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. More background information on Se More data on Se Te Discovered by von Reichenstein in 1782, tellurium is a brittle metalloid that is relatively rare. Tellurium can be alloyed with some metals to increase their machinability and is a basic ingredient in the manufacture of blasting caps. Elemental tellurium is occasionally found in nature but is more often recovered from various gold ores, all containing AuTe 2 .