THE HALOGEN FAMILY 209 



was first examined by Liebig, he thought it was an unstable com- 

 pound of chlorine and iodine, and so missed gaining the credit 

 of its discovery as an element. 



The activity of the halogens, as is evident from their action on 

 hydrogen, decreases in the order of increasing atomic weight. 

 This is seen also in their displacement reactions in solutions. 

 Thus we have found 'that chlorine displaces bromine from bromides 

 and that bromine displaces iodine from iodides. Fluorine is 

 able to displace even chlorine from chlorides : 2C1+ + F 2 2F~ + 

 Cl2. When we note also that the halogens displace sulphur from 

 sulphides (compare p. 205) and that oxygen displaces iodine from 

 hydriodic acid, we are able to draw up an order of activity for 

 the non-metals, similar to the activity order for the metals. This 

 order of activity expresses the order of preference of the non- 

 metals for assuming the ionic state. Starting from the top, it is 

 F, Cl, Br, O, I, S. 



A last point worthy of mention is that some of the properties 

 of fluorides are peculiar. Thus hydrogen fluoride at low tem- 

 peratures has the formula H 2 F 2 , and its solution in water is not 

 highly ionized. We shall see later that it is usual for the first 

 member of a family of elements or compounds to exhibit a few 

 peculiarities. In fact we have already noted, at the beginning of 

 this chapter, one peculiarity of the first of all the elements, hydro- 

 gen. Although a non-metal, it gives a positive ion H + . 



Exercises. 1. Tabulate the properties of fluorine, chlorine, 

 bromine and iodine, and of their compounds with hydrogen. 



2. How should you distinguish by chemical reactions the 

 chloride, bromide, iodide, and fluoride, (a) of hydrogen, (b) of 

 sodium from one another? 



3. Write equations for the action, (a) of chlorine upon a solu- 

 tion of hydrogen sulphide, (6) of bromine upon a solution of 

 hydrogen iodide, (c) of oxygen upon a solution of hydrogen iodide, 

 (d) of fluorine upon a solution of hydrogen iodide. 



