HALOGENS. This term is applied to fluorine, chlorine, bromine and iodine, on account of the great similarity of their sodium salts to ordinary sea-salt (Gr. &Xs, salt, and yEvi' v, to produce). These four elements show a great resemblance to one another in their general chemical behaviour, and in that of their compounds, whilst their physical properties show a gradual transi tion. Thus, as the atomic weight increases, the state of aggrega tion under ordinary conditions changes from that of a gas in the case of fluorine and chlorine, to that of a liquid (bromine) and finally to that of the solid (iodine). The halogen of lower atomic weight can displace one of higher atomic weight from its hydrogen compound, or from the salt derived from such hydrogen com pound, while, on the other hand, the halogen of higher atomic weight can displace that of lower atomic weight, from the halogen oxy-acids and their salts; thus iodine will liberate chlorine from potassium chlorate, and also from perchloric acid. All four of the halogens unite with hydrogen, but the affinity for hydrogen de creases as the atomic weight increases, hydrogen and fluorine uniting explosively at very low temperatures (even at — 253°, where all other chemical action has apparently ceased), and in the dark, whilst hydrogen and iodine unite only with difficulty and even then the resulting compound is very readily decomposed by heat. The hydrides of the halogens are all colourless, strongly fuming gases, readily soluble in water to give a strongly acid solution; they react readily with basic oxides, forming in most cases well-defined crystalline salts which resemble one another very strongly. On the other hand the stability of the known oxygen compounds increases with the atomic weight ; thus iodine pentoxide is, at ordinary temperatures, a well-defined crystalline solid, which is decomposed only when heated strongly, whilst chlorine monoxide, chlorine peroxide, and chlorine heptoxide are unstable, even at ordinary temperatures, decomposing at the slightest shock. Binary compounds of bromine and oxygen have not yet been isolated. In some respects there is a very marked difference between fluorine and the other members of the group, for, whilst sodium chloride, bromide and iodide are readily soluble in water, sodium fluoride is much less soluble; again, silver chloride, bromide and iodide are practically insoluble in water, whilst, on the other hand, silver fluoride is appreciably soluble in water. Further, although calcium fluoride is practically insoluble in water, the other halides of calcium are very soluble. Again, fluorine shows a great tendency to form double salts which have no counterpart among the compounds formed by the other members of the family.
All four halogens apparently possess seven valency electrons, and by combination they strive to obtain the stable 8-electron grouping, thereby becoming negatively charged.