Selenium is a non-metal element required by humans and other organisms in trace amounts. Valence describes an element’s combining power with other atoms and is often represented by the oxidation state. This state shows the number of electrons an atom gains, loses, or shares when forming a chemical bond. Selenium’s combining power determines the specific compounds it forms, which dictates its behavior in the environment and biological systems, influencing factors like solubility and potential for toxicity.
Understanding Selenium’s Chemical Nature
Selenium (Se) sits in Group 16 of the periodic table, placing it among the elements known as chalcogens, directly below oxygen and sulfur. This position means a selenium atom has six electrons in its outermost valence shell. The presence of these six valence electrons determines the element’s chemical reactivity and its range of possible valence states.
Like other chalcogens, selenium can achieve a stable electron configuration by either gaining two electrons (its most reduced state) or by sharing or losing four or all six outer electrons (its oxidized states). This flexibility in electron sharing allows selenium to exhibit multiple common valence states, enabling it to participate in a wide variety of chemical reactions.
The Three Primary Valence States
The most significant and common valence states for selenium are -2, +4, and +6, each forming distinct chemical species.
The most reduced state is a valence of -2, known as selenide (\(Se^{2-}\)), which occurs when the selenium atom gains two electrons. Selenide is found in compounds like hydrogen selenide (\(H_2Se\)) and, importantly, in the biologically active amino acid selenocysteine.
A valence of +4 is found in selenite, such as the selenite ion (\(SeO_3^{2-}\)), which is a common inorganic form. This state occurs when selenium shares or formally loses four of its valence electrons, as seen in selenium dioxide (\(SeO_2\)). Selenite is one of the primary forms of selenium absorbed through the diet.
The most oxidized state is a valence of +6, known as selenate. This occurs when the selenium atom shares all six of its outer electrons, forming the selenate ion (\(SeO_4^{2-}\)), which is chemically analogous to the sulfate ion. Selenate is highly soluble and is often the predominant form of selenium in alkaline soils and water, making it another major dietary source.
How Valence Dictates Biological Function
The varying valence states are fundamental to selenium’s function and metabolism within the body. The most biologically active form is the -2 valence state, selenide, which serves as the direct precursor for the amino acid selenocysteine. Selenocysteine is incorporated into the active sites of enzymes known as selenoproteins, such as glutathione peroxidase.
The incorporation of selenide into selenoproteins gives them antioxidant properties that protect cells from damage. In contrast, the +4 (selenite) and +6 (selenate) forms are the primary inorganic species absorbed from food and water. The body must chemically reduce these higher-valence states down to the -2 selenide form before they can be used for selenoprotein synthesis.
The difference in valence also determines the element’s chemical reactivity and safety profile. The highly oxidized selenite and selenate are more reactive, and consuming them in excess can lead to toxicity. The body maintains a narrow concentration range between the required amount and the harmful level, which is linked to the interconversion of these different valence species.