Krypton is an element. It sits at atomic number 36 on the periodic table, belongs to group 18 (the noble gases), and consists entirely of krypton atoms. It is not a compound, which would require atoms of two or more different elements bonded together, nor is it a mixture, which would involve two or more substances blended without chemical bonding. That said, the story of krypton is far more interesting than the straightforward classification suggests, because this supposedly inert gas can, under the right conditions, be coaxed into forming genuine chemical compounds.
What Makes Krypton an Element
Every sample of pure krypton contains only one type of atom: atoms with 36 protons in their nuclei. That single fact is what defines an element. Whether krypton exists as a gas at room temperature, a liquid at very low temperatures, or a solid frozen even colder, it remains the same element. Its physical state changes, but its identity does not. The atoms do not bond to atoms of other elements under ordinary conditions, so a jar of krypton gas is about as pure an elemental substance as you can get.
Krypton is one of six noble gases, a family that also includes helium, neon, argon, xenon, and radon. Noble gases share a defining trait: they have fully occupied outer electron shells, which makes them extremely reluctant to participate in chemical reactions. For most of the twentieth century, chemists considered this reluctance to be absolute, calling the group “inert gases” and treating their inability to react as a rule of nature. That changed in 1962 when xenon was shown to form stable compounds with fluorine, and krypton followed not long after.
How Krypton Differs from a Compound or a Mixture
The distinction between elements, compounds, and mixtures is one of the most fundamental ideas in chemistry. A compound forms when atoms of different elements bond in a fixed ratio. Water, for instance, always contains two hydrogen atoms for every one oxygen atom. A mixture, on the other hand, is a physical combination of substances where each component retains its own chemical identity and can, in principle, be separated by physical means. The air you breathe is a mixture of nitrogen, oxygen, argon, carbon dioxide, and trace gases including krypton.
Krypton in Earth’s atmosphere is present at roughly 1.1 parts per million by volume, a figure pinned down through careful mass spectrometry using isotope dilution techniques.1International Journal of Mass Spectrometry and Ion Physics. Determination of the abundance of krypton in the earth’s atmosphere by isotope dilution mass spectrometry At that concentration, krypton is a trace component of a mixture (the atmosphere), but it is still an element within that mixture. Separating it out, typically by fractional distillation of liquefied air, gives you pure elemental krypton.
Physical Properties of the Pure Element
In its pure form, krypton is a colorless, odorless, tasteless gas. It becomes a liquid at about −153 °C and freezes into a solid at around −157 °C, depending on pressure. Detailed calorimetric studies have pinned its triple point temperature at approximately 115.8 K (about −157.4 °C), with a heat of fusion of roughly 392 calories per mole and a heat of vaporization at the normal boiling point of about 2,162 calories per mole.2Proceedings of the Physical Society. Thermodynamic Properties of Krypton. Vibrational and Other Properties of Solid Argon and Solid Krypton These are small energy values compared to substances with strong intermolecular bonds, which reflects the weak van der Waals forces holding krypton atoms together in the liquid and solid phases.
One property that gave krypton a moment of fame in measurement science is the precision of its spectral lines. Krypton-86, a specific isotope, produces an extremely narrow and well-defined orange-red spectral line. In 1960, the international definition of the meter was actually redefined in terms of the wavelength of this line, replacing the old platinum-iridium bar that had served as the standard. Preliminary wavelength measurements of krypton-86 lines were carefully compared against cadmium lamp standards using interferometric methods.3Journal of the Optical Society of America. Preliminary Measurements of Some Wavelengths of Krypton 86 and Mercury 198 Lines The krypton-based meter lasted until 1983, when the meter was redefined in terms of the speed of light, but the episode highlights how the clean spectral behavior of a noble gas can have surprisingly practical consequences.
When Krypton Does Form Compounds
The classification of krypton as an element does not mean krypton atoms never appear in compounds. They do, though these compounds are rare, often unstable, and require aggressive chemistry to produce. The most well-known krypton compound is krypton difluoride (KrF₂), first synthesized in the 1960s. KrF₂ is a powerful fluorinating agent and one of the few stable molecules in which krypton is genuinely bonded to another element.
Researchers have characterized the crystal structures of KrF₂ and several of its salt derivatives, including compounds containing the KrF⁺ and Kr₂F₃⁺ cations, using low-temperature X-ray diffraction.4PubMed. X-ray crystal structures of alpha-KrF(2),[KrF]MF(6),[Kr(2)F(3)][SbF(6).KrF(2), [Ke(2)F(3)2[SbF(6)]2.KrF(2), and [Kr(2)F(3)][AsF(6)].[KrF][AsF(6)]; synthesis and characterization of [Kr(2)F(3)][PF(6).nKrF(2); and theoretical studies of KrF(2), KrF+, Kr(2)F(3)+, and the [KrF]MF(6) ion pairs Computational studies have also predicted the thermodynamic properties of higher krypton fluorides like KrF₄ and KrF₆, though these remain theoretical targets rather than laboratory realities.5PubMed. Heats of formation of krypton fluorides and stability predictions for KrF4 and KrF6 from high level electronic structure calculations
More recently, a krypton difluoride coordination compound was synthesized in which KrF₂ molecules coordinate to a bromine center, producing a structure described as [BrOF₂][AsF₆]·2KrF₂. The crystal structure, determined at −173 °C, represents a rare example of KrF₂ coordinated to a main-group atom.6PubMed. A rare example of a krypton difluoride coordination compound: [BrOF2][AsF6] x 2 KrF2 These compounds are fascinating to chemists precisely because they defy the old “inert gas” label, but they require extreme conditions to make and are unstable enough that you would never encounter one outside a specialized laboratory.
The important distinction here: when krypton difluoride forms, it becomes a compound (KrF₂ contains both krypton and fluorine atoms bonded together). The krypton within it is still the element krypton, with its 36 protons, but the substance KrF₂ itself is a compound. Krypton the element can participate in compounds, but krypton gas sitting in a sealed tube is not a compound any more than a block of iron sitting on a shelf is.
Krypton Under Extreme Pressure
If fluorine chemistry seems exotic, the things krypton does under extreme pressures are stranger still. When squeezed in diamond-anvil cells to pressures millions of times atmospheric pressure, krypton forms compounds and structures that would be unthinkable under everyday conditions.
One example is the van der Waals compound Kr(H₂)₄, first synthesized at pressures above about 5.3 gigapascals. In this substance, krypton atoms sit at face-centered cubic lattice sites while hydrogen molecules occupy the spaces around them. The structure has been characterized by single-crystal X-ray diffraction under pressure and studied to above 50 gigapascals.7Scientific Reports. New high-pressure van der Waals compound Kr(H2)4 discovered in the krypton-hydrogen binary system This is not a conventional chemical compound in the way water is. The krypton and hydrogen are held together by weak van der Waals interactions rather than strong covalent or ionic bonds, and the material would fall apart immediately if you released the pressure.
Even more dramatic are the krypton oxides predicted and studied at pressures above 280 gigapascals, conditions found deep inside giant planets. Computational studies identified stable krypton-oxygen phases in which krypton and oxygen form zigzag or linear chains with short, directional Kr-O contacts. At 300 gigapascals, the shortest Kr-O distance in the most stable phase is only about 1.81 ångströms, merely 14% longer than a standard single covalent bond between the two atoms. That closeness indicates genuine chemical bonding between krypton and oxygen, not just atoms being squished together.8Scientific Reports. Krypton oxides under pressure These are conditions so far removed from anything on Earth’s surface that they are primarily interesting for understanding planetary interiors and the fundamental limits of chemical bonding.
Krypton’s Isotopes and Why They Matter
Like most elements, krypton exists as a mixture of isotopes: atoms with the same number of protons but different numbers of neutrons. Naturally occurring krypton has six stable isotopes, with mass numbers 78, 80, 82, 83, 84, and 86. Krypton-84 is the most abundant. Two radioactive isotopes, krypton-81 and krypton-85, are particularly useful for very different reasons.
Krypton-81 has a half-life of about 229,000 years, which makes it an ideal tracer for dating very old groundwater and ice. It is produced in the atmosphere by cosmic ray interactions and distributes uniformly around the globe, giving it an advantage over carbon-14, which depends on the presence of organic material and can only date samples up to about 50,000 years old. Krypton-81 dating extends the range to roughly 30,000 to 1.5 million years, and it has revealed that paleo-groundwater in many of the world’s major aquifers is over a million years old, much older than previously expected.9Nature Reviews Earth & Environment. Trapping atoms of krypton-81 to date groundwater and ice cores
This dating technique has been applied successfully to ancient polar ice. Researchers extracted krypton from air bubbles trapped in roughly 350-kilogram samples of ice from Taylor Glacier in Antarctica and used a method called Atom Trap Trace Analysis to measure the krypton-81 content. The radiometric ages they obtained agreed with independent estimates from other dating techniques, confirming that the ice was about 120,000 years old.10PubMed Central. Radiometric 81Kr dating identifies 120,000-year-old ice at Taylor Glacier, Antarctica The fact that a noble gas dissolved in ancient ice can serve as a clock stretching back hundreds of thousands of years is a remarkable application for an element that was once dismissed as completely unreactive and functionally useless.
Krypton-85, by contrast, has a much shorter half-life of about 10.8 years and is produced almost entirely by human activity. It forms during the fission of uranium and plutonium in nuclear reactors and enters the atmosphere mainly when spent fuel rods are reprocessed, releasing the gas during chopping and dissolution of the rods.11PubMed. Update and improvement of the global krypton-85 emission inventory Because krypton-85 disperses through the atmosphere as a chemically inert tracer, it has two practical uses. First, atmospheric scientists use it to test and validate models of how gases move through the atmosphere. Second, intelligence and nonproliferation analysts monitor krypton-85 levels as a potential indicator of clandestine nuclear reprocessing, since any facility dissolving spent fuel will inevitably release some of this isotope.
Long-term monitoring of krypton-85 concentrations in ground-level air has been carried out at various sites, including measurements in Prague spanning from 1983 to 1993 that tracked time-related changes in atmospheric contamination from the nuclear industry.12PubMed. The measurement of low concentrations of Kr-85 in atmospheric air samples The background level has been rising steadily for decades, a direct consequence of the global expansion of nuclear fuel reprocessing.
Where Krypton Comes From on a Cosmic Scale
Krypton atoms were not present at the beginning of the universe. Hydrogen, helium, and trace amounts of lithium formed in the Big Bang, but everything heavier had to be forged in stars or stellar explosions. Krypton’s isotopes carry signatures of multiple nucleosynthetic processes, and teasing apart those signatures has helped scientists reconstruct the history of the material that eventually became our solar system.
Studies of noble gas compositions in primitive meteorites have identified a component whose krypton isotope ratios point to the slow neutron capture process (the s-process), which takes place in the interiors of certain evolved stars. Analysis of this component, known as Kr-P3, also revealed that a second process, contributing the krypton isotopes not made by the s-process, must have added material to the mix before the solar system’s composition was established.13Geochimica et Cosmochimica Acta. “Planetary” noble gas components and the nucleosynthetic history of solar system material In other words, the krypton in your atmosphere arrived from at least two different stellar sources, mixed together long before the Earth formed. Every breath you take contains trace atoms whose histories diverge billions of years into the past.
Why People Confuse Krypton with a Compound or Mixture
Part of the confusion is cultural. For many people, the word “krypton” first brings to mind the fictional planet Krypton from the Superman franchise, not a chemical element. The name sounds exotic and fictional, which can make it feel like it belongs to a different category than familiar elements like gold or oxygen. In reality, krypton was discovered in 1898 by William Ramsay and Morris Travers, who isolated it from liquefied air. Its name comes from the Greek word kryptos, meaning “hidden,” a nod to how difficult it was to find.
Another source of confusion is the existence of krypton compounds like KrF₂. If krypton can bond with fluorine, does that make krypton itself a compound? No. Iron can bond with oxygen to form rust (iron oxide), but that does not make a lump of pure iron a compound. The element and the compounds it participates in are different substances. Pure krypton gas, the stuff you would buy in a cylinder from a gas supplier, is an element. Krypton difluoride is a compound that happens to contain krypton atoms.
A subtler confusion comes from the fact that naturally occurring krypton is a mixture of isotopes. Six stable isotopes coexist in every natural sample. Does that make krypton a mixture? In the strict chemical sense, no. All krypton isotopes are the same element, with the same number of protons and the same chemical behavior. A mixture, in chemistry, refers to a combination of different substances. Isotopic variation within an element does not qualify. A tank of natural krypton gas is a pure substance, not a mixture, even though it contains atoms with slightly different masses.
Practical Uses Beyond the Laboratory
Despite being scarce and relatively expensive, krypton has a handful of real-world applications that take advantage of its physical properties. The most common is in lighting. Krypton gas is used to fill certain types of incandescent and fluorescent bulbs, where its low thermal conductivity allows the filament to operate at a higher temperature, producing a brighter and more efficient light. Krypton-filled bulbs are more expensive than those filled with the far cheaper argon, but they last longer and waste less energy as heat.
Krypton also finds use in some window insulation. Double- and triple-pane windows sometimes have their gaps filled with krypton gas instead of air or argon, because krypton’s lower thermal conductivity provides better insulation. The trade-off is cost: krypton is roughly a hundred times more expensive than argon, so its use tends to be reserved for high-performance windows where space is at a premium and thinner gaps are needed.
In medicine, krypton isotopes have been used in lung ventilation imaging, where inhaled krypton gas helps doctors visualize airflow patterns in the lungs. And in lasers, the krypton fluoride excimer laser (which uses a short-lived KrF molecule) operates in the ultraviolet range and is used in applications from semiconductor lithography to corrective eye surgery. Here again, krypton temporarily forms part of a compound during the laser’s operation, but the krypton gas fed into the system is an element.