The Sublime Element Selenium

The Element Selenium

Selenium is an interesting element of the periodic table, with a wide variety of uses. For example, it is a vital micronutrient, excellent photoconductor, and great catalyst for chemical reactions. Learn more about this unique nonmetal down below!

Cool Facts About Selenium

  1. Selenium is named after the Greek goddess of the moon, Selene.
  2. Selenium salts are often used to combat mercury poisoning and dandruff.
  3. The element is often produced as a byproduct of copper refining.
  4. It has many different allotropes and they interconvert depending on temperature.
  5. The element is also an essential micronutrient for animals but is toxic in large doses.

Selenium on the Periodic Table

Selenium, symbol Se, can be found in group sixteen on the periodic table, to the right of arsenic, and to the left of bromine. As a chalcogen, it shares similar properties to tellurium below it, and sulfur above it. The selenium atom has an electron configuration of [Ar] 3d10 4s2 4p4

It has five allotropic forms which can be divided into three types: red Se, gray Se, and black Se. Traditionally, selenium is classified as a nonmetal, but some of its allotropes feature metalloid characteristics.

Unique Properties of Selenium 

Due to its semiconductor abilities, it has been used for almost a century to rectify circuits. Additionally, it is photoconductive and photovoltaic which means the presence of light increases the production/conduction of electricity. For this reason, it is often used in electronics such as photocells and solar cells.

Allotropes of Selenium

Selenium has three main allotropic forms: red Se, gray Se, and black Se. The normal state for the element is red Se and it appears as a red amorphous powdery substance. When heated rapidly, it becomes black Se. This vitreous allotrope is both physically brittle and lustrous. On the other hand, when red Se is heated slowly, it forms gray Se. Moreover, the red and black allotropic forms are great insulators, whereas gray Se is a great semiconductor. Gray Se is also the most stable form while red Se is the most reactive.

Selenium in Real Life

What is Selenium Used for?

Staining Glass with Selenium

Around 1910, the use of selenium in creating glassware began to become more common. As of now, it is the largest commercial use of the element and comprises about 50% of its consumption. In small concentrations, selenium produces a pink color which is used to decolorize glass by offsetting the green/yellow tint that is often created by iron impurities. Typically, selenite and selenate salts are used for this. On the other hand, large concentrations can create a nice red tint on the glass. The compound used for this is often cadmium selenide.

Cancer versus Selenium

Though the research is technically still inconclusive, there has been a long-held debate on whether selenium intake and cancer have any correlation. Selenium is an essential trace element and too much or too little exposure to this mineral can be detrimental to the wellbeing of an individual. In the 1960s, various observational studies were conducted and concluded an inverse relationship where increased consumption of selenium led to a reduced risk of blood cancer.

However, more recent studies have challenged this idea. Instead, they are now suggesting that high consumption has the potential to increase the risk of certain cancers such a colon cancer. In the end, research for the relationship between cancer and selenium is still inconclusive.

Selenium Mining – Where is Selenium Found?

Selenium ores do not occur, but minerals are found in trace amounts, usually where sulfides occur, or in copper deposits known as porphyry copper deposits. Most selenium is produced in Germany, Japan, the USA, and China.

Coal mining can lead to bioaccumulation of selenium which can become an environmental hazard. Applications exist that can detect selenium at levels as low as 2 ppt in solids and liquids.

History of Selenium

Selenium was discovered in 1817 by Jöns Jacob Berzelius. It all started when the sulfuric acid factory he partially owned has a sudden spur in illness among its workers. Ultimately, it was discovered that a particular substance had contaminated the chemical solution. Berzelius hypothesized the contamination had occurred due to impurities within the newly imported ores. The substance shared properties similar to both tellurium and sulfur but was ultimately determined to be a new element. Berzellious’s friend, Martin K had discovered the element tellurium and named it after the goddess of the earth. To match this, Berzelius named this new element selenium, after the Greek goddess of the moon, Selene.

Although selenium wasn’t officially discovered until the 19th century, it is believed it was known beforehand. For example, Arnold of Villanova, a medieval Spanish alchemist, once referenced a mystery substance in his writing and dubbed it “red sulfur”. This is believed to have been selenium because its most common allotrope is an amorphous red substance and it has properties similar to that of sulfur. Moreover, American biochemist Thressa Stadtman paved the path for selenium research by identifying the elements’ various biological significances.

Selenium – compounds, reactions, synthesis and oxidation states

Selenium reactions

Because it is a fairly reactivity element, selenium is often used as a catalyst in chemical reactions. It most easily combines hydrogen and halogens such as fluorine, chlorine, and bromine. Moreover, it reacts with nitric and sulphuric acids. It will also react with many different metals to form compounds called selenides.

Selenium compounds

When forming compounds, selenium likes to form acids in higher oxidation states. Common oxidation states of selenium compounds are -2, +4, and +6. These compounds are often toxic in large quantities. Additionally, selenium can react with most metals to form selenides. An example of this is aluminum selenide, which is a precurser to hydrogen selenide. It also likes to combine with oxygen to form selenium dioxide. When this compound reacts with water, it forms selenous acid, H2SeO3. Many selenium compounds feature oxygen and a halogen attached. Selenium also forms cations called the selenate ion, SeO4-2 and the selenite ion, SeO3-2, which are analogous to the sulfate and sulfite ion. Selenite compounds can be brightly colored, and both can be very toxic.

Isolation of Selenium

Although rare, selenium exists in nature as the minerals selenide, selenate, and selenite. The element can also be found in amino acids such as selenomethionine, selenocysteine, and methylselenocysteine. However, more often than not, selenium is produced from selenide in sulfide ores like copper and nickel. In fact, it is often produced as a byproduct of copper refining. Here, selenium dioxide is produced as sodium carbonate is oxidized. When this compound is combined with water, it is acidified into selenous acid. With the help of sulfur dioxide as a reducing agent, elemental selenium is formed.

Selenium oxidation states

The oxidation states for this element are Se-2, Se+6, Se+4.

Selenium Physical Properties

  • Atomic Symbol Melting point: 221 °C
  • Boiling point: 685 °C
  • Density: 4.79 g/mL
  • Atomic weight: 78.96 g/mol
  • Atomic number: 34
  • Electronegativity: 2.4
  • Classification: Metalloid, Group 16 metal, Chalcogen
  • Crustal Abundance: 0.05 ppm
  • Electron shell configuration: [ Ar ] 3d10 4s2 4p4
  • Isotopes : 9 total, 4 are radioactive
  • Found naturally in the minerals clausthalite, naumannite, and selenium.
  • Toxicity: Selenium toxicity occurs from excessive ingestion.

Where can I buy the Element Selenium?

You can purchase selenium nearly anywhere in the form of supplements. However, for laboratory purposes, selenium can be purchased from specialty shops for about 2.5 cents a gram.

Further Reading