Noble gases do have electronegativity, and chemists have spent decades trying to figure out exactly how much. The common shorthand that noble gases “don’t react” and therefore have no meaningful electronegativity is outdated. Assigning reliable values has proven surprisingly difficult, though, because the standard methods for measuring electronegativity run into unusual problems when applied to elements with completely filled outer electron shells.
Why Assigning Values Has Been So Difficult
Electronegativity is a measure of how strongly an atom pulls on shared electrons when bonded to another atom. For most elements, you can figure this out through straightforward experiments and calculations. For the noble gases, the process is considerably more awkward. One widely used approach averages an element’s ionization energy (how hard it is to pull an electron away) with its electron affinity (how much energy is released when it gains an electron). The ionization energy side is easy: helium, neon, argon, krypton, xenon, and radon all have well-measured ionization energies. The electron affinity side is the problem. Noble gas atoms do not want to gain an extra electron. Their outer shells are already full, so any added electron would have to occupy a higher, less stable energy level. The resulting negative ion is unstable, and calculating the energy involved in forming it requires very careful computational methods.1PubMed. The noble gases: how their electronegativity and hardness determines their chemistry
For decades, many textbooks either left the noble gas electronegativity cells blank or filled them in by extrapolating from neighboring elements on the periodic table. Those extrapolations tended to place the noble gases at the very top of their respective rows, above even the halogens (fluorine, chlorine, and so on). This always seemed a bit strange: if noble gases were truly the most electronegative elements in each period, you would expect them to form bonds more readily than they do. The extrapolations were a mathematical artifact of assuming a smooth trend across each row without actual noble gas bonding data to anchor the curve.
Where Noble Gases Actually Land on the Scale
A computational study using the Mulliken definition of electronegativity tackled the problem head-on by carefully calculating the electron affinities that experiments alone cannot cleanly deliver. The results reshuffled the conventional picture in three significant ways. First, noble gas electronegativities decrease steadily as you move down the group: helium is the highest, then neon, argon, krypton, xenon, and radon. Second, the halogens sit above the noble gases in electronegativity within each period, which matches chemical intuition far better than the old extrapolated scales that put noble gases on top. Third, the noble gas values land close to those of the chalcogens, elements like oxygen, sulfur, and selenium.2ACS Publications. The Noble Gases: How Their Electronegativity and Hardness Determines Their Chemistry
That last point may be the most surprising. Oxygen and sulfur are famously reactive. Saying noble gases have comparable electronegativities does not mean they are equally reactive, though. Reactivity depends on more than electronegativity alone. The noble gases have very high hardness values, meaning their electron clouds resist being distorted or shared. Even though a xenon atom can pull on electrons about as strongly as a sulfur atom in principle, it takes much more energy to get xenon into a bonding situation in the first place. Electronegativity tells you about the tug-of-war over electrons once a bond exists; hardness tells you how reluctant the atom is to enter that tug-of-war at all.
Noble Gas Compounds and What They Reveal
If noble gases truly had zero electronegativity or simply could not participate in bonding, you would expect zero compounds. In reality, xenon chemistry has been known since the early 1960s, and the list of noble gas compounds has grown considerably since then. Xenon difluoride (XeF₂) and xenon tetrafluoride (XeF₄) are well-characterized solids that can be synthesized in a laboratory. Krypton difluoride exists too, though it is much less stable. Even argon has been coaxed into bonding. Researchers demonstrated that HArF, a compound of hydrogen, argon, and fluorine, is genuinely stable. Computational work confirmed that this molecule holds together thanks to significant ionic and covalent contributions to its bonding, with properties resembling those of the analogous xenon and krypton compounds.3PubMed. A stable argon compound
The existence of these compounds is what makes noble gas electronegativity more than an academic exercise. When xenon bonds with fluorine, the two atoms share electrons unevenly. Fluorine, the most electronegative element on the periodic table, pulls electron density toward itself, and xenon’s electronegativity determines how much it pulls back. Without an electronegativity value for xenon, you cannot predict bond polarity, molecular geometry, or whether a proposed noble gas compound will actually be stable. The electronegativity values that place noble gases near the chalcogens help explain why xenon bonds most readily with the most electronegative partners like fluorine and oxygen: the electronegativity difference is large enough to create a strong polar bond, which compensates for xenon’s reluctance to share electrons in the first place.
Aerogen Bonding
Beyond traditional covalent and ionic bonds, noble gases participate in a more recently recognized type of interaction called aerogen bonding. When a noble gas atom is covalently bonded to very electronegative partners (like fluorine), the electron density around the noble gas gets pulled away unevenly. This creates a small region of positive electrostatic potential on the opposite side of the noble gas atom, a so-called sigma-hole. That positive spot can then attract a nearby negative site, such as a lone pair on another molecule or an anion.4PubMed. Aerogen Bonding Interaction: A New Supramolecular Force?
The energy involved in aerogen bonds is comparable to that of hydrogen bonds, which makes them significant for understanding how noble gas compounds pack together in crystals. They are somewhat less directional than hydrogen bonds, meaning the geometry is a bit more flexible. A study of xenon fluoride derivatives confirmed the importance of these non-covalent noble gas bonds in the solid state, examining compounds where xenon’s oxidation state ranged from +2 to +6.5PubMed Central. Covalent and Non-covalent Noble Gas Bonding Interactions in XeF Derivatives
Aerogen bonding is essentially the noble gas version of a phenomenon already well known for halogens and chalcogens. The fact that it exists at all reinforces the computational finding that noble gas electronegativities sit in the same neighborhood as those of chalcogens. When you bond fluorine to sulfur or to xenon, similar sigma-hole physics emerge on the less electronegative partner, and the resulting non-covalent interactions follow similar rules.
Why Helium and Neon Are Different From the Rest
Not all noble gases are equally cooperative. Xenon forms dozens of known compounds. Krypton forms a handful. Argon has a few isolated examples. But helium and neon have essentially zero stable compounds under ordinary conditions. The reason is not that their electronegativity values are too low; it is actually the opposite. Helium and neon have the highest ionization energies and the smallest, most tightly held electron clouds in the group. They are so hard that no bonding partner can provide enough energy to compensate for the cost of disturbing their electron arrangement.
Under extreme pressures, however, the rules shift. Computational and experimental work has shown that helium can be inserted into ionic crystal lattices at very high pressures, around 300 gigapascals (roughly three million times atmospheric pressure). In compounds like MgF₂He and MgOHe, helium atoms sit inside the crystal structure, but topological analysis of the electron density reveals that no covalent bonds form between helium and any of its neighbors. The helium atoms remain essentially neutral, with calculated charges of only 0.04 to 0.07. The stabilization comes from changes in the way the surrounding ions interact electrostatically, not from helium participating in bonding in any traditional sense.6PubMed Central. Reactivity of He with ionic compounds under high pressure
This is a useful illustration of the limits of electronegativity as a concept. Helium has a well-defined electronegativity value, but that value is essentially irrelevant to its chemistry under any conditions humans normally encounter. Even at pressures that crush atoms into unfamiliar arrangements, helium holds onto its electrons so tightly that its neighbors cannot coax it into sharing.
Oganesson Breaks the Pattern
At the very bottom of the noble gas group sits oganesson, element 118. It was first synthesized in 2002, and only a few atoms have ever been produced, so direct chemical experiments are currently impossible. Everything we know about oganesson’s properties comes from relativistic quantum-mechanical calculations, and those calculations paint a picture that looks nothing like the rest of the group.
Relativistic effects become enormous for superheavy elements. As the nuclear charge grows, inner-shell electrons move at speeds approaching a significant fraction of the speed of light, and this alters the energies and shapes of all electron orbitals. For oganesson, the consequences are dramatic. Calculations of the band structure of noble gas solids from neon down to oganesson show that the band gap, the energy difference between occupied and unoccupied electronic states, follows a smooth periodic trend from neon through xenon and even radon. Radon’s predicted band gap is about 7.1 electron volts. Oganesson’s, however, comes in at only about 1.5 electron volts, a value small enough to classify solid oganesson as a semiconductor rather than an insulator.7PubMed Central. Oganesson Is a Semiconductor: On the Relativistic Band‐Gap Narrowing in the Heaviest Noble‐Gas Solids
A semiconducting noble gas is a remarkable contradiction in terms. Every other noble gas solid is a wide-gap insulator, which is exactly what you would expect from atoms that hold their electrons tightly and barely interact with their neighbors. Oganesson’s relativistic electron cloud is so diffuse and distorted that it probably behaves more like a metalloid than a noble gas. Its electronegativity, if it could ever be directly measured, would likely be lower than any other member of the group, consistent with the general downward trend. But the practical meaning of that number would be radically different from what it means for, say, xenon, because oganesson’s electron cloud is qualitatively unlike anything else in Group 18.
Noble Gases in Interstellar Chemistry
One of the more unexpected places where noble gas bonding matters is in space. Interstellar and circumstellar environments contain noble gas atoms alongside dust grains, ice particles, and large carbon-based molecules called polycyclic aromatic hydrocarbons (PAHs). Computational studies have shown that noble gas atoms can be adsorbed onto the surfaces of PAHs through van der Waals interactions with the molecules’ electron clouds. The interaction gets stronger as the PAH gets larger, meaning bigger dust-grain molecules are better at trapping noble gas atoms. An incoming reactive partner could then bond to the noble gas atom while detaching it from the PAH surface.8PubMed Central. Formation of Potential Interstellar Noble Gas Molecules in Gas and Adsorbed Phases
This matters for planetary science because noble gases serve as cosmic thermometers. Since they are chemically inert under most conditions and have distinct condensation temperatures, measuring how much of each noble gas got incorporated into cometary ices tells researchers about the temperatures those ices formed at and the thermal history they experienced afterward.9The Planetary Science Journal. The EXCITING Experiment Exploring the Behavior of Nitrogen and Noble Gases in Interstellar Ice Analogs The fact that noble gas atoms can weakly bond to PAH surfaces and potentially form molecules in space complicates that picture. If some fraction of noble gases in a comet are locked up in molecular forms rather than simply trapped as free atoms, the inferences about temperature history might need adjusting.
The chemistry here is subtle and still being worked out. These interstellar noble gas molecules, if they exist in significant quantities, would involve bonding driven by the same electronegativity values that govern noble gas chemistry in Earth-based laboratories. The electron-pulling power of a partner like fluorine or a charged ion determines whether a noble gas atom can be coaxed out of its shell, and that is just as true on a dust grain in the Orion Nebula as it is in a reaction vessel on a lab bench.
Why Textbooks Still Get It Wrong
Walk into an introductory chemistry class and you will likely be told one of two things: that noble gases have no electronegativity, or that they have the highest electronegativity in each period. Both are wrong, though in different ways. The “no electronegativity” version confuses the practical difficulty of measuring the value with the value not existing. An element that can form bonds, even reluctantly, has an electronegativity. The “highest in the period” version comes from naive extrapolation of the Pauling scale, which was built on thermochemical data from compounds that noble gases mostly do not form. When you try to extend a curve that was fit to data from lithium through fluorine all the way to neon without any neon bonding data, the math happily generates a number, but that number is not anchored to anything real.
The corrected picture, where noble gases sit near the chalcogens rather than above the halogens, is more consistent with the chemistry we actually observe.2ACS Publications. The Noble Gases: How Their Electronegativity and Hardness Determines Their Chemistry Xenon bonds with fluorine and oxygen, not the other way around, which means xenon must be less electronegative than those partners. If xenon were truly the most electronegative element in its period, it would pull electron density away from fluorine, which it clearly does not. The fact that many periodic tables still show no values for noble gas electronegativity mostly reflects how slowly textbook publishing catches up with computational chemistry, not any genuine scientific uncertainty about whether the values exist.
For a student or curious reader, the takeaway is that electronegativity is a spectrum, not a binary. Noble gases sit on that spectrum at moderate-to-high values, held in check by extreme hardness. They are not the electron-hungry elements their high ionization energies might suggest, but they are not zeros either. Their chemistry is rare and conditional, existing mostly at the heavier end of the group where electron clouds are large enough and soft enough to participate. That conditional chemistry is precisely what makes their electronegativity values worth calculating: those numbers predict which noble gas compounds should be stable, which bonding partners are needed to coax a reaction, and how the resulting molecules will behave.