Selenium
![Relationship between survival of juvenile salmon and concentration of selenium in their tissues after 90 days (Chinook salmon[106]) or 45 days (Atlantic salmon[107]) exposure to dietary selenium. The 10% lethality level (LC10=1.84 µg/g) was derived by applying the biphasic model of Brain and Cousens[108] to only the Chinook salmon data. The Chinook salmon data comprise two series of dietary treatments, combined here because the effects on survival are indistinguishable.](/uploads/202502/10/Se_dose-response_curve_for_juvenile_salmon_mortality_-_percent_scale4113.jpg)
Selenium is a chemical element with symbol Se and atomic number 34. It is a nonmetal with properties that are intermediate between those of its periodic table column-adjacent chalcogen elements sulfur and tellurium. It rarely occurs in its elemental state in nature, or as pure ore compounds. Selenium (Greek σελήνη selene meaning "Moon") was discovered in 1817 by Jöns Jacob Berzelius, who noted the similarity of the new element to the previously known tellurium (named for the Earth).