ARTICLES 



57 



Returning to the consideration of the oxides of nitrogen, 

 we find that the octet theory accounts for N3O2 as O = N - N = 

 O, NaOa as 0=N-0-N=0 or 0=N-N=0, N^O^ as 



II 



O 

 = N-N = O, N^Og as O = N-O-N = O. For N^Oe and 

 II II II II 



00 



higher values of x, the number of electrons required becomes 

 greater than can be formed from the available electrons in the 

 atoms, except on the unlikely supposition of the formation of 

 rings or strings of oxygen atoms. The formation of NO and 

 NO2 is more difficult to account for. Let us first consider the 

 structure of the nitrogen molecule. The properties of ele- 

 mentary nitrogen are in many ways extraordinary, and the 

 contrast between its properties and those of carbon, which is 

 adjacent to it in the periodic table, could hardly be more 

 striking. Carbon does not melt at 3,700° C, and combines readily 

 with oxygen at moderate temperatures. On the other hand, 

 nitrogen has a lower boiling-point than any element except 

 hydrogen, helium, and neon, and combines with oxygen only 

 at exceedingly high temperatures. Moreover, the nitrogen 

 molecule is not appreciably dissociated even at 3,900° C. This 

 stability of the molecule indicates some unusual kind of struc- 

 ture. Supposing the nitrogen molecule contains two octets, 

 we have w = 2, e = 10, /> = 3, giving a structure N = N, 

 resembling that of acetylene, a substance which is endothermic, 

 relatively unstable, and easily forms addition products. Such 

 a structure could not account for the nitrogen molecule. 



Many considerations show that the absolute boiling-points 

 of substances give a measure of the external field of force of 

 their molecules, while the freezing-points appear to be largely 

 dependent on the symmetry of the molecules. In many of its 

 properties nitrogen resembles argon. Let us compare these 

 properties of O2, Ng, and Ar : 



Freezing-point .... 



Boiling-point ..... 

 Difference ...... 



Its boiling-point indicates that nitrogen has a weaker field 

 than either oxygen or argon, while the difference between the 

 freezing-point and boiling-point shows that nitrogen is much 

 more symmetrical than oxygen and more like argon. The 

 evidence suggests, therefore, that the nitrogen molecule is 

 surrounded by a single octet like that in the outer shell of 

 argon. Actually two nitrogen atoms have 14 electrons, of 



