CHAPTER XVIII 

 VALENCE 



THE differing number of charges on different ions has called 

 our attention vaguely to a subject which must now be explored 

 and set forth more clearly. 



Valence. The formulae of a number of common compounds, 

 including some that we have met with, are as follows: 



NaCl ZnCl 2 A1C1 3 SnCl 4 

 NaBr ZnBr 2 AlBr 3 SnBr 4 

 Nal ZnI 2 A1I 3 



We observe that one atomic weight of sodium appears to unite 

 with only one unit of another element, one unit weight of zinc 

 with only two units of another element, a unit of aluminium with 

 not more than three, and a unit of tin with only four units. 



It seems that an atomic weight of each element has a fixed ca- 

 pacity for combining with not more than a certain number of atomic 

 weights of other elements. 



Other compounds of only two elements have the formulae: 



HC1 H 2 O NH 3 CH 4 Li 3 N CC1 4 . 



So far as we may judge from this limited list, Cl combines with 

 only one atom of another element, O with two atoms, N with 

 three atoms, C with four atoms. Also an atom of hydrogen 

 combines with not more than one atom of another element, 

 although it may take more than one atom of hydrogen to satisfy 

 the atom of that other element (H 2 O, CH 4 , etc.). 



This limited combining capacity of each kind of atomic weight 

 (or atom) is called its valence. 



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