230 NON-METALS AND THEIR COMBINATIONS. 



18. CHLOEINE. 



Cl' = 35 (35.18). 



Halogens. The four elements, fluorine, chlorine, bromine, and 

 iodine, which form a natural group of elements, are known as halogens, 

 the term meaning producers of salt. The relation shown by the atomic 

 weights of these four elements has been mentioned in connection with 

 the consideration of natural groups of elements generally (see page 

 125). In many other respects a resemblance or relation can be dis- 

 covered. For instance- : While the haloids as a general rule act as 

 univalent elements, they all form compounds into which they enter 

 with a valence of either 3, 5, or 7 ; they combine with hydrogen, 

 forming the acids HF, HC1, HBr, HI, all of which are colorless 

 gases, soluble in water ; they combine directly with most metals, 

 forming fluorides, chlorides, bromides, and iodides. The relative 

 combining energy lessens as the atomic weight increases ; fluorine 

 with the lowest atomic weight having the greatest, iodine with the 

 highest atomic weight the smallest, affinity for other elements. The 

 first two members of the group are gases, the third (bromine) is a 

 liquid, the last (iodine) a solid, at ordinary temperature. They all 

 show a distinct color in the gaseous state, have a disagreeable odor, 

 and possess disinfecting properties. 



Occurrence in nature. Chlorine is found chiefly as sodium 

 chloride or common salt, NaCI, either dissolved in water (small 

 quantities in almost every spring water, larger quantities in some 

 mineral waters, and the principal amount in sea-water), or as solid 

 deposits in the interior of the earth as rock salt. 



Other chlorides, such as those of potassium, magnesium, calcium, 

 also are found in nature. As common salt, chlorine enters the animal 

 system, taking there an active part in many of the physiological and 

 chemical changes. 



Preparation of chlorine. Most methods of liberating chlorine 

 depend on an oxidation of the hydrogen of hydrochloric acid by 

 suitable oxidizing agents, the hydrogen being converted into water, 

 while chlorine is set free. 



As oxidizing agents, may be used potassium chlorate, potassium 

 dichromate, potassium permanganate, chromic acid, nitric acid, and 

 many other substances. 



The most common and cheapest mode of obtaining chlorine is to 

 heat manganese dioxide, usually called black oxide of manganese, 



