Selenium is classified as a nonmetal on most modern periodic tables, but it sits so close to the metal-nonmetal dividing line that plenty of reputable sources still call it a metalloid. The confusion is not a mistake or a sign that someone has it wrong. Selenium genuinely straddles the boundary, and its physical behavior shifts depending on which of its several solid forms you are looking at. Where it lands on the label depends less on any intrinsic truth about the element and more on which set of properties the classifier decides to emphasize.
Why the Classification Is Genuinely Ambiguous
The periodic table does not come with a built-in rulebook that sorts every element neatly into “metal,” “nonmetal,” or “metalloid.” Those categories are human conventions based on clusters of properties: electrical conductivity, luster, crystal structure, chemical reactivity, and how an element behaves when forming compounds. Most elements land clearly on one side. Copper conducts electricity, is shiny, and forms positive ions easily, so it is a metal. Nitrogen is a gas at room temperature, does not conduct electricity, and forms negative ions or covalent bonds, so it is a nonmetal. Selenium, however, checks some boxes in each column, and that is where the argument starts.
In its most stable crystalline form, grey (trigonal) selenium has a metallic sheen and conducts electricity, though poorly. It is a semiconductor, meaning its conductivity sits between that of a true metal and a true insulator. It also responds dramatically to light: shine a beam on grey selenium and its conductivity jumps, a property called photoconductivity that was exploited in early photocopiers and light meters. These traits sound metalloid-like. But selenium also forms compounds that are overwhelmingly nonmetallic in character. It bonds covalently in most of its chemistry, forms acids rather than bases, and its electronegativity is high for an element sometimes called a metalloid. Its oxides are acidic, not amphoteric, which is one of the classic litmus tests that separates nonmetals from metalloids.
The Allotrope Problem
A big part of the confusion is that selenium does not come in just one form. Like carbon (which can be diamond, graphite, or fullerene), selenium exists in several allotropes, and these allotropes behave very differently from each other.
The two most commonly discussed forms are red selenium and grey selenium. Red selenium is amorphous, meaning its atoms are not arranged in any repeating crystalline pattern. It looks like a red powder, does not conduct electricity, and behaves in every observable way like a straightforward nonmetal. Grey selenium, by contrast, forms needle-like crystalline structures arranged in a trigonal lattice. Research on selenium nanoparticles has confirmed these structural differences at the nanoscale: red selenium nanoparticles are spherical and amorphous, while grey selenium nanoparticles form elongated, needle-like crystals with a trigonal structure.1PubMed Central. Allotrope-dependent physicochemical and optical properties of red and grey selenium nanoparticles This structural difference matters because the trigonal crystal arrangement is what gives grey selenium its semiconducting and photoconductive properties.
So when someone says “selenium is a metalloid,” they are usually thinking about grey selenium. When someone says “selenium is a nonmetal,” they are thinking about the element’s overall chemical behavior and its amorphous forms. Both descriptions capture something real. The disagreement is partly about which form of the element you treat as representative.
How Different Authorities Handle It
There is no single global authority that settles the metal-nonmetal-metalloid question once and for all. IUPAC, the closest thing chemistry has to a governing body, has never issued a formal, universally adopted definition of “metalloid.” That leaves room for different textbooks and periodic table publishers to draw the line in slightly different places.
Most widely used general chemistry textbooks published in the last two decades classify selenium as a nonmetal. The logic is straightforward: selenium’s chemical behavior, including its electronegativity, its tendency to form covalent bonds, its acidic oxides, and its role as an anion in ionic compounds, aligns it firmly with the nonmetals. The semiconductor behavior is noted as a physical curiosity rather than a classification-defining feature.
Other references, particularly those focused on materials science, toxicology, or nutrition, sometimes call selenium a metalloid. You will find published biomedical literature that casually refers to selenium as “the essential metalloid,” reflecting its borderline physical properties and its position on the periodic table near arsenic and tellurium, which are widely accepted metalloids. This usage is not wrong per se; it just prioritizes physical properties over chemical ones.
If you need a single answer for a chemistry exam or a standardized test, “nonmetal” is the safer choice and the one that aligns with the majority of current textbooks. If you are reading a materials science or biomedical paper that calls selenium a metalloid, the authors are not confused. They are just using a different but defensible classification framework.
Where Selenium Sits on the Periodic Table and Why That Matters
Selenium occupies Group 16 (the chalcogens) in Period 4, directly below sulfur and above tellurium. The chalcogen group shows a clean gradient from nonmetal to metal as you move down: oxygen and sulfur are clearly nonmetals, tellurium is usually called a metalloid, and polonium is a metal. Selenium sits right at the transition point in that gradient, which is another reason it gets pulled in both directions.
The diagonal band of metalloids on the periodic table, running roughly from boron through silicon, germanium, arsenic, antimony, and tellurium, skirts right past selenium. Some periodic tables include selenium in that band. Others draw the line just below it, placing selenium on the nonmetal side and tellurium on the metalloid side. The fact that selenium’s neighbor directly below is a commonly accepted metalloid and its neighbor directly above is an unambiguous nonmetal puts it in an inherently awkward spot.
The Semiconductor Angle
Grey selenium’s semiconducting behavior deserves more attention because it is the single property most responsible for the metalloid label. A semiconductor has electrical conductivity between that of a conductor and an insulator, and that conductivity can be tuned by temperature, light, or the addition of small amounts of other elements (doping). Silicon and germanium, the two most famous semiconductors, are both classified as metalloids, so it is natural to assume that any elemental semiconductor must be a metalloid too.
But that assumption does not hold up. The band gap, which is the energy barrier electrons need to overcome to conduct electricity, varies between semiconductor materials, and selenium’s electronic structure has specific features tied to how its helical chains of atoms interact in the trigonal crystal. Density functional theory studies have shown that the band gap of trigonal selenium is shaped by anisotropic interactions between its atomic chains, with charge transfer between different orbital bands playing a key role.2ACS Publications. Role of Interchain Interaction in Determining the Band Gap of Trigonal Selenium: A Density Functional Theory Study with a Linear Combination of Bloch Orbitals This means selenium’s semiconducting behavior is real and well-characterized, but it arises from a crystal structure that is unique to one allotrope, not from the kind of bonding that defines metalloids like silicon or germanium across all their common forms.
Being a semiconductor is a physical property, not a chemical classification. Plenty of compounds that no one would call metalloids are semiconductors (gallium arsenide, cadmium telluride). The confusion arises because, for elements specifically, the handful that are semiconductors in their standard states happen to cluster near the metal-nonmetal boundary. Selenium fits that pattern physically but not chemically.
What Happens Under Extreme Pressure
If you squeeze selenium hard enough, its character shifts even further toward metallic. At pressures above about 100 gigapascals (roughly a million times atmospheric pressure), selenium’s crystal structure transforms and it becomes a superconductor, carrying electrical current with zero resistance at temperatures below about 9.4 K.3Scientific Reports. Superconductivity in bcc-selenium under megabar pressure Superconductivity is an exclusively metallic property. No nonmetal in its normal state is a superconductor. So under extreme conditions, selenium does not just inch toward metalloid territory; it crosses all the way into metal behavior.
This is interesting for physics but does not change the practical classification. Plenty of elements behave differently under extreme pressures. Hydrogen, the quintessential nonmetal, is predicted to become a metallic superconductor at sufficiently high pressures. The classification on the periodic table refers to behavior under normal conditions, not what happens inside a diamond anvil cell. Still, the fact that selenium transitions to a superconductor at achievable laboratory pressures underscores that its electrons are closer to metallic behavior than those of a “true” nonmetal like nitrogen or chlorine.
Selenium’s Chemical Personality
If you set aside the physical properties and look purely at how selenium acts in chemical reactions, the case for nonmetal is strong. Selenium dioxide, its most common oxide, dissolves in water to produce selenious acid, an acidic solution. This is textbook nonmetal behavior. Metalloids typically form amphoteric oxides, meaning they can act as either an acid or a base depending on what they react with. Selenium’s oxides are not amphoteric; they are straightforwardly acidic.
Selenium also forms hydrogen selenide (H₂Se), which is a toxic, foul-smelling gas analogous to hydrogen sulfide. The pattern of forming volatile hydrides is characteristic of nonmetals. And in its most common oxidation states, selenium acts as an anion (selenide, Se²⁻) in ionic compounds, gaining electrons rather than losing them, another hallmark of nonmetals.
Where things get murkier is in selenium’s ability to act as both an oxidizing agent and a reducing agent depending on the context. This flexibility is more common among metalloids and transition metals than among deep nonmetals. Selenium dioxide, for instance, is a useful selective oxidizing agent in organic chemistry, capable of reactions that simpler nonmetal oxides cannot perform. But this versatility alone is not enough to push selenium out of the nonmetal category. Sulfur, which no one classifies as a metalloid, is similarly versatile in its oxidation chemistry.
Selenium in Biology
In living systems, selenium behaves entirely as a nonmetal. Your body uses trace amounts of selenium to build selenoproteins, a family of proteins that contain the amino acid selenocysteine.4PubMed Central. Selenium Metabolism and Biosynthesis of Selenoproteins in the Human Body Selenocysteine is structurally almost identical to the common amino acid cysteine, except that a selenium atom replaces the sulfur atom. This substitution works precisely because selenium and sulfur have similar chemical personalities. Both are nonmetals in Group 16, both form covalent bonds in biological molecules, and both can shuttle electrons in redox reactions.
The selenoproteins that depend on selenium include antioxidant enzymes like glutathione peroxidases and thioredoxin reductases, which protect cells from oxidative damage.5PubMed. Selenium homeostasis and antioxidant selenoproteins in brain: implications for disorders in the central nervous system In all of these biological roles, selenium is forming covalent bonds, participating in redox chemistry through electron sharing, and behaving exactly the way a nonmetal does. None of its metalloid-like physical traits, the semiconductivity, the metallic luster, come into play in biology. If you judged selenium solely by its biochemistry, you would never even consider calling it a metalloid.
Practical Consequences of the Label
For most people, the metalloid-versus-nonmetal debate about selenium has no practical consequences at all. Whether you are taking a selenium supplement, working with selenium compounds in a lab, or studying environmental selenium contamination, the label does not change how the element behaves. The chemistry is the chemistry regardless of which box on the periodic table someone places it in.
Where the label does matter is in materials science. If you are engineering a semiconductor device and someone tells you selenium is “just a nonmetal,” you might overlook its useful semiconducting and photoconductive properties. If you are told it is a metalloid, you might expect it to behave more like silicon or germanium in contexts where it actually behaves like sulfur. The most accurate mental model is to think of selenium as a nonmetal that happens to have one allotrope with semiconductor properties, rather than treating it as a member of the metalloid club alongside silicon, germanium, and arsenic.
How Tellurium Compares
It helps to look at selenium’s heavier neighbor, tellurium, which sits directly below it in the periodic table. Tellurium is almost universally classified as a metalloid, and the contrast with selenium is instructive. Tellurium’s most stable form is a silvery-white crystalline solid that conducts electricity better than selenium does (though still poorly by metal standards). Tellurium’s oxides are amphoteric, showing the mixed acid-base character that is a hallmark of metalloids. And tellurium’s electronegativity is lower than selenium’s, placing it closer to the metals on that scale.
The step from selenium to tellurium is the step across the nonmetal-metalloid boundary in Group 16. Selenium is on one side; tellurium is on the other. But the step is a small one, and selenium leans toward the edge. If you imagine a continuous spectrum from “pure nonmetal” to “pure metal,” sulfur is well into nonmetal territory, selenium is near the border on the nonmetal side, and tellurium is near the border on the metalloid side. The periodic table forces a hard line where nature draws a gradient.
Red Selenium in Nanotechnology
The allotrope distinction is not just academic. Red and grey selenium nanoparticles are being explored for different applications precisely because their physical properties diverge so sharply. Red selenium nanoparticles, which are spherical and amorphous, have optical properties that differ from the grey form’s needle-like trigonal crystals.1PubMed Central. Allotrope-dependent physicochemical and optical properties of red and grey selenium nanoparticles The amorphous red form absorbs light differently and has a distinct electronic structure, making it a candidate for biomedical imaging and antioxidant therapies. The grey crystalline form, with its semiconducting properties, is more relevant to electronics and sensor design.
This split personality at the nanoscale mirrors the larger classification debate. The same element, when arranged differently in space, produces materials that belong to different practical categories. Red selenium nanoparticles behave like a nonmetal product. Grey selenium nanoparticles behave like a semiconducting, borderline-metalloid material. Neither version is more “truly” selenium than the other; they are both just arrangements of the same atoms. The classification question, at its heart, is really a question about which arrangement you treat as defining the element.