Selenium

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).
Symbol: Se
Atomic number: 34
Electron configuration: [Ar] 3d10 4s2 4p4
Electrons per shell: 2,8,18,6
Atomic mass: 78.96 ± 0.03 u
Discoverers: Johan Gottlieb Gahn, Jöns Jacob Berzelius