Is Astatine a Metalloid or a Nonmetal?

Astatine does not fit neatly into either category, and the honest answer is that scientists still argue about it. Most modern periodic tables label element 85 a metalloid, but a growing body of computational and experimental work suggests it behaves more like a metal in some contexts and more like a nonmetal (specifically, a halogen) in others. The confusion is not a failure of chemistry; it stems from astatine being the rarest naturally occurring element on Earth and so intensely radioactive that nobody has ever assembled enough of it to observe basic bulk properties like color, luster, or electrical conductivity.

Why Astatine Is So Hard to Classify

When chemists decide whether an element is a metal, nonmetal, or metalloid, they typically look at a handful of measurable traits: does it conduct electricity? Does it form positive ions (cations) easily? Does it have a shiny, metallic appearance in solid form? For most elements, these questions have straightforward answers backed by decades of lab work. Astatine breaks that process entirely.

Every isotope of astatine is radioactive, and the longest-lived one, astatine-210, has a half-life of only about 8.1 hours. The most studied isotope, astatine-211, has a half-life of roughly 7.2 hours. At any given moment, the total amount of astatine present in the Earth’s crust is estimated at well under a gram. Scientists first synthesized it in 1940 by bombarding bismuth with alpha particles in a cyclotron at the University of California, and the quantities produced were so tiny that the researchers could only characterize it through its radioactive decay and a handful of chemical reactions performed at the tracer level, meaning individual atoms rather than visible amounts.¹

This is the root of the classification problem. You cannot measure the electrical conductivity or band structure of a substance you have never held in bulk. Everything known about astatine’s physical character in a condensed phase comes from theoretical predictions, not direct measurement. And different theoretical approaches sometimes point in different directions.

The Case for Nonmetal

Astatine sits at the bottom of Group 17, the halogens, directly below fluorine, chlorine, bromine, and iodine. Every other member of that group is unambiguously a nonmetal. From a purely positional standpoint, astatine should follow the same pattern.

Its chemistry in many situations supports this. Astatine forms halogen bonds, the weak attractive interactions that halogens use to stick to electron-rich partners. A 2018 study published in Nature Chemistry provided the first direct experimental and computational evidence that astatine acts as a halogen-bond donor, and a stronger one than iodine, because of its more electrophilic character.² Follow-up work confirmed astatine monoiodide (AtI) as bearing “the most potent halogen-bond donor atom” among the halogens.³ This is textbook halogen behavior, just turned up louder.

Astatine also forms interhalogen compounds. It reacts with iodine, and researchers have synthesized pseudohalogen compounds with astatine behaving as the electronegative partner, fitting the halogen mold. Gas-phase studies have identified elemental astatine, astatine oxide, and hypo-astatic acid as distinct species, each with measurable adsorption enthalpies on quartz and gold surfaces.⁴ The existence of an oxide and an oxyacid is consistent with nonmetal chemistry, where elements bond covalently with oxygen rather than forming ionic metal oxides.

The Case for Metal (or at Least Metalloid)

Here is where astatine starts behaving strangely for a halogen. In aqueous solution, astatine can exist as At⁺ and AtO⁺, positively charged cations that form complexes with inorganic ligands like chloride, bromide, and thiocyanate. Computational studies validated with experimental data show that these metallic forms of astatine produce both 1:1 and 1:2 complexes, and the calculated thermodynamic constants match what is measured in the lab.⁵

Forming stable cations in solution is a hallmark of metals, not halogens. Chlorine and bromine do not float around as Cl⁺ or Br⁺ in water. Iodine can form I⁺ under extreme conditions, but astatine’s cationic forms appear far more readily. This cation chemistry is one of the strongest arguments for pushing astatine toward the metallic side of the periodic table, or at least into the borderland territory that defines metalloids.

Theoretical work using the Goldhammer-Herzfeld criterion offers another angle. This approach compares an element’s atomic polarizability to the volume its atoms occupy in a condensed phase. When the polarizability is large enough relative to the volume, the electron cloud around each atom effectively “overlaps” with its neighbors, and the element should behave as a metal. A review of this framework applied across the periodic table found it to be a surprisingly effective predictor of metallic versus nonmetallic behavior.⁶ While the exact prediction for astatine depends on assumptions about its condensed-phase density (which, again, nobody has measured), several analyses using this criterion suggest astatine would be metallic or at least sit right on the metal-nonmetal boundary.

Relativistic Effects and Why Heavier Elements Get Weird

Much of astatine’s dual personality traces to something called relativistic effects. In heavy atoms, inner-shell electrons orbit the nucleus so fast that their mass increases according to Einstein’s relativity, which causes those inner orbitals to contract. The contraction, in turn, changes the shielding felt by outer electrons and reshapes the energy landscape of the entire atom. For light elements, these effects are negligible. For an element as heavy as astatine (85 protons), they are large enough to meaningfully alter chemical bonding.

One consequence is that astatine’s outermost electrons are easier to remove than you would expect from a simple extrapolation down the halogen group. Its ionization energy is lower than iodine’s, and its electron affinity is weaker. Both trends push astatine away from typical halogen character and toward more metallic behavior. At the same time, the same relativistic effects strengthen the electrophilic “hole” on the astatine atom’s surface, which is why it forms such strong halogen bonds. Relativistic effects do not push astatine uniformly toward metal or nonmetal; they amplify some halogen-like traits while undermining others.

Recent computational work has explored how these effects play out in simple bonded systems involving astatine and tennessine (element 117, the next halogen down, which is even more extreme). The study used halogen-bonded model compounds to track how bond features change as you descend the group, confirming that relativistic influences grow dramatically at these atomic numbers and alter what the elements can do chemically.⁷

What the Periodic Table Labels Actually Mean

Part of the confusion stems from the fact that “metalloid” is not a rigorous category with a universally agreed-upon definition. Different textbooks use different criteria, and the list of elements classified as metalloids varies from source to source. The most commonly cited metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium. Astatine sometimes makes the list and sometimes does not, depending on which properties the author prioritizes.

If you define metalloid as “an element that shows a mix of metallic and nonmetallic properties,” astatine fits easily. Its cation chemistry is metallic; its halogen bonding is nonmetallic. If you define metalloid more strictly as “a semiconductor with intermediate electrical conductivity,” the classification breaks down because nobody has ever measured astatine’s conductivity.

The International Union of Pure and Applied Chemistry (IUPAC) does not officially designate any element as a metalloid in its periodic table recommendations. It recognizes the term informally but has not drawn a definitive line. This means the labels you see on classroom periodic tables reflect the choices of whoever designed that particular chart, not a universal scientific consensus. For astatine, the designer is making an educated guess.

How Chemists Actually Work With Astatine

Despite the classification debate, astatine has real and increasingly important applications, all centered on its radioactivity rather than its position on the metal-nonmetal spectrum. Astatine-211 emits alpha particles as it decays, and those particles are devastatingly effective at destroying cells over very short distances (a few cell diameters). This makes At-211 one of the most promising candidates for targeted alpha therapy, a cancer treatment strategy that delivers radiation directly to tumor cells while sparing surrounding tissue.⁸

Clinical interest has grown rapidly in recent years. Researchers have attached At-211 to antibodies and small proteins that seek out specific markers on cancer cells, particularly in prostate cancer. Preclinical studies using At-211-labeled minibodies targeting a prostate cancer marker called PSCA have shown promising antitumor activity in both standard tumor models and bone microtumors, which are notoriously difficult to treat.⁹ The short half-life of At-211 is actually an advantage here: it delivers its radiation burst quickly and then largely disappears, limiting prolonged exposure to healthy tissue.

Translation to human patients is still in early stages. While preclinical data on tumor suppression and limited toxicity are encouraging, clinical trials remain relatively few, and researchers have stressed that more work is needed before At-211 therapies become standard.¹⁰ For practical purposes, chemists working with astatine care far more about its nuclear properties and solution-phase behavior than about whether it belongs in the metal or nonmetal column.

Why the Debate Persists and Probably Will Not Be Settled Soon

To definitively classify astatine, you would need to do something no laboratory has managed: produce enough of it in one place to measure bulk physical properties. At current production capabilities, the most astatine ever generated at once amounts to micrograms, and it decays so fast that accumulating a visible sample is physically impossible with any known isotope. Unless a significantly longer-lived isotope is discovered or an entirely new way of characterizing condensed-phase behavior from trace quantities is developed, the question of whether astatine is “really” a metalloid or a nonmetal will remain a matter of which theoretical framework you trust more.

This is not as unsatisfying as it sounds. The periodic table’s neat categories work beautifully for most elements but start to fray at the edges, particularly for heavy elements where relativistic effects blur traditional boundaries. Astatine is the most dramatic example of this blurring in the halogen group, but it is not unique. Bismuth, once considered a stable element, turned out to be very slightly radioactive. Polonium, astatine’s neighbor, is technically a metal but behaves oddly compared to other metals in its group. The bottom of the periodic table is a place where familiar rules start to bend.

What Textbooks Will Probably Look Like in a Decade

The trend in recent literature leans toward treating astatine as a halogen with significant metallic character rather than as a straightforward metalloid. The halogen-bonding evidence is strong and growing, which anchors astatine firmly in the halogen family’s chemical behavior.² At the same time, nobody denies its cationic chemistry in solution, which no other halogen replicates so readily.⁵ Some researchers have proposed that astatine might best be described as a “metallic halogen,” a label that captures both sides without forcing it into a box designed for lighter, better-behaved elements.

The real action in astatine research right now is not about settling the classification question. It is about understanding its chemistry well enough to exploit it medically. Every study that pins down how astatine bonds to carrier molecules, how it behaves in biological fluids, and how its compounds stick to surfaces feeds directly into the effort to turn At-211 into a practical cancer therapy. The element’s identity crisis is fascinating to periodic-table enthusiasts, but the researchers spending the most time with astatine have moved on to questions with more immediate stakes.