VALENCE 215 



recovers the valence of an element or radical, when needed, by 

 recalling the formula of a substance containing this element or 

 radical in combination with a more familiar element or radical, 

 such as Cl 1 or H 1 . 



f 



Elements with More than One Valence. The rule of 



valence is not so simple as it has thus far appeared to be. A 

 number of the elements have more than one valence. In other 

 words, the capacity of an atomic weight of such an element may 

 have two (or even more) values, according to the circumstances 

 under which it is combining with other elements. 



Thus, antimony is usually trivalent, and gives compounds like 

 SbCl 3 , Sb 2 3 , SbBr 3 . But it can also form compounds in which it 

 is quinquivalent, like SbCl 5 . Similarly, iron forms two complete 

 series of compounds: 



Bivalent: FeCl 2 , FeO, Fe(OH) 2 , FeSO 4 . 

 Trivalent: FeCl 3 , Fe 2 O 3 , Fe(OH) 3 , Fe 2 (SO 4 ) 3 . 



Even the halogens, although uniformly univalent in their com- 

 pounds with hydrogen and other positive radicals, show oxygen 

 compounds of higher valence, such as chlorine dioxide C1O 2 , 

 iodine pentoxide I 2 O 5 . When an element does give more than one 

 series of compounds, however, we always make a strong point of 

 this fact, so that it may not be overlooked. 



No simple rule, for telling, in advance, which valence will be 

 used in a given action, can be stated. But the ions Fe++ and 

 Fe+++, for example, have different properties, and are easily 

 recognized in practice. 



As a rule, an element passes from one form of combination to 

 another without change of valence. But compounds of elements 

 like tin or manganese can undergo changes in the course of which 

 the valence alters. A case of this kind has already been en- 

 countered in the preparation of chlorine (p. 143). 



Mn0 2 + 4HC1 - 2H 2 + MnCl 2 + C1 2 . 



