CHAPTER XXXIII.
THE HALOGENS.
169. Halogens Compared.—The elements F, Cl, Br, I, form a natural group. Their properties, as well as those of their compounds, vary in a step-by-step way, as seen below. F is sometimes an exception. They are best remembered by comparing them with one another. Notice:
1. Similarity of name-ending. Each name ends in ine.
2. Similarity of origin. Salt water is the ultimate source of all, except F.
3. Similarity of valence. Each is usually a monad.
4. Similarity of preparation. Cl, Br, I, are obtained from their salts by means of MnO2 end H2SO4.
5. Variation in occurrence. Cl occurs in sea-salt, Br in sea- water, I in sea-weed.
6. Variation in color; F being colorless, Cl green, Br red, I violet.
7. Gradation in sp. gr.; F 19, Cl 35.5, Br 80, I 127.