Selenium is a trace element and a chemical element with the atomic symbol Se and atomic number 34. It has an atomic mass of 78.971 unified atomic mass units. and it is found in the human body (which requires only small amounts) inside the proteins and it produces selenoproteins important antioxidant and anti-inflammatory functions.

The name "selenium" comes from the Greek word "σελήν", which means "moon". It was discovered in 1817 by Jöns Jacob Berzelius, a Swedish chemist, who found the element while studying sulfuric acid from a factory in Sweden.

It's a nonmetal with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium.

Selenium is not synthesized in a laboratory or factory setting because it's a basic chemical element. Instead, it's extracted from the minerals in which it occurs naturally. 

The extraction process usually involves roasting the minerals in the presence of oxygen or air to convert selenium to selenium dioxide, which is then dissolved in water to produce selenous acid. This can then be reduced to selenium using sulfur dioxide.

What it is for and where

Selenium plays an important role, together with iodine, in thyroid function, for production of thyroid hormones and in brain development.

• Electronics. Selenium is used in the electronics industry to fabricate rectifiers, solar cells, and photocopiers. It's also used in surge protectors and light meters for cameras.
• Glass Manufacturing. Selenium is used in the glass industry to decolorize glass and to make red and yellow pigments. It can also be used to reduce the transmission of sunlight in architectural glass, giving it a bronze tint.
• Health and Nutrition. Selenium is an essential trace element in the human diet and is a component of several major metabolic pathways, including thyroid hormone metabolism and immune function. It's often included in dietary supplements.
• Photography. Selenium toners are used to improve the appearance of black and white prints and delay the onset of fading.
• Agriculture. Selenium is sometimes used to boost the selenium content of agricultural soils in regions where dietary selenium is low.
• Metallurgy. Selenium is used in the production of steel and lead to improve their machinability.
• Rubber Industry. Selenium is used as a vulcanizing agent in the production of rubber.
• Solar Energy. Selenium is used in thin-film solar cells in combination with copper, indium, and gallium to form the semiconductor material.

Safety

Please note that while selenium has many beneficial uses, it is toxic in large amounts, and care must be taken in its use and handling.

Chemical safety

Selenium studies

• Molecular Formula: Se
• Molecular Weight: 78.971g/mol
• CAS: 7782-49-2
• EC Number: 231-957-4
• MDL number: MFCD00134090
• PubChem Substance ID: 24852583

 Here are some of its properties:

Thermodynamic Properties:

• Phase at STP: Solid
• Melting point (gray selenium): 217 °C
• Boiling point (gray selenium): 684.9 °C

Electromagnetic Properties:

• Magnetic type: Diamagnetic
• Color: Gray
• Refractive index (gray selenium): 1.000895
• Reactivity:
• Valence: 6
• Electronegativity: 2.55
• Electron affinity: 2.021 eV
• First ionization energy: 9.753 eV

Material Properties:

• Mass density: 4.28 g/cm³ (black selenium), 4.39 g/cm³ (red selenium), 4.81 g/cm³ (gray selenium)
• Mohs hardness: 2 (≈ gypsum)
• Young modulus: 10 GPa
• Sound speed: 3350 m/s
• Thermal conductivity: 0.52 W/(m K)

Atomic Properties:

• Term symbol: ³P_2
• Atomic radius: 115 pm

Abundances:

• Universe abundance: 3×10^-6 mass% (rank: 32nd)
• Crust abundance: 5×10^-6 mass% (rank: 69th)
• Human abundance: 5×10^-6 mass% (rank: 33rd)

Nuclear Properties:

• Half-life: Stable
• Stable isotopes: Selenium-80 (49.61%)
• Unstable isotopes: Selenium-79 (0.295 Myr)