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.
8. Gradation in state, corresponding to sp. gr.; F being a light
gas, Cl a heavy gas, Br a liquid, I a solid.
9. Corresponding gradation in their usual chemical activity; F
being most active, then Cl, Br, and I.
10. Corresponding gradation in the strength of the H acids; the
strongest being HF, the next, HCl, etc.
11. Corresponding gradation in the explosibility of their N
compounds; the strongest NCl3, the next, NBr3, etc.
12. Corresponding gradation in the number of H and O acids; Cl 4,
Br 3, I 2.
170. Compounds.--The following are some of the oxides, acids, and
salts of the halogens. Name them.
CI2O (+H2O=) 2 HClO. The salts are hypochlorites, as Ca(ClO)2.
Cl2O3 (+H20=) 2 HClO2. The salts are chlorites, as KClO2.
Cl2O4
-- HClO3 The salts are chlorates, as KClO3.
-- HClO4 The salts are perchlorates, as KClO4,
-- HBrO The salts are ? KBrO,
-- -- The salts are wanting.
-- HBrO3. The salts are ? KBrO3,
-- HBrO4. The salts are ? KBrO4,
-- -- The salts are wanting.
-- -- The salts are wanting.
I2O5 (+H2O=) 2 HIO3. The salts are ? KIO3.
-- HIO4. The salts are ? KIO4.
F forms no oxides, and no acids except HF. HF, HCl, HBr, HI, are
striking illustrations of acids with no O. HClO4 is a very strong
oxidizing agent. A drop of it will set paper on fire, or with
powdered charcoal explode violently. This is owing to the ease
with which it gives up 0. Notice why its molecule is broken up
more readily than HC103. The higher the molecular tower, or the
more atoms it contains, the greater its liability to fall. Some
organic compounds contain hundreds of atoms, and hence are easily
broken down, or, as we say, are unstable. Inorganic compounds
are, as a rule, much more stable than organic ones. It is not
always true, however, that the compound with the least number of
atoms is the most stable. SO2 is more stable than SO3, but H2SO3
is less so than H2SO4.