![Selenium element card: symbol Se, atomic number 34, atomic mass 78.971, electron configuration [Ar] 3d¹⁰ 4s² 4p⁴, oxidation states +6 +4 +2 -2, Nonmetal, group 16, period 4, liquid at room temperature.](https://stage.chemistry-dictionary.com/wp-content/uploads/2026/09/selenium-chemical-element.webp)
Selenium sits in group 16 of the periodic table, right between sulfur and tellurium. It shares many important chemical characteristics with both of these neighboring elements. Selenium takes on several different physical forms, which chemists call allotropes.
The most stable allotrope acts as a solid semiconductor at room temperature. A semiconductor is a material that conducts electricity only under certain specific conditions. This common allotrope looks like a dense, purplish-gray network of atoms.
In nature, selenium almost never exists in a completely pure elemental state. Miners usually find it mixed deeply into sulfide ores like pyrite and chalcopyrite. Chemically, selenium reacts strongly with very active metals to form compounds called selenides.
It also reacts easily with oxygen gas to produce oxides like selenium dioxide. High amounts of concentrated selenium compounds are highly toxic to most life forms. A sudden large dose can quickly cause severe illness or even death.
However, living things still need very small amounts of selenium to survive. Humans and many other animals use these vital trace amounts every single day. It acts as an essential micronutrient to support healthy cellular function. The body uses it to build important antioxidant enzymes that protect delicate cells.