212 SMITH'S INTERMEDIATE CHEMISTRY 



Marking the Valence. Until we are familiar with their 

 values in each case, it may be well to mark the valences thus : 



Na J Zn Al m Sn IV Cl 1 Br 1 I 1 O N m C IV 



As we should expect, an atom with the double capacity can 

 combine with two of the single capacity, or with one of the double 

 capacity, and so forth. Thus we have compounds of oxygen: 



Zn n O n Sn IV O 2 u Al 2 ra 3 n 



Briefly stated, the quantities of the two elements which com- 

 bine must have equal total combining capacities. Thus Sn IV has 

 the capacity four, and O 2 n has the total capacity of 2 X 2 or 4: 

 Al 2 m has a total capacity of 2 X 3 (or 6) and so has 3 n (3X2 

 = 6). 



iThe unit of combining capacity of an atomic weight (or atom) 

 s called a valence. The atomic weights of H and Cl are said to 

 >e univalent; those of Zn and O, bivalent; those of Al and N, 

 trivalent; those of Sn and C, quadrivalent. The highest valence 

 known is eight. 



Valence and Ionic Charges. Comparison with the formulae 

 of the ions already given will now show that the valence is equal 

 to the number of charges on the corresponding ions: IPCl 1 gives 

 H+ + Cl- and Zn n Cl 2 T gives Zn++ -f 2C1~. Also, of course, the 

 total number of each kind of charges (positive and negative) 

 was equal, just as the total valences of each of two constituents 

 of each compound are equal. 



Valence of Radicals. What has been said applies to com- 

 pounds of not more than two elements so called, binary com- 

 pounds. We cannot, by inspection, tell the valences in a com- 

 pound of three or more elements, like H 2 S0 4 . But, as we have 

 seen, all electrolytes behave like binary compounds, because they 

 divide into radicals, which move as wholes from one state of com- 

 bination to another. Hence we can assign a valence to the radical 



