Sulfur is the tenth most abundant element in the known universe. There are three allotropic forms of sulfur. This last form gradually reverts to one of the more stable crystalline forms. Other than sulfuric acid, perhaps the most familiar compound of sulfur in the chemistry lab is the foul-smelling hydrogen sulfide gas, H 2 S, which smells like rotten eggs. More background information on S More data on S Se Selenium was discovered by Berzelius in 1818.
Most selenium is recovered from the electrolytic copper refining process.
This is usually in the form of the red allotrope. There are at least two other allotropes of the element, including a semi-metallic state. Selenium is an important semi-conductor which is particularly sensitive to light. Thus it finds applications that range from light sensors to xerography. In large amounts the element is toxic, but it is a trace element in living organisms.