Selenium

Selenium is one of those elements that sits right on the knife-edge between essential and toxic. Your body needs a tiny amount, approx. 55 micrograms per day, for critical enzymes that protect your cells. Yet just a little too much causes hair loss, nail damage and serious illness. It is also the element that makes photocopiers work and gives certain types of glass a vivid red colour.

  • Atomic Number3434 protons, 34 electrons
  • Atomic Mass78.97 uAbout 79× heavier than hydrogen
  • State at Room TempSolidgrey solid (stable form)
  • Density4.809 g/cm³About 4.8× denser than water
  • Melting / Boiling220.5°C / 684.9°CMelts at 221°C
  • Discovered1817Jöns Jacob Berzelius, 1817

Selenium sits in Group 16: the chalcogens, below sulfur.

It shares the group with oxygen, sulfur, tellurium and polonium, all with six outer electrons.

Atomic Mass Comparison
Sulfur32 u
Selenium79.0 u
Tellurium128 u
Iron55.8 u
Oxygen16 u

Selenium (79 u) is approx. 2.5 times heavier than sulfur (32 u) and half the mass of tellurium (128 u). All Group 16 elements have six outer electrons and tend to form two bonds, but their properties shift dramatically from gas (oxygen) through solids (sulfur, selenium) to metalloid (tellurium).

What is selenium?

Selenium is a non-metal in Group 16 of the periodic table, sitting directly below sulfur. It has 34 protons and exists in several allotropic forms: the most stable is the grey, metallic-looking crystalline form that conducts electricity but only when light shines on it. This property, photoconductivity, was crucial to early photocopier technology. Amorphous red selenium looks like a deep red powder and is the form commonly encountered in selenium chemistry.

Selenium gets its name from Selene, the ancient Greek goddess of the Moon, chosen by the Swedish chemist Jöns Jacob Berzelius who discovered it in 1817. Berzelius named it this because it was found alongside tellurium, already named after Tellus (Earth), and the moon seemed an appropriate companion to the earth. The symbol Se comes from the name.

Fact Selenium makes the beautiful deep red colour in some antique red glass and glazed ceramics. Cadmium selenide and selenium compounds were used as pigments in glass and ceramics for centuries, producing colours ranging from amber to vivid red. Some stained glass windows still contain selenium-coloured panes over a century old.

Where you find selenium

On Earth

Selenium is relatively rare in the Earth's crust, approx. 0.05 parts per million, and is distributed very unevenly, which is why some soils and regions are selenium-deficient while others have too much.

  • By-product of copper refining. Almost all commercial selenium comes from the anode slime left over when copper is refined electrolytically. The slime concentrates selenium, tellurium and other rare elements.
  • Selenium-rich soils. Parts of the Great Plains of the USA have selenium-rich soils derived from selenium-containing shale. Livestock in these areas can develop selenium toxicity (selenosis).
  • Seafood. Fish and shellfish are among the richest dietary sources of selenium, accumulating it from the food chain.

How we use selenium

  • Photocopiers (historically). Selenium drums were the basis of the Xerox photocopier: the most commercially important use of selenium for decades. When light hit the selenium drum, it became conductive in the illuminated areas, allowing a toner image to form. Organic photoconductors have largely replaced selenium today.
  • Glass and pigments. Selenium compounds decolourise iron-tinted glass and are used as red pigments in glass, ceramics and rubber.
  • Dietary supplements. Selenium supplements are widely used where soils are selenium-deficient, particularly in parts of Europe, China and New Zealand. Selenium is essential for the thyroid gland and immune system.
  • Thin-film solar cells. CIGS (copper indium gallium diselenide) solar cells are a promising thin-film technology with efficiency comparable to silicon cells.
Did you know? Brazil nuts contain extraordinarily high levels of selenium, sometimes 100 times more than other foods. A single Brazil nut can provide the entire recommended daily intake of selenium. The trees absorb selenium from selenium-rich soils of the Amazon basin. Eating too many Brazil nuts can actually cause selenium toxicity, so they should be enjoyed in moderation.

How it was discovered

Selenium was discovered in 1817 by Jöns Jacob Berzelius and Johan Gottlieb Gahn while investigating a red deposit forming in the lead chambers of a sulfuric acid factory in Sweden. The deposit had been thought to contain tellurium, but Berzelius showed it was a new element with properties similar to, but distinct from, tellurium and sulfur. Because it accompanied tellurium (named after Earth), he named the new element after the Moon.

Deeper dive: selenium in biology and the selenoproteins

Selenium is incorporated into proteins in a unique way: instead of being attached as a co-factor like most minerals, selenium is actually encoded genetically as a specific amino acid called selenocysteine, sometimes called the "21st amino acid". Selenocysteine replaces cysteine in a family of approx. 25 selenoproteins, proteins that need selenium at their active site to function.

The most important selenoproteins include glutathione peroxidases, which protect cells from oxidative damage by destroying hydrogen peroxide; thioredoxin reductases, involved in DNA synthesis and repair; and deiodinases, which convert inactive thyroid hormone (T4) to its active form (T3). Without selenium, thyroid function, immune responses and fertility are all impaired.

Selenium deficiency is surprisingly common worldwide. Keshan disease, a severe cardiomyopathy (heart muscle disease) affecting children in selenium-deficient regions of China, was shown in the 1970s to be caused by selenium deficiency and is now largely prevented by selenium supplementation. Parts of Europe, particularly Finland, have selenium-poor soils, and selenium is routinely added to fertilisers there to prevent deficiency in the food supply.

Selenium sits right on the boundary between essential nutrient and toxic element. Moving to 35 protons brings us to bromine, the only non-metal that is a liquid at room temperature.