Selenium

Selenium is a chemical element with the symbol Se and atomic number 34. It can look like a brick-red powder, a dark solid, or a grey material. Selenium was found in 1817 by Jöns Jacob Berzelius, who saw that it looked like another element called tellurium.

We often find selenium in metal sulfide ores, where it replaces sulfur. It is usually made as a byproduct when refining these ores, because pure selenium minerals are rare. Today, selenium is mainly used in glassmaking and to make colorful pigments. It also works as a semiconductor and is used in photocells. In the past, selenium had many uses in electronics, but now silicon does most of those jobs. Selenium is still used 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. Selenium is added to many multivitamins and infant formula, and it helps in important processes inside our bodies. Plants need different amounts of selenium.

Characteristics

Selenium can look different depending on how it is made or changed. In a lab, it is usually a brick-red powder. If melted quickly, it becomes a black, shiny solid often sold as small beads. This black form is brittle and does not dissolve 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, which makes 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 different forms, called oxidation states, such as −2, +2, +4, and +6. It is a nonmetal, meaning it is not a metal. 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. 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. Selenium also forms compounds with metals, like zinc selenide. 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 found a red substance while making sulfuric acid and realized it was a new element.

In 1873, people learned that grey selenium could change how well it conducts electricity when light hit it. This helped make early light-sensing devices. Selenium was used in inventions like the photophone and light meters. Though it was used in electronic parts for many years, newer materials took its place. Later, scientists found that selenium is important for living things and helps keep us healthy.

Occurrence

See also: Category:Selenide minerals

Native selenium is a rare mineral. When it forms crystals, they look like tiny steep shapes or very small hair-like crystals. It is hard to separate selenium because it is often mixed with other materials.

Selenium is found in nature in a few inorganic forms, such as selenide, selenate, and selenite. These minerals are uncommon. The mineral called selenite is not a selenium mineral; it is a type of gypsum (calcium sulfate hydrate). Selenium is most often found in small amounts 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. Human activities, like burning coal and mining, can add more selenium to the environment.

Production

Selenium is made from materials found in rocks that contain metals like copper, nickel, or lead. When these metals are cleaned and made pure, selenium is often left over. One common way to get selenium is from materials left after making copper very pure.

In factories, selenium is taken from leftovers 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.

Selenium is used in many things, such as metal work, making glass, farming, chemicals, paints, and electronic devices.

Applications

Selenium has many useful purposes. One big way it is used is in making glass. Adding selenium to glass gives it a red color. This helps remove unwanted green or yellow colors 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 the past, selenium was used in the first solar cells. Today, scientists are still studying selenium for use in new types of solar cells that could work with silicon cells.

Selenium is good at changing light into electricity. This makes it useful in special cameras that see X-ray images. 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. 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 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. It 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.

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 too much can be harmful. Animals need it, and it is found in small amounts in plants. In our bodies, selenium helps protect cells and supports the thyroid gland, which controls important body processes.

We get selenium from foods like meat, nuts, cereals, and mushrooms. Brazil nuts have a lot of selenium. Our bodies need a small amount each day to stay healthy. Not enough selenium can affect our health, especially the thyroid gland.