246 ELECTRO-MOTIVE SERIES OF THE METALS. 



Hence for two metatis, ATi and M^, which are to be converted 

 into salts ty oxidation and su'bsequent treatment with acid, we 

 have the equations 



I. (a) ]\ri -]- oxygen = Mi oxide. 



(b) [Ml oxide -|- same acid =: Mi salt 4- wat' . , 

 II. (a) M2 + oxygen -- M2 oxide. 



(b) M2 oxide -|- acid=: M2 salt + water. 



In these cases the tendency to undergo reaction is practically 

 the same in lb and 11/?, the formation of water being as it were 

 the driving force in each case. Hence if the tendencies of the 

 total reaction Metal ^^ Salt are different in the two cases (as 

 they actually are), the cause of the difference is to be sought in 

 the processes represented by la and Ila, i.e., in the different 

 tendency of the two metals to undergo oxidation. Thus we see 

 that the relative tendency of a metal to undergo oxidation is also 

 its relative tendency to form salts. Hence the metals high up in 

 the Electro-motive Series wnll never occur native on account of 

 their salt-forming tendencies, whereas the metals at the bottom 

 of the series will generally ibe found uncombined in nature owing 

 to the difficult}' with which they form salts. But a metal may 

 be converted into a salt by another process, z'i::., by the direct 

 action of an acid. In this case we have ('taking the metal -as 

 divalent) Mi + 2 HCl r= Mi CI2 -f Ho. 



But a solution of HCl. consist^ chiefly of hydrogen and 



I __ 



chlorine ions, H and CI. Similiarlv Mi Clc. in solution consists 



largely of Mi and CI ions. \\"e can therefore write the above 

 equation in the form, 



+ ++ - 



Ml + 2 H + 2 CI = Ml -i- 2 CI + H2. 

 Metal ions ions ions ions gas (unionised). 



+ + + 



or Ml + 2 H ^ M 4- H2. 



Metal ions ions gas. 



Thus the action of an acid on a metal to form a salt really 

 consists (on the ionic theory) in the taking away of the electric 

 charge from the hydrogen ion by the metal atom. The metal 

 atom must thus have a greater affinity for an electric charge 

 than the hydrogen atom, and with different metals the relative 

 tendency of these to dissolve in acids to form salts must be the 

 same as the relative tendency oif the metals to rob the H ions of 

 their charges. The electric charge on an ion, in fact, behaves 

 as if it were a chemical element, and the different metals have 

 different affinities for this ionic charge, just as they have different 

 affinities for oxygen, chlorine, etc. The Electro-motive Series 

 expresses the relative value of this affinity, which, moreover, can 

 be expressed definitely in numbers, which are obtained as the 

 results of physical measurements. We are thus able to give to 



