| ATOMIC WEIGHT | MELTING POINT | BOILING POINT | COLOR AND STATE | |
| Fluorine (F) | 19.00 | -223° | -187° | Pale yellowish gas. |
| Chlorine (Cl) | 35.45 | -102° | -33.6° | Greenish-yellow gas. |
| Bromine (Br) | 79.96 | -7° | 59° | Red liquid. |
| Iodine (I) | 126.97 | 107° | 175° | Purplish-black solid. |
The family. The four elements named in the above table form a strongly marked family of elements and illustrate very clearly the way in which the members of a family in a periodic group resemble each other, as well as the character of the differences which we may expect to find between the individual members.
1. Occurrence. These elements do not occur in nature in the free state. The compounds of the last three elements of the family are found extensively in sea water, and on this account the name halogens, signifying "producers of sea salt," is sometimes applied to the family.
2. Properties. As will be seen by reference to the table, the melting points and boiling points of the elements of the family increase with their atomic weights. A somewhat similar gradation is noted in their color and state. One atom of each of the elements combines with one atom of hydrogen to form acids, which are gases very soluble in water. The affinity of the elements for hydrogen is in the inverse order of their atomic weights, fluorine having the strongest affinity and iodine the weakest. Only chlorine and iodine form oxides, and those of the former element are very unstable. The elements of the group are univalent in their compounds with hydrogen and the metals.
FLUORINE
Occurrence. The element fluorine occurs in nature most abundantly as the mineral fluorspar (CaF2), as cryolite (Na3AlF6), and in the complex mineral apatite (3 Ca3(PO4)2·CaF2).
Preparation. All attempts to isolate the element resulted in failure until recent years. Methods similar to those which succeed in the preparation of the other elements of the family cannot be used; for as soon as the fluorine is liberated it combines with the materials of which the apparatus is made or with the hydrogen of the water which is always present. The preparation of fluorine was finally accomplished by the French chemist Moissan by the electrolysis of hydrofluoric acid. Perfectly dry hydrofluoric acid (HF) was condensed to a liquid and placed in a U-shaped tube made of platinum (or copper), which was furnished with electrodes and delivery tubes, as shown in Fig. 52. This liquid is not an electrolyte, but becomes such when potassium fluoride is dissolved in it. When this solution was electrolyzed hydrogen was set free at the cathode and fluorine at the anode.
Fig. 52