ELECTRICAL FORCES 



43 



TABLE 2-2. Dependence of Force and Energy of Attraction Upon Distance 

 Between Particles 



Name 



Interaction 



Force Energy 



Proportional to 



Coulombic 



London-van der Waals 



London-van der Waals in 

 long-chain molecular 

 associations 



The Hydrogen Bond 



In the covalent bond two atoms are said to be held together by "shared 

 pairs" of electrons, and the postulate that the electron of a pair can spend 

 part of its time around each atom is thought to confer extra stability on the 

 bond. This is the process known as "exchange." In a similar manner the 

 hydrogen ion of an — OH group, if it finds itself in the vicinity of a second, 

 somewhat negative oxygen, halogen, or nitrogen group may, by thermal agi- 

 tation jump the gap to this second group. Ideally it may continuously os- 

 cillate between the two, and on the average assume a position half-way be- 

 tween them. When this occurs, the strong positive charge is equidistant 

 from two negative charges, is attracted to them both, and so forms a bridge 

 — a weak bond. This is the currently fashionable "hydrogen bond" (Fig- 

 ure 2-6). It is very versatile in the sense that, in tissues especially, which are 

 80 per cent water, it can be credited with much of the secondary structure 



,a^ v 



■0^ 



About 5 kcol needed to break 

 I mole of hydrogen bonds I 



Figure 2-6. Hydrogen Bond — a 

 Shared Proton. 



