VALENCE 217 



valences of some elements. Many of these elements, however, 

 possess other regular valences, in addition to tnose shown, so that 

 the list does not pretend to be complete. 



Where no charges are indicated, the element, by itself, does not 

 ordinarily form an ion. 



V Valence and Electrons.^- We stated in our discussion on 

 the source of the charges on ions (p. 195) that the outermost shell 

 of an atom of any element contained a definite small number of 

 negative electrical units (electrons), relatively loosely held. We 

 also assumed that, when atoms of unlike elements combined or 

 reacted, a transfer of electrons from one atom, or group of atoms, 

 to another might take place. 



On the basis of electrons, our idea of valence becomes some- 

 what more definite. The valence of an element is the (number 

 of^dectrgnsjihat an atom of that eUmentloses, or takes up, in entering 

 inio combination with atoms of other elements.) An atom of hydro- 

 gen has only one electron to lose, the hydrogen ion H + consists 

 of nothing but the residual proton. Consequently hydrogen is 

 univalent. Zn, however, can lose two electrons to form Zn++, 

 and Al can lose three to give A1+++. Cl can gain one electron to 

 form Cl~, S can gain two to give S = . 



The relation of valence to atomic structure in the case of these 

 and other elements will be taken up in detail in the final chapter. 



A Suggestion. Having just discussed the conception of 

 valence, we have now considered all the laws of chemical com- 

 position. At this point the reader should pause and review 

 thoroughly the subjects of the first eighteen chapters. The under- 

 standing of the fundamental principles which this retrospect will 

 give will greatly lighten the task of understanding the new and 

 more complex substances we shall have to consider, and the new 

 kinds of reactions and new conceptions we shall encounter, in the 

 chapters immediately following. 



