WYCKOFF: FORCES BETWEEN ATOMS IN SOLIDS 587 



thoroughly saturated. With ammonia the tendency for nitrogen 

 to acquire electrons is so much greater than the ability of the 

 hydrogen to hold them that the electron of hydrogen is dis- 

 placed quite a distance towards the nitrogen atom. Doublets 

 of very considerable moment are thus set up within the ammonia 

 molecule and are of large effect upon other atoms or molecules 

 nearby. In water, doublets of still greater moment are to be 

 distinguished on account of the more pronounced tendency of 

 oxygen to acquire electrons. In hydrogen fluoride the single 

 doublet is of even greater moment. 



In the water molecule each of the two doublets is of shghtly 

 greater moment than each of the three in ammonia. However, 

 the fields about the molecule of water are much more important 

 than those about the ammonia molecule largely because of the 

 comparative simpUcity of the former. The hydrogen atoms 

 in ammonia must lie in three different directions; the water 

 molecule requires at most two directions, and probably both 

 hydrogens would be in a line (the symmetrical arrangement). 

 Consequently the turning of the water molecule in, order that 

 it may exert its maximum effect is much simpler a process than 

 the corresponding process for ammonia.^ Concerning the fields 

 about hydrogen fluoride there is relatively little information. 

 The molecule is not much simpler than the molecule of water; 

 the single doublet does not possess a very much greater moment 

 than either of the doublets of the water molecule. 



The fields about the water molecule seem to be the strongest 

 of those considered. In liquid water the combination between 

 two molecules due to the interlocking of these doublet fields 

 draws in the fields so that those about the "dihydrol" molecule 

 are much lessened. The dihydrol is a fairly stable compound. 

 Such a liquid, in which combination with itself is possible, is 

 said to be "associated." The fields are intense enough and 

 sufficiently localized to make possible combination between two, 

 occasionally more, molecules but not strong enough to cause 

 sohdification. Organic acids associate by reason of the intensity 

 of the combinations between the — COOH groups of different 

 molecules, and alcohols because of the association of — OH groups. 



