482 PBINCJPLKS OF CHEMISTRY 



or nitrogen in this respect differs widely from the halogens. Although 

 the form C1O 2 corresponds with KO._, and C0 2 , yet the latter is the 

 highest oxide of carbon, whilst that of nitrogen is N.,O-, and for chlorine, 

 if there was an anhydride of perchloric acid, its composition would be 

 C1. 2 O 7 , which is quite different from that of carbon. In respect to 

 the forms of their compounds the halogens, like all elements of out- 

 family or group, are perfectly analogous to each other, as is seen from 

 their hydrogen compounds : 



HF, HC1, HBr, HI. 



It is the same with their oxygen compounds. Only fluorine does 

 not give any oxygen compounds. The iodine and bromine compounds 

 corresponding with HC1O 3 and HC1O, are HBrO 3 and HBrO 4 , HIO ;j 

 and HIO 4 . On comparing the properties of these acids we can even 

 foresee t;he fact that fluorine will not form any oxygen compound. In 

 fact, iodine is easily oxidised for instance, by nitric acid whilst 

 chlorine is not directly oxidised. The oxygen acids of iodine are com- 

 paratively more stable than the acids of chlorine ; and, generally 

 speaking, the affinity of iodine for oxygen is much greater than that of 

 chlorine. Here also bromine occupies an intermediate position. In 

 fluorine we may expect a still smaller affinity for oxygen than in 

 chlorine and up to now it has not been combined with oxygen. If 

 any oxygen compound of fluorine be obtained, it will be exceedingly 

 unstable. The relation of these elements to hydrogen is the reverse 

 of the above. Fluorine has so great an affinity for hydrogen that it 

 decomposes water at the ordinary temperature ; whilst iodine has so 

 little affinity for hydrogen that hydriodic acid, HI, is formed with diffi- 

 culty, is easily decomposed, and acts as a reducing agent in a number 

 of cases. 



From the form of their compounds the halogens are nnivalent 

 elements with respect to hydrogen and septivalent with respect to 

 oxygen, if N be trivalent to hydrogen (it gives NH 3 ) and quinquivalent 

 to oxygen (it gives N. 2 O 5 ), and if C be quadrivalent to both H and O as 

 it forms CH 4 and CO.;. 



As not only their oxygen compounds, but also their hydrogen com- 

 pounds, have acid properties, the halogens are elements of an exclusively 

 acid character. Such metals as sodium, potassium, barium only give 

 basic oxides. In the case of nitrogen, although it forms acid oxides, 

 still in ammonia we find that capacity to give an alkali with hydrogen 

 which indicates a less distinctly acid character than in the halogens. 

 In no other elements are the acid characters so strongly developed as 

 in the halogens. 



