The element Selenium (Se), atomic number 34, is classified on the periodic table into one of three categories: metals, nonmetals, or metalloids. While classification is usually straightforward, elements near the dividing line often exhibit ambiguous characteristics. Selenium is one such element, displaying a mix of traits that causes confusion about its true identity. Determining whether it is a metalloid or a nonmetal requires examining the defining properties of each group.
What Defines a Metalloid
Metalloids are elements that possess properties intermediate between those of typical metals and nonmetals. They are clustered along the “stair-step” line that separates metals on the left side of the periodic table from nonmetals on the right side. This intermediate positioning reflects their mixed behavior, leading them to sometimes be called semimetals.
Physically, metalloids often display a metallic luster and appear shiny, but they are usually brittle and cannot be easily hammered into sheets or drawn into wires. Their most distinctive physical trait is their electrical conductivity, which falls between the high conductivity of metals and the insulating properties of nonmetals. This characteristic allows them to function as semiconductors, making elements like silicon and germanium invaluable in electronics.
Chemically, metalloids tend to behave more like nonmetals, often forming covalent bonds and acidic oxides. Their electronegativity values are moderate, sitting between the low values of metals and the high values of nonmetals. Because the criteria for strict classification are not always absolute, the exact list of metalloids can vary slightly among chemists.
Selenium’s Physical and Chemical Behavior
Selenium’s complex behavior stems from its ability to exist in multiple structural forms, or allotropes, which possess widely different physical properties. The most stable allotrope is gray or trigonal selenium, which has a distinct, somewhat metallic luster. This specific form is the primary reason the element is often mistaken for a metalloid.
Gray selenium is a well-known semiconductor, conducting electricity better than a nonmetal but less efficiently than a metal. This allotrope also exhibits photoconductivity, meaning its electrical conductivity increases significantly when exposed to light. This light-sensitive property was historically important in early photocells and light meters.
Despite these metallic-like physical characteristics, Selenium’s chemical profile strongly aligns with nonmetals. Selenium is located in Group 16, the Chalcogens, directly below the nonmetal Sulfur. Like Sulfur, it readily forms compounds by gaining two electrons to achieve a stable octet, creating the selenide anion (Se²⁻). Its compounds often behave similarly to those of Sulfur, and its acidic oxide, selenium dioxide, is characteristic of a nonmetal.
The Official Classification of Selenium
Despite the semiconducting properties of its gray allotrope, Selenium (Se) is officially classified by most sources as a nonmetal. This determination is based on a holistic view of the element’s chemistry and its position within the periodic table. Its strong chemical resemblance to Sulfur, which sits directly above it, is a significant factor in this classification.
The tendency of Selenium to form acidic oxides and stable anions confirms its nonmetallic chemical nature. While the electrical conductivity of gray selenium suggests metalloid behavior, it is considered an exception within its overall nonmetal profile. The six elements most consistently recognized as metalloids—Boron, Silicon, Germanium, Arsenic, Antimony, and Tellurium—form a group that Selenium generally sits outside.
The consensus places Selenium firmly in the nonmetal category, despite debate over its unique electrical properties. Its overall chemical reactivity and tendency to form covalent bonds outweigh the single physical trait of semiconductivity found in only one allotropic form. The official nonmetal classification reflects its dominant chemical behavior.