THE PERIODIC SYSTEM 283 



of chemical substances which was complete in its scope, and ideal 

 in its exactness. This, however, is far from being the case. 



The order of activity of the metals (p. 54) and of the non-metals 

 (p. 209) summarizes many properties, and explains many features 

 of the chemical behavior of the elements. This list is scattered 

 through the periodic table (compare both), without any trace of 

 regularity. 



The periodic system concentrates attention too largely on one 

 of the valences of each element. Thus, for manganese, it focuses 

 attention on the septivalent form in the permanganates. But 

 manganous salts are more like the ferrous, the cobaltous, the 

 chromous, and other sets of salts, none of which are in the same 

 column of the table. Similarly, the manganic salts are like the 

 ferric salts and the salts of aluminium. Again, copper is univalent 

 in one series of salts, but in its better known salts it is bivalent. 

 Silver, which belongs to the same periodic family, is always 

 univalent, while gold, also in the same family, is univalent or 

 trivalent, and in the latter case is almost wholly a non-metallic 

 element. If it were possible to place each element in several 

 different columns, one for each of the valences that it shows, the 

 table would then include far more of the properties of the elements. 

 But this cannot be done, for, according to the order of magnitude 

 of the atomic weights, there is but one place for each element. 

 In other words, the periodic system largely ignores the variety 

 of different classes of chemical relations which an element with 

 several valences always shows. 



The position of hydrogen in the system is still a matter of dis- 

 pute. It is more familiar to us as a univalent positive radical 

 resembling the alkali metals in forming compounds with negative 

 radicals such as chlorine, but it can also function as a univalent 

 negative radical, resembling the halogens in forming hydrides 

 with the alkali metals which are analogous to chlorides. Most 

 chemists shelve the difficulty by giving hydrogen a position all to 

 itself at the top of the table. 



