The clement selenium usually receives scant[3] attention in elementary textbooks, probably becausc it is of little importance commercially. Nevertheless, it is an interesting substancc and well worth studying.
Selenium was discovered by the Swedish giant among chemists, Bcrzclius.
The element is not abundant, but it is to be found in various ores.
Selenium is the sister element of sulphur, forming with tellurium the elements occurring in Group VI. It has an atomic weight of 78.96 as compared with 32 for sulphur. From the fact that the atomic weight is more, we may infer that selenium should be less active than sulphur. Its valences arc: +2, +4, and +6, the same as those of sulphur. It can be found in several allotropic forms, just as sulphur docs. It will be helpful to remember that the two elements are very much alike in their chemical properties and so the reactions of sulphur arc similar to those of selenium.
A piece of amorphous selenium is rather hard and quite brittle, just as sulphur is. The dark colour of the clement, the silver-grey coating on its surface arc characteristic. Another variety of the element is red.
The clement is both odourless and tasteless. It burns as readily as sulphur docs, with a reddish-blue flame and the peculiar odour. In working with selenium, beware[4] of the odour of its hydrogen compound; it is worse than that of hydrogen sulphide.
One curious property of selenium should be mentioned. The substance varies in its electrical conductivity according to the amount of light that falls upon it. We should remember that sulphur is a nonconductor. An experiment shows that selenium differs in this respect.
Under proper conditions selenium can form a colloid. One gram of selenium dioxide is dissolved in 500 ml of water. To 50 ml of this solution we add, after heating, 10 ml of a one-percent solution of gelatin, and then, drop by drop,* 60 ml of hydrazine hydrate (1:2,000 of water). We must remember to keep it just below the boiling point for 16 minutes. The beautiful pcach-pink" colour of the colloid is to be observed. The colloid can be made without gelatin, but the protective colloid serves to prolong the life of the colloidal suspension.
