Selenium (Se)


  • Symbol: Se
  • Atomic Number: 34
  • Atomic Weight: 78.971
  • Element Classification: Non-metal
  • Discovered By: Jöns Jacob Berzelius
  • Discovery Date: 1817
  • Name Origin: Greek: ‘selene’ (moon), due to its similarity to tellurium, whose name comes from ‘tellus’, meaning earth
  • Density(g/cc): 4.809 (gray form)
  • Melting Point: 221°C (gray form)
  • Boiling Point: 685°C (gray form)
  • Appearance: Red powder, metallic gray, or black vitreous solid (depending on the allotrope)
  • Atomic Radius(pm): 116


Selenium was discovered in 1817 by Swedish chemist Jöns Jacob Berzelius. The discovery occurred when Berzelius was working with lead chamber plates from a sulfuric acid plant and noticed a substance with properties that were similar to tellurium and sulfur. He initially thought it was tellurium due to its similarity in odor when heated, but further investigation revealed it to be a new element. Berzelius named it selenium after ‘selene’, the Greek goddess of the moon, to complement the name of tellurium, which comes from ‘tellus’, the Latin word for earth.

Relation to Other Elements

Selenium is a non-metal that resides in the oxygen family (group 16) of the periodic table, alongside oxygen, sulfur, tellurium, and polonium. This group is characterized by its members having six electrons in their outermost shell. Selenium can exist in several allotropic forms, including amorphous red powder, black vitreous form, and metallic gray form. It shares chemical properties with both sulfur and tellurium, including the ability to form compounds in the -2, +4, and +6 oxidation states. Selenium is less reactive than sulfur and is known for its photovoltaic (conversion of light into electricity) and photoconductive (change in electrical resistance when exposed to light) properties.

Natural Occurrence

Selenium is rarely found in its elemental form in nature. It mostly occurs in sulfide minerals where it partially replaces sulfur, in minerals such as pyrite, chalcopyrite, and other copper and nickel ores. Selenium is widely distributed in the Earth’s crust, but at very low concentrations. It is also found in the ash of coal and in the residues from the processing of sulfide ores.


Selenium has diverse applications, benefiting from its unique properties:

  • Glass Industry: Selenium is used to decolorize glass and to make red-colored glasses and enamels.
  • Electronics: Due to its photovoltaic and photoconductive properties, selenium is used in solar cells, photocopiers, and light meters.
  • Metallurgy: It is added to stainless steel and other alloys to improve machinability.
  • Agriculture: Selenium is an essential micronutrient for animals and is added to animal feeds to prevent and treat selenium deficiency.
  • Chemical Industry: Selenium compounds are used in the formulation of pigments, in anti-dandruff shampoos, and as a catalyst in chemical reactions.

Selenium’s discovery was significant for chemistry and technology, illustrating the complexity of elements and their allotropes. Despite its toxicity in high doses, selenium’s essential role in biology and its applications in industry highlight the dual nature of many elements as both beneficial and potentially harmful, depending on their use and exposure levels.


Arsenic (As)

Bromine (Br)