Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with intermediate properties between the elements above and below in the periodic table, sulfur and tellurium, and has similarities to arsenic. It rarely occurs in its elemental state or as pure ore compounds in the Earth's crust. 

Pnictogens and Chalcogens

A pnictogen is one of the chemical elements in group 15 of the periodic table. This group is also known as the nitrogen family. The chalcogens are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family.

Pnictogens consist of the elements nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi),  and perhaps the chemically uncharacterized synthetic element moscovium (Mc).