Description
In the stirring times of rapid chemical development during the nineteenth century, the use of halogens in advancing chemical theory has been the fore- runner of the diversity in hydrocarbon uses and products which have distinguished the twentieth century for its multiplicity of industrial output. The presentations of both Laurent, in 1837, and Dumas, in 1839, of the new principle of substitution in a hydrocarbon by which the electropositive halogen replaces the electropositive hydrogen in the molecule, thereby producing modifications and additions in the properties of the derivative, have given stimulus to and influenced the direction of chemical research. The term “halogen” is applied to five elements- fluorine, chlorine, bromine, iodine, and the recently discovered astatine. Berzelius named them halogens from the Greek word meaning “salt-producer” because of the similarity of their sodium salts to the salt of the sea. These elements and their compounds resemble each other in their general chemical behavior while, in relation to their atomic weights, they show a gradual transition in their physical properties. Thus, as the atomic weight increases, fluorine and chlorine occur as gases, bromine as a liquid, and iodine as a solid, fluorine being the lightest and iodine the heaviest halogen, except for the rare astatine.
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