Selenium (Se) is a naturally occurring element fundamental to both industrial technology and human health. As a trace mineral, it is essential for biological processes and is also utilized extensively in electronics. This dual nature highlights its unique position in both the physical and biological worlds.
Selenium’s Place on the Periodic Table
Selenium’s chemical identity is defined by its position on the periodic table, where it holds the atomic number 34. The element’s chemical symbol is Se, and it is located in Group 16, also known as the Chalcogens or the Oxygen Family. Selenium resides in Period 4, placing it between sulfur and tellurium, with which it shares several chemical properties. Its electron configuration includes six valence electrons in its outermost shell, explaining its tendency to form compounds with oxidation states ranging from \(-2\) to \(+6\). While classified as a nonmetal, selenium exhibits properties close to metalloids, particularly its ability to display semiconductor behavior.
Distinct Physical Properties and Industrial Applications
Selenium is notable for its ability to exist in multiple structural forms, known as allotropes, which include red, black, and gray varieties. The most stable form under ordinary conditions is the gray, crystalline allotrope, which displays a metallic sheen and acts as an effective semiconductor. The most distinguished physical property of gray selenium is its photoconductivity, where its electrical conductivity increases significantly when exposed to light. This light-sensitive characteristic made selenium indispensable in early electronic applications, such as photocells and light meters. Its use in xerography led to its extensive application in photocopiers and laser printers. Beyond electronics, selenium compounds are used in glass manufacturing to decolorize glass and to impart a deep ruby-red color to other glass products.
Essential Role in Human Biology
Selenium is an essential trace element required by the human body for proper function. Its biological role is primarily carried out after it is incorporated into a specialized group of proteins called selenoproteins. The thyroid gland, in particular, has a high concentration of selenium, utilizing it in several selenoproteins. These proteins act as powerful antioxidants, protecting cells from damage caused by free radicals and oxidative stress.
Enzymes like glutathione peroxidase and thioredoxin reductase rely on selenium to neutralize harmful reactive oxygen species within the body. Selenoproteins are also necessary for regulating thyroid hormone metabolism. Specifically, enzymes called iodothyronine deiodinases, which contain selenium, are involved in converting the inactive thyroxine (T4) into the biologically active triiodothyronine (T3).
Understanding Deficiency and Toxicity
The amount of selenium in food sources can vary widely depending on the concentration of the element in the soil where the food was grown or raised. Common food sources that can help maintain balanced intake include:
- Brazil nuts
- Seafood
- Organ meats
- Whole grains
The therapeutic window for selenium is quite narrow, meaning a small margin exists between the amount required for health and the amount that becomes harmful. A severe lack of selenium can lead to conditions such as Keshan disease, a form of heart muscle disease. Signs of insufficient intake can also include a weakened immune response, muscle weakness, and hair loss. Conversely, chronic excessive intake leads to a toxic state called selenosis, which is most often caused by over-supplementation. Symptoms of selenosis typically involve:
- Hair loss
- Brittle nails
- A metallic taste in the mouth
- A distinct garlic odor on the breath