Polonium is a metal. It has a silvery metallic appearance, conducts electricity, and behaves chemically like a metal in most reactions. Yet it shows up on metalloid lists in some textbooks and periodic tables, and its position in Group 16 of the periodic table, sharing a column with oxygen and sulfur, makes the classification feel less obvious than it should be. The disagreement is real enough that a survey of nearly 200 published metalloid lists found polonium’s status worth singling out for discussion, and the confusion persists in classrooms and online reference material today.
Where Polonium Sits and Why That Causes Confusion
Polonium occupies position 84 in the periodic table. It is the heaviest member of the chalcogen group, which includes oxygen, sulfur, selenium, and tellurium, all of which are nonmetals or metalloids. There is a well-known trend in the periodic table where elements become more metallic as you move down a group. Oxygen is a gas. Sulfur is a brittle yellow solid. Selenium straddles the metal-nonmetal line. Tellurium is usually called a metalloid. Following that downward trend, polonium should be the most metallic of the bunch, and it is.
The confusion arises because people sometimes assume that all elements in a “nonmetal group” must themselves be nonmetals. That assumption breaks down for the heavier elements in several groups. Bismuth in Group 15 is unambiguously a metal even though nitrogen and phosphorus above it are nonmetals. Polonium is an analogous case in Group 16. Its metallic character is strong enough that a 2023 review article straightforwardly calls it “a silvery metal” and lists its melting point at 254 °C and boiling point at 962 °C, values typical of a low-melting metal rather than a nonmetal or metalloid.1Journal of Environmental Radioactivity. Polonium on the 125th anniversary of its discovery: its chemistry, radiotoxicity and application
The Physical Evidence for Metal
When researchers have managed to isolate visible quantities of polonium, the element looks and acts like a metal. It is silvery-gray with a metallic luster. It conducts electricity, though not as well as copper or silver. It is soft enough to be scratched and can be dissolved in acids, both of which are characteristic metal behaviors. Its density is roughly 9.3 grams per cubic centimeter, placing it in the same general ballpark as bismuth and lead, its neighbors on the periodic table.
Polonium also forms compounds that look metallic in character. It readily forms polonium dioxide and reacts with halogens. In solution, it can exist as Po²⁺ and Po⁴⁺ cations, which is a hallmark of metallic elements. Nonmetals typically form anions (gaining electrons) rather than cations (losing them). The fact that polonium prefers to lose electrons in chemical reactions puts it firmly on the metal side of the ledger.
Its melting point of 254 °C is low for a metal but falls well within the range where metals sit. Gallium melts at about 30 °C, and tin melts at 232 °C; nobody questions whether those are metals. A low melting point is not the same as being a nonmetal. Meanwhile, polonium’s boiling point of 962 °C is unremarkable for a metal and far higher than what you see in molecular nonmetals, which tend to boil at much lower temperatures.
Why Some Lists Call It a Metalloid
The metalloid label for polonium has a long shelf life in educational materials, and it is not entirely baseless. A survey of 194 published metalloid lists found that polonium’s classification was one of the contested cases worth reviewing, along with selenium and astatine.2Journal of Chemical Education. Which Elements Are Metalloids? The reason is partly historical and partly chemical.
Historically, polonium’s placement in Group 16 led early periodic-table designers to shade it as a metalloid, since its lighter neighbors selenium and tellurium are often classified that way. Once an element lands on a metalloid list in a widely used textbook, it tends to stay there for decades, even if the scientific consensus shifts. Textbook inertia is a real force in chemistry education.
Chemically, polonium does have a couple of properties that lean slightly toward metalloid territory. It can form a hydride (polonium hydride, H₂Po), echoing the behavior of sulfur and tellurium. And some of its oxide chemistry has amphoteric character, meaning its oxides can react with both acids and bases. Amphoteric behavior is a classic metalloid trait, though it also shows up in undeniable metals like aluminum and zinc. So while these properties keep polonium on the edge of some classification schemes, they are not strong enough to override the preponderance of metallic evidence.
The survey of metalloid lists found that the classifications most consistent with established criteria tend to restrict the metalloid label to about six or seven elements: boron, silicon, germanium, arsenic, antimony, and tellurium, with occasional inclusion of selenium or astatine. Polonium does not make the cut in the most carefully defined systems. It falls on the metal side of the dividing line.
An Unusual Crystal Structure
One of the most scientifically interesting things about polonium is its crystal structure. At room temperature, polonium crystallizes in a simple cubic lattice. This is extraordinary. Of all the elements, polonium is the only one that adopts a simple cubic structure under ordinary conditions. Most metals pack into more efficient arrangements like face-centered cubic or body-centered cubic structures. The simple cubic structure leaves a lot of empty space, and physicists have spent decades trying to explain why polonium prefers it.
The current understanding points to spin-orbit coupling, a quantum mechanical effect that becomes very strong in heavy atoms. Polonium has 84 protons, making it heavy enough for the interaction between an electron’s spin and its orbital motion to reshape the energy landscape of the crystal. Computational studies show that when spin-orbit effects are included in simulations of polonium’s electronic structure, the energy bands shift in ways that stabilize the simple cubic arrangement.3PubMed Central. Two-dimensional square lattice polonium stabilized by the spin–orbit coupling Without those relativistic effects, the calculations predict that polonium should adopt a different, more conventional crystal structure.
This detail matters for the metal-versus-metalloid question in an indirect way. The simple cubic structure gives polonium a lower packing efficiency and contributes to some of its unusual physical properties, like its relatively low density compared to what you might expect for such a heavy atom. These quirks can make polonium look less conventionally metallic to someone scanning a data table. But crystal structure alone does not determine whether something is a metal. Plenty of metals have unusual crystal structures, and polonium’s electrical conductivity and chemical behavior still place it squarely in metal territory.
Why Studying Polonium Is So Difficult
Part of the reason polonium’s classification has remained fuzzy for so long is that the element is ferociously radioactive and extremely rare. The most common isotope, polonium-210, has a half-life of only about 138 days. It decays by emitting alpha particles, and the radiation is intense enough that a milligram-sized sample glows blue from the excitation of surrounding air. The heat output is staggering: a single gram of polonium-210 generates about 140 watts of thermal energy, enough to noticeably warm any container holding it.
This makes hands-on chemistry with polonium a logistical nightmare. Samples self-destruct over months, contaminate equipment, and pose severe health hazards. Much of what we know about polonium’s physical and chemical properties comes from work done in the mid-twentieth century at government nuclear laboratories, often under wartime or early Cold War conditions. The measurements are not always as precise or reproducible as what we have for safer elements, and relatively few research groups have revisited them since.
The practical upshot is that some of the borderline properties people point to when arguing for metalloid status may simply reflect measurement uncertainty rather than genuine ambiguity in the element’s nature. When you can only work with micrograms of a material that is actively decaying and irradiating everything around it, your measurements of things like electrical resistivity and thermal conductivity carry wider error bars than usual. Modern computational chemistry has helped fill the gaps, and the computational results consistently describe polonium as metallic in its bonding and electronic behavior.
How Different Authorities Classify Polonium Today
There is no single governing body that hands down official element classifications. Different organizations and reference works make their own calls, which is why you can find conflicting answers depending on where you look.
- IUPAC: The International Union of Pure and Applied Chemistry, the closest thing chemistry has to an international standards body, does not maintain an official metal/nonmetal/metalloid list. It defines the terms but leaves the application to individual elements somewhat open.
- Most university-level textbooks: Increasingly classify polonium as a metal, though older editions and introductory texts sometimes still shade it as a metalloid on periodic table diagrams.
- The Royal Society of Chemistry: Lists polonium as a metal on its online periodic table.
- Los Alamos National Laboratory: Describes polonium as a metal in its element fact sheets, consistent with the nuclear-science community’s longstanding treatment.
The trend over the past few decades has been toward consensus that polonium is a metal. The metalloid classification appears most often in sources that are either older, aimed at younger students, or using a broad definition of metalloid that sweeps in borderline cases. If you are taking a chemistry exam and the question asks whether polonium is a metal, nonmetal, or metalloid, the safest modern answer is metal, but you should check which periodic table your instructor uses because some classroom charts still mark it differently.
Polonium Compared to Its Neighbors
Looking at the elements immediately surrounding polonium on the periodic table helps put its classification in context. Directly above it in Group 16 sits tellurium, which is genuinely a metalloid. Tellurium has some metallic luster and conducts electricity, but only poorly, and its chemical behavior is a true mix of metal and nonmetal traits. Polonium goes further: better conductivity, cation formation, dissolution in acids, and a higher density. The jump from tellurium to polonium parallels the jump from antimony to bismuth in Group 15, where a recognized metalloid gives way to a recognized metal.
To polonium’s left on the periodic table sits bismuth (element 83), a metal with no classification controversy. To its right sits astatine (element 85), another element with a contested classification, though astatine’s extreme rarity and even shorter half-life make it even harder to study than polonium. Below polonium in the periodic table sits livermorium (element 116), a synthetic superheavy element that has only been produced in atom-quantities. Theoretical predictions suggest livermorium should also be metallic, though its properties have never been measured directly.
The pattern across this region of the periodic table is clear: the heavier you go, the more metallic the elements become. Polonium fits that pattern without breaking it.
The Role of Relativistic Effects in Heavy-Element Chemistry
Polonium’s classification touches on a broader theme in chemistry that often gets overlooked in introductory courses. In heavy atoms, electrons near the nucleus move at speeds that are a meaningful fraction of the speed of light. At those speeds, relativistic effects change the energies and shapes of electron orbitals in ways that alter an element’s chemistry.
For polonium, the most important consequence is that its outermost electrons are arranged in a way that favors metallic bonding. The 6p electrons that determine most of polonium’s chemistry are split by spin-orbit coupling into two sub-groups with different energies.3PubMed Central. Two-dimensional square lattice polonium stabilized by the spin–orbit coupling This splitting affects how the electrons are shared in the solid, contributing to the simple cubic crystal structure mentioned earlier and to the element’s overall metallic character.
Gold’s yellow color, mercury’s liquid state at room temperature, and lead’s relative inertness compared to tin are all consequences of relativistic effects. Polonium’s metallic character, somewhat stronger than a naive extrapolation from lighter chalcogens might predict, is another example. Without relativity, the periodic table’s trends would be smoother and more predictable. With relativity, the heavy elements develop personalities that sometimes surprise even experienced chemists.
Polonium in Periodic Table Diagrams
If you have ever noticed that different periodic table posters color polonium differently, you are not imagining things. The color-coding of elements into metals, nonmetals, and metalloids is one of the most inconsistent features across published periodic tables. A study examining metalloid lists found wide variation in which elements get included, with polonium being one of the most inconsistently treated.2Journal of Chemical Education. Which Elements Are Metalloids?
Some periodic tables draw a bold staircase line separating metals from nonmetals, with metalloids sitting on the line. Where exactly that staircase falls in the lower rows varies from one publisher to another. Tables that draw the line to include tellurium as a metalloid sometimes automatically shade polonium the same way, treating the entire right side of the staircase as nonmetal or metalloid territory. Tables that draw the line more carefully, reflecting actual properties, place polonium on the metal side.
For students, the practical advice is straightforward: do not rely on color-coding alone. If a periodic table marks polonium as a metalloid, check whether the same table also marks aluminum or bismuth in ambiguous ways. Older or simplified tables tend to over-include elements in the metalloid category. The element’s actual measured and computed properties point clearly toward metal, and that is the classification gaining ground in modern references.
Polonium’s Radioactivity and Real-World Encounters
Most people first hear about polonium not through chemistry class but through its notoriety as a poison. Polonium-210 gained international attention in 2006 after it was used to fatally poison Alexander Litvinenko, a former Russian intelligence officer, in London. The case highlighted just how dangerous alpha-emitting isotopes are when ingested or inhaled. Alpha particles cannot penetrate skin, but inside the body they shred nearby cells with devastating efficiency.
Outside of espionage, polonium-210 exists in trace amounts in the environment. It is part of the uranium-238 decay chain and accumulates in tobacco leaves, soil, and certain seafoods. The quantities involved are minuscule, far below the levels needed to cause acute radiation sickness, but they contribute a small fraction of the background radiation dose that everyone receives.
Polonium-210’s intense heat output has also found a niche engineering use. Soviet lunar rovers and some space probes used polonium-based radioisotope heater units to keep instruments warm during the lunar night. The high wattage per gram makes it efficient for generating heat in a compact package, though the short half-life means the heat source runs down within a couple of years, limiting its usefulness for long-duration missions.
None of these applications depend on whether polonium is classified as a metal or metalloid. But they do underscore why the element is so hard to study in the laboratory and why its basic physical chemistry has remained less thoroughly characterized than that of its periodic table neighbors. The properties we can measure and calculate point to a metal. The practical difficulty of working with it is the main reason the question stayed open as long as it did.