Selenium contains four unpaired electrons in its outermost orbital. These electrons can form bonds with other elements and are called valence electrons. On the periodic table, selenium is element number 34; it is located in group 16 below oxygen and sulfur. This group of elements is sometimes called the chalcogens.
The term chalcogen loosely translates to copper former or ore former. Given its position on the periodic table, selenium is usually classified as a non-metal or metalloid. As with oxygen or sulfur, selenium needs to gain only 2 electrons to fill its 4p orbital to capacity. Just as oxygen and sulfur anions (ions that have gained electrons) have a 2- charge, so too does selenium, which forms a selenide ion.
Nonetheless, selenium mimics a metal when it forms compounds with non-metals like fluorine, chlorine and other halogens (group 17 elements). Because of these versatile properties, selenium is often used as a semiconductor in microchips and solar energy panels.