Although all halogens are highly reactive and toxic, the magnitude of re activity and toxicity generally decreases from fluorine to iodine. Most of the halides can be classified into two categories. The fluorides and chlorides of many metallic elements, especially those belonging to the alkali metal and alkaline earth metal (except beryllium) families, are ionic compounds. Most of the halides of nonmetals such as sulfur and phosphorus are covalent compounds. As Figure 3.8 shows, the oxidation number of the halogens can vary from 1 to +7 . The only exception is fluorine. Because it is the most electronegative element, it can have only oxidation numbers, 0 (as in F2) and -1, in its compounds.
The chemistry of fluorine differs in some ways from that of the rest of the halogens. The following are some of the differences: