Selenium is a chemical element with the symbol Se and atomic number 34, belonging to Group 16 of the periodic table, alongside oxygen and sulfur. It is classified as a metalloid, meaning it exhibits properties of both metals and non-metals. Despite its history of being considered toxic, Selenium is now recognized as an essential trace element required for human health, playing an important role in numerous biological systems even in small, microgram quantities.
Calculating Selenium’s Atomic Mass
The atomic mass of an element represents the mass of a single atom of that element. For Selenium, the standard atomic mass is 78.971 atomic mass units (u). This value is the weighted average of the masses of all naturally occurring isotopes, taking into account their relative abundance on Earth. This standard value allows researchers to calculate the mass of a mole of Selenium, which is 78.971 grams per mole (g/mol), providing a consistent reference point for laboratory work.
The Influence of Naturally Occurring Isotopes
Selenium’s atomic mass is approximately 79 because it exists as a mixture of six stable, naturally occurring isotopes. Isotopes are atoms of the same element that contain the identical number of protons but a varying number of neutrons. The most prevalent stable isotope is Selenium-80 (\(^{80}\text{Se}\)), which accounts for about 49.61% abundance. The other significant stable forms include Selenium-78 (\(^{78}\text{Se}\)) at 23.77%, and Selenium-76 (\(^{76}\text{Se}\)) at 9.37%. The remaining isotopes, Selenium-74 (\(^{74}\text{Se}\)), Selenium-77 (\(^{77}\text{Se}\)), and Selenium-82 (\(^{82}\text{Se}\)), occur in lower percentages.
Biological Function and Dietary Importance
The biological role of Selenium is linked to its incorporation into selenoproteins. There are 25 known selenoproteins in humans, which are fundamental to various physiological processes. Selenoproteins, such as glutathione peroxidases, function as antioxidants that help protect cells from damage caused by oxidative stress. Selenium also supports the thyroid gland, which holds a higher concentration of the element than any other organ. Selenoproteins are necessary for the conversion of inactive thyroid hormone (\(T_4\)) into its active form (\(T_3\)), regulating metabolism, and adequate levels also support immune function.
The recommended daily intake for most adults is 55 micrograms (\(\mu\text{g}\)) per day, obtainable through diet. Rich sources include Brazil nuts, seafood, and organ meats, though content in plant-based foods depends on soil concentration. While deficiency leads to health issues, excessive intake can result in selenosis, emphasizing the importance of maintaining balance.