Halogens may have interested chcmists sincc early times, for they possess unique properties. They must have interested scientists as all of them, F2, Cl>, Br2 and I2, consist of diatomic molcculcs. We arc going to touch upon some of their properties in what follows.
a)
Chlorinc (from Greek "chloros", greenish-yellow), the most common of the halogens is a greenish-yellow gas with a sharp irritating odour. It
was first made by the Swedish chemist K. W. Scheele in 1774, by the action of manganese dioxide on hydrochloric acid. It is now manufactured on a large scalc by the clcctrolysis of a strong solution of sodium chloride.
The element bromine (from Greek "bromos", stench) occurs in the form of compounds in small quantities in scawatcr and in natural salt deposits. It is an easily volatile, dark rcddish-browrn liquid with a strong disagreeable odour and an irritating effect on the eyes and throat. It may producc painful sores when spilled on the skin. The free element can be made by treating a bromide with an oxidizing agent, such as chlorine.
The element iodine (from Greek "iodes", violet) occurs as iodide ion, 1% in very small quantities in scawatcr, and, as sodium iodatc, NalOj, in deposits of Chile saltpeter. It is made commercially from sodium iodatc obtained from saltpeter, from kelp, which concentrates it from scawatcr, and from oil-well brines.
The free element is an almost black crystalline solid with a slightly metallic lustre. On gentle wrarming it gives a beautiful blue-violet vapour. Its solutions in chloroform, carbon tetrachloride, and carbon disulphidc arc also blue-violet in colour, indicating that the molecules I2 in these solutions closely resemble the gas molecules. The solutions of iodine in water containing potassium iodide and in alcohol (tincture of iodine) arc brown; this change in colour suggests that the iodine molecules have undergone chemical reaction in these solutions. The brown compound KI3, potassium triiodide, is present in the first solution, and a compound with alcohol in the second.
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