulae of the oxide and sul- 

 phate of aluminium, we should have looked these up separately 

 also. 



But now, all we have to do is to find out the valence of aluminium. 

 Knowing already the valences of Cl 1 and O n and (S0 4 ) n , we have 



