NITROGEN. 171 



water to give nitrous and nitric acid. Conversely, when the acids are 

 deprived of the elements of water, the respective oxides of nitrogen 

 are obtained. The monoxide corresponds to hyponitrous acid, 

 H 2 N 2 O 2 , but does not yield the acid with water. It is, hence, not an 

 anhydride. All of the oxides are obtained from nitric acid, directly 

 or indirectly. The last one is formed by abstraction of water from 

 nitric acid, the others involve reduction of nitric acid or, in reality, 

 of nitrogen pentoxide. 



While our knowledge of the structure of the oxides of nitrogen is unsatis- 

 factory, the following graphic formulas, in which the valence of nitrogen is 

 assumed to be either 1, 3, or 5, have been proposed to show the manner in which 

 the atoms may be linked together : 



Nitrogen monoxide, N O N or N N 



Nitric oxide, N = O 



Nitrogen trioxide, O = N O N=Oor 



Nitrogen tetroxide, 



Nitrogen pentoxide, 



Nitrogen tetroxide, at high temperature, has the composition N0 2 , and it is 

 possible that in NO 2 , and nitric oxide, NO, the valence of nitrogen is 4 and 2 

 respectively. The truth is that we have not sufficient knowledge of the struc- 

 ture of the oxides of nitrogen to make any positive statement as to the valence 

 of nitrogen in them. 



The structure of the nitrogen acids may be represented thus : 



N OH 

 Hyponitrous acid, N OH, or possibly II ^ 



Nitrous acid, O = N - OH, or N \OH 



//O 

 Nitric acid, ^N OH, or 



Nitrogen tetroxide, at low temperature, has the formula N 2 O 4 , but at 

 elevated temperatures this splits up into 2NO 2 , to form again N 2 O 4 , when 

 the temperature is decreased. There are many other cases like 

 chemistry. 



