554 



SMITH S INTERMEDIATE CHEMISTRY 



with a compound, such as LiF (Fig. 124), in which the constitu- 

 ent atoms are apparently separate, being held together only by the 

 attraction of their opposite charges. An atom may also com- 

 plete its stable ring of eight, however, by sharing electrons with 

 another atom, as in the case of the fluorine molecule F 2 (Fig. 125). 



FIG. 125 



Similarly in carbon tetrachloride CC1 4 , we may assume that all 

 five atoms have completed their " octets," each chlorine atom 

 holding a pair of electrons in common with the central carbon 

 atom. 



The number of pairs of electrons which an atom of an element 

 can thus share with other atoms is called its co-valence. 



Polar and Non-Polar Compounds. Compounds like LiF 

 or NaCl, in which an electron has already passed from one atom 

 to another, are evidently potentially ionized, and if we can diminish 

 the attractive forces between the two charged atoms sufficiently 

 to enable them to break away from one another, we obtain im- 

 mediately the free ions (as in aqueous solution). Such com- 

 pounds are termed polar. In substances like CCU, on the other 

 hand, where electrons are held in common, the molecule will not 

 tend to break up in this way. Such substances do not, conse- 

 quently, ionize in solution, and are termed non-polar. 



Strictly speaking, however, the distinction between polar 

 and non-polar substances is not fundamental, but one of degree 

 only. In no non-polar compound, probably, are pairs of eleo- 



