Selenium

Selenium was discovered by Baron Jöns Jacob Berzelius, a Swedish chemist who is also credited with discovering cerium. Now considered one of the founders of modern chemistry, Berzelius stumbled upon the element quite by accident in 1817, when he noticed a reddish mud collecting in vats used to manufacture sulfuric acid.

He began experimenting on the substance and initially thought it was tellurium, in part because it apparently smelled like radishes when heated. Berzelius ultimately named this element selenium, after the Greek word selene (moon), as a complement to tellurium, which is named after the Latin word for the Earth.

In 1873, an English engineer named Willoughby Smith made a surprising discovery while testing materials for underwater telegraph cables. He was working with selenium, that still wasn’t well understood at the time. He was surprised when he noticed that selenium’s ability to conduct electricity increased when it was exposed to light. This was the first clear observation of photoconductivity, and it opened the door to a whole new way of thinking about light and electricity. Thanks to this simple observation, selenium became one of the earliest materials used in light sensors, solar cells, and even early photocopiers. It was a big moment for science, sparked by curiosity and a little bit of luck.

Selenium exists in several allotropic forms and can be prepared with either an amorphous or a crystalline structure. Amorphous selenium is either red (as a powder) or black (in vitreous form). Crystalline monoclinic selenium is a deep red; crystalline hexagonal selenium, the most stable variety, is a metallic gray.

Selenium exhibits both photovoltaic action (where light is converted directly into electricity) and photoconductive action (where the electrical resistance decreases with increased illumination). It can convert AC electricity to DC.

Selenium is a semiconductor, meaning it conducts some electricity (just not as well as conductors do). It becomes a significantly better semiconductor when light is shining on it; selenium is used in solar cells because it can produce electricity directly from sunlight.

As sodium selenate, the element can decolorize glass and make the red glass needed for traffic lights. It has many other uses, such as in photography, livestock feed, and drug production.

Humans need to consume trace amounts of selenium to stay healthy. It’s found in organ meats like kidneys and liver, but if liver pate doesn’t sound appealing, rest assured that it’s also in nuts, chicken, eggs, spinach, fish, and many other common foods.

Selenium is best enjoyed in moderation; consuming far too much can make your nails fall off. A toxic dose can also lead to brain damage, hair loss, and strong garlic breath.

Selenium forms few inorganic compounds, including selenious acid (H₂SeO₃), selenium dichloride (SeCl₂) and selenium oxychloride (SeOCl₂). Hydrogen selenide and other selenium compounds are extremely toxic and resemble arsenic in their physiological reactions.

Selenium is found in many foods and is a byproduct of refining metal sulfide ores. Today, most of the world's selenium comes from the muds produced during the electrolytic refining of copper.