Krypton is a nonmetal. More specifically, it belongs to the noble gases, the elements in Group 18 of the periodic table that sit at the far right column and share a reputation for chemical aloofness. At room temperature and normal pressure, krypton is a colorless, odorless gas with no metallic properties whatsoever: it does not conduct electricity, it has no luster, and it does not form the kinds of crystal lattices that metals do. The question of whether krypton might be a metalloid is equally straightforward, because metalloids occupy a diagonal band between metals and nonmetals on the periodic table, and krypton is nowhere near that band. But the full story has some surprising wrinkles, especially when extreme conditions enter the picture.
What Makes Krypton a Nonmetal
The classification of an element as a metal, nonmetal, or metalloid rests on a cluster of physical and chemical properties. Metals conduct heat and electricity well, tend to be shiny solids at room temperature, and lose electrons readily to form positive ions. Nonmetals generally do none of those things. They are poor conductors, often exist as gases or brittle solids, and tend to gain or share electrons rather than give them up.
Krypton checks every nonmetal box. It is a gas at standard temperature and pressure, with a boiling point of roughly −153 °C. It does not conduct electricity under ordinary conditions. Its atoms have a complete outer electron shell, which is the defining feature of noble gases and the reason they resist forming chemical bonds. This filled shell gives krypton an extremely high ionization energy, meaning it takes a lot of effort to strip an electron away, and virtually no electron affinity, meaning it has no interest in picking up extra electrons either. Both traits place it firmly in nonmetal territory.
Why Krypton Is Not a Metalloid
Metalloids are sometimes called “semimetals” because they straddle the line between metals and nonmetals. Elements like silicon, germanium, and arsenic fall into this category: they can conduct electricity under some conditions but not others, making them useful as semiconductors. They sit along a staircase-shaped boundary on the periodic table that runs roughly from boron down to astatine.
Krypton is several columns to the right of that boundary. No mainstream chemistry classification has ever placed krypton among the metalloids. It lacks the intermediate electrical conductivity, the semi-lustrous appearance, and the amphoteric oxide chemistry that characterize that group. The confusion, when it arises, seems to come from a general uncertainty about where noble gases fit, since they are so chemically inert that they can seem like they belong in their own category. They do belong in their own category, but that category is a subset of nonmetals, not something between metals and nonmetals.
Krypton’s Surprising Chemistry
For decades after their discovery, the noble gases were called “inert gases” because scientists believed they could not form chemical compounds at all. That changed in 1962 when xenon was shown to react with fluorine, and researchers soon turned their attention to krypton. Krypton difluoride (KrF₂) was synthesized not long after, proving that krypton could, under the right conditions, be coaxed into bonding.
The chemistry is severely limited, though. Computational studies predict that KrF₂, along with hypothetical higher fluorides like KrF₄ and KrF₆, are all thermodynamically unstable, meaning they tend to fall apart and release fluorine gas.1PubMed. Heats of formation of krypton fluorides and stability predictions for KrF4 and KrF6 from high level electronic structure calculations KrF₂ can be made and stored at low temperatures, but it decomposes readily at room temperature. This fragility underscores how reluctant krypton is to participate in chemical bonding at all.
Researchers have managed to push krypton chemistry a bit further by building coordination compounds, structures where KrF₂ molecules attach to a central metal-containing unit. One such compound pairs two KrF₂ molecules with a bromine-oxygen-fluorine cation and an arsenic-fluorine anion. Analysis of its electronic structure shows that a small but measurable amount of electron charge, about 0.10 electrons per KrF₂ molecule, transfers away from the krypton-containing ligands.2PubMed. A rare example of a krypton difluoride coordination compound: [BrOF2][AsF6] x 2 KrF2 That is a tiny amount, and the bonding is dominated by electrostatic attraction rather than the robust electron sharing you see in typical covalent bonds. Still, the very existence of such a compound illustrates that “nonmetal” does not mean “completely inert.”
None of this chemistry nudges krypton toward metallic or metalloid behavior. Forming a fragile fluoride compound is a far cry from conducting electricity or behaving like a semiconductor. Krypton’s limited reactivity is more a curiosity of noble-gas chemistry than evidence of any metallic character.
Can Extreme Pressure Make Krypton Metallic?
Here is where the story gets genuinely interesting. Under everyday conditions, krypton is about as far from a metal as an element can be. But physics has a way of rewriting the rules when you squeeze matter hard enough. At sufficiently extreme pressures, even noble gases can be forced into metallic states, where their electrons delocalize and the material begins to conduct electricity.
The theoretical framework for understanding this goes back to 1927, when physicist Karl Herzfeld proposed a simple criterion: an element becomes metallic when its atoms are packed closely enough that their electron clouds overlap and merge into a shared “sea” of electrons. The key variable is the element’s polarizability, a measure of how easily its electron cloud can be distorted. Elements with high polarizability, like rubidium and cesium, are naturally metallic at ambient conditions because their electron clouds are large and easily deformed. Elements with low polarizability, like hydrogen, need extreme compression before their electrons start to overlap.3PubMed Central. ‘… a metal conducts and a non-metal doesn’t’
Krypton’s polarizability is modest, placing it in a middle ground where metallization is theoretically possible but requires enormous pressures. Theoretical calculations have examined the conditions under which krypton’s band gap would close and it would transition to a metallic state. Studies of the krypton-metallization problem indicate that electronic f-bands do not play a significant role in that transition, unlike what some earlier models suggested.4ScienceDirect. B.c.c.-f.c.c. allotropy, F-bands, and metallization in xenon and krypton More recent experiments have probed the reflectivity of shock-compressed liquid krypton at around 63 gigapascals, comparing measured reflection coefficients to various theoretical models of a dense krypton plasma.5JETP Letters. Reflectivity of a Dense Krypton Plasma at a Pressure of P = 63 GPa When a material starts to reflect light like a mirror, that is a hallmark of metallic behavior, because free electrons at the surface are bouncing photons back.
To put 63 gigapascals in perspective, that is roughly 630,000 times atmospheric pressure, comparable to conditions deep inside giant planets. No one would call krypton a metal based on behavior that only emerges under such crushing forces. But the research confirms something philosophically interesting: the metal-nonmetal boundary is not an absolute property of an element’s identity. It depends on context. Under the right conditions, even a noble gas can be pushed across that boundary.
Krypton in Host-Guest Structures
Even without forming traditional chemical bonds, krypton atoms can be physically trapped inside molecular cages. The best-known examples are clathrate hydrates, ice-like structures where water molecules arrange themselves into a lattice of cages, and gas atoms or small molecules sit inside those cages as “guests.” Krypton forms such hydrates, and their crystal structures have been studied by X-ray diffraction. The host cages adjust their size depending on which noble gas they are encapsulating, even when the overall unit-cell dimensions stay the same.6PubMed. Structure and Density Comparison of Noble Gas Hydrates Encapsulating Xenon, Krypton and Argon
Under high pressure, krypton can also form van der Waals compounds with hydrogen. In these structures, krypton and hydrogen molecules pack together in an ordered crystal, held in place not by covalent bonds but by the same weak intermolecular forces responsible for things like the boiling points of liquids. A compound with the formula Kr(H₂)₄ was discovered in the krypton-hydrogen binary system under high pressure, and its stability involves a subtle interplay of van der Waals forces and possibly weak charge-transfer interactions.7Scientific Reports. New high-pressure van der Waals compound Kr(H2)4 discovered in the krypton-hydrogen binary system
These structures are not evidence of metallic or metalloid character. They are more akin to molecular imprisonment than chemical combination: the krypton atoms are physically enclosed rather than chemically bonded in the traditional sense. But they matter to researchers studying planetary interiors and the behavior of matter under extreme conditions, environments where noble gases do not have the luxury of floating around as isolated atoms.
Practical Uses That Depend on Krypton Being a Nonmetal
Many of krypton’s real-world applications exist precisely because it is a chemically inert nonmetal. In lighting, krypton fills some incandescent and fluorescent bulbs. Its presence slows the evaporation of the tungsten filament, extending the bulb’s life. It can do this job because it does not react with the hot filament, something a metal vapor would be poorly suited for.
In high-performance windows, krypton gas is sandwiched between panes of glass as an insulating layer. It works well for this because its thermal conductivity is lower than that of air, and because it is chemically inert and will not degrade the window seals over time. Argon is used for the same purpose in less expensive windows; krypton provides better insulation but costs more because it is far rarer in Earth’s atmosphere.
Krypton fluoride also finds use in excimer lasers. A KrF excimer laser produces ultraviolet pulses at 248 nm and is used in applications ranging from semiconductor manufacturing to biochemistry research, where those nanosecond UV pulses can generate hydroxyl radicals for studying protein structure.8ScienceDirect. Mass Spectrometry-based Methods to Study Macromolecular Higher Order Structure and Interactions – Section: 3.1.1 Laser The “excimer” in the name refers to an excited dimer, a short-lived molecule that only exists in an energetically excited state. KrF is unstable in its ground state, which is entirely consistent with krypton’s reluctance to bond.
Krypton as a Scientific Measuring Stick
One of krypton’s more unexpected roles has been in metrology, the science of measurement. In 1960, the international definition of the meter was redefined in terms of a specific wavelength of light emitted by the isotope krypton-86. The orange-red spectral line produced by a particular electronic transition in Kr-86 was adopted as the primary standard of length, replacing the old platinum-iridium meter bar that had been the reference since 1889.9Applied Optics. The International Length Standard This krypton-based definition served for over two decades before being superseded in 1983 by the current definition tied to the speed of light. The reason krypton worked so well for this purpose was that a single isotope of a noble gas, with its simple and well-characterized atomic structure, produces extremely sharp and reproducible spectral lines.
Krypton isotopes also serve as natural clocks in Earth science. Krypton-81 is a radioactive isotope with a half-life of about 229,000 years, making it useful for dating groundwater and geological formations on timescales from tens of thousands to over a million years. Researchers have used Kr-81 dating to determine when deep groundwater systems were recharged by surface water, providing insights into how ancient climate shifts drove water circulation through rock formations.10Geophysical Research Letters. Krypton‐81 Dating Constrains Timing of Deep Groundwater Flow Activation One study found that formation waters above certain evaporite deposits had been recharged since the Late Pleistocene, while waters below those deposits, at depths of up to three kilometers, contained relatively young meteoric water components dating from roughly 0.4 to 1.1 million years ago. Krypton is particularly well suited to this kind of work because, as a noble gas, it does not react with the surrounding rock or water. It just sits there, decaying at a known rate, which is exactly what you want from a geological clock.
Krypton Beyond Earth
Krypton is produced by stellar nucleosynthesis, primarily through a process in which neutrons are captured slowly by atomic nuclei inside aging stars. This means krypton is scattered throughout the galaxy, albeit in very small amounts. Astronomers have measured the ratio of krypton to hydrogen in the interstellar medium using ultraviolet absorption lines observed by the Hubble Space Telescope. Across 50 lines of sight extending up to about 5,900 parsecs from the Sun, the krypton-to-hydrogen ratio is remarkably consistent, though a ring-shaped region between roughly 600 and 2,500 parsecs away shows some variation, hinting at differences in the local nucleosynthetic history of the galaxy.11The Astrophysical Journal. Interstellar Krypton Abundances: The Detection of Kiloparsec-scale Differences in Galactic Nucleosynthetic History
Krypton has also been detected in the atmospheres of certain white dwarfs, the dense remnants of dead stars. The first detection of krypton in a white dwarf came from ultraviolet spectroscopy of a hot DO-type white dwarf, where about 20 spectral lines from highly ionized krypton were identified.12The Astrophysical Journal Letters. FIRST DETECTION OF KRYPTON AND XENON IN A WHITE DWARF In those environments, krypton atoms have been stripped of five or six electrons by extreme heat, giving them high positive charges. That ionization state might sound metal-like, since losing electrons is what metals do, but it is driven entirely by temperature, not by any intrinsic metallic tendency. Every element can be ionized if you heat it enough. The krypton in a white dwarf atmosphere is a plasma, not a metal.
These astrophysical detections are valuable because they help scientists trace the chemical evolution of the galaxy. Since krypton is produced by specific nucleosynthetic pathways, its abundance in different environments tells a story about which generations of stars contributed material to a given region of space. The very inertness that makes krypton unreactive on Earth makes it a clean tracer in space: it does not get locked up in minerals or molecules the way more reactive elements do, so its gas-phase abundance is a relatively undistorted record of how much was produced by stellar processes.
How Krypton Compares to Its Noble Gas Neighbors
Krypton sits between argon and xenon in Group 18. All three are nonmetals, but they differ in how far their chemistry can be pushed. Argon, which is lighter and has a smaller, less polarizable electron cloud, has essentially no confirmed stable neutral compounds under ordinary conditions. Krypton, as discussed, forms a handful of fragile fluorides. Xenon, heavier and more polarizable, has a substantially richer chemistry: xenon difluoride is stable enough to be stored in a bottle at room temperature, and xenon forms compounds with oxygen and even with carbon.
This trend reflects the increasing size and looseness of the outer electron cloud as you move down the group. A larger electron cloud is easier to distort and easier to pull into a bonding arrangement with an aggressive partner like fluorine. The same trend shows up in metallization pressures: xenon is predicted to become metallic at lower pressures than krypton, and krypton at lower pressures than argon. Radon, the next noble gas down, is radioactive and rare enough that its chemistry is difficult to study, but it is expected to be even more reactive than xenon.
None of this progression moves any of these elements into metalloid or metal territory at ambient conditions. It simply means that the noble gas label covers a spectrum of reluctance: helium and neon at the top are essentially untouchable, while xenon and radon at the bottom are willing to participate in chemistry if you offer them the right partner. Krypton falls on the reluctant end of that spectrum, closer in behavior to argon than to xenon, but with just enough flexibility to form a few exotic compounds when researchers work hard enough to make them.