A gray metalloid element
found in group IVA of the periodic table;
it occurs as rare selenides with heavy-metal sulfides,
and is obtained as a by-product of copper
refining or the lead-chamber process. It was discovered (1817) by Jöns
Selenium has three allotropes (see allotropy),
the most stable being the gray, metallic form. Its chemistry is analogous
to that of sulfur. It is used to make photoelectric
cells, solar cells, and rectifier,
in xerography, and as a semiconductor;
also to make ruby glass and to vulcanize rubber.
Its most common isotope is 80Se (49.82%).
|relative atomic mass
||217°C (423°F) (gray)
||685°C (1,265°F) (gray)
Selenium and life
Selenium plays an important role in the metabolic functions of the body
and is essential for maintaining optimum health in humans and other animals.
It is an important antioxidant and is
significant in cell formation, healthy reproductive processes, and adequate
immune response. However, even small amounts of selenium above normal levels
can lead to chronic or acute toxicity in humans and other animals and can
have a destructive effect on the environment. The riches dietary sources
of selenium are meat, fish, whole grains, and dairy products.