Selenium

Selenium is a chemical element with the symbol Se and atomic number 34. It can appear in different forms, such as a brick-red powder, a dark glassy solid, or a grey metallic-looking material. Selenium was discovered in 1817 by Jöns Jacob Berzelius, who noticed it resembled another element called tellurium.

We often find selenium in metal sulfide ores, where it takes the place of sulfur. It is usually made as a byproduct when refining these ores, since pure selenium minerals are uncommon. Today, selenium is mainly used in glassmaking and to make colorful pigments. It also works as a semiconductor and is used in photocells. Though it had many uses in electronics in the past, most of those jobs are now done by silicon. Selenium is still found in some DC power surge protectors and a special kind of fluorescent quantum dot.

Small amounts of selenium are important for living things, including humans, but too much can be harmful. Too much selenium can cause problems like stomach issues, tiredness, hair loss, and more. Selenium is added to many multivitamins, infant formula, and helps in important processes inside our bodies. Plants need different amounts of selenium, with some needing a lot and others needing very little.

Characteristics

Selenium can look different depending on how it is made or changed. When made in a lab, it is usually a brick-red powder. If it is melted quickly, it becomes a black, shiny solid that is often sold as small beads. This black form is brittle and not very soluble in a special liquid called CS₂.

When heated, the black form becomes soft at about 50 °C and turns into a gray form at 180 °C. The gray form is the most stable and dense type of selenium. Unlike the other forms, the gray selenium can let electricity flow when light shines on it, making it useful in some technologies. It does not dissolve in CS₂, and it does not react with air or certain acids.

Selenium has several versions called isotopes. Most of these stay the same forever, but one changes very slowly over a very long time. There are also some isotopes made in labs that do not stay the same and change over time.

Chemical compounds

Selenium can be found in several different forms, called oxidation states, such as −2, +2, +4, and +6. It is a nonmetal, meaning it is not a metal, and it has properties similar to the elements above and below it in the periodic table, like sulfur and tellurium.

Selenium makes two kinds of oxides, or compounds with oxygen: selenium dioxide and selenium trioxide. Selenium dioxide is made when selenium burns in oxygen, and it can dissolve in water to make an acid called selenous acid. Selenium trioxide is less stable and breaks down easily into selenium dioxide.

Selenium also mixes with other elements to form different compounds. For example, it reacts with fluorine to make selenium hexafluoride, which is more reactive than a similar compound made from sulfur. Selenium also forms compounds with metals, like zinc selenide, which are used in electronics. Some of these compounds can be used in everyday products, like shampoo.

History

Selenium, named after the Moon, was discovered in 1817 by two chemists, Jöns Jacob Berzelius and Johan Gottlieb Gahn, in Sweden. They noticed a red substance forming in their sulfuric acid production process and later identified it as a new element.

In 1873, it was found that grey selenium's ability to conduct electricity changed with light, leading to its use in early light-sensing devices. Selenium was used in important inventions like the photophone and light meters. Though selenium was used in electronic parts for many years, newer materials eventually replaced it. Scientists later learned that selenium plays important roles in living things and is needed for health.

Occurrence

See also: Category:Selenide minerals

Native selenium is a rare mineral that does not usually form nice crystals, but when it does, they look like tiny steep shapes or very small hair-like crystals. It is hard to separate selenium because other materials are often mixed in.

Selenium is found in nature in a few inorganic forms, such as selenide, selenate, and selenite, but these minerals are uncommon. The mineral called selenite is not actually a selenium mineral; it is a type of gypsum (calcium sulfate hydrate) and was named long before selenium was discovered. Selenium is most often found as a tiny amount mixed in with sulfide ores of many metals.

In living things, selenium is part of certain amino acids like selenomethionine, selenocysteine, and methylselenocysteine. In these compounds, selenium works similarly to sulfur. Another natural organoselenium compound is dimethyl selenide.

Some soils have a lot of selenium, and certain plants can take in more selenium from these soils. In soils, selenium is often found in soluble forms such as selenate, which can easily wash into rivers. Ocean water also contains a good amount of selenium.

Typical natural amounts of selenium are very small: not more than 1 ng/m³ in the air, 1 mg/kg in soil and plants, and 0.5 μg/L in fresh and ocean water. The average amount in the Earth’s crust is between 0.05 and 0.09 mg/kg.

Human activities, like burning coal and mining and processing sulfide ores, can add more selenium to the environment.

Production

Selenium is mainly made from materials found in certain rocks, especially those containing metals like copper, nickel, or lead. When these metals are cleaned and made pure, selenium is often found as a leftover product. One common way to get selenium is from special leftover materials left after making copper very pure.

In factories, selenium is usually taken from leftovers left after cleaning copper. This process starts by turning these leftovers into a substance called selenium dioxide. This is then mixed with water and a special liquid to make another substance, which is later changed back into pure selenium.

In 2011, about 2,000 tonnes of selenium were made around the world, mostly in Germany, Japan, Belgium, and Russia. Selenium is used in many things, such as metal work, making glass, farming, chemicals, paints, and electronic devices. China uses the most selenium, about 1,500 to 2,000 tonnes each year.

Applications

Selenium has many useful purposes. One big way it is used is in making glass. Adding selenium to glass gives it a nice red color, which helps cancel out unwanted green or yellow colors that come from impurities in the glass.

Selenium is also used in making alloys, which are mixtures of metals. It can be mixed with metals like bismuth and brass to replace more harmful materials. In batteries, there was interest in using selenium for storing energy, and it was used in the very first solar cells made a long time ago. Today, scientists are still studying selenium for use in new types of solar cells that could work together with silicon cells.

Selenium is good at changing light into electricity, which makes it useful in special cameras that see X-rays. These cameras use thin layers of selenium to turn X-ray images into electric signals that can be seen on a screen. Selenium was also used in old electrical parts called rectifiers, but these are now mostly made from silicon. Even so, selenium is still used in some devices that protect electricity from big power surges.

Other uses for selenium include making copying machines, special lights called LEDs, tiny particles called quantum dots, and even helping to make photographic prints look better. It is also found in some shampoos that help with dandruff.

Pollution

Selenium pollution can harm some water animals and may come from farming and factories. Eating fish might help people stay healthy, but too much selenium in water can be bad for animals.

Selenium can build up in water animals, making them sick. Some tiny water animals can hold a lot of selenium, and bigger animals can get sick by eating them. Too much selenium in water can hurt fish and birds, making it hard for them to have healthy babies. Some fish can survive, but many get very sick and cannot have babies.

For example, in Belews Lake in North Carolina, fish were lost because of wastewater from a power plant. At the Kesterson National Wildlife Refuge in California, many fish and birds were harmed by selenium in farm water.

Biological role

Selenium is important for living things, but it can be harmful if there is too much of it. It is needed by animals and is found in small amounts in plants. In our bodies, selenium helps protect cells from damage and supports the thyroid gland, which controls important body processes.

We get selenium from foods like meat, nuts, cereals, and mushrooms. Brazil nuts are especially rich in selenium. The body needs a small amount of selenium each day to stay healthy. Too little selenium can affect our health, especially the thyroid gland.