Selenium
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Selenium is a chemical element. It has the chemical symbol Se. It has the atomic number 34. Selenium is not found in the free (just as pure selenium) state in nature. It is a nonmetal. Its chemistry is in some ways like sulfur and tellurium. Selenium is found most in sulfide ores llke pyrite.
It is toxic in large amounts. Trace (very small) amounts of it are needed in all cells and probably in all organisms. In cells it is part of the active center of some enzymes.
Isolated selenium (selenium atoms only) has several different forms. The most stable of these is a dense gray semimetal (semiconductor). The way the atoms are put together is a named a trigonal polymer chain. It lets electricity pass through it better in the light than in the dark. This form is used in photocells. Selenium also has many nonconductive forms: a black glass-like substance, as well as several red crystalline forms. In the red form the eight atoms of selenium form ring to make a molecule. These rings then stack together to make the solid red selenium. The way that red seleniums atoms are put together is similar to sulfur.
