================================================================== | | | | | ATOMIC | MELTING | BOILING | COLOR AND STATE | WEIGHT | POINT | POINT | ______________|________|_________|_________|______________________ | | | | Fluorine (F) | 19.00 | -223 deg. | -187 deg. | Pale yellowish gas. Chlorine (Cl) | 35.45 | -102 deg. | -33.6 deg. | Greenish-yellow gas. Bromine (Br) | 79.96 | -7 deg. | 59 deg. | Red liquid. Iodine (I) | 126.97 | 107 deg. | 175 deg. | 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 (CaF_{2}), as cryolite (Na_{3}AlF_{6}), and in the complex mineral apatite (3 Ca_{3}(PO_{4})_{2}.CaF_{2}).