Selenium Se

Discovery

In 1817 the Swedish chemist Jöns Jacob Berzelius, working with Johan Gottlieb Gahn, investigated a reddish deposit from the lead chambers of a sulfuric acid works. At first they took it for tellurium, but closer analysis showed it was a new element.

In 1873 the English telegraph engineer Willoughby Smith noticed that the electrical resistance of selenium bars fell dramatically under illumination. This photoconductivity was soon exploited in early light sensors, experimental image-transmission devices and photographic exposure meters.

By the mid-twentieth century, drums coated with amorphous selenium had become the heart of xerography, the dry copying process behind the office photocopier. Organic photoconductors later replaced it, but selenium still appears in digital X-ray detectors used in hospitals.

Origin of the name

Berzelius named the element after selene, Greek for the Moon. Its close chemical cousin tellurium had been named after tellus, Latin for Earth, so the new partner received a lunar name. The symbol is Se.

Main uses

• Glass: selenium decolourises glass by masking the green tint from iron impurities, and it is also used to make red and bronze-tinted glass.
• Metallurgy: it is added in the electrolytic production of manganese.
• Electronics and energy: copper indium gallium selenide (CIGS) thin-film solar cells and flat-panel X-ray detectors rely on selenium.
• Agriculture and health: it is added to fertilisers and animal feed in selenium-poor regions, and selenium sulfide is the active ingredient in some anti-dandruff shampoos.

Isotopes

Natural selenium has six isotopes. Selenium-74, -76, -77, -78 and -80 are stable, while selenium-82 undergoes double beta decay with a half-life of around 10²⁰ years. Selenium-80 is the most abundant, making up nearly half of the total. The artificial isotope selenium-75, with a half-life of about 120 days, is used for industrial gamma radiography and in a medical test of bile acid absorption.

In everyday life

The body uses selenium to build selenoproteins, including antioxidant enzymes such as glutathione peroxidase. Keshan disease, a heart muscle disorder described in a selenium-poor region of northeast China, is closely linked to dietary selenium deficiency.

Excess selenium causes selenosis, with hair and nail loss and a garlic odour on the breath. Brazil nuts are unusually rich in selenium, and eating many of them daily can exceed safe limits. Supplements should stay within recommended amounts. Seafood, eggs, meat and grains are the usual dietary sources.