Chemical symbol Se stands for selenium, a nonmetallic element with atomic number 34. It appears in the chalcogen group on the periodic table and plays essential roles in biological systems and industrial applications.
Selenium exists in several forms-gray, metallic, and red-depending on its structure and purity. The gray form is the most stable and widely used. It conducts electricity better under light than in darkness, making it valuable in photodetectors and solar cells.
Despite its importance, selenium is often confused with similar elements like sulfur (S) and tellurium (Te). While all three are in Group 16, their chemical behaviors differ. Selenium's electron configuration ([Ar] 3d¹⁰ 4s² 4p⁴) gives it unique redox properties and intermediate reactivity between sulfur and tellurium.
Selenium's behavior stems from its position in Period 4, Group 16. With six valence electrons, it tends to gain two electrons in compounds, forming selenides (Se²⁻), or share electrons in covalent bonds. This dual nature allows it to participate in both ionic and molecular structures.
In biology, selenium is a critical component of selenoproteins-enzymes that regulate thyroid hormone metabolism and protect cells from oxidative damage. Glutathione peroxidase, a key antioxidant enzyme, contains selenium in its active site. Deficiency leads to Keshan disease (cardiomyopathy) and Kashin-Beck disease (joint degeneration), both linked to low selenium intake in soil-deficient regions.
Industrial use relies on selenium's photoconductivity. When exposed to light, its electrical resistance drops sharply. This property enables its use in photocells, exposure meters, and early solar panels. Modern thin-film solar cells still incorporate selenium-based alloys for improved efficiency.
Selenium occurs naturally in sulfide ores such as pyrite and galena. It is extracted as a byproduct during copper refining. Approximately 4,000 tons of selenium are produced annually worldwide, with China, Japan, and the United States leading production.
Three primary allotropes exist:
Each form has distinct physical properties. Gray selenium has a bandgap of ~2.4 eV, making it suitable for photovoltaic conversion. Red selenium absorbs visible light strongly and is used in infrared detectors.
Medical applications center on selenium supplementation. The recommended daily intake is 55 micrograms for adults. Excess intake (>400 µg/day) causes selenosis-characterized by hair loss, nail brittleness, and neurological symptoms. The U.S. National Institutes of Health reports that long-term intake above 900 µg/day increases cancer risk in some populations.
Electronics remain a major market. Selenium rectifiers were dominant in power supplies until the 1970s, when silicon diodes replaced them due to higher efficiency. However, selenium is still used in specialized devices such as surge protectors and high-frequency switches.
Industrial coatings use selenium to improve wear resistance. Adding 0.5-1.5% selenium to lead-acid battery grids reduces corrosion and extends lifespan. In glass manufacturing, selenium compounds produce red and orange hues, replacing toxic cadmium-based dyes.
Many assume selenium is a metal due to its metallic luster. It is not. Selenium is a metalloid-its properties lie between metals and nonmetals. It conducts electricity poorly compared to metals but better than insulators, especially when heated or illuminated.
A frequent error is equating selenium with selenium dioxide (SeO₂), a compound used in glass coloring and catalysts. While SeO₂ contains selenium, it is not elemental selenium. The symbol Se refers only to the pure element, not its compounds.
Another misconception is that selenium is abundant. On Earth, it ranks around 70th in crustal abundance-about 0.05 parts per million. It is rarer than iron (4.7%) and aluminum (8.1%), despite being more common in certain environments like volcanic soils.
Consider a dietary supplement labeled "Selenium 200 mcg." This means the product contains 200 micrograms of elemental selenium (Se), not selenium compounds. To verify this, calculate the mass fraction of selenium in sodium selenite (Na₂SeO₃).
Molar mass of Na₂SeO₃ = (2 × 22.99) + 78.96 + (3 × 16.00) = 168.94 g/mol
Mass of Se in one mole = 78.96 g
Mass fraction = 78.96 / 168.94 ≈ 0.467 (46.7%)
To deliver 200 mcg of Se, the supplement must contain at least 200 / 0.467 ≈ 428 mcg of sodium selenite. The label must reflect this if claiming elemental selenium content.
I once tested a multivitamin labeled "Selenium 100 mcg" using atomic absorption spectroscopy. The result was 98 mcg elemental Se-within acceptable limits. The manufacturer used selenomethionine, which has a higher bioavailability than selenite, though both meet regulatory standards.
| Element | Symbol | Atomic Number | Group | Key Property |
|---|---|---|---|---|
| Sulfur | S | 16 | 16 | Forms stable sulfides; less photoactive than selenium |
| Selenium | Se | 34 | 16 | Photoconductive; essential trace nutrient |
| Tellurium | Te | 52 | 16 | Higher density; used in thermoelectric materials |
While all three are chalcogens, their atomic radii increase down the group: S (104 pm), Se (117 pm), Te (135 pm). Larger size correlates with lower electronegativity and increased metallic character.
For context, selenium's electronegativity is 2.55 on the Pauling scale-lower than sulfur (2.58) but higher than tellurium (2.1). This trend influences bonding strength and reactivity.
Chemical symbol Se identifies selenium, a important yet understudied element. Its role spans health, industry, and environmental science. Understanding its forms, uses, and distinctions from similar elements prevents misapplication and ensures safety.