Selenium ($Se$)
The light-sensitive non-metal that captures the sun's energy, protects our cells from damage, and bridges the gap between Earth and Moon.
Selenium is an element that owes its name to the heavens. It was discovered in 1817 by the legendary Swedish chemist Jöns Jacob Berzelius while he was investigating a red residue left behind in a sulfuric acid plant. He initially thought it was tellurium (named after Earth), but soon realized it was a new substance. He named it Selenium after Selene, the Greek goddess of the Moon, to symbolize its relationship with tellurium.
Occupying Group 16 (the Chalcogens) in Period 4, selenium sits directly below sulfur. It is a non-metal that shares many chemical properties with its neighbor, but it possesses unique physical characteristics—most notably its sensitivity to light—that have made it a cornerstone of modern electronics and photography.
Atomic & Physical Properties
Selenium is a semiconductor that exists as several allotropes. Its physical state and electrical conductivity change dramatically based on its structural arrangement and its exposure to light.
| Property | Value |
|---|---|
| Atomic Number | 34 |
| Standard Atomic Weight | 78.971 |
| Electron Configuration | $[Ar] 3d^{10} 4s^2 4p^4$ |
| Phase at STP | Solid |
| Common Oxidation States | -2, +4, +6 |
| Melting Point (Grey) | 494 K (221 °C) |
| Boiling Point | 958 K (685 °C) |
The Colors of Selenium: Allotropy
Selenium is famous for its distinct allotropic forms, each with unique properties:
1. Grey Selenium
The most stable and metallic form. It consists of long helical chains and is a photoconductor—its electrical conductivity increases when exposed to light.
2. Red Selenium
An amorphous, glassy form produced by rapid cooling. It has a beautiful deep crimson color and consists of $Se_8$ rings, similar to sulfur.
3. Black Selenium
A brittle, lustrous solid that is the primary commercial form. It is often referred to as "vitreous" or glassy selenium.
Chemical Reactivity
Like sulfur, selenium reacts with oxygen, hydrogen, and metals. Its compounds often have a distinct, unpleasant odor similar to rotting radishes.
1. Formation of Selenium Dioxide
Selenium burns in air with a blue flame to produce selenium dioxide, a white crystalline solid that is a powerful oxidizing agent in organic chemistry.
2. Selenides
Selenium reacts with metals to form selenides, which are used as semiconductors and pigments.
3. Reaction with Acids
Selenium is not attacked by non-oxidizing acids but dissolves in concentrated nitric acid to form selenous acid ($H_2SeO_3$).
The "Electric Eye": Photoconductivity
In 1873, Willoughby Smith discovered that the electrical resistance of grey selenium decreased significantly when exposed to light. This photoconductive effect was revolutionary. It led to the development of the first photoelectric cells, light meters for cameras, and early television systems. Selenium was also the primary material used in xerography (the technology behind photocopiers) for decades before being replaced by organic photoreceptors.
Biology: The Antioxidant Defense
Selenium is a unique element in biology. It is the only element that is encoded into the genetic code in the form of a specific amino acid: Selenocysteine.
- Antioxidant Enzymes: It is a vital component of glutathione peroxidase, an enzyme that protects our cells from oxidative damage caused by free radicals.
- Thyroid Health: Selenium is required for the proper functioning of the thyroid gland.
- The Goldilocks Element: Selenium has a very narrow margin between being essential and being toxic. Too little causes Keshan disease, while too much (selenosis) can cause hair loss and nerve damage.
Industrial Applications
Beyond electronics and biology, selenium plays a major role in several industries:
- Glass Manufacturing: Selenium is used to decolorize glass (by neutralizing the green tint of iron impurities) or to create brilliant red "ruby" glass.
- Solar Energy: Copper Indium Gallium Selenide (CIGS) solar cells are a high-efficiency thin-film alternative to traditional silicon panels.
- Metallurgy: Added to steel and copper to improve their machinability.
This is the thirty-fourth part of our "Elements and Their Properties" series. From the moon's name to the cells of your body, selenium is everywhere! To master the trends of Group 16 and the chemistry of life, follow our Success Blueprint.
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