Tennessine sits in Group 17 of the periodic table, the column traditionally reserved for the halogens, yet no one has measured enough of its properties to say definitively whether it behaves as a nonmetal, a metalloid, or a metal. Only a handful of atoms have ever been produced, each decaying in tens of milliseconds, so the classification rests almost entirely on theoretical calculations. Those calculations suggest tennessine departs sharply from lighter halogens like chlorine and iodine, largely because of extreme relativistic effects on its electrons, and most current predictions place it somewhere in the metalloid-to-semimetal range.
Why Direct Measurement Is Not an Option
Tennessine, element 117, was first synthesized in 2010 by a joint Russian-American team at the Joint Institute for Nuclear Research in Dubna. The experiment smashed calcium-48 ions into a berkelium-249 target and detected a total of six atoms. Since then, the count of observed tennessine atoms has grown only modestly. Every atom created so far has decayed through alpha emission within roughly 50 to 100 milliseconds, which rules out nearly every conventional method for determining whether a substance is a metal, a metalloid, or a nonmetal.
Classifying an element normally depends on measuring bulk properties: electrical conductivity, luster, crystal structure, the way it bonds with other elements, and whether it tends to gain or lose electrons. You need at least enough material to form a visible sample, and ideally a stable one. Tennessine fails on both counts. Researchers cannot accumulate even a microgram, let alone test conductivity or observe a crystal. That leaves classification in the hands of computational chemists who model how tennessine’s electrons should arrange themselves and how the resulting atom should interact with neighbors.
The Halogen Trend and Why It Points Toward Metallic Character
Group 17 elements follow a clear trend: as you move down from fluorine to chlorine to bromine to iodine, the atoms get larger, hold onto their outermost electrons less tightly, and become progressively less “nonmetallic.” Fluorine is among the most reactive nonmetals in existence. Iodine, four rows down, already shows some metallic luster in solid form and conducts electricity slightly under pressure. Astatine, one row above tennessine, is so rare and short-lived that its classification has been debated for decades, but many chemists treat it as a metalloid because its chemistry seems to sit at the boundary between metallic and nonmetallic behavior.
Following this gradient, tennessine would be expected to lean even further toward metallic character than astatine. In a simplified picture, a heavier atom in Group 17 has more electron shells and a larger atomic radius, which weakens the nucleus’s grip on the outermost electrons. Weaker electron affinity and lower electronegativity both push an element away from classic nonmetal behavior. If the periodic table’s trends held perfectly, tennessine would look more like a metal than any halogen above it.
Relativistic Effects Complicate the Picture
The simple “heavier means more metallic” story breaks down for superheavy elements because their innermost electrons move at a significant fraction of the speed of light. At those speeds, relativistic effects kick in and reshape the entire electronic structure of the atom in ways that lighter elements never experience. For tennessine, these effects are not minor corrections; they dominate the atom’s chemistry.
One major consequence is the large spin-orbit splitting of tennessine’s outermost p-electrons. In lighter halogens, the difference in energy between the two spin states of p-electrons is small enough to mostly ignore. In tennessine, that splitting is enormous. The lower-energy p orbital (called p₁/₂) contracts sharply toward the nucleus, becoming more tightly bound, while the higher-energy p orbital (p₃/₂) expands outward and becomes less tightly bound. Computational studies on halogen-group elements confirm that relativistic effects lead to pronounced shrinkage of the p₁/₂ orbital and expansion of the p₃/₂ orbital, and that these effects are even more dramatic in tennessine than the already-strong relativistic effects seen in gold atoms.1Physica Scripta. Relativistic effects on properties of halogen group elements/ions
This reshuffling has a practical consequence: tennessine’s outermost electrons do not sit in the same kind of environment as those in chlorine or iodine. The p₁/₂ subshell effectively closes off, becoming almost inert, while the p₃/₂ electrons are the ones available for bonding. Researchers studying fluorination reactions of heavy p-block ions have noted that this kind of enhanced spin-orbit splitting can stabilize low-oxidation states, meaning the atom becomes less eager to gain electrons and form negative ions the way lighter halogens do.2Journal of Physics G: Nuclear and Particle Physics. Comparing the fluorination of Pb+ and Po+: insights into p-block relativistic effects for superheavy elements That reluctance to accept extra electrons is a distinctly un-halogen-like trait, and it nudges tennessine’s predicted behavior toward the metalloid or even metallic end of the spectrum.
Predicted Electron Affinity and Electronegativity
Two properties that matter most for the metal-versus-nonmetal question are electron affinity (how much energy an atom releases when it gains an electron) and electronegativity (how strongly it pulls electrons toward itself in a bond). Nonmetals typically have high values for both; metals have low values.
For tennessine, computational work predicts an electron affinity of about 2.4 electron volts.1Physica Scripta. Relativistic effects on properties of halogen group elements/ions That number is positive, meaning tennessine should still be willing to accept an extra electron, unlike a true metal. But it is substantially lower than the electron affinities of lighter halogens: chlorine sits near 3.6 eV, bromine at about 3.4 eV, and iodine around 3.1 eV. The steady decline continues through astatine, estimated at roughly 2.4 to 2.8 eV depending on the method, and reaches tennessine at the low end of that range. An electron affinity of 2.4 eV is not far from some metalloids and is well below what you would expect for a confident nonmetal.
Electronegativity follows a similar downward path. Tennessine is expected to be the least electronegative halogen by a wide margin. That does not necessarily make it a metal, since many metalloids also have moderate electronegativities, but it does mean tennessine should not form the strongly negative ions that define halogen chemistry. Fluorine rips electrons away from almost everything; tennessine, if it could be studied, would likely form bonds that are far more covalent and far less ionic.
What Physical Properties Have Been Estimated
Beyond electron affinity, researchers have tried to estimate a range of physical properties for tennessine by extrapolating from the known behavior of chlorine, bromine, iodine, and astatine, and then adjusting for relativistic effects. One study estimated the covalent radius of tennessine at about 160 pm, its polarizability at roughly 11.3 × 10⁻²⁴ cm³, and, for the hypothetical molecule tennessine hydride (TsH), a bond distance of about 197 pm with a bond energy of approximately 163 kJ/mol.3Chemical Physics Letters. Estimation of some physical properties for tennessine and tennessine hydride (TsH)
The bond energy of TsH, if accurate, would be notably weaker than that of hydrogen iodide (about 297 kJ/mol) and weaker still than hydrogen chloride (about 431 kJ/mol). This continues the halogen trend of progressively weaker bonds with hydrogen, and it suggests that tennessine’s grip on bonding partners is considerably looser than what you see in lighter halogens. The estimated dipole moment of TsH is also very small (about 0.24 debye), implying that the bond would be nearly nonpolar. A hydrogen halide that barely polarizes is, again, unusual behavior for a halogen.
Some models have also attempted to predict whether tennessine would be a solid, liquid, or gas at room temperature. Given the trend where fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids, tennessine is universally expected to be a solid. Its melting and boiling points are predicted to be higher than those of astatine, though the exact numbers vary substantially between models. The key uncertainty is whether solid tennessine would form a metallic lattice (like a metal), a covalent network (like a metalloid), or a molecular solid (like the diatomic crystals of iodine). Nobody knows, and given the difficulty of producing even a single atom, nobody is likely to find out anytime soon.
Where Periodic Tables Disagree
If you look at different periodic tables, you will find tennessine labeled differently depending on who made the chart. Some tables color it as a halogen and a nonmetal, simply extending Group 17 membership all the way down. Others shade it as a metalloid, reflecting the theoretical predictions that its properties sit at the metal-nonmetal boundary. A few classify it as a post-transition metal, leaning on the argument that relativistic effects have pushed it past the metalloid zone entirely.
IUPAC, the international body that governs chemical nomenclature and standards, does not currently assign tennessine a definitive metal/nonmetal/metalloid classification. The element is placed in Group 17 as a matter of electronic configuration (it has seven valence electrons, at least formally), but IUPAC has not taken a position on whether its bulk properties would be metallic. This ambiguity is not unusual for elements at the bottom of the periodic table; even astatine’s classification is listed inconsistently across reference sources.
The disagreement matters less than it might seem, because the question “metal or nonmetal” is really a question about bulk behavior, and tennessine cannot be produced in bulk. The labels are projections, not measurements. Different computational approaches, different treatments of relativistic corrections, and different assumptions about crystal structure lead to different conclusions. Until someone develops a way to produce tennessine in weighable quantities (which would require isotopes far more stable than any known), the classification will remain a best guess.
How Astatine Foreshadows the Difficulty
Tennessine’s classification problem is a more extreme version of the one that has dogged astatine for decades. Astatine, element 85, sits directly above tennessine in Group 17 and is the rarest naturally occurring element on Earth, with estimates suggesting only about 25 grams exist in the planet’s crust at any given moment. Its most stable isotope has a half-life of about 8.1 hours, which gives researchers considerably more time than tennessine’s milliseconds but still makes bulk experiments almost impossible.
Astatine’s chemistry has been studied primarily through tracer techniques, where tiny numbers of atoms are dissolved in solution and their behavior is inferred from what they stick to and how they migrate. These experiments suggest astatine can act as both a halide (forming At⁻ ions, like a halogen) and a cation (forming At⁺ ions, like a metal), depending on conditions. That dual personality is exactly what you would expect of a metalloid: an element at the boundary, capable of going either way depending on its chemical environment.
Gas chromatography studies have been used to examine how astatine species adsorb onto gold surfaces, and similar experiments have been proposed for tennessine once enough atoms can be produced at accelerator facilities.4Chemical Physics Letters. Adsorption of the astatine species on a gold surface: A relativistic density functional theory study The idea is that the volatility of an element and how readily it deposits on a metal surface reveal something about its bonding character: a volatile, weakly interacting species behaves more like a nonmetal, while one that bonds strongly to gold behaves more metallically. Researchers have already used this approach with flerovium (element 114) and are working toward applying it to tennessine, though the practical challenges are immense.
Classifying Superheavy Elements More Broadly
Tennessine is not the only superheavy element whose classification is uncertain. The entire seventh row of the periodic table, from francium (element 87) through oganesson (element 118), is subject to increasing relativistic distortions that blur traditional periodic table categories. Oganesson, for instance, sits in Group 18 with the noble gases but is predicted to be a solid rather than a gas, and its outer electron shell may be so smeared out by relativistic effects that it does not truly have a closed-shell noble gas configuration.
Attempts to classify these elements rigorously have led researchers to develop new approaches that go beyond simple group membership. One recent study used a combination of thermodynamic parameters, including formation enthalpies, cohesive enthalpies, and bond dissociation energies, to map heavy and superheavy elements (from atomic number 85 through 118) onto scatter plots and classify them based on where they cluster.5Computational and Theoretical Chemistry. On the classification of actinoid and superheavy elements These methods aim to replace the traditional “which column are you in” approach with something more grounded in actual (or predicted) chemistry, acknowledging that periodic table position becomes a less reliable guide as atomic numbers climb.
For elements in the seventh period, the relativistic contraction of s and p₁/₂ orbitals is so strong that the shell structure itself changes shape. Studies on superheavy elements have shown that the 7s shell contracts to a radius comparable to that of the 6d shell, and the 8s shell contracts to sit alongside the 7p₃/₂ shell, drastically altering the energetic landscape of the atom compared to what a simple extrapolation from lighter elements would predict.6Nuclear Physics A. Relativistic and quantum electrodynamic effects in superheavy elements This means the “personality” of each superheavy element is shaped less by its column in the periodic table and more by the specific way relativity reshapes its electrons.
Why the Label Matters Less Than You Might Think
For practical purposes, whether you call tennessine a metalloid, a metal, or a nonmetal changes almost nothing, because you will never encounter it outside of a nuclear physics facility. There is no tennessine wire, no tennessine compound in a battery, no industrial use waiting in the wings. The element exists solely as a subject of fundamental research, produced a few atoms at a time in particle accelerators and studied for what it reveals about the limits of nuclear stability and the behavior of matter under extreme conditions.
The classification question is genuinely interesting, though, because it tests how far the periodic table’s organizing principles stretch. The table was built on patterns observed in lighter elements: alkali metals on the left are soft and reactive, halogens on the right are aggressive nonmetals, noble gases at the far right barely react at all. Those patterns emerge from the way electrons fill orbitals, and they hold up remarkably well through the first six rows. The seventh row is where cracks appear. Relativistic effects break the neat periodicity, and elements start defying the expectations set by their lighter cousins. Tennessine’s ambiguous classification is one of the clearest examples of that breakdown: an element in the halogen column that might not actually behave like a halogen at all.
This is part of a broader open question in chemistry: does the periodic table need to be redesigned for superheavy elements, or does it remain useful as a rough guide even when the details go sideways? Some chemists argue for keeping the traditional layout and simply noting where predictions deviate, while others have proposed alternative arrangements that account for relativistic shell restructuring. The debate is unlikely to be resolved by experiment anytime soon, given that producing and studying superheavy elements remains one of the most resource-intensive undertakings in all of science. For tennessine specifically, the honest answer to “metal, metalloid, or nonmetal?” is that the best theoretical evidence points toward metalloid, but the question is still genuinely open, and it will stay open until someone figures out how to make enough of the stuff to actually test.