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HALOGENS
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HALOGENS . The word halogen is derived from the See also:Greek aAs (See also:sea-See also:salt) and yevvav (to produce), and consequently means the sea-salt producer. The See also:term is applied to the four elements See also:fluorine, See also:chlorine, See also:bromine and See also:iodine, on See also:account of the See also:great similarity of their See also:sodium salts to See also:ordinary sea-salt. These four elements show a great resemblance to one another in their See also:general chemical behaviour, and in that of their compounds, whilst their See also:physical properties show a See also:gradual transition. Thus, as the atomic See also:weight increases, the See also:state of See also:aggregation changes from that of a See also:gas in the See also:case of fluorine and chlorine, to that of a liquid (bromine) and finally to that of the solid (iodine); at the same See also:time the melting and boiling points rise with increasing atomic weights. The halogen of See also:lower atomic weight can displace one of higher atomic weight from its See also:hydrogen See also:compound, or from the salt derived from such hydrogen compound, while, on the other See also: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 See also:potassium chlorate and also from perchloric See also:acid. All four of the halogens unite with hydrogen, but the See also:affinity for hydrogen decreases as the atomic weight increases, hydrogen and fluorine uniting explosively at very See also:low temperatures and in the dark, whilst hydrogen and iodine unite only at high temperatures, and even then the resulting compound is very readily decomposed by See also:heat. The hydrides of the halogens are all colourless, strongly fuming gases, readily soluble in See also:water and possessing a strong acid reaction; 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 See also:oxygen compounds increases with the atomic weight, thus iodine pentoxide is, at ordinary temperatures, a well-defined crystalline solid, which is only decomposed on See also:heating strongly, whilst chlorine monoxide, chlorine peroxide, and chlorine heptoxide are very unstable, even at ordinary temperatures, decomposing at the slightest See also:shock. Compounds of fluorine and oxygen, and 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 See also:group, for, whilst sodium chloride, bromide and iodide are readily soluble in water, sodium fluoride is much less soluble; again, See also:silver chloride, bromide and iodide are practically insoluble in water, whilst, on the other hand, silver fluoride is appreciably soluble in water. Again, fluorine shows a great tendency to See also:form See also:double salts, which have no counterpart among the compounds formed by the other members of the See also:family. Additional information and CommentsThere are no comments yet for this article.
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