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The Element Selenium

[Click for Isotope Data]





Atomic Number: 34

Atomic Weight: 78.971

Melting Point: 493.65 K (220.5°C or 428.9°F)

Boiling Point: 958 K (685°C or 1265°F)

Density: 4.809 grams per cubic centimeter

Phase at Room Temperature: Solid

Element Classification: Non-metal

Period Number: 4    Group Number: 16    Group Name: Chalcogen

What's in a name? From the Greek word for the moon, selene.

Say what? Selenium is pronounced as si-LEE-nee-em.

History and Uses:

Selenium was discovered by Jöns Jacob Berzelius, a Swedish chemist, in 1817 after analyzing an impurity that was contaminating the sulfuric acid (H2SO4) being produced at a particular factory in Sweden. Originally believing the material was tellurium, Berzelius eventually realized that it was actually a previously unknown element. Selenium occurs in minerals such as eucairite (CuAgSe), crooksite (CuThSe) and clausthalite (PbSe), but these minerals are too rare to use as a major source of selenium. Today, most selenium is obtained as a byproduct of refining copper.

Selenium's resistance to the flow of electricity is greatly affected by the amount of light shining on it. The brighter the light, the better selenium conducts electricity. This property has made selenium useful in devices that respond to the intensity of light, such as electric eyes, photo cells, light meters for cameras and copiers. Selenium can also produce electricity directly from sunlight and is used in solar cells. Selenium is also a semiconductor and is used in some types of solid-state electronics as well as in rectifiers, devices which convert alternating current electricity into direct current electricity. In addition to its use in electrical devices, selenium is also used to make a ruby-red color in glasses and enamels, as a photographic toner and as an additive to stainless steel.

Selenium forms few inorganic compounds, none of which are commercially important. They include selenious acid (H2SeO3), selenium dichloride (SeCl2) and selenium oxychloride (SeOCl2).

Estimated Crustal Abundance: 5×10-2 milligrams per kilogram

Estimated Oceanic Abundance: 2×10-4 milligrams per liter

Number of Stable Isotopes: 6   (View all isotope data)

Ionization Energy: 9.752 eV

Oxidation States: +6, +4, -2

Electron Shell Configuration:


2s2   2p6

3s2   3p6   3d10

4s2   4p4