Two allotropes of oxygen are known. Ozone occurs naturally in the upper atmosphere where it acts as a shield protecting the earth's biosphere from ultraviolet radiation. Closer to the surface ozone can be a nuisance as it is very reactive. Pure sulfur is tasteless and odorless with a light yellow color. Samples of sulfur often encountered in the lab have a noticeable odor.
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. Additional Information About Sulfur Selenium was discovered by Berzelius in 1818. Most selenium is recovered from the electrolytic copper refining process.