On the periodic table, the element named after the Moon, from the Greek word selene, is selenium.
Jöns Jacob Berzelius and Johan Gottlieb Gahn discovered selenium in 1817 while investigating a reddish residue from a sulfuric-acid plant. Berzelius named it selenium after Selene, the Greek Moon goddess, because he saw a relationship with tellurium, whose name comes from Earth.
Selenium is a nonmetal in group 16 and has atomic number 34. It occurs in several allotropes, including a gray form that conducts electricity better when exposed to light. This photoconductivity helped make selenium useful in photocells and early copying technology.
Selenium is also an essential trace nutrient in very small amounts because it forms part of certain enzymes. Too little can be harmful, but excessive selenium is toxic. Its lunar name is sometimes confused with titanium, whose name refers to the Titans of Greek mythology.