Name: selenium
Symbol: Se
Atomic number: 34
Atomic weight: 78.96 (3)
Group in periodic table: 16
Group name: Chalcogen
Period in periodic table: 4
Block in periodic table: p-block
CAS registry ID: 7782-49-2
Selenium can be prepared with either an amorphous or crystalline structure. Crystalline monoclinic selenium is deep red; crystalline hexagonal selenium, the most stable variety, is a metallic grey. Elemental selenium is relatively nontoxic and is considered to be an essential trace element. However, hydrogen selenide (H2Se) and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions. Hydrogen selenide in a concentration of 1.5 ppm is intolerable to man. Selenium occurs in some soils in amounts sufficient to produce serious effects on animals feeding on plants such as locoweed (an American plant) grown in such soils.