A surprising moment in mid-20th-century chemistry changed how scientists described the noble gases: once thought entirely inert, these elements were shown to form real chemical species under extreme conditions. That shift set chemists hunting for the limits of bonding, and krypton—atomic number 36—turned out to be one of the more intriguing cases.
Krypton is far less reactive than most elements, but under the right recipes it participates in a range of interactions from true covalent compounds to excited-state excimers and simple physical trapping. Those behaviors matter: they power ultraviolet light sources used in microfabrication and sterilization, they inspire novel materials like endohedral fullerenes, and they teach us about bonding at the margins.
If you search for elements krypton reacts with, you’ll find a short, varied list—fluorine tops it, but oxygen, hydrogen ions, halogens in excited states, carbon cages, water lattices, and metal surfaces all show interesting interactions. Below I’ll walk through those groups and explain when the interactions are true chemical bonds and when they’re fleeting or physical inclusions.
Covalent and Ionic Partners
Krypton forms bona fide chemical species only under carefully controlled conditions. The examples below represent the strongest, best-documented bonding involving krypton—either covalent-like or ionic—and most require specialized apparatus, reactive fluorine, extreme pressures, or ion-beam methods.
1. Fluorine (F)
Fluorine is the element with which krypton forms the most firmly established compound: krypton difluoride, KrF2. Chemists synthesize KrF2 in the lab by exposing krypton to elemental fluorine under electrical discharge or photochemical activation; the result has been isolated and characterized spectroscopically and crystallographically.
KrF2 is written as KrF2 and behaves as a true chemical compound rather than a mere physical complex. Fluorine’s atomic number is 9, and its extreme electronegativity helps stabilize bonding to the otherwise unreactive krypton atom.
Related excited-state chemistry—KrF* excimer emission—is central to KrF excimer lasers, which emit at about 248 nm and have been widely used in semiconductor photolithography. Those applications exploit transient, high-energy states rather than bulk Kr–F materials, but they show how krypton–fluorine chemistry scales from lab curiosities to industrial tools.
2. Oxygen (O)
Oxygen-related krypton species have been detected or predicted, but they’re far less robust than KrF2. Experimental evidence comes mostly from matrix-isolation spectroscopy, where trace Kr–O signals appear when krypton is trapped with oxygen at cryogenic temperatures.
Oxygen has atomic number 8, and theoretical studies suggest that krypton oxides or krypton–oxygen bonding could be stabilized at multi-gigapascal (GPa) pressures. Those high-pressure predictions expand our models of bonding, but they remain primarily academic rather than practical.
In short, Kr–O interactions are typically transient or pressure-stabilized; they teach us about how extreme environments force new types of bonding more than they give us everyday chemicals.
3. Hydrogen (H)
Hydrogen’s atomic number is 1, and its chemistry with krypton appears mainly through ions and fleeting species rather than a stable neutral hydride. Mass-spectrometry studies detect ions like KrH+ formed in gas-phase ion chemistry experiments.
Infrared spectroscopy in rare-gas matrices and careful mass-spec experiments assign lines to protonated krypton species, which are useful probes of fundamental ion–molecule reactions. These aren’t bulk hydrides comparable to alkali metal hydrides; they’re short-lived, ionic, and require specialized set-ups.
Excited-State Reactions and Excimers
Excimers are complexes that exist only in excited electronic states; they have no bound ground state. Krypton forms excimers with halogens and even with other noble gases, and those fleeting species are technologically important for UV light sources and lasers.
Because excimers live only while an excited state persists, they don’t represent stable covalent chemistry, but they are genuine chemical phenomena with measurable spectra and practical uses.
4. Chlorine (Cl)
Chlorine (atomic number 17) is a frequent excimer partner for krypton, forming KrCl* in discharge plasmas. The KrCl excimer emits near 222 nm, a wavelength used in germicidal lamps and some specialized UV photochemistry.
Commercial KrCl excimer lamps are manufactured for disinfection and surface-treatment applications; they operate by creating excited-state Kr–Cl complexes and collecting the emitted UV light rather than isolating a stable Kr–Cl compound.
5. Bromine (Br)
Bromine (atomic number 35) forms analogous excited complexes with krypton. Spectroscopic studies reveal KrBr* emission bands in laboratory discharges and plasma experiments, providing insight into energy-transfer and collisional dynamics.
These Kr–Br excimers are mostly of academic or niche technological interest, but they’re valuable for researchers studying plasma photochemistry, lamp physics, and short-wavelength UV sources.
6. Noble-gas partners (Argon, Xenon)
Krypton interacts weakly with other noble gases like argon (Ar, atomic number 18) and xenon (Xe, atomic number 54). At low temperatures or in matrix-isolation experiments, van der Waals dimers such as Kr–Ar and Kr–Xe are observable.
Mixed-gas discharge lamps and controlled plasma mixtures also exhibit mixed excimer behaviors useful in spectroscopy and lamp research. These are physical interactions—weak and reversible—but they’re measurable and instructive about interatomic forces.
Trapping, Adsorption, and Weak Interactions
Not every interaction needs to be a new covalent bond. Krypton frequently participates in physical trapping (endohedral fullerenes), clathrate hydrates formed by water, and surface adsorption on metals. These interactions matter for materials science, gas separation, and surface physics.
7. Carbon (C)
Carbon, in the particular form of fullerenes, can physically trap krypton atoms to form endohedral complexes such as Kr@C60. Carbon’s atomic number is 6, and the encapsulated krypton remains essentially atom-like inside the carbon cage.
Kr@C60 has been isolated and characterized by mass spectrometry, NMR, and X-ray methods. Encapsulation is a physical inclusion rather than a new covalent bond to carbon, but these materials are studied for unique electronic and magnetic properties and for fundamental tests of atomic confinement.
8. Water (H/O) — clathrate hydrates (elemental H and O)
Krypton forms clathrate hydrates when trapped inside the cage-like lattice of water molecules, so the relevant partners are hydrogen and oxygen in the water framework. These hydrates require low temperature and elevated pressures—often near 0 °C and tens of atmospheres—to form in the lab.
Researchers characterize krypton hydrates by X-ray and neutron diffraction and by thermal-desorption studies. Clathrates are useful models for guest–host interactions and are relevant to gas storage and natural trapping of volatiles in cold environments.
9. Metal surfaces (e.g., Gold, Au)
Krypton physisorbs on metal surfaces such as gold (Au, atomic number 79) and platinum, interacting via weak van der Waals forces and tiny charge-transfer effects. Surface-science experiments—temperature-programmed desorption (TPD), adsorption isotherms, and electron spectroscopy—map how krypton layers form and desorb.
Experimental setups often use krypton as a chemically inert probe gas to test surface cleanliness or to calibrate instruments. These are reversible physical interactions, not bulk krypton–metal compounds under normal conditions.
Summary
- Krypton’s chemistry spans true compounds (chiefly KrF2 with fluorine), excited-state excimers that emit UV, and physical trapping inside cages or on surfaces.
- Fluorine is the primary element that forms an established krypton compound; KrF2 and related excimer chemistry underpin technologies like 248 nm photolithography.
- Many interactions—Kr–O species, KrH+ ions, Kr–halogen excimers, Kr@C60, and clathrate hydrates—involve transient states, high pressures, low temperatures, or specialized instrumentation.
- Studying elements krypton reacts with highlights how extreme conditions and modern techniques expand the boundaries of chemical bonding and yield useful tools for optics, materials, and surface science.
