Selenium is a chemical element that has an atomic number (Z) 34. Its standard atomic mass is 78.971 and it is represented by the symbol Se. It is a nonmetal but considered a metalloid. In the periodic table, it is in group 16 (Oxygen family) and period 4.  It is a p-block element and its electronic configuration is [Ar] 3d10 4s2 4p4. At standard temperature and pressure, it is solid. Its melting point is 494 K (221 °C, 430 °F) and its boiling point 958 K ?(685 °C, ?1265 °F). It has a density of 4.81 g/cm3. Selenium forms several allotropes with change in temperature. It has grey-metallic beads like appearance. The electronegativity of selenium is 2.55 (Pauling scale) and has a 1st ionization energy of 941.0 kJ/mol. The oxidation stares of selenium is −2, −1, +1, +2, +3, +4, +5, +6 mostly forming strong acidic oxides. Selenium has five stable isotopes, Selenium-80 is found with 49.80% abundance. Selenium was isolated and discovered by Jöns Jakob Berzelius and Johann Gottlieb Gahn in 1817.

Selenium is a rare element and does not found free in nature. Isolation is complicated because of the presence of other compounds and elements. It is found in a number of minerals such as selenide, selenate, and selenite. But these minerals are very rare mostly found in sulfide ores. In living systems, it could be found attached with some amino acids like methyl selenocysteine,  selenomethionine and selenocysteine etc.

Selenium produced as by-products during refining copper or sulfuric acid production. The main producers of Selenium are Germany (650 t), Japan (630 t), Belgium (200 t), and Russia (140 t). The other two major producers are the United States and China.

Selenium has a wide range of applications.

The lithium–selenium (Li–Se) battery is used as an alternative for lithium-sulfur battery which has more electrical conductivity. Other uses are in chemicals and pigments,  electronics, metallurgy, glass manufacturing and agriculture.


How many valence electrons does selenium have?

Selenium has 6 valence electrons in its outermost shell. It is a p-block element and its electronic configuration is [Ar] 3d10 4s2 4p4. (1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p4). There are 34 electrons, 34 protons and 45 neutrons in an electrically neutral selenium atom.

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What is selenium used for?

Selenium has a wide range of uses. Some of the following are its uses.


Manganese electrolysis:

Selenium dioxide is used in electrowinning of manganese. It decreases the power needed for electrolysis cells.


Lithium–selenium batteries:

Lithium–selenium (Li–Se) is used in batteries. The Li–Se battery is used as an alternative for lithium–sulfur battery. Li-Se batteries has high electrical conductivity.


Solar cells:

Copper indium gallium selenide is used in solar cells which converts sunlight into electrical energy.


Used in Glass production:

Various selenite and selenate salts are added to glass for removing iron impurities which changes it colour to red and other desired colours.


Electronics industry;

The demand for selenium in electronics industry is decreasing but it is still used in photocopying, photocells, light meters and solar cells.