Selenium

Selenium (Se) is a chemical element with atomic number 34. It is located in group 16 of the periodic table, in the same group as sulfur and tellurium. Because of this, it has chemical properties similar to sulfur, particularly in the way it bonds with other elements.

Selenium has several different forms (allotropes). The most common form is black or dark red; the metallic form has a special property: its electrical conductivity increases when exposed to light. This property, known as photoconductivity, was the basis for its industrial use in the twentieth century.

History of Discovery

The element was identified by the Swedish chemist Jöns Jacob Berzelius in the early 1810s while working with slag from a sulfuric acid production facility. The name comes from the Greek word 'selene', meaning moon, by which it was distinguished from tellurium, whose name derives from the Earth.

Biological Role

Selenium is a trace element essential to humans and many animals. It is part of certain proteins called selenoproteins, which include enzymes that protect cells from oxidative damage. It also plays a role in the regulation of thyroid hormones. The amino acid 'selenocysteine' is conventionally called the 21st amino acid of proteins.

The margin between the amount needed and the amount that causes harm is narrow. Selenium deficiency is associated with diseases in regions where soil selenium is low; excessive intake causes a condition known as selenosis, whose symptoms include hair loss and brittle nails. Foods rich in it include fish, eggs, meat, and Brazil nuts.

Uses and Production

Most selenium is obtained as a byproduct in the electrolytic refining of copper, since it rarely occurs as a standalone ore. Its uses include glass manufacturing—where it is used to remove the green tint that copper impurities create or to produce red glass—treatment of dandruff and addition to animal feed as a supplement. Historically it was widely used in photocopiers and light-sensitive devices.