Selenium is a nonmetal with properties intermediate between the elements above and below in the periodic table.
Core Principles & Analysis
Joens Jacob Berzelius discovered selenium in 1817 in residues from a sulfuric acid plant and named it after Selene, the Moon, to pair with tellurium, named for the Earth.
Its electrical conductivity rises sharply in light, a photoconductivity that made early photocells and light meters possible, and it is an essential trace nutrient that turns toxic at modest doses.
Selenium-coated drums enabled xerographic photocopying, and the element still tints glass ruby red and treats dandruff as selenium sulfide shampoo.